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  • Dark Energy Push / Expansion

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  • Encompassing Gravity Pull

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    2 66.67%
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Thread: Dark Energy Push/Expanssion or Encompassing Gravity Pull?

  1. #1 Dark Energy Push/Expanssion or Encompassing Gravity Pull? 
    Forum Freshman IamIan's Avatar
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    Newbie to forum ... curious about what others thoughts about an issue I've pondered off and on for a little while.

    - - - - - - - -

    What evidence ... or how would an experiment show the difference between these two?

    Pushed apart or expansion from the inside ... or Pulled apart from the outside?

    - - - - - - -

    Dark Energy:
    Described as a pushing force... or 'expansion of space itself' on the inside.

    Gravitational Pull from the outside:
    This seems like it would also manifest observationally in a similar fashion ... even though the objects are being pulled from the outside.

    - - - - - - -

    Dark Energy:
    the traditional view I've read of Dark Energy ... is as a counter push force of 'empty space' ... or an expansion of empty space itself ... not really empty empty ... but you get the idea ... it accelerates the expansion of the observable objects in the universe and such.

    - - - - - - -

    Pull of Infinite Universe Gravity:

    If the gravitational pull of objects on the outside is larger than the gravitational pull of objects on the inside ... than it gets pulled apart.

    Sense the observed gravitational force at a point decreases with distance ... there would exist a ratio of how far from that point a given larger encompassing mass would have to be ... in order for it to be the net larger pull apart type of force.

    AFAIK there is not yet any known 'minimum' for gravity ... the smallest of masses experiences a weakening gravitational force over increasing distance ... and it it sometimes said to be soo small to be negligible , etc ... but AFAIK there is no minimum ... there is no point where the tiny mass is so far from another tiny mass that the gravity between them completely goes to zero... not just very tiny ... but actually completely zero.

    If it doesn't go to actual 100% zero fast enough ... than just by the scale of an very large universe of other mass encompassing a point ... that creates the outward gravitational pull.

    So ... if there is ever a big enough 'local' gap between objects ... than that external gravitational pull would become dominate... and pull the objects apart.

    - - - - - - - - -

    But I don't see Dark Energy ever described as even the possibility of it being that kind of external pull apart ... which leaves me to wonder ... how could anyone know if the force was a push / expansion from the inside out ... as is often described ... and not a encompassing pull from the outside?

    - - - - - - - - -

    Thanks.


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  3. #2  
    Moderator Moderator Janus's Avatar
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    The problem with this outside pull idea is Newton's shell theorem. Newton proved that if you have a hollow sphere of uniform thickness and density, the gravitational force within it is zero at all points. In other words, no matter how massive the shell, it exerts no "outward pull" on anything inside of it regardless of where it was located in the shell.

    Now you can consider the universe surrounding any given part of space as a series of concentric hollow spheres, which have no gravitational influence on the space they enclose. Ergo, no matter how much mass is on the "outside" it exerts no outward pull on anything on the "inside".


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  4. #3  
    Forum Freshman IamIan's Avatar
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    Quote Originally Posted by Janus View Post
    The problem with this outside pull idea is Newton's shell theorem. Newton proved that if you have a hollow sphere of uniform thickness and density, the gravitational force within it is zero at all points. In other words, no matter how massive the shell, it exerts no "outward pull" on anything inside of it regardless of where it was located in the shell.

    Now you can consider the universe surrounding any given part of space as a series of concentric hollow spheres, which have no gravitational influence on the space they enclose. Ergo, no matter how much mass is on the "outside" it exerts no outward pull on anything on the "inside".
    thanks.

    But , a thought.

    I was under the impression ... correct me if I'm wrong ... that Newton's Shell theorem proved that there was no 'net' gravitational force pulling any more in one direction than any other direction.

    Which would not be the same as their being no gravitational force at all.

    Just like if I had a tug of war where both sides pulled equally ... there is no 'net' force ... but there is still the pull force ... even if you would only be able to observe it ... when that pull was net greater than the binding force holding the rope together.

    Or like if you had a mass at a sweet spot between two larger masses ... a sweet spot where the gravitational pull from the two sides was exactly equal ... you would experience no 'net' force ... but you are still being attracted and have a pull force in both directions ... just neither one is winning... and as long as the local atomic and molecular forces hold you together you might not even obverse the gravitational tug of war trying to pull you apart.
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  5. #4  
    Forum Radioactive Isotope MagiMaster's Avatar
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    Doesn't work like that. Gravity applies uniformly to every point within the object unlike your rope analogy, so no net force is the same as no force.

    This is also a significant difference between the uniform shell and the point between two massive objects. In the uniform shell, the force is zero everywhere, so it's zero on both ends of an object. In between the two massive objects, it's only zero right at the balance point and is non-zero everywhere else, which means the two ends of an object would experience different pulls creating a net tension.
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  6. #5  
    Forum Freshman IamIan's Avatar
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    Quote Originally Posted by MagiMaster View Post
    Doesn't work like that. Gravity applies uniformly to every point within the object unlike your rope analogy, so no net force is the same as no force.
    Bold Added.

    This is the part that doesn't seem correct to me.

    I understand that we would expect to observe the same effects weather is was no force or no net force ... but I don't see them as = , or the same, as each other ... in one case there is actually no force ... and in the other there are forces.

    - - - - - - - - - - - -

    On a side note:
    How is the force not uniform in the rope example?
    I would have expected the force to be uniform ... one dimensional maybe along that rope line ... with no net force ... neither side wins ... it looks like there is no force at all ... until the force is high enough to break the bonds holding the rope together ... when that happens it becomes clear there was a force ... but until then the two sides are balanced and it would look the same as if it were no force at all ... but there still is a force pulling both directions on the rope ... even if there is no net force.
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  7. #6  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by IamIan View Post
    Just like if I had a tug of war where both sides pulled equally ... there is no 'net' force ... but there is still the pull force ... even if you would only be able to observe it ... when that pull was net greater than the binding force holding the rope together.

    Or like if you had a mass at a sweet spot between two larger masses ... a sweet spot where the gravitational pull from the two sides was exactly equal ... you would experience no 'net' force ... but you are still being attracted and have a pull force in both directions ... just neither one is winning... and as long as the local atomic and molecular forces hold you together you might not even obverse the gravitational tug of war trying to pull you apart.
    You seem confused. "No net force" means the two forces balance: there is no force. The rope does not move in the first example, there would be no force in your "sweet spot" to move something away from it, in the second.

    As you say, you would not be able to observe the forces. Because they cancel out.

    If there is no net force from mass outside the universe then it cannot pull on anything - that is what "no net force" means.
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  8. #7  
    Moderator Moderator Markus Hanke's Avatar
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    Gravitational Pull from the outside:
    This seems like it would also manifest observationally in a similar fashion ... even though the objects are being pulled from the outside.
    The problem with this is twofold :

    1. There is no "outside" of the universe
    2. A gravitational "pull" is not the same as a dark energy "push" - the former would lead only to local movement of objects, which is something we can rule out observationally with something like 23 sigmas, whereas the latter leads to accelerated metric expansion of space itself, which doesn't involve any local motion. These two cases can be observationally distinguished. Furthermore a gravitational pull is synonymous with a high degree of space-time curvature, yet observationally our universe appears flat, or at least very nearly so.
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  9. #8  
    Forum Radioactive Isotope MagiMaster's Avatar
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    Quote Originally Posted by IamIan View Post
    Quote Originally Posted by MagiMaster View Post
    Doesn't work like that. Gravity applies uniformly to every point within the object unlike your rope analogy, so no net force is the same as no force.
    Bold Added.

    This is the part that doesn't seem correct to me.

    I understand that we would expect to observe the same effects weather is was no force or no net force ... but I don't see them as = , or the same, as each other ... in one case there is actually no force ... and in the other there are forces.

    - - - - - - - - - - - -

    On a side note:
    How is the force not uniform in the rope example?
    I would have expected the force to be uniform ... one dimensional maybe along that rope line ... with no net force ... neither side wins ... it looks like there is no force at all ... until the force is high enough to break the bonds holding the rope together ... when that happens it becomes clear there was a force ... but until then the two sides are balanced and it would look the same as if it were no force at all ... but there still is a force pulling both directions on the rope ... even if there is no net force.
    Actually, yeah, I think I was wrong about the rope. F = ma can be rearranged to a = F/m, so if there's no acceleration, there's no force, so there's no force on the rope. Someone correct me if I'm wrong, but I think it's stress, not force, that would eventually break the rope. Stress is created when neighboring particles push or pull on one another which doesn't happen in a uniform gravitational field (but does happen in a gravitational gradient).
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  10. #9  
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    Quote Originally Posted by Markus Hanke View Post
    Gravitational Pull from the outside:
    This seems like it would also manifest observationally in a similar fashion ... even though the objects are being pulled from the outside.
    The problem with this is twofold :

    1. There is no "outside" of the universe
    2. A gravitational "pull" is not the same as a dark energy "push" - the former would lead only to local movement of objects, which is something we can rule out observationally with something like 23 sigmas, whereas the latter leads to accelerated metric expansion of space itself, which doesn't involve any local motion. These two cases can be observationally distinguished. Furthermore a gravitational pull is synonymous with a high degree of space-time curvature, yet observationally our universe appears flat, or at least very nearly so.
    1. Depends only on how you define the universe. I define our universe which includes all the galaxies and stars as our local universe. Now that leaves a lot to be guessed at beyond the edges of our local universe.

    2.If our universe was an expanding bubble inside the greater universe with roughly the same gravity pulling from all directions, that expanding bubble from our point of view would look a great deal like you are describing what we are calling dark energy creating more space between all objects in our local universe.
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  11. #10  
    Forum Radioactive Isotope MagiMaster's Avatar
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    You don't really get to redefine words and claim you're still part of the same argument. What you call the universe, most people call the visible universe.
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  12. #11  
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    Quote Originally Posted by MagiMaster View Post
    You don't really get to redefine words and claim you're still part of the same argument. What you call the universe, most people call the visible universe.

    The visible universe is limited by the cosmological horizon. However that doesn't mean the galaxies and stars stop at that distance. So what you are claiming as the visible universe is only based on our point of view on Earth, and we can see out to the same distance in all directions and I'm pretty sure no one thinks we are at the center of the universe. I prefer to use terminology that will include all the galaxies and stars whether they are visible to us or beyond our visibility limits, hence the local universe.

    Cosmological horizon - Wikipedia, the free encyclopedia
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  13. #12  
    Forum Freshman IamIan's Avatar
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    Quote Originally Posted by MagiMaster View Post
    Actually, yeah, I think I was wrong about the rope. F = ma can be rearranged to a = F/m, so if there's no acceleration, there's no force, so there's no force on the rope. Someone correct me if I'm wrong, but I think it's stress, not force, that would eventually break the rope. Stress is created when neighboring particles push or pull on one another which doesn't happen in a uniform gravitational field (but does happen in a gravitational gradient).
    Perhaps a term like ... Pressure might be a better fit to the concept I am trying to describe.... I had thought force would be ... but if a different term better describes the concept ... so be it... I'm interested in exploring / better understanding the concept.

    Would you say the gravitation force ceases to exist when there is no net force ... because the ground exerts an equal counter force?

    I've had classes where we were taught to use F=MA to calculate the force of gravity ... even when the ground was providing a equal counter force ... ie there was no net force ... but , that gravitational force was still there ... no net force was not the same as no force.

    Quote Originally Posted by Strange View Post
    You seem confused. "No net force" means the two forces balance: there is no force. The rope does not move in the first example, there would be no force in your "sweet spot" to move something away from it, in the second.

    As you say, you would not be able to observe the forces. Because they cancel out.

    If there is no net force from mass outside the universe then it cannot pull on anything - that is what "no net force" means.
    I don't agree with ... no net force = the same thing as no force.
    I do agree they might look similar ... but I don't see them as the same thing.

    I do agree force might not be the best term to use ... pressure might be better for describing what I was thinking of.

    Quote Originally Posted by Markus Hanke View Post
    The problem with this is twofold :

    1. There is no "outside" of the universe
    I wasn't thinking of being outside the universe.

    I'm sorry I didn't describe it better in the first attempt in the beginning.

    I was thinking that there is and outside to the local area ... be it a solar system , a galaxy , or even a super cluster of galaxies , even the entire observable universe ... is still itself inside an encompassing gravitational influence from all the mass that is outside of it.

    Quote Originally Posted by Markus Hanke View Post
    2. A gravitational "pull" is not the same as a dark energy "push" - the former would lead only to local movement of objects, which is something we can rule out observationally with something like 23 sigmas, whereas the latter leads to accelerated metric expansion of space itself, which doesn't involve any local motion. These two cases can be observationally distinguished. Furthermore a gravitational pull is synonymous with a high degree of space-time curvature, yet observationally our universe appears flat, or at least very nearly so.
    What is the observational basis that shows the Dark Energy is Pushing from the Inside .... and is not the result of a pull from the outside?

    Why would the gravitational force from every mass everywhere in the universe be required to only have a local influence? ... Gravity is a very long range influence ... I am not aware of any data showing it to have an upper end limit on it's range ... nor any minimum lower end on how weak it can get ... unlike other things we have found some lower and/or upper end limits for.
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  14. #13  
    Forum Radioactive Isotope MagiMaster's Avatar
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    @Bad Robot, I might have read your description wrong, but I'm not sure that anything outside the visible universe can have an effect on us due to there being too little time since the big bang for that effect to reach us. (Again, someone correct me if I'm wrong.)

    @IamIan, The thing is, standing on a planet, the ground pushes against your feet while gravity pulls on your whole body, so there is internal stress. For an object inside a hollow sphere, the gravitational force is zero everywhere, so there is no internal stress and nothing else to differentiate it from simply no gravity. So I'm not saying no force is the same as no net force in general, but the hollow massive shell is a special case.
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  15. #14  
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    Quote Originally Posted by MagiMaster View Post
    @Bad Robot, I might have read your description wrong, but I'm not sure that anything outside the visible universe can have an effect on us due to there being too little time since the big bang for that effect to reach us. (Again, someone correct me if I'm wrong.)
    I'm just like everyone else when it comes to speculating on what's beyond what we can see of the universe or guesstimating what black energy might be. But I do choose to believe the big bang that created our universe didn't happen as a one of a kind event. It is part of a natural process that's been going on before our universe and most likely going on in other locations distant from our universe and will continue going on long after our universe. But then it can't be proved one way or the other by anyone so it's really not worth arguing about anymore than arguing about which religion is the right one when they are all wrong.
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  16. #15  
    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by IamIan View Post
    What is the observational basis that shows the Dark Energy is Pushing from the Inside .... and is not the result of a pull from the outside?
    A gravitational pull from the outside would lead to distant objects moving locally; however, such local movements can be ruled out through something called magnitude-redshift comparisons of observable supernovae. We are currently in a position to tell with an error margin of about 23 sigmas that distant object are not in fact locally moving; this leaves only metric expansion as a consistent explanation for the observed recession speeds. Further details can be found here :

    http://www.mso.anu.edu.au/~charley/p...neweaver04.pdf
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  17. #16  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by IamIan View Post
    Would you say the gravitation force ceases to exist when there is no net force ... because the ground exerts an equal counter force?
    You need to distinguish between gravitation and force caused by it.

    For example, as a result of the shell theorem, as you move towards the centre of the Earth, the force of gravity decreases. This is because it is only caused by the mass inside a sphere with the same radius as your distance from the centre. But, as you move to the centre, gravitational potential (space-time curvature) increases.
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  18. #17  
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    Quote Originally Posted by MagiMaster View Post
    @IamIan, The thing is, standing on a planet, the ground pushes against your feet while gravity pulls on your whole body, so there is internal stress. For an object inside a hollow sphere, the gravitational force is zero everywhere, so there is no internal stress and nothing else to differentiate it from simply no gravity. So I'm not saying no force is the same as no net force in general, but the hollow massive shell is a special case.
    How do we know it's a special case?

    If it doesn't apply to other cases of opposing forces ... isn't that itself evidence that is what we should expect with gravity as well? ... At least until there is experimental data that could somehow show that gravity is an exception ? ... which leads back to the problem of the two looking observationally similar to each other.

    Quote Originally Posted by Strange View Post
    You need to distinguish between gravitation and force caused by it.
    And in the same path of the concept described ... thinking of gravity as a curvature of space instead of a force ... isn't that external gravitation still there? ... in that reference there is a curvature instead of a force ... but the curvature is still there on all sides from the encompassing mass ... doesn't that just give the same concept using a different method of viewing gravity?

    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by IamIan View Post
    What is the observational basis that shows the Dark Energy is Pushing from the Inside .... and is not the result of a pull from the outside?
    A gravitational pull from the outside would lead to distant objects moving locally; however, such local movements can be ruled out through something called magnitude-redshift comparisons of observable supernovae. We are currently in a position to tell with an error margin of about 23 sigmas that distant object are not in fact locally moving; this leaves only metric expansion as a consistent explanation for the observed recession speeds. Further details can be found here :

    http://www.mso.anu.edu.au/~charley/p...neweaver04.pdf
    Sorry if I'm being dense ... but ...
    What do you mean by 'moving locally' ?
    Why are you imposing a 'locally' restriction to gravity ? ... AFAIK Gravity has no known range limit.

    I read through your reference ... but I don't see how it explains why the expansion is considered to be caused by a more internal push and not an external pull ... the 23 sigmas is about:

    "The interpretation of the cosmological redshift as an SR Doppler effect is ruled out at more than 23 sigmas compared with the ACDM concordance model."


    Which as far as I can tell is just about the redshift supporting an expansion ... not weather the expansion is caused by a push or a pull.

    Which leads me back to the same question ... what is the observational basis for thinking it is a push and not a pull?

    A crude example:
    I can expand a ballon with internal pressure (push)... Or I can expand a ballon with external negative pressure (pull) ... either way the balloon expands ... watching the ballooon expand ... would look very very similar between the two... either push or pull.
    Last edited by IamIan; October 20th, 2013 at 12:16 PM. Reason: crude example
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    Forum Radioactive Isotope MagiMaster's Avatar
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    AFAIK, gravity is a special case because it's monopolar. Gravity only attracts whereas other forces can also repel. I may be mixing some details up here, so I'll leave the full explanation to people who understand this better, but you can't make a static bottle out of electromagnetic forces, but you can out of gravitational forces (any massive body qualifies).

    Edit: Also, gravity, unlike your rope example affects each atom individually, which is a significant difference.
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  20. #19  
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    Simply put, anything moving due to gravity has what is known as peculiar motion, which would be subject to Special Relativity and thus the redshift of distant galaxies would be an SR Doppler effect. This has been ruled out.

    The redshift of distant galaxies is not, therefore, due to them moving through space. The fact that galaxies with redshifts of more than z~1.4 have apparent recessions speeds faster than light (which gives the observable universe a radius of 46 billion light-years after only 13.7 billion years, where everything started off in the same place) would seem to confirm this. This means that it is not an "outside" gravity effect that is moving them.

    And as has already been stated, Newton's shell theorem would preclude "outside" gravity from moving them anyway.
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    Quote Originally Posted by IamIan View Post
    I don't agree with ... no net force = the same thing as no force.
    I do agree they might look similar ... but I don't see them as the same thing.
    It doesn't really matter whether you agree or see it that way or not. Because .... you are wrong.
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  22. #21  
    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by IamIan View Post
    What do you mean by 'moving locally' ?
    It basically just means that the objects themselves are moving, as opposed to them being stationary and space between us and the object metrically expanding. They are not the same thing, and can be observationally distinguished.

    Which as far as I can tell is just about the redshift supporting an expansion ... not weather the expansion is caused by a push or a pull.
    No, it means exactly what the sentence states - the distant objects aren't in fact moving, hence the observed redshift is due solely to the metric expansion of space and not due to a Doppler effect. Since they are not moving, there is no external gravity acting on them.

    Which leads me back to the same question ... what is the observational basis for thinking it is a push and not a pull?
    There is neither push nor pull at play here.

    Or I can expand a ballon with external negative pressure (pull)
    Frankly, I would like to see this being done in a way such that that both the process itself and the end results are observationally indistinguishable. I doubt very much that this can be done. And even if it can, the resulting model would contain many more assumptions ( i.e. external space, uniform pull, etc etc ) than accelerated metric expansion, which relies only on the geometry of space-time itself as modelled by General Relativity ( which is experimentally well verified ) and the presence of dark energy. It does not need any embedding into higher dimensional spaces, nor does it require any mysterious outside forces acting.
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    Quote Originally Posted by MagiMaster View Post
    AFAIK, gravity is a special case because it's monopolar. Gravity only attracts whereas other forces can also repel. I may be mixing some details up here, so I'll leave the full explanation to people who understand this better, but you can't make a static bottle out of electromagnetic forces, but you can out of gravitational forces (any massive body qualifies).

    Edit: Also, gravity, unlike your rope example affects each atom individually, which is a significant difference.
    Would this mean that any balanced ( net zero ) monopolar forces are the same as no force? ... even if the monopolar force was not from gravity.

    stresses suddenly disappear the instant the forces balance perfectly....


    Quote Originally Posted by SpeedFreek View Post
    Simply put, anything moving due to gravity has what is known as peculiar motion, which would be subject to Special Relativity and thus the redshift of distant galaxies would be an SR Doppler effect. This has been ruled out.

    The redshift of distant galaxies is not, therefore, due to them moving through space. The fact that galaxies with redshifts of more than z~1.4 have apparent recessions speeds faster than light (which gives the observable universe a radius of 46 billion light-years after only 13.7 billion years, where everything started off in the same place) would seem to confirm this. This means that it is not an "outside" gravity effect that is moving them.

    And as has already been stated, Newton's shell theorem would preclude "outside" gravity from moving them anyway.
    What if it's not outside gravity moving the objects ... outside gravity being the cause of the expansion of the space itself.

    It's the same expansion of space weather it is caused by inside push or outside pull... how does seeing an expansion tell you if it was from a inside push or an outside pull? ... same amount of expansion of space either way.

    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by IamIan View Post
    What do you mean by 'moving locally' ?
    It basically just means that the objects themselves are moving, as opposed to them being stationary and space between us and the object metrically expanding. They are not the same thing, and can be observationally distinguished.
    Weather it is from an inside influence or an outside influence ... it's the same expansion either way.

    We know Gravity can cause a curving / stretching of space itself ... that in itself would seem to prove that gravity is not limited to just 'move locally' ... as you call it ... that gravity can curve and stretch the space also.

    Quote Originally Posted by Markus Hanke View Post
    Which as far as I can tell is just about the redshift supporting an expansion ... not weather the expansion is caused by a push or a pull.
    No, it means exactly what the sentence states - the distant objects aren't in fact moving, hence the observed redshift is due solely to the metric expansion of space and not due to a Doppler effect. Since they are not moving, there is no external gravity acting on them.
    I don't see it claiming no external gravity acting on them ??? ... where do you get that?
    I don't see it claiming distant objects aren't moving ??? ... where do you get that?
    I don't see it claiming the observed redshift is due solely to the expansion ?? ... where do you get that?

    I saw it showing a gap in the redshift that supports there is a expansion of the space itself ... that tells you nothing about weather that expansion is caused from the inside of the expansion or the outside of the expansion ... just that it supports there is an expansion.

    Quote Originally Posted by Markus Hanke View Post
    Which leads me back to the same question ... what is the observational basis for thinking it is a push and not a pull?
    There is neither push nor pull at play here.
    What term would you prefer to use to describe the influence that is causing the expansion of the space?

    What evidence is there that this cause is originating from inside the expansion and not from the outside of the expansion?

    If there is neither involved ... how do you account for other influences ... such as gravitational objects in closer proximity to each other having an influence on the expansion of the space itself? ... AFAIK every model of the expansion has gravity at close ranges able to counter / nullify the expansion of the space itself.

    If anything that seems to suggest some level of interaction between gravity and the expansion of the space itself.
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    Quote Originally Posted by IamIan View Post
    I don't agree with ... no net force = the same thing as no force.
    I do agree they might look similar ... but I don't see them as the same thing.

    I do agree force might not be the best term to use ... pressure might be better for describing what I was thinking of.
    If Newton's shell theorem isn't good enough for you, then there's Birkhoff's theorem that basically says that the spacetime geometry external¹ to a spherical matter distribution is Schwarzschild, from which one can conclude that the spacetime inside a hollow spherical shell of matter is flat. This means there is no gravitation at all inside a hollow spherical shell.


    ¹ "External" means external to the matter itself and includes the region inside a hollow spherical shell.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by IamIan View Post
    Weather it is from an inside influence or an outside influence ... it's the same expansion either way.
    No - an "outside" gravitational influence would not lead to metric expansion, only local movement, and even that only provided that there is indeed a net influence, which wouldn't actually be the case ( shell theorem ). That is what I have been trying to explain to you.

    We know Gravity can cause a curving / stretching of space itself ... that in itself would seem to prove that gravity is not limited to just 'move locally' ... as you call it ... that gravity can curve and stretch the space also.
    Gravity from an external source does not cause global metric expansion, it only causes curvature. These are not the same things. Space can metrically expand without being curved, which is exactly what we observe.

    I don't see it claiming distant objects aren't moving ??? ... where do you get that?
    I don't see it claiming the observed redshift is due solely to the expansion ?? ... where do you get that?
    Page 10 of the PDF, top left. It wouldn't hurt to actually read the links we provide, you know.

    I don't see it claiming no external gravity acting on them ??? ... where do you get that?
    You will not find this statement anywhere simply because there is no "outside" to the universe.

    What term would you prefer to use to describe the influence that is causing the expansion of the space?
    Both expansion and gravity are geometric properties of space-time, rather than forces.

    What evidence is there that this cause is originating from inside the expansion and not from the outside of the expansion?
    Quite frankly, this is about the 3rd time you ask this now, and I see no reason why I need to repeat what has already been explained. See my earlier posts.

    such as gravitational objects in closer proximity to each other having an influence on the expansion of the space itself?
    Metric expansion does not affect gravitationally bound systems.

    If anything that seems to suggest some level of interaction between gravity and the expansion of the space itself.
    The connection is that they are both consequences of the same underlying physical law, being the field equation of General Relativity. That does not mean that one cause the other.
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    If on the other hand gravity IS a force, the problem studying it is that it's so diffuse (compared to electromagnetism.)
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    Quote Originally Posted by KJW View Post
    If Newton's shell theorem isn't good enough for you, then there's Birkhoff's theorem that basically says that the spacetime geometry external¹ to a spherical matter distribution is Schwarzschild, from which one can conclude that the spacetime inside a hollow spherical shell of matter is flat. This means there is no gravitation at all inside a hollow spherical shell.


    ¹ "External" means external to the matter itself and includes the region inside a hollow spherical shell.
    It is not that Newton's shell theorem isn't good enough ... it's just that ... it is about net forces ... so I recognize it for what it is.

    The part that isn't 'good enough' for me ... if we even want to use that kind of phrase ... is an assumption that no net force is the same or = to no force at all ... I see problems with that kind of claim.

    Thanks for the Birkhoff pointer ... I will have to look more into it.

    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by IamIan View Post
    Weather it is from an inside influence or an outside influence ... it's the same expansion either way.
    No - an "outside" gravitational influence would not lead to metric expansion, only local movement, and even that only provided that there is indeed a net influence, which wouldn't actually be the case ( shell theorem ). That is what I have been trying to explain to you.
    That comment is not about gravity ... it is about the source of the influence that causes the expansion.

    Gravity seemed to be tripping you up ... and preventing you from seeing the question being asked ... and words like push and pull also seemed to be tripping you up and preventing you from seeing the question being asked ... So I specifically tried that time to avoid those trip ups.

    If you have an expansion of space ... you observe evidence of the expansion ... as we have ... by what method can you tell weather that expansion is caused by an inside influence ... meaning inside the space that is expanding ... or an outside influence ... meaning outside the space that is expanding but still inside the universe ?

    Quote Originally Posted by Markus Hanke View Post
    I don't see it claiming distant objects aren't moving ??? ... where do you get that?
    I don't see it claiming the observed redshift is due solely to the expansion ?? ... where do you get that?
    Page 10 of the PDF, top left. It wouldn't hurt to actually read the links we provide, you know.

    I don't see it claiming no external gravity acting on them ??? ... where do you get that?
    You will not find this statement anywhere simply because there is no "outside" to the universe.
    Not outside the universe ... outside the space that is expanding.

    Despite your quib I did read it ... I did even went back and read the page you indicate here again ... and I still do not see them making these 3 claims you are making ... if anything , there are several parts of the referenced work that disagree with those claims you made there ... there are others ... but here are two quick examples:

    The graph on page 8 of the pdf ... page 104 of the paper ... shows a considerable amount of the RedShift observed that can be explained by SR ( not solely the expansion )... not all of the RedShift ... but it does not seem like they are claiming solely the expansion as you claimed. Link

    The portion of the RedShift explained by SR does not come from the expansion ... but instead comes from Gravity , or the objects moving , or some combination of both ... any of that disagrees with those 3 claims of yours.

    Page 1 of the pdf page 97 of the paper ... in the abstract ... they write:
    "We show that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light."

    If the light from there can get to us here ... the only way your claim of gravity not effecting it ... that is solely the expansion would be if you somehow think those galaxies don't have any gravity ... ??? ... and for your claim of them to not be moving to work means you somehow think the light emitting objects in those galaxies are not moving either ????

    So ... I do not see it making these 3 claims of yours ... if anything it looks like the reference disagrees with these 3 claims of yours.

    Quote Originally Posted by Markus Hanke View Post
    What term would you prefer to use to describe the influence that is causing the expansion of the space?
    Both expansion and gravity are geometric properties of space-time, rather than forces.
    That doesn't say what causes the expansion ... ie doesn't answer the question asked.

    Quote Originally Posted by Markus Hanke View Post
    What evidence is there that this cause is originating from inside the expansion and not from the outside of the expansion?
    Quite frankly, this is about the 3rd time you ask this now, and I see no reason why I need to repeat what has already been explained. See my earlier posts.
    Which post? ... I'm happy to go back and re-read it.

    AFAIK you've posted other things... but not yet answered this... other than me reminding you that outside the expansion is still inside the universe... this one is not about gravity, don't get tripped up on that again.

    Quote Originally Posted by Markus Hanke View Post
    such as gravitational objects in closer proximity to each other having an influence on the expansion of the space itself?
    Metric expansion does not affect gravitationally bound systems.

    If anything that seems to suggest some level of interaction between gravity and the expansion of the space itself.
    The connection is that they are both consequences of the same underlying physical law, being the field equation of General Relativity. That does not mean that one cause the other.
    Gravity can influence the expansion ... even stop it entirely ... If we already know the influence goes one way ... what rules out the influence going the other way ?

    Even if one does not cause the other ... they do influence each other.
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    Quote Originally Posted by IamIan View Post
    Quote Originally Posted by MagiMaster View Post
    AFAIK, gravity is a special case because it's monopolar. Gravity only attracts whereas other forces can also repel. I may be mixing some details up here, so I'll leave the full explanation to people who understand this better, but you can't make a static bottle out of electromagnetic forces, but you can out of gravitational forces (any massive body qualifies).

    Edit: Also, gravity, unlike your rope example affects each atom individually, which is a significant difference.
    Would this mean that any balanced ( net zero ) monopolar forces are the same as no force? ... even if the monopolar force was not from gravity.

    stresses suddenly disappear the instant the forces balance perfectly....
    I don't think we have any other monopolar forces to test this with, but I think the more important point is that gravity acts on every particle individually and, inside a hollow shell, every particle experiences no net force. That means two particles right next to each other will not experience any force trying to move them apart no matter how massive the shell. This is true whatever scale you're measuring things at too.

    This doesn't apply in either of your other examples, although in the two massive bodies example, if they're far enough apart the difference would be fairly small.
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    Quote Originally Posted by IamIan View Post
    is an assumption that no net force is the same or = to no force at all ... I see problems with that kind of claim.
    Thanks for the Birkhoff pointer ... I will have to look more into it
    Yes, in that case I'd agree with KJW that Birkhoff's theorem is a better notion to use.

    If you have an expansion of space ... you observe evidence of the expansion ... as we have ... by what method can you tell weather that expansion is caused by an inside influence ... meaning inside the space that is expanding ... or an outside influence ... meaning outside the space that is expanding but still inside the universe ?
    There is no "inside" and "outside" as all of space expands everywhere. It must be remembered though that this expansion isn't sufficient to overcome gravity in gravitationally bound systems.

    The graph on page 8 of the pdf ... page 104 of the paper ... shows a considerable amount of the RedShift observed that can be explained by SR ( not solely the expansion )... not all of the RedShift ... but it does not seem like they are claiming solely the expansion as you claimed. Link
    It says exactly the same as I did ( quote ) : "The interpretation of the cosmological redshift as an SR Doppler effect is ruled out more than 23 sigma (...)".

    The portion of the RedShift explained by SR does not come from the expansion ... but instead comes from Gravity , or the objects moving , or some combination of both ... any of that disagrees with those 3 claims of yours.
    No portion of the text says anything like this - see the quote above. It is quite clear in concluding that any SR Doppler effect explanation can be effectively ruled out.

    "We show that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light."
    Which means of course that this cannot be due to local motion of the galaxies. As I have stated multiple times now.

    So ... I do not see it making these 3 claims of yours ... if anything it looks like the reference disagrees with these 3 claims of yours.
    I suggest you read more carefully then, because the text is in exact agreement with what I said, or else I would not have quoted it as a supporting reference.

    That doesn't say what causes the expansion ... ie doesn't answer the question asked.
    The question was what term to use for the origin of the expansion, if not "force" - I answered it is due to the geometry of space-time, and that is exactly what it is. In the absence of a cosmological constant the relevant solution to the GR field equation will always predict metric expansion, it is inherent in the ( global ! ) geometry of space-time, and its equations of motion. You need a term in the field equations to counteract this to obtain a static solution - which is what Einstein set out to do when he introduced the cosmological constant.

    AFAIK you've posted other things... but not yet answered this... other than me reminding you that outside the expansion is still inside the universe... this one is not about gravity, don't get tripped up on that again.
    There is no "outside" and "inside" - metric expansion is a global phenomenon that affects all of space-time, as can easily be seen in the metric tensor of the Lambda-CDM model. Metric expansion is not the same as gravity, but it is the result of the same geometric laws, the Einstein field equations. I don't know how else to explain this. What questions haven't I answered ?

    Gravity can influence the expansion ... even stop it entirely
    That is true, but gravity is not the cause of expansion. As explained earlier, the expansion is inherent in the equations of motion of space-time itself. In other words - even in a completely flat universe ( i.e. no global gravitational net force ), space would still undergo metric expansion.

    Which post? ... I'm happy to go back and re-read it.
    Posts 24, 21, 19, 16, 15, 7, 6...you get the idea.
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    Quote Originally Posted by IamIan View Post
    The part that isn't 'good enough' for me ... if we even want to use that kind of phrase ... is an assumption that no net force is the same or = to no force at all ... I see problems with that kind of claim.
    From Newton's laws of motion, F = ma. If there is no acceleration, then there is no (net) force.
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    Quote Originally Posted by MagiMaster View Post
    I don't think we have any other monopolar forces to test this with
    Do you know of any tests that have been done to verify gravity is this kind of special case / exception?

    Quote Originally Posted by Strange View Post
    Quote Originally Posted by IamIan View Post
    The part that isn't 'good enough' for me ... if we even want to use that kind of phrase ... is an assumption that no net force is the same or = to no force at all ... I see problems with that kind of claim.
    From Newton's laws of motion, F = ma. If there is no acceleration, then there is no (net) force.
    exactly ... but that doesn't itself tell you anything about weather ... no net force is the same thing as no force.

    The net force is the combination of all other forces ... no force means there are no forces at all ... not just that the +forces balance the -forces to reach a net of zero... when there is a + and a - force even if it is net zero ... there are still + and - forces ... ie not no force.... even though it is no net force.

    I like how MagiMaster refereed to in terms of stress ... paraphrasing ( if I understood correctly ) ... no forces means no stress ... no net force can still have stress.

    Quote Originally Posted by Markus Hanke View Post
    If you have an expansion of space ... you observe evidence of the expansion ... as we have ... by what method can you tell weather that expansion is caused by an inside influence ... meaning inside the space that is expanding ... or an outside influence ... meaning outside the space that is expanding but still inside the universe ?
    There is no "inside" and "outside" as all of space expands everywhere. It must be remembered though that this expansion isn't sufficient to overcome gravity in gravitationally bound systems.
    Not all space expands everywhere ... if gravity stops it from expanding anywhere ... at those places it is not expanding.

    There are points that are inside the expanding space event ... and there are points that are on the outside of that expanding space event ... ie ... there is an inside to it ... and an outside to it ... every individual expanding space event you look at.

    A distant galaxy can have an expanding space event inside the space between it and us ... thus we and that other distant galaxy are outside of that one specific expanding space event ... but there are points inside of that space between us and that distant galaxy that are inside that expanding space event.

    Quote Originally Posted by Markus Hanke View Post
    It is quite clear in concluding that any SR Doppler effect explanation can be effectively ruled out.
    Does this mean ... that you are retracting or abandoning your earlier three absolute type of claims?

    Quote Originally Posted by Markus Hanke
    the distant objects aren't in fact moving, hence the observed redshift is due solely to the metric expansion of space and not due to a Doppler effect. Since they are not moving, there is no external gravity acting on them.
    And your moving toward some other claim? ... something that doesn't claim the distant galaxy has no gravity , and no movement? ... something that makes the expansion redshift add on to the other known redshift effects ... and not be solely expansion?

    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by IamIan
    Quote Originally Posted by Markus Hanke
    Quote Originally Posted by IamIan
    What evidence is there that this cause is originating from inside the expansion and not from the outside of the expansion?
    Quite frankly, this is about the 3rd time you ask this now, and I see no reason why I need to repeat what has already been explained. See my earlier posts.
    Which post? ... I'm happy to go back and re-read it.
    Posts 24, 21, 19, 16, 15, 7, 6...you get the idea.
    I re-read them ... but I don't see this answered in any of those posts.... sorry if I'm being blind ... but can you be more specific?

    See above for explanation of how to be inside or outside.

    Quote Originally Posted by Markus Hanke View Post
    What questions haven't I answered ?
    Same kind of thing as above.

    What evidence is there and/or how would we tell if the expansion of space was/is caused from inside or outside influences?

    See above for explanation of how to be inside or outside.
    Last edited by IamIan; October 23rd, 2013 at 08:23 PM. Reason: gramer
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    I like how MagiMaster refereed to in terms of stress ... paraphrasing ( if I understood correctly ) ... no forces means no stress ... no net force can still have stress.
    I don't really know what the relevance of stresses is to this, as stress without local motion does not lead to redshift.

    Quote Originally Posted by IamIan View Post
    Not all space expands everywhere ... if gravity stops it from expanding anywhere ... at those places it is not expanding.
    No, space still expands everywhere, but that expansion cannot overcome gravitationally bound systems. In other words - even if the underlying space expands, gravitationally bound systems within that space do not.

    Does this mean ... that you are retracting or abandoning your earlier three absolute type of claims?
    My "claim" has been all along that the observed data is to be explained by metric expansion of space as opposed to local movement of those objects ( i.e. as opposed to SR Doppler effects due to local motion ). I have not changed anything in what I am saying.
    I am starting to get the feeling though that you yourself are not clear on the difference between metric expansion and SR Doppler effects - do you need clarification on this matter ?

    And your moving toward some other claim?
    No, my "claim" remains exactly the same - that the observed empirical data is best explained by metric expansion, whereas local movement ( SR Doppler effects ) can be effectively ruled out. And if there is no local movement, there are also no net gravitational forces.

    See above for explanation of how to be inside or outside.
    The issue is that we have already explained to you that there is no "inside" or "outside" - metric expansion is a global phenomenon that happens everywhere. Are you familiar with the basics of differential geometry ? If so I can show you the metric of the FLRW solution, and you will see immediately what is meant by that.

    What evidence is there and/or how would we tell if the expansion of space was/is caused from inside or outside influences?
    The question is meaningless, because there is no "inside" or "outside", since metric expansion happens everywhere. As for the question on whether or not those distant objects move locally, we have already answered that - they don't, and the evidence is to be found in the magnitude-redshift data as explained in detail in the sources provided.

    So what else do you want us to explain or answer ?
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    I mentioned stress to point out why a rope will break even though there would be no force measured along the rope just before that (although that's actually assuming an infinitely stiff rope as most ropes will stretch a bit).

    Again though, there's a huge difference between a rope (or the space between two massive objects) and the inside of a hollow shell. The pressures at the ends of the rope create stresses within the rope because it is applied to a few particles which then pull on their neighbors (etc.) while the massive shell does not as is acts on each particle individually.
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    Quote Originally Posted by IamIan View Post
    exactly ... but that doesn't itself tell you anything about weather ... no net force is the same thing as no force.
    Of course it does. If there were a force acting on the object it would move. No movement, no force.
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    Quote Originally Posted by MagiMaster View Post
    I mentioned stress to point out why a rope will break even though there would be no force measured along the rope just before that (although that's actually assuming an infinitely stiff rope as most ropes will stretch a bit).

    Again though, there's a huge difference between a rope (or the space between two massive objects) and the inside of a hollow shell. The pressures at the ends of the rope create stresses within the rope because it is applied to a few particles which then pull on their neighbors (etc.) while the massive shell does not as is acts on each particle individually.
    If you're looking for a 'perfect' analogy ... sorry , I don't know how to give you that.

    All I can do is call it an 'ideal' rope ... no stretch , equal distribution, etc ... and try to describe the concept ... or the other one used is opposition between gravity and the normal force... either way ... the goal is to grasp the concept.

    As the expression goes ... you have to see the forest past the trees.

    If you see no difference at all between a tug of war ideal rope with no net motion / winner ... and that same ideal rope just laying flat on the floor with noone pulling on either end of it ... than ok ... but I can't give you a 'perfect' analogy.

    Gravity might be an exception to the implications for other cases ... as you already suggested.... I don't know of test data that shows it is or is not.

    - - - - - - - - - - -

    Quote Originally Posted by Strange View Post
    Quote Originally Posted by IamIan View Post
    exactly ... but that doesn't itself tell you anything about weather ... no net force is the same thing as no force.
    Of course it does. If there were a force acting on the object it would move. No movement, no force.
    If some amount of time passed ... but at change of time = 0 ... even if there is a net force ... there has not yet been any time for it to accelerate ... it is not yet moving ... even though it does can have a net force acting on it at that instant... the application of force and the acceleration caused by it are not necessarily always 100% perfectly instantaneously in sync ... one such example is seen in AC electrical flow voltage vs current being out of phase with each other.

    If you aren't comfortable using the term 'force' ... for describing things acting in opposition ... what would you prefer to call the action of pulling on a rope that doesn't move? Pressure , Stress, what?

    And .. if it doesn't move ... is that the actually completely the same as not pulling on it at all? .. a Tired Arm will claim they are not the same.

    - - - - - - - - - -

    Quote Originally Posted by Markus Hanke View Post
    I like how MagiMaster refereed to in terms of stress ... paraphrasing ( if I understood correctly ) ... no forces means no stress ... no net force can still have stress.
    I don't really know what the relevance of stresses is to this, as stress without local motion does not lead to redshift.
    That wasn't about redshift.
    Not every conversation happening in this thread is about redshift.

    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by IamIan View Post
    Not all space expands everywhere ... if gravity stops it from expanding anywhere ... at those places it is not expanding.
    No, space still expands everywhere, but that expansion cannot overcome gravitationally bound systems. In other words - even if the underlying space expands, gravitationally bound systems within that space do not.
    That is entirely different than I have ever heard it explained.

    From that kind of masses on infinite 'space' tread mills kind of concept ... why do we see the light from the sun at all if the space between us and the sun is expanding faster than the speed of light?

    Every version of the expansion of space I've heard ... has the expansion itself slowing or being stopped by stronger gravity ... ie the space between us and our sun is not expanding, because the gravity has effectively stopped it from expanding at any significant rate ... and when the gravity between objects gets too weak ... that the expansion is not stopped or slowed ... ie a distant galaxy with more 'empty' space between.

    Quote Originally Posted by Markus Hanke View Post
    See above for explanation of how to be inside or outside.
    The issue is that we have already explained to you that there is no "inside" or "outside" - metric expansion is a global phenomenon that happens everywhere.
    Weather it is global or not , I don't see that preventing an inside and outside.

    Especially because I was classifying it for specific expansion events ... and not all of expansion everywhere.

    For example:
    If I pick our galaxy , or the earth , etc ... that defines a space ... there is a defined space inside of it... and a defined space outside of it ... expansion that happens inside of it is inside ... expansion that happens outside of it is outside... you could do the same thing with 2 points for 1D , or 3 points for 2D, 4 points for 3D

    Quote Originally Posted by Markus Hanke View Post
    Does this mean ... that you are retracting or abandoning your earlier three absolute type of claims?
    My "claim" has been all along that the observed data is to be explained by metric expansion of space as opposed to local movement of those objects ( i.e. as opposed to SR Doppler effects due to local motion ). I have not changed anything in what I am saying.
    I am starting to get the feeling though that you yourself are not clear on the difference between metric expansion and SR Doppler effects - do you need clarification on this matter ?
    When I noticed the wording of 'Effectively Ruled Out' ... I was hoping you were moving away from those previous extreme claims ... more toward something like ... the expansion redshift mechanism can eventually dwarf the other redshift mechanisms ... that the other redshift contributions become comparatively small in some cases.

    I see a rather MASSIVE difference between saying something like ... the expansion based redshift can eventually dwarf other redshift mechanisms ... compared to ... that is entirely different from the claim that the redshift is solely from the expansion.

    Claiming the Redshift is solely from the expansion ... means you are ruling out the existence of ANY other redshift mechanism ... As if Gravity can't cause a redshift ... and as though movement can't cause a redshift ... Sense we have strong evidence for the existence of those other non=expansion redshift mechanism ... the red shift can't be solely from the expansion.

    You claimed there was no gravity there ... why wouldn't a distant star have gravity?

    You claimed there was no movement there ... why wouldn't a distant star traveling around a distant galaxy be moving?

    This is the extreme claim I was referring to:
    Quote Originally Posted by Markus Hanke
    the distant objects aren't in fact moving, hence the observed redshift is due solely to the metric expansion of space and not due to a Doppler effect. Since they are not moving, there is no external gravity acting on them.
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    Quote Originally Posted by IamIan View Post
    why do we see the light from the sun at all if the space between us and the sun is expanding faster than the speed of light?
    Because that is not what happens. From us to the sun is an extremely small distance on cosmological scales; metric expansion only becomes noticeable on very large scales. It is a cumulative effect - the more space there is between two points, the more expansion is observed. In other words - the further away an object is, the faster it is seen to recede. Within our solar system the effect is so small as to be unmeasurable due to the tiny scales involved, hence there is no effect to the light from the sun.

    Every version of the expansion of space I've heard ... has the expansion itself slowing or being stopped by stronger gravity ... ie the space between us and our sun is not expanding, because the gravity has effectively stopped it from expanding at any significant rate ... and when the gravity between objects gets too weak ... that the expansion is not stopped or slowed ... ie a distant galaxy with more 'empty' space between.
    This is technically a misconception - the space does expand, even in the solar system, but the expansion on small scales such as the solar system is an extremely tiny effect, and thus not strong enough to overcome the gravitational bond between the sun and the earth. The end result is the same - expansion is not observed on such small scales as the solar system.

    Weather it is global or not , I don't see that preventing an inside and outside.
    If it is global, there is no "outside".

    If I pick our galaxy , or the earth , etc ... that defines a space ... there is a defined space inside of it... and a defined space outside of it ... expansion that happens inside of it is inside ... expansion that happens outside of it is outside... you could do the same thing with 2 points for 1D , or 3 points for 2D, 4 points for 3D
    I'm afraid I don't understand what you are trying to say here.

    You claimed there was no gravity there ... why wouldn't a distant star have gravity?
    You claimed there was no movement there ... why wouldn't a distant star traveling around a distant galaxy be moving?
    Ok, I concede that you have a point here - stating that there is no local movement is not technically correct, so I do apologise. Of course galaxies do drift slowly, as do the stars within those galaxies. The point is that this local movement is entirely insubstantial as compared to the observed recession velocities, and does not even begin to account for the observed redshifts. Remember that z=1.4 would correspond to a local speed of light, and we observe objects receding at z > 8, so local movement is obviously not a valid explanation for this, and can in any case be ruled out via the aforementioned magnitude redshift relation.

    means you are ruling out the existence of ANY other redshift mechanism
    As stated, all other mechanisms are entirely negligible on the scales considered here.

    You claimed there was no gravity there ... why wouldn't a distant star have gravity?
    Of course stars have gravity, but the point is that no local gravitational effect will ever lead to redshifts on the order of the ones we observe. Also remember that redshift isn't the only evidence for metric expansion, in fact it is only a small part of the puzzle. You need to consider the bigger picture as well. Are you aware of the other supporting evidence for metric expansion ?
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    Quote Originally Posted by IamIan View Post
    Quote Originally Posted by MagiMaster View Post
    I mentioned stress to point out why a rope will break even though there would be no force measured along the rope just before that (although that's actually assuming an infinitely stiff rope as most ropes will stretch a bit).

    Again though, there's a huge difference between a rope (or the space between two massive objects) and the inside of a hollow shell. The pressures at the ends of the rope create stresses within the rope because it is applied to a few particles which then pull on their neighbors (etc.) while the massive shell does not as is acts on each particle individually.
    If you're looking for a 'perfect' analogy ... sorry , I don't know how to give you that.

    All I can do is call it an 'ideal' rope ... no stretch , equal distribution, etc ... and try to describe the concept ... or the other one used is opposition between gravity and the normal force... either way ... the goal is to grasp the concept.

    As the expression goes ... you have to see the forest past the trees.

    If you see no difference at all between a tug of war ideal rope with no net motion / winner ... and that same ideal rope just laying flat on the floor with noone pulling on either end of it ... than ok ... but I can't give you a 'perfect' analogy.

    Gravity might be an exception to the implications for other cases ... as you already suggested.... I don't know of test data that shows it is or is not.
    It's your analogy. I'm pointing out why it isn't a good one and won't help you understand the problem you're trying to understand.

    Of course there's a difference between a rope being pulled and a rope lying on the ground, but in an ideal rope with infinite breaking strength and no stretch those differences go away, but that's a physically impossible object anyway so again, not a good analogy.

    Think about just three particles within the rope lined up in a row and bonded together (although loosely). If you pull on the two ends with little enough force you won't break things, but you will stretch them out a bit. This internal motion is stress. When it gets large enough, the bonds break.

    Now take the same three particles and pull on each individual particle all at the same time. No matter how hard you pull, you'll never break anything, just move the group around as a whole.
    That applies to three molecules within a rope, three atoms within a molecule or three quarks within an atom. If you're pulling on everything all at once, there's no internal motion and no stress.

    In the case of the hollow sphere, it's pulling on all the particles in all directions equally, so again, no internal stresses and nothing will ever break no matter how massive the shell.

    Finally, if there's no observable/measurable difference between nothing and a massive shell, there's no difference. Science only cares about things that are observable/measurable.

    Then there's the relativistic definition of gravity, as the curvature of spacetime. In that case, the only way of getting no net gravitational force is for spacetime in that area to be flat. If it's flat, it's flat whether it's because there's nothing there or because the hollow shell smooths out all the curves. It has all the same effects either way.
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    Quote Originally Posted by Markus Hanke View Post
    Ok, I concede that you have a point here
    Thanks.

    And Thanks for the additional explanations.


    Quote Originally Posted by Markus Hanke View Post
    Weather it is global or not , I don't see that preventing an inside and outside.
    If it is global, there is no "outside".

    If I pick our galaxy , or the earth , etc ... that defines a space ... there is a defined space inside of it... and a defined space outside of it ... expansion that happens inside of it is inside ... expansion that happens outside of it is outside... you could do the same thing with 2 points for 1D , or 3 points for 2D, 4 points for 3D
    I'm afraid I don't understand what you are trying to say here.
    No outside to the entire global effect , yes I agree ... just like there isn't an outside to the whole universe ... but weather it is a global effect or not ... does not prevent me from defining a region / piece of the whole ... and in that piece there is an inside to it and and outside to it.

    Just like If I define the region as our galaxy ... there is an inside to our galaxy and there is an outside to our galaxy... and I can talk about the inside influences and the outside influences to that defined region of space.

    If I talk about the expansion that happens inside that region ... ie the expansion that happens inside of our galaxy ... there is an inside and and outside still to that region and to the expansion refered to that is inside of that region... is inside of it ... and the influences from outside that defined region at outside of it.

    The points thing is nothing fancy ... just basic geometry.

    Any two unique points defines a line ... a 1 dimentional region ... the points along that line inside of those two defined points are inside the defined region ... and the points on that line but outside of those two points are outside of that interior defined region.

    If I add a 3rd point that is any where not on that 1 dimensional line ... I have now refined a 2 dimensional region ... a triangle to be specific ... and there is an inside to that region and and outside to that region along that 2 dimensional plane.

    If I add a 4th point that is not in that 2 Dimensional plane ... I have now defined a 3 Dimensional region of space .. and there is an inside to that region and an outside to that region.

    Quote Originally Posted by Markus Hanke View Post
    Are you aware of the other supporting evidence for metric expansion ?
    AFAIK there is also the arguments from the quantum mechanics view of the energy of 'empty space' itself ... and the expansion allows observed data to not violate the current estimated age of the universe ... and without expansion the flat universe model falls apart with observed data ... those are the three others that come to mind ... although I have no doubt there are also others.

    - - - - - - - -

    Quote Originally Posted by MagiMaster View Post
    It's your analogy. I'm pointing out why it isn't a good one and won't help you understand the problem you're trying to understand.
    As I said I don't know of a 'perfect' analogy.

    I know it isn't a perfect analogy ... I don't have a perfect one for you... if you know of one for this concept ... by all means let me know ... the shell theorem itself is a flawed analogy ... no such shell is possible ... but that less than perfect analogy doesn't have to prevent the concept one is trying to describe from being explored ... similar to how he did with the Shell Theorem.

    Quote Originally Posted by MagiMaster View Post
    Of course there's a difference between a rope being pulled and a rope lying on the ground,
    Which is a piece of what I was trying to get at.

    If there is a difference ... then opposing influences / forces that balance out to zero ... is not the same as there not being a influence or force at all... at least in a general sense.

    As you already said before ... Gravity might be an exception to the general case ... I know of no test data proving gravity is or is not such an exception.

    Quote Originally Posted by MagiMaster View Post
    In the case of the hollow sphere, it's pulling on all the particles in all directions equally, so again, no internal stresses and nothing will ever break no matter how massive the shell.
    I thought the previous argument / position was that it did not pull in all directions equally ... ie Newton's basis for his shell theorem is incorrect ... that the pull itself actually ceased to exist ... not just pulled equally in all directions to balance all out , as the shell theorem goes for.

    Quote Originally Posted by MagiMaster View Post
    Finally, if there's no observable/measurable difference between nothing and a massive shell, there's no difference. Science only cares about things that are observable/measurable.
    I don't know about that.

    AFAIK Gravitons have never yet been observed ... nor measured ... but there is still a good bit of science about them.

    There was also lot and lots of science about Higgs Boson for many many decades before any type of actual observation / measurement.

    etc... etc... there are many such examples.

    Quote Originally Posted by MagiMaster View Post
    Then there's the relativistic definition of gravity, as the curvature of spacetime. In that case, the only way of getting no net gravitational force is for spacetime in that area to be flat. If it's flat, it's flat whether it's because there's nothing there or because the hollow shell smooths out all the curves. It has all the same effects either way.
    But in one there is a shell that exists ... and in the other there is nothing... no shell exists.
    Weather it seems like an observable difference or not ... they are not the same thing... and I am very cautious about claiming they are the same thing.
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    Quote Originally Posted by IamIan View Post
    does not prevent me from defining a region / piece of the whole ... and in that piece there is an inside to it and and outside to it.
    I don't doubt that, but I don't really see the significance. You see, the model upon which the idea of metric expansion is built is the Friedmann-Lemaitre-Robertson-Walker metric, which is an exact solution of the field equations describing a globally homogeneous and isotropic universe. And that is really all we can apply the concept of metric expansion to - a homogeneous and isotropic universe as a whole. Once you consider only a small subregion of a such a universe, you will find in general that this region is no longer homogeneous and/or isotropic, and hence the FLRW metric isn't a valid description of such a region, rendering the entire notion of metric expansion in such a volume of space meaningless, even if it is valid concept on global cosmological scales.

    although I have no doubt there are also others.
    Yes, and I think it is really important that we don't forget the overall context in all of this. Redshift is only one piece of the evidence ( albeit an important one ), and there are quite a few others; once all of these are considered together, we find that the Lambda-CDM model with metric expansion is the best available model that fits all available evidence.

    And Thanks for the additional explanations.
    No problem, it's a pleasure. It is often too easy to get carried away in the heat of a discussion ( and I am as guilty of that as anyone ); sometimes one must take a step back and examine just what it is that the other party bases their arguments on.
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    @IamIan, the shell theorem is not an analogy. It is a piece of mathematics. And it is relevant. A solid sphere can be viewed as a series of nested shells (see calculus) which has implications for the internal workings of stars and geologically active planets.

    With gravity, "pulled equally in all directions" and "no gravity at all" are the same due to the nature of gravity. The same would be true of electromagnetism except that (AFAIK) you can't make a magnetic field pull equally in all directions over an area (as opposed to at a single point).

    And what do gravitons have to do with anything?
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    Quote Originally Posted by IamIan View Post
    then opposing influences / forces that balance out to zero ... is not the same as there not being a influence or force at all... at least in a general sense.
    The issue here is that gravity is not a force; it is a geometric property of space-time. The notion of forces and force vectors stems from Newtonian physics, but what we are discussing here are cosmological models, all of which are routed in General Relativity.

    To make a long story short - for the interior region of a thin shell, Birkhoff's theorem tells us that the geometry of space-time must be Minkowskian. In other words, space-time is completely flat and thus gravity is completely absent. It is not the "balancing out" of otherwise large forces.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by IamIan View Post
    does not prevent me from defining a region / piece of the whole ... and in that piece there is an inside to it and and outside to it.
    I don't doubt that, but I don't really see the significance. You see, the model upon which the idea of metric expansion is built is the Friedmann-Lemaitre-Robertson-Walker metric, which is an exact solution of the field equations describing a globally homogeneous and isotropic universe. And that is really all we can apply the concept of metric expansion to - a homogeneous and isotropic universe as a whole. Once you consider only a small subregion of a such a universe, you will find in general that this region is no longer homogeneous and/or isotropic, and hence the FLRW metric isn't a valid description of such a region, rendering the entire notion of metric expansion in such a volume of space meaningless, even if it is valid concept on global cosmological scales.
    It's just the definition of 2 dimensional space or 3 dimensional space ... Define points , and you have defined an inside that is between those points and an outside that is outside those points... 3 points can give a 2 dimensional plane (triangle) ... 4 points can give a 3 dimension space.

    The core question / concept I have been trying to get at from the beginning ... is weather the expansion is caused by influences from inside that specific region that is expanding ... or if it is caused by influences from outside of that specific region that is expanding ... so it is significant because it is a core part of the concept I'm curious about... yes there are other regions ... a whole universe of them ... the question was about the point of view of asking about a specific region ... any one specific region.

    Quote Originally Posted by Markus Hanke View Post
    although I have no doubt there are also others.
    Yes, and I think it is really important that we don't forget the overall context in all of this. Redshift is only one piece of the evidence ( albeit an important one ), and there are quite a few others; once all of these are considered together, we find that the Lambda-CDM model with metric expansion is the best available model that fits all available evidence.
    AFAIK ... that still just leaves us with ... there is an expansion ... and doesn't get at weather the cause of the expansion for a given region is caused from the inside of that region or from the outside of that region.

    Quote Originally Posted by Markus Hanke View Post
    And Thanks for the additional explanations.
    No problem, it's a pleasure. It is often too easy to get carried away in the heat of a discussion ( and I am as guilty of that as anyone ); sometimes one must take a step back and examine just what it is that the other party bases their arguments on.
    +1
    I also , am not a PHD in physics ... or anything like that ... purely amateur with a little interest ... so it no doubt doesn't help if I don't use 100% exactly the right (industry / academic) phrasing or terminology , in efforts to explain a concept in my head.

    - - - - - - - - - - - -

    Quote Originally Posted by MagiMaster View Post
    @IamIan, the shell theorem is not an analogy. It is a piece of mathematics. And it is relevant. A solid sphere can be viewed as a series of nested shells (see calculus) which has implications for the internal workings of stars and geologically active planets.
    Of course it's relevant ... never said it wasn't ... if anything it is a prime example of an analogy built on flaws but despite those flaws is still useful to explore the concept itself being examined... which is why it is useful for those other things you listed ... despite it's flaws ... not because it has no flaws.

    Of course it is an analogy.
    If you have __ and ___ and ___ etc ... then he applied the mathematics to that... it was not just math.

    For those stars and such ... the Shell theorem is again used as an analogy for those other star and such systems.

    He assumed a perfect sphere ... no doubt because it made the math easier etc ... which would in itself not be possible due to fluctuations of atoms and subatomic particles ... he assumed a uniform density ... which is also not possible ... atoms and atomic structures don't have uniform density ... he assumed it was arbitrarily thin ... which is also not possible ... etc ... etc. ... but the important thing ... is not the flaws ... but the concept being explored ... seeing the forest past the trees ... and despite those flaws ... the concept is explored... and was/is useful... even though the assumptions used in building the analogy are flawed and it would not actually be possible in the real world.

    Quote Originally Posted by MagiMaster View Post
    With gravity, "pulled equally in all directions" and "no gravity at all" are the same due to the nature of gravity. The same would be true of electromagnetism except that (AFAIK) you can't make a magnetic field pull equally in all directions over an area (as opposed to at a single point).
    That might very well be the case ... But as I said the last time we came to this particular point.

    AFAIK there is no observational test data to confirm this either way ... true or not true about gravity being a special case ... it seems as a general case , it is not true for other non-gravity interactions.

    I do not know if it is or is not an exception ... I am not aware of any test data either way... therefore , in the lack of that test data ... I do not feel confident in making either claim.

    Quote Originally Posted by MagiMaster View Post
    And what do gravitons have to do with anything?
    My reference to Gravitons and Higgs Boson were both a reply to your comment :

    "Science only cares about things that are observable/measurable."

    Instead of just saying I wasn't sold on your claim ... I was trying to explain why I am not sold on that claim about science ... I see examples of there being lots of science done about things that were not observed or measured ... and might not even exist... and yet there is still science about them.

    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by IamIan View Post
    then opposing influences / forces that balance out to zero ... is not the same as there not being a influence or force at all... at least in a general sense.
    The issue here is that gravity is not a force; it is a geometric property of space-time. The notion of forces and force vectors stems from Newtonian physics, but what we are discussing here are cosmological models, all of which are routed in General Relativity.

    To make a long story short - for the interior region of a thin shell, Birkhoff's theorem tells us that the geometry of space-time must be Minkowskian. In other words, space-time is completely flat and thus gravity is completely absent. It is not the "balancing out" of otherwise large forces.
    Which would make gravity the exception to the general case ... which very well might be correct ... it does seem to agree with current models and accepted theories ... but I am not aware of any observed test data yet confirming or denying this gravitational exception ... as such I do not feel confident to claim it is or is not such an exception... instead I'll say , I don't know yet.
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    Quote Originally Posted by IamIan View Post
    Of course it is an analogy.
    If you have __ and ___ and ___ etc ... then he applied the mathematics to that... it was not just math.

    For those stars and such ... the Shell theorem is again used as an analogy for those other star and such systems.
    Perhaps you are confusing the words "analogy" and "idealisation".

    The shell theory is not an analogy, it is an accurate mathematical description of how gravity works. Now, it might only be perfectly accurate in a perfectly idealised model. That is probably true of all theories. But that doesn't stop it being useful as an accurate model of the real world.

    If it were an analogy it would (a) not have any math and (b) would involve things like apples or unicorns.

    "Science only cares about things that are observable/measurable."

    Instead of just saying I wasn't sold on your claim ... I was trying to explain why I am not sold on that claim about science ... I see examples of there being lots of science done about things that were not observed or measured ... and might not even exist... and yet there is still science about them.
    There is a big difference between observable and observed. Science only deals with the former, even if it can't (yet) do the latter.

    There are many, many examples of this. The discovery of Neptune (the original "dark matter") or and neutrinos (the other original "dark matter"). These were both part of science, based on indirect observations, long before they could be observed. But they were both, in principle, observable from the moment they were hypothesized.
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    @IamIan, I was going to say about the same thing Strange said, but he said it better.

    If the shell theorem (or more accurately, Birkhoff's theorem) were false, I'm pretty sure it would have significant consequences for the inner workings of massive stars and planets. After all, you can conceptually separate an imperfect sphere into a smaller perfect sphere plus some surface imperfections, and in the case of a star, the gravity from those surface imperfections is going to be pretty minuscule. The motions of individual atoms will have a completely imperceptible effect on things (theoretically calculable, but probably not measurable).
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    Furthermore (following on from what Strange and MagiMaster said), the curvature of spacetime inside a hollow shell results only from the difference between the actual matter distribution and the best-fit spherical idealisation of the actual matter distribution. In other words, an approximately spherical matter distribution will still produce an approximately flat interior spacetime. Invoking atomic scale differences between the actual distribution and the perfectly spherical distribution is not going to lead to a significant interior curvature.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by IamIan View Post
    the question was about the point of view of asking about a specific region ... any one specific region.
    If we isolate a specific region of space-time, and demand it to be approximately homogeneous and isotropic ( as would the space between galaxies be ), then the FLRW metric would still be a valid solution within that region. What this solution to the field equations is telling us is that the metric expansion is a geometric property of space-time within that region itself, and not due to outside influences.

    So long as we consider a system that is homogeneous and isotropic, the FLRW metric can be applied both to a subregion and the universe as a whole; in both cases it is a solution to the Einstein field equations, so no "outside" influences are relied on.

    AFAIK ... that still just leaves us with ... there is an expansion ... and doesn't get at weather the cause of the expansion for a given region is caused from the inside of that region or from the outside of that region.
    Metric expansion is a geometric property of space-time, and a direct result of the Einstein field equations which form the basic law of geometrodynamics. There are no "inside" and "outside" influences, only pure geometry.
    Tell me, how much do you really know about the meaning of the Einstein field equations ?

    Which would make gravity the exception to the general case
    Well, gravity is the only one of the fundamental interactions which is described geometrically; all the others are formulated in terms of quantum field theories.

    but I am not aware of any observed test data yet confirming or denying this gravitational exception
    The currently accepted theory of gravity, General Relativity, has been exhaustively tested and found to be in accordance with experiment and observation. There is a sticky in the "Physics" section of this forum which details some of these tests, you may want to take a look there.
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by IamIan View Post
    Of course it is an analogy.
    If you have __ and ___ and ___ etc ... then he applied the mathematics to that... it was not just math.

    For those stars and such ... the Shell theorem is again used as an analogy for those other star and such systems.
    Perhaps you are confusing the words "analogy" and "idealisation".

    The shell theory is not an analogy, it is an accurate mathematical description of how gravity works. Now, it might only be perfectly accurate in a perfectly idealised model. That is probably true of all theories. But that doesn't stop it being useful as an accurate model of the real world.

    If it were an analogy it would (a) not have any math and (b) would involve things like apples or unicorns.
    Bold added ... Exactly my point... We choose to look past the flaws to make good use of it despite those flaws ... not because it doesn't have any flaws.

    As for the analogy part ... I disagree ... weather something does or does not contain math is not the criteria for being or not being an analogy.

    Dictionary ... definition of Analogy:
    "a similarity between like features of two things, on which a comparison may be based: the analogy between the heart and a pump."


    By definition ... The shell theorem is an analogy any time and every time anyone ever uses it to compare like features between it and other things ... such as stars ... uniform gravitation in all directions ... etc.

    The very act of claiming that being in that shell is gravitationally the same as there being no shell at all ... is itself an analogy... math or no math ... that's what it means to be an analogy.

    Quote Originally Posted by Strange View Post
    "Science only cares about things that are observable/measurable."

    Instead of just saying I wasn't sold on your claim ... I was trying to explain why I am not sold on that claim about science ... I see examples of there being lots of science done about things that were not observed or measured ... and might not even exist... and yet there is still science about them.
    There is a big difference between observable and observed. Science only deals with the former, even if it can't (yet) do the latter.

    There are many, many examples of this. The discovery of Neptune (the original "dark matter") or and neutrinos (the other original "dark matter"). These were both part of science, based on indirect observations, long before they could be observed. But they were both, in principle, observable from the moment they were hypothesized.
    It is very difficult ... if not impossible ... to say what is or is not ever possible to be observable... even if we do not now see how it could be done today.

    For example ... in theory ... if gravitons do exist ... and can be detected ... than one would be able to observe via those gravitons the difference between no gravity from no shell ... and no net gravity from the a perfect shell around you.

    Or ... for example ... if Anti-Gravity can ever be created in a directional method like seen in some sci-Fi shows ... than that too would make it possible to be able to do a test and observe the difference between no shell and shell... as you could then create anti-gravity in one direction but not others.

    - - - - - - - - - - - - - - - - -

    Quote Originally Posted by MagiMaster View Post
    If the shell theorem (or more accurately, Birkhoff's theorem) were false, I'm pretty sure it would have significant consequences for the inner workings of massive stars and planets. After all, you can conceptually separate an imperfect sphere into a smaller perfect sphere plus some surface imperfections, and in the case of a star, the gravity from those surface imperfections is going to be pretty minuscule. The motions of individual atoms will have a completely imperceptible effect on things (theoretically calculable, but probably not measurable).
    And maybe ... someday that will lead to someone devising and performing a series of proper scientific peer reviewed tests ... that would show data that gravity is or is not an exception to the general case.... I look forward to that ... but I am not aware of it having yet been done.

    Even if there is something off with those theorems ... it might be a very tiny difference ... like using F=MA for the motion of a arrow instead of relativistic equations combined with quantum mechanical equations.

    - - - - - - - - - - - - - - - -

    Quote Originally Posted by KJW View Post
    Furthermore (following on from what Strange and MagiMaster said), the curvature of spacetime inside a hollow shell results only from the difference between the actual matter distribution and the best-fit spherical idealisation of the actual matter distribution. In other words, an approximately spherical matter distribution will still produce an approximately flat interior spacetime. Invoking atomic scale differences between the actual distribution and the perfectly spherical distribution is not going to lead to a significant interior curvature.
    No argument there... but I will add a note:

    significance is relative ... And any difference at all. On any scale. Is still a difference.... even if that difference is not viewed as significant.

    - - - - - - - - - - - - - - - - -

    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by IamIan View Post
    the question was about the point of view of asking about a specific region ... any one specific region.
    If we isolate a specific region of space-time, and demand it to be approximately homogeneous and isotropic ( as would the space between galaxies be ), then the FLRW metric would still be a valid solution within that region. What this solution to the field equations is telling us is that the metric expansion is a geometric property of space-time within that region itself, and not due to outside influences.

    So long as we consider a system that is homogeneous and isotropic, the FLRW metric can be applied both to a subregion and the universe as a whole; in both cases it is a solution to the Einstein field equations, so no "outside" influences are relied on.
    So after only 10 days of trying to explain the concept in my head ... I can finally get an answer ... those field equations claim its from the inside.
    thanks

    Quote Originally Posted by Markus Hanke View Post
    Metric expansion is a geometric property of space-time, and a direct result of the Einstein field equations which form the basic law of geometrodynamics. There are no "inside" and "outside" influences, only pure geometry.
    Tell me, how much do you really know about the meaning of the Einstein field equations ?
    As I said ... I'm no PHD in physics ... purely an armature with some interest ... So I am slightly aware of the field equations , but I am no expert.

    If those field equations do not account for inside or outside influences on a specific region of study ... than are they still accurate to be used to study a specific region ... if we happen to know ... via other methods ... that the region is being influenced by things from outside of that specific region of study? ... and how well does it continue to work as it looses Isotropy and homogeneous on smaller scale regions?

    Quote Originally Posted by Markus Hanke View Post
    but I am not aware of any observed test data yet confirming or denying this gravitational exception
    The currently accepted theory of gravity, General Relativity, has been exhaustively tested and found to be in accordance with experiment and observation. There is a sticky in the "Physics" section of this forum which details some of these tests, you may want to take a look there.
    AFAIK there have been tests that show less than perfect matches with GR and test data ... thus the on going work to find a Grand Unified Theory.

    I am also not saying GR is wrong ... the question was about observational data to confirm or deny gravity is an exception ... even if GR is 100% correct ... I would still be interested in that test data ... maybe I'll have it / find it someday.
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    You're wrong on at least two points.

    First, significant here does not mean "something someone thinks is important" but statistically significant which is something that is measurably different from random chance. So no, the difference is not significant no matter what anyone thinks.

    Second, though similarly, it is often very possible, and sometimes even trivial, to distinguish things that are theoretically observable from those that aren't based on the correspondence principal if nothing else.

    Also, we know GR is wrong (in a sense) not from any specific observations but because it doesn't work with QM even though they are both highly accurate models of reality, just at different scales.
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    Quote Originally Posted by IamIan View Post
    So after only 10 days of trying to explain the concept in my head ... I can finally get an answer ... those field equations claim its from the inside.
    thanks
    Ok, I still don't agree with your way of trying to distinguish "inside" and "outside", but I suppose I can live with the above statement. The field equations are local equations at each point in space-time, so in that sense they are "inside".

    than are they still accurate to be used to study a specific region
    Yes. As mentioned above, the field equation is a geometric law which is valid everywhere in space-time; you can consider any region of your choice, provided you have the ability to define proper boundary conditions, and a proper expression for the distribution of energy-momentum. For example, one of the simplest solutions is the Schwarzschild metric, which describes space-time inside ( interior SM ) and around ( exterior SM ) an isolated static, spherically symmetric body without angular momentum or electric charge. Of course in reality no such "ideal" body exists, but very many systems can be approximated as such to a very high degree. If more accuracy is needed one can start considering things like angular momentum, non-isotropic mass distributions, and the influences of gravitational waves, and thus arrive at different ( more complicated ) solutions. However, all these multitudes of metrics obey the same underlying geometric law - the Einstein field equations, which are valid everywhere.

    that the region is being influenced by things from outside of that specific region of study?
    A net flow of energy in and out of system can also be modelled through the field equations; what happens is simply that the resulting solutions are no longer static, i.e. that the metric coefficients now depend on time, or some other affine parameter. Plane gravitational waves are one example of such a solution.

    nd how well does it continue to work as it looses Isotropy and homogeneous on smaller scale regions?
    It still works, but becomes more difficult to handle. Generally speaking, the more symmetries your system has ( such as homogeneity, isotropy, spherical symmetry etc ), the better your chances of finding a closed analytical solution to the field equations. As you abandon symmetries, the maths become more and more complicated, and oftentimes all you can do then is try and find numerical solutions which can then be plotted with appropriate software on a computer. But again, the underlying geometric law remains always valid, no matter how simple or complicated the system is.

    AFAIK there have been tests that show less than perfect matches with GR and test data
    Such as...?

    thus the on going work to find a Grand Unified Theory.
    I think you mean quantum gravity. Such a model is needed in specific circumstances, like for example the gravitational collapse of a body, where quantum effects become important. Standard GR does not incorporate such quantum effects, it is a purely classical theory, and as such it will fail in all scenarios where such effects cannot be neglected. The realm of quantum physics is outside the domain of applicability of GR, and vice versa also - quantum physics cannot adequately model gravitational effects and interactions. Unifying these two is one of the ultimate goals of physics, and it is an area of active and ongoing research.

    the question was about observational data to confirm or deny gravity is an exception
    I am not quite sure what you mean here. To tell whether something is an exception, we must first know the rule - in this case, that would be a unified model of all fundamental interactions. We don't have such a model yet, and even if we did, there wouldn't be any way to experimentally test it. Our current technology and resources would fail to probe the required energy domain by many orders of magnitude. But maybe someday in the future, who knows...
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    The very act of claiming that being in that shell is gravitationally the same as there being no shell at all ... is itself an analogy... math or no math ... that's what it means to be an analogy.
    It still holds true, though. An observer cannot distinguish ( in his own local frame ) being inside a thin shell from being far away outside all gravitational influences, based on just observations about his gravitational environment. In both cases space-time is completely flat, which can be directly tested, given sensitive enough instruments. One way would for example be to "stake out" a very large triangle around you, and measure the sum of its angles; in both cases it will be exactly 180 degrees.

    For example ... in theory ... if gravitons do exist ... and can be detected ... than one would be able to observe via those gravitons the difference between no gravity from no shell ... and no net gravity from the a perfect shell around you.
    No, there would be no difference. In neither one of the two cases would you detect any gravitons at all.

    than that too would make it possible to be able to do a test and observe the difference between no shell and shell
    Again, it would not be possible to devise such a test, since the outcome would be the exact same with or without the shell.

    Even if there is something off with those theorems
    In order for Birkhoff's theorem to be false, GR must be wrong. And not only GR - even Newton's theory would have to be wrong, because the Shell Theorem is a direct result of it. Both Newton and GR agree on the outcome in this instance. How likely, do you think, is this really, as opposed to the possibility of yourself perhaps not being entirely correct ? I don't wish to be condescending or anything, but in cases of "me vs basic laws of physics", all of us are generally well advised to take a step back and examine our own understanding of things. Usually it turns out that we haven't completely understood something, and it presents us with an opportunity to learn. Actual instances where basic laws of physics are shown to be incorrect are mercifully rare these days, and for good reasons.

    Also remember that Birkhoff's theorem is not just of physical significance, it has a purely mathematical interpretation as well, and can be rigorously proven in that context.
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    Quote Originally Posted by MagiMaster View Post
    You're wrong on at least two points.

    First, significant here does not mean "something someone thinks is important" but statistically significant which is something that is measurably different from random chance. So no, the difference is not significant no matter what anyone thinks.
    Your #1 is completely off base.

    A> I don't know who you are quoting there ... but that is not what I wrote about significance... as such has no barring on my comment about significance.

    B> Bold ... Disagreeing with something I never claimed ... has nothing to do with what I claimed.

    C> My comment about significance was in reply to KJW ... who was not in his post using the term as you describe it there ... he specifically described a situation ... and claimed that atomic scale differences doesn't lead to significant interior curvature ... this use of the term ... is not about random chance ... in that comment he was defining a scale that he thought difference would not be significant.

    D> I agreed with KJW's post... and posted such.

    Quote Originally Posted by MagiMaster View Post
    Second, though similarly, it is often very possible, and sometimes even trivial, to distinguish things that are theoretically observable from those that aren't based on the correspondence principal if nothing else.
    AFAIK this agrees with statements I've already made ... so I don't see your point ?

    - - - - - - - - - - - -

    Quote Originally Posted by Markus Hanke View Post
    AFAIK there have been tests that show less than perfect matches with GR and test data
    Such as...?
    AFAIK ... some of the tests that have supported QM... you point out this same concept in your next comment.

    Quote Originally Posted by Markus Hanke View Post
    thus the on going work to find a Grand Unified Theory.
    I think you mean quantum gravity. Such a model is needed in specific circumstances, like for example the gravitational collapse of a body, where quantum effects become important. Standard GR does not incorporate such quantum effects, it is a purely classical theory, and as such it will fail in all scenarios where such effects cannot be neglected. The realm of quantum physics is outside the domain of applicability of GR, and vice versa also - quantum physics cannot adequately model gravitational effects and interactions. Unifying these two is one of the ultimate goals of physics, and it is an area of active and ongoing research.
    Quote Originally Posted by Markus Hanke View Post
    the question was about observational data to confirm or deny gravity is an exception
    I am not quite sure what you mean here. To tell whether something is an exception, we must first know the rule - in this case, that would be a unified model of all fundamental interactions. We don't have such a model yet, and even if we did, there wouldn't be any way to experimentally test it. Our current technology and resources would fail to probe the required energy domain by many orders of magnitude. But maybe someday in the future, who knows...
    Exactly.

    Quote Originally Posted by Markus Hanke View Post
    The very act of claiming that being in that shell is gravitationally the same as there being no shell at all ... is itself an analogy... math or no math ... that's what it means to be an analogy.
    It still holds true, though.
    The Comment wasn't about weather it was true or not ... The comment was in reply to claims that it was not an analogy ... which it is an analogy.

    Quote Originally Posted by Markus Hanke View Post
    For example ... in theory ... if gravitons do exist ... and can be detected ... than one would be able to observe via those gravitons the difference between no gravity from no shell ... and no net gravity from the a perfect shell around you.
    No, there would be no difference. In neither one of the two cases would you detect any gravitons at all.
    I have to go re-read about gravitons then ... that is different than what I've been told previously.

    Quote Originally Posted by Markus Hanke View Post
    if Anti-Gravity can ever be created in a directional method like seen in some sci-Fi shows ... than that too would make it possible to be able to do a test and observe the difference between no shell and shell
    Again, it would not be possible to devise such a test, since the outcome would be the exact same with or without the shell.
    You forgot the most important part of that quote of mine you are referencing ... so I put it back in.

    If that directional Anti-Gravity technology were ever created ... it would no longer be the same outcome ... because it would allow a test where you could turn off the gravity from one direction.

    For example ... you could effectively make the sphere no longer gravitationally a sphere ... even though it was actually still a sphere ... you could remove the gravitational effect of the mass from say 25% section of the sphere ... which would be gravitationally the same as making an object that was almost a sphere but leaving one end open , with no mass in that open area ... in which case it is not the same any more... and there would be places inside of that actual sphere that would have gravity ... because a section of the sphere's mass would gravitationally no longer exist.... IF we ever had such a type of technology... that's a big IF.

    Quote Originally Posted by Markus Hanke View Post
    Even if there is something off with those theorems... it might be a very tiny difference ... like using F=MA for the motion of a arrow instead of relativistic equations combined with quantum mechanical equations.
    In order for Birkhoff's theorem to be false, GR must be wrong. And not only GR - even Newton's theory would have to be wrong, because the Shell Theorem is a direct result of it.
    You cut off the important part again ... so again I put it back.

    F=MA from Newton is by itself wrong ... we know know better ... but just like I wrote above ... the difference between them can be tiny in some cases ... an archer may not care at all about the tiny difference... which is why ... in some cases today F=MA is 'good enough'... and is still used.

    Just because something is wrong ... does not mean it is useless ... or 100% completely wrong ... just like the tiny difference if you only use F=MA for the arrow... it might not be significant to an archer ... but such differences might be significant to a quantum physicist.
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    Quote Originally Posted by IamIan View Post
    Quote Originally Posted by MagiMaster View Post
    You're wrong on at least two points.

    First, significant here does not mean "something someone thinks is important" but statistically significant which is something that is measurably different from random chance. So no, the difference is not significant no matter what anyone thinks.
    Your #1 is completely off base.

    A> I don't know who you are quoting there ... but that is not what I wrote about significance... as such has no barring on my comment about significance.

    B> Bold ... Disagreeing with something I never claimed ... has nothing to do with what I claimed.

    C> My comment about significance was in reply to KJW ... who was not in his post using the term as you describe it there ... he specifically described a situation ... and claimed that atomic scale differences doesn't lead to significant interior curvature ... this use of the term ... is not about random chance ... in that comment he was defining a scale that he thought difference would not be significant.

    D> I agreed with KJW's post... and posted such.
    Quote Originally Posted by IamIan
    Quote Originally Posted by KJW
    ...
    significance is relative ... And any difference at all. On any scale. Is still a difference.... even if that difference is not viewed as significant.
    My point is, significance in the technical sense is not so relative and differences can be insignificant. And I'm pretty sure the gravity of a few atoms at a great distance is insignificant no matter what scale you're looking at.

    Quote Originally Posted by IamIan
    Quote Originally Posted by MagiMaster View Post
    Second, though similarly, it is often very possible, and sometimes even trivial, to distinguish things that are theoretically observable from those that aren't based on the correspondence principal if nothing else.
    AFAIK this agrees with statements I've already made ... so I don't see your point ?
    Quote Originally Posted by IamIan
    It is very difficult ... if not impossible ... to say what is or is not ever possible to be observable... even if we do not now see how it could be done today.
    You seem to be saying exactly the opposite here, and that's my point. It is rarely difficult to distinguish what could theoretically be observed from what couldn't.
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    Quote Originally Posted by IamIan View Post
    AFAIK ... some of the tests that have supported QM... you point out this same concept in your next comment.
    QM and GR don't actually contradict each other, they just aren't compatible. What I mean by this is that GR cannot model any quantum effects, and QM cannot model gravity. There are no experiments which actually contradict GR within its domain of applicability (!).

    I have to go re-read about gravitons then ... that is different than what I've been told previously.
    It boils down to the fact that gravitons ( if they exist ) would mediate only changes in gravity; this is why static gravitational fields can act instantaneously, whereas changes in the field propagate at or below the speed of light.

    If that directional Anti-Gravity technology were ever created ... it would no longer be the same outcome ... because it would allow a test where you could turn off the gravity from one direction.
    I don't get this - are you talking about anti-gravity, or about gravitational shielding ? They aren't the same things; the former would amount to a concentration of "exotic matter", which locally curves space-time in a way opposite to that of "normal" energy-momentum, as is done for example in the Alcubierre metric. The latter simply shields the effect of any existing gravity, i.e. it "flattens out" space-time. Which one are you talking about ?
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    Quote Originally Posted by MagiMaster View Post
    Quote Originally Posted by IamIan
    Quote Originally Posted by KJW
    ...
    No argument there... but I will add a note:
    significance is relative ... And any difference at all. On any scale. Is still a difference.... even if that difference is not viewed as significant.
    My point is, significance in the technical sense is not so relative and differences can be insignificant. And I'm pretty sure the gravity of a few atoms at a great distance is insignificant no matter what scale you're looking at.
    Seems like 3 claims to me.

    #1> I disagree.
    If anything it seems to me, the more technical you get the more it is required of you to specifically define exactly what you mean when you use a relative terms like 'significance'... it is that defining that narrows it ... not anything about the term itself... technically, the term itself is always relative ... just by the definition of the word.

    #2> I agree
    But I do not see an insignificant difference being the same as no difference.

    #3> I disagree
    The phrasing is to absolute and all encompassing for my taste ... Many, most, etc ... I'd be fine with ... but not the absolute way you phrased it... just a personal preference.

    Quote Originally Posted by MagiMaster View Post
    Quote Originally Posted by IamIan
    Quote Originally Posted by MagiMaster View Post
    Second, though similarly, it is often very possible, and sometimes even trivial, to distinguish things that are theoretically observable from those that aren't based on the correspondence principal if nothing else.
    AFAIK this agrees with statements I've already made ... so I don't see your point ?
    Quote Originally Posted by IamIan
    It is very difficult ... if not impossible ... to say what is or is not ever possible to be observable... even if we do not now see how it could be done today.
    You seem to be saying exactly the opposite here, and that's my point. It is rarely difficult to distinguish what could theoretically be observed from what couldn't.
    I didn't see them as exactly the opposite ... just a different emphasis ... you used qualifiers to soften the claim coming from one side ... I used qualifiers to soften it on the other ... both seemed to me to be pointing in the same general direction ... but thanks for at least explaining yourself further ... Now I see a difference more clearly.

    I disagree with the bold ... rarely difficult? ... that goes too far for my taste ... and is on the wrong side of assumed knowledge of the future and every possible situation.

    - - - - - - - - - - - - - - - - -

    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by IamIan View Post
    AFAIK ... some of the tests that have supported QM... you point out this same concept in your next comment.
    QM and GR don't actually contradict each other, they just aren't compatible. What I mean by this is that GR cannot model any quantum effects, and QM cannot model gravity. There are no experiments which actually contradict GR within its domain of applicability (!).
    Bold
    Is the key part of that.

    AFAIK I don't recall any part of the actual theory of GR that lists any limitation on it's 'domain of applicability', that would prevent it from being used at QM scales... I was under the impression , that people chose not to do so, because they knew it gave the wrong answers compared to observations ... not because of anything in the theory itself that prevented it from being used there.

    Is this 'applicability' you a limiting GR to ... actually part of the theory itself somewhere that I missed?

    Quote Originally Posted by Markus Hanke View Post
    I have to go re-read about gravitons then ... that is different than what I've been told previously.
    It boils down to the fact that gravitons ( if they exist ) would mediate only changes in gravity; this is why static gravitational fields can act instantaneously, whereas changes in the field propagate at or below the speed of light.
    Correct me if I'm wrong ... but two things occur to me.

    #1> If the observer inside the shell moved ... the change in the location of their own personal gravitational field would be mediated by those gravitons ... (if they exist) ... at or bellow the speed of light ... that propagation inside of the shell would have two different gravitational fields ... the field for where it was at point A and the field for where it was at point B ... thus a flux , ie change in the 'flatness' of the gravitational field of the shell interior ... for the duration of the graviton propagation.

    #2> If the shell itself where expanding , or rotating quickly ... wouldn't that too ... also create a propagation front?

    Quote Originally Posted by Markus Hanke View Post
    If that directional Anti-Gravity technology were ever created ... it would no longer be the same outcome ... because it would allow a test where you could turn off the gravity from one direction.
    I don't get this - are you talking about anti-gravity, or about gravitational shielding ? They aren't the same things; the former would amount to a concentration of "exotic matter", which locally curves space-time in a way opposite to that of "normal" energy-momentum, as is done for example in the Alcubierre metric. The latter simply shields the effect of any existing gravity, i.e. it "flattens out" space-time. Which one are you talking about ?
    I wasn't thinking of either of those two.
    I was thinking of something different.

    Two different kinds of thing ... both along the same 'path' of logic I was thinking of.

    #1> A Gravity effecting beam ... a variation of the gravity gun in AllStarSuperman Animated movie ... Basically it can increase or decrease the gravity of the mass of objects that are hit with it's beam ... If you fired such a fictional beam out in an arch toward the shell ... you could make some of the mass from pieces of the shell gravitationally no longer there ... they would still be there physically ... but they would no longer make any contribution to gravity... you could still move around the physical shell ... but piece of it would no longer be contributing to creating a flat gravitational space ... ie their would be places inside that shell that would now have gravity ... but if I fired the same beam out and there was no shell I will still have no gravity ... ie a test method that could see the difference.... as I already wrote though , it is a big IF.

    #2> A gravity object effecting ability like Graviton from Marvel Comics has ... he can increase or decrease the gravity of objects ... and he can do it directionally ... so he can use gravity to pull something toward him in one direction but have no change to the gravity in other directions ... using a fictional thing like that ... you could again ... gravitationally remove piece of the shell from existence ... they actually do still exist ... but no longer contribute toward the creation of the gravitational flat space inside the shell ... if you reached out with such a 'turn gravity off' effect and there was no shell there to turn the gravity off ... you would be able to have an observable test that gravitationally could detect the difference between sphere / shell and no shell.

    So those are today just purely sci-fi ... but if that kind of concept were ever to be possible ... it would allow for a method of gravitationally testing and observing the difference between a shell and no shell.

    At least that is the kind of concept I was thinking of... if that makes any more sense in understanding the concept.
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    Quote Originally Posted by IamIan View Post
    My point is, significance in the technical sense is not so relative
    Seems like 3 claims to me.

    #1> I disagree.
    If anything it seems to me, the more technical you get the more it is required of you to specifically define exactly what you mean when you use a relative terms like 'significance'... it is that defining that narrows it ... not anything about the term itself... technically, the term itself is always relative ... just by the definition of the word.
    You seem to be missing the point. Words can have more than one definition. In science, and particularly statistics, "significance" has a very specific meaning which is not the same as the common use of the word. It does not just mean something like, "I think that is important". It is an exact and formal way of determining when a value or effect can be ignored.
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    @IamIan, I don't really see how you get "absolute and all encompassing" from "a few atoms at a great distance," but like Strange said, you're still misunderstanding what I meant when I said insignificant.

    Also, it requires practically no knowledge of the future to tell whether something is even theoretically observable. Designing and building the tests to actually observe it might be far out of our reach and we may have to wait for various other breakthroughs to build such things, but just working out whether it could be observed is not so hard. Basically, by the time we know enough about something to ask whether or not it would be theoretically possible to observe it, we know enough to answer that (specific) question. (Feel free to point me at some historical instances where this hasn't been the case though.)

    Take Hawking radiation for example. We've never observed it and have no idea whether it really exists or not, but it's pretty easy to tell that it is theoretically observable. Just put a probe in near a black hole and look for energy in the appropriate spectrum. Close enough and the faintness of the signal won't matter. Actually getting a probe that close to a black hole is another matter entirely and we'll likely come up with better ways of detecting it long before then anyway.
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    Quote Originally Posted by IamIan View Post
    AFAIK I don't recall any part of the actual theory of GR that lists any limitation on it's 'domain of applicability', that would prevent it from being used at QM scales... I was under the impression , that people chose not to do so, because they knew it gave the wrong answers compared to observations ... not because of anything in the theory itself that prevented it from being used there.

    Is this 'applicability' you a limiting GR to ... actually part of the theory itself somewhere that I missed?
    As I have mentioned before, GR is a purely classical theory; as such it can only be meaningfully applied to classical systems, i.e. systems in which quantum effects can be neglected. This defines the theory's domain of applicability - it basically spans all macroscopic systems, the description of which does not rely on quantum mechanics. Any scenario where QM effects can not be neglected can't be meaningfully described by GR - so yes, this limitation is part of the theory itself, since it doesn't contain any mechanism to account for quantum effects.

    the change in the location of their own personal gravitational field would be mediated by those gravitons
    No, there would be no change in the gravitational field, since it is uniform everywhere within the interior region of the shell.

    that propagation inside of the shell would have two different gravitational fields ... the field for where it was at point A and the field for where it was at point B ... thus a flux , ie change in the 'flatness' of the gravitational field of the shell interior ... for the duration of the graviton propagation.
    No, that's wrong. The overall metric structure is of the general form



    where J(r) is determined by the boundary conditions at infinity ( i.e. the Minkowski metric ). In the interior region of the shell the Riemann curvature tensor for this metric vanishes everywhere, whereas outside the shell this reduces to the standard Schwarzschild metric. In other words - the interior region of the shell is completely flat and uniform, i.e. all points have the exact same curvature ( which is zero ), and there are no gradients anywhere. Hence there is no propagation or flux.

    If the shell itself where expanding , or rotating quickly ... wouldn't that too ... also create a propagation front?
    This is a difficult and highly non-trivial scenario, and you are now moving far away from the original setup. Basically what would happen is that you'd get a very specific form of gravitational radiation ( called Rossby-Haurwitz waves ), as well as frame-dragging effects at the interior and exterior boundaries of the shell. See here for a basic treatment of this. Needless to say that this destroys the original symmetries, so the Birkhoff theorem is not valid under these conditions, and space-time in the interior is not flat and not static either.

    At least that is the kind of concept I was thinking of... if that makes any more sense in understanding the concept.
    I'm sorry, but I can't really comment on comic book physics. I can only explain what GR tells us about real-world physics in these circumstances.
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    Quote Originally Posted by Strange View Post
    You seem to be missing the point. Words can have more than one definition. In science, and particularly statistics, "significance" has a very specific meaning which is not the same as the common use of the word. It does not just mean something like, "I think that is important". It is an exact and formal way of determining when a value or effect can be ignored.
    Bold is not what I was getting at at all... in the comment I made about significance.

    I'll try again with a different set of examples.

    For example:
    To the scientist doing QM the distance across an electron can be a significant distance.

    To the scientist doing Chemistry that electron distance might not be significant anymore, but other things are significant to the chemist that might not be significant to the scientist doing QM... or not equally significant.

    For the scientist doing geology ... or psychology ... or astrophysics ... etc... what is significant is relative... 1fm is not a universally significant distance to every scientist who does any kind of experiment or test ... weather that 1fm is or is not significant is relative.

    -------

    Quote Originally Posted by MagiMaster View Post
    Basically, by the time we know enough about something to ask whether or not it would be theoretically possible to observe it, we know enough to answer that (specific) question. (Feel free to point me at some historical instances where this hasn't been the case though.)
    The first that comes to mind is the multi-year back and forth about uncertainty Einstein had.

    A close second is the more recent 'black hole wars' with Hawking.

    I don't see either of those at that time being 'easy' or 'trivial' etc ... despite knowing enough at that time to be able to ask the question... they were debated ... and there was a back and forth ... even among people , who at the time had a vast amount of knowledge about the issues.

    ---------

    Quote Originally Posted by Markus Hanke View Post
    If the shell itself where expanding , or rotating quickly ... wouldn't that too ... also create a propagation front?
    This is a difficult and highly non-trivial scenario, and you are now moving far away from the original setup. Basically what would happen is that you'd get a very specific form of gravitational radiation ( called Rossby-Haurwitz waves ), as well as frame-dragging effects at the interior and exterior boundaries of the shell. See here for a basic treatment of this. Needless to say that this destroys the original symmetries, so the Birkhoff theorem is not valid under these conditions, and space-time in the interior is not flat and not static either.
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    Quote Originally Posted by Markus Hanke View Post
    I'm sorry, but I can't really comment on comic book physics. I can only explain what GR tells us about real-world physics in these circumstances.
    But do you see how such a case would allow for a test of shell or no shell to work? ... even if the device / ability needed to do such a test does not itself exist... and might never exist ... might not even ever be possible ... but if it ever did , we could do that test.
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    Quote Originally Posted by IamIan View Post
    Bold is not what I was getting at at all... in the comment I made about significance.

    I'll try again with a different set of examples.

    For example:
    To the scientist doing QM the distance across an electron can be a significant distance.

    To the scientist doing Chemistry that electron distance might not be significant anymore, but other things are significant to the chemist that might not be significant to the scientist doing QM... or not equally significant.

    For the scientist doing geology ... or psychology ... or astrophysics ... etc... what is significant is relative... 1fm is not a universally significant distance to every scientist who does any kind of experiment or test ... weather that 1fm is or is not significant is relative.
    OK. That is true. However, it might just be your careless use of language, but you still seem to be missing an important point. Significance (in this technical sense) is precisely quantifiable. So it is not just someone causally saying, "oh I don't care about that", rather they have done an analysis and determined that, for the conditions they are studying, it can be safely ignored.
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    @IamIan, neither of your examples of scientific arguments have anything to do with the question of whether or not the theories in question could in principle be observed. The arguments were which of the two theories were better given the absence of actual observations, which is the hard part.
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    Quote Originally Posted by IamIan View Post
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    No problem.

    But do you see how such a case would allow for a test of shell or no shell to work?
    Not really, no. Gravity shielding would have no effect at all, since there is no gravity inside the shell to start with; anti-gravity would simply create a local disturbance in space-time. Both of these would work the exact same way in the interior of a shell, and in free space, so you couldn't use this to distinguish between these cases.
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    hmmmm, anyone feel like indulging me in some questions on the effects of a spinning uniform shell on the gravity of the region within it? As well as the effect of a symmetric non-uniform shell on the gravity of the region within?

    For the first, In my mind, what makes sense is for the gravitational force inside of a hollow uniform shell spinning about an axis is a a tending for gravity to pull objects into the plane perpendicular to the axis of spin, and at the "equator" of the shell. I'm wondering if that would be accurate?

    The second question, for a non-uniform shell, I'm envisioning a shell that is uniform and symmetric across a cross-section about it's axis where the cross section would be a crescent. This shell, however, would not be rotating. For this, I would imagine the same result as the case above, though I suspect there are important differences that make this case different, with a different result. Any insight?
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    Quote Originally Posted by Strange View Post
    OK. That is true. However, it might just be your careless use of language, but you still seem to be missing an important point. Significance (in this technical sense) is precisely quantifiable. So it is not just someone causally saying, "oh I don't care about that", rather they have done an analysis and determined that, for the conditions they are studying, it can be safely ignored.
    I don't see how I'm missing that point ... as I'm not claiming that casual concept.

    - - - - - - - - -

    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by IamIan
    Quote Originally Posted by Markus Hanke
    I'm sorry, but I can't really comment on comic book physics. I can only explain what GR tells us about real-world physics in these circumstances.
    But do you see how such a case would allow for a test of shell or no shell to work?
    Not really, no. Gravity shielding would have no effect at all, since there is no gravity inside the shell to start with; anti-gravity would simply create a local disturbance in space-time. Both of these would work the exact same way in the interior of a shell, and in free space, so you couldn't use this to distinguish between these cases.

    Not about the effects you listed ... I was specifically referencing your reply about the effects I listed ... which were different from these two that you listed.

    Do you see how the two that I listed could allow for that test result?

    - - - - - - - - -

    Quote Originally Posted by MagiMaster View Post
    Quote Originally Posted by IamIan
    Quote Originally Posted by MagiMaster
    Basically, by the time we know enough about something to ask whether or not it would be theoretically possible to observe it, we know enough to answer that (specific) question. (Feel free to point me at some historical instances where this hasn't been the case though.)
    The first that comes to mind is the multi-year back and forth about uncertainty Einstein had.

    A close second is the more recent 'black hole wars' with Hawking.
    @IamIan, neither of your examples of scientific arguments have anything to do with the question of whether or not the theories in question could in principle be observed. The arguments were which of the two theories were better given the absence of actual observations, which is the hard part.
    Bold = Well that should make 100% sense ... as that was not what was asked to have examples of.

    This particular discussion came from ... and is about ... the claim about how easy or hard it is to be able to know what things are or are not observable .... you on the one side of it ... where you claiming it is usually fairly easy "rarely difficult" ... in the above you claiming once we can ask we already know the answer ... and me on the other side of it were I claim it can be very hard to determine what is or is not observable.

    AFAIK ... The two examples I gave are directly about this issue ... both are about the observation of something ... both showing a clear example of it being hard ( not easy ) ... even for highly knowledgeable people on such issues ... who disagreed for years in both of the examples given ... both are well documented historical examples ... as such they are both 100% completely on point ... and are both examples that meet what you asked me to provide you examples of.
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    No, neither of your examples are about whether the theory could be observed at all. The arguments were about which is most worth pursuing in the absence of observations, but in none of the cases were there any arguments about whether it would be possible in principle, which is what I'm saying is easy. Actually performing the observations is often very difficult, but that's not what I'm saying.

    And you may not be claiming that you're using the common definition of significant, but your use of the word suggests you don't understand the technical definition either. The differences you have been talking about are insignificant in a measurable, technical sense.

    But at this point, I think it's safe to say nothing we can say will convince you of your errors, so I'm inclined to leave it at that.
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    Quote Originally Posted by Arcane_Mathematician View Post
    For the first, In my mind, what makes sense is for the gravitational force inside of a hollow uniform shell spinning about an axis is a a tending for gravity to pull objects into the plane perpendicular to the axis of spin, and at the "equator" of the shell. I'm wondering if that would be accurate?
    No, at least not to the best of my knowledge. What happens is that the rotating shell "drags along" space-time in the interior regions, thereby creating both Coriolis and centrifugal forces, which can in principle be detected. There is, however, no force present which would result in an accumulation of mass in the equatorial plane. Michal Heller gives a basic overview of the main results of such a scenario here.

    The second question, for a non-uniform shell, I'm envisioning a shell that is uniform and symmetric across a cross-section about it's axis where the cross section would be a crescent. This shell, however, would not be rotating. For this, I would imagine the same result as the case above, though I suspect there are important differences that make this case different, with a different result. Any insight?
    I must admit that I am not completely certain what it is you are suggesting here - can you explain once again how density is distributed in your shell ?
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    Quote Originally Posted by Markus Hanke View Post
    What happens is that the rotating shell "drags along" space-time in the interior regions, thereby creating both Coriolis and centrifugal forces, which can in principle be detected.
    I'll add that the rotation of the spherical shell reduces the symmetry from spherical to axial, thus the specified conditions of Birkhoff's theorem no longer hold. However, frame-dragging is an extremely tiny effect, much smaller in magnitude than the effects of spacetime curvature associated with gravitation. However, it has been successfully detected by the Gravity Probe B experiment. It should be noted that the rotation represents an extremely small deviation from spherical symmetry, thus Birkhoff's theorem still holds apart from the frame-dragging (assuming that the shape of the spherical shell is not distorted by the rotation).
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    Quote Originally Posted by MagiMaster View Post
    No, neither of your examples are about whether the theory could be observed at all.The arguments were about which is most worth pursuing in the absence of observations, but in none of the cases were there any arguments about whether it would be possible in principle, which is what I'm saying is easy. Actually performing the observations is often very difficult, but that's not what I'm saying.
    Reminder of what you asked for: (Bold added)
    Quote Originally Posted by MagiMaster
    Basically, by the time we know enough about something to ask whether or not it would be theoretically possible to observe it, we know enough to answer that (specific) question. (Feel free to point me at some historical instances where this hasn't been the case though.)
    This issue was NOT about the theory itself being able to be observed or not.
    It was NOT about how easy it is to perform the observation itself or not.

    The Issue was about how easy or hard it is to know if some specific thing is or is not itself ever observable even theoretically so ... with any arbitrary amount of scientific advancement in the future ... you claiming it is easy to know ... that if we can ask the question we already know the answer .... I claim it can be hard to know, even if we know enough to be able to ask the question... I am NOT claiming it is always hard.

    I gave 2 examples that are 100% on point about this... that is can be hard to know.

    #1> Einstine's multi-year debates on quantum uncertainty.
    #2> Hawkings more recent multi-year debate colloquially known as the "the black hole war".

    Both examples are well known historical cases ... as you asked for.

    Both examples are directly about weather some specific thing could be observed or not ... even if just in theory... as you asked for.

    Both examples clearly demonstrate experts in the field debating for many years ... despite knowing enough to be able to ask the question ... it was still debated and researched for many years ... the answer was NOT known, just because they knew enough to ask the question ... it was HARD... again examples exactly what you asked for.

    Quote Originally Posted by MagiMaster View Post
    And you may not be claiming that you're using the common definition of significant, but your use of the word suggests you don't understand the technical definition either. The differences you have been talking about are insignificant in a measurable, technical sense.

    But at this point, I think it's safe to say nothing we can say will convince you of your errors, so I'm inclined to leave it at that.
    I am convinced that you misunderstood ... read into a comparatively simple statement I made ... read into it something I never claimed ... and then ... despite me many times now trying to correct that misconception ... and trying to point out what I have and have not claimed ... the same misconception persists as a piece of fiction.

    For the heck of it ... because I like a long shot / underdog ... I'll try one last time ... although I have doubts if there is any point ... after this ... sure ... think whatever you like ... even if it is just a piece of fiction that I have never claimed.

    About 'significance', I claim:
    significance is relative ... the same results of 1fm ( for example) will NOT be significant in every context ... the same results that are significant in QM are not necessarily significant in SR , or biology , etc ... I HAVE NEVER ONCE , claimed that the determination of significance is just some casual personal opinion , where everyone is correct , no matter what the result is ... weather a particular result is or is not significant will be determined by the relevant scientific community ... individual scientist do not necessarily speak for the whole of that relevant scientific community ... The whole of that relevant scientific community finds in the past for what counted for being significant or not , form a historical pattern that scientists in that field are able to use as a basis to have a reasonably accurate idea of weather that relevant scientific community will find some other specific result significant or not , even before those specific results are given via peer review to that relevant scientific community..... = significance is relative

    I honestly do hope you can see it.
    But ... Either way ... I'm done with it.
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    Significance has nothing to do with a consensus, even within a specific field. It is a quantifiable, numerical, statistical property of a specific measurement or series of measurements and is basically related to the concept of a signal-to-noise ratio.

    The differences you have been talking about, those of atomic variations in a shell (i.e. the difference between an idealized shell and a somewhat realistic one), are going to be insignificant to any measuring device period because they will be drowned out even by the thermal noise from the cosmic microwave background.

    Also, the arguments Einstein had against uncertainty had nothing to do with whether or not such things could in principle be observed. Einstein simply didn't like the idea that the universe was fundamentally random and kept looking for ways to make it deterministic again (without success), but at no point did he argue that it was unobservable. Bohr-Einstein debates

    The question of observability almost always seems to be very easy to answer compared to all of the other questions that come up and especially compared to the engineering challenges involved in actually making the observations.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by Arcane_Mathematician View Post
    For the first, In my mind, what makes sense is for the gravitational force inside of a hollow uniform shell spinning about an axis is a a tending for gravity to pull objects into the plane perpendicular to the axis of spin, and at the "equator" of the shell. I'm wondering if that would be accurate?
    No, at least not to the best of my knowledge. What happens is that the rotating shell "drags along" space-time in the interior regions, thereby creating both Coriolis and centrifugal forces, which can in principle be detected. There is, however, no force present which would result in an accumulation of mass in the equatorial plane. Michal Heller gives a basic overview of the main results of such a scenario here.
    I could be mistaken, but from the inertial reference frame of any point within the shell, wouldn't the density distribution of the rotating shell change, due to the different speeds at different points along the shell, going from a minimum at the 'pole' and maximum at the 'equator'? In my mind, due to general relativity, the mass of the shell would increase the faster it rotated, but that increase in mass wouldn't be uniform across the shell, due to the different speeds. Not sure if that's right or not, or if that would have a subsequent effect on the gravitational field within the shell.


    The second question, for a non-uniform shell, I'm envisioning a shell that is uniform and symmetric across a cross-section about it's axis where the cross section would be a crescent. This shell, however, would not be rotating. For this, I would imagine the same result as the case above, though I suspect there are important differences that make this case different, with a different result. Any insight?
    I must admit that I am not completely certain what it is you are suggesting here - can you explain once again how density is distributed in your shell ?
    Yeah, rereading that I can see how convoluted it is. The density would be symmetric across the sphere, about an arbitrary line passing through the center of the sphere. The density would be at a minimum at the 'poles' of this line of symmetry, and at a maximum at the 'equator' of the shell about that line of symmetry, if that makes any sense.
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    Quote Originally Posted by Arcane_Mathematician View Post
    I could be mistaken, but from the inertial reference frame of any point within the shell, wouldn't the density distribution of the rotating shell change, due to the different speeds at different points along the shell, going from a minimum at the 'pole' and maximum at the 'equator'?
    Hm, interesting question. So long as the observer in the interior is at rest with respect to the shell, everything is easy, because all points of the shell have the same angular velocity for him, so there is no distortion. Things become more complicated if the observer moves inertially at relativistic speeds, because now the radius of the shell becomes length-contracted along his direction of motion, so the angular velocity vector of the surrounding shell would become a function of position relative to the observer. This would require some maths to treat properly.

    In my mind, due to general relativity, the mass of the shell would increase the faster it rotated, but that increase in mass wouldn't be uniform across the shell, due to the different speeds.
    That depends how you define "mass"; personally I don't like the notion of relativistic mass since it leads to all sorts of misconceptions if not treated and understood properly. I thus like to stick with the concept of "rest mass", which is a relativistic invariant, i.e. it is the same for all observers. What happens then is that the mass doesn't increase, only the total energy of the system does. Having said that, I don't know exactly what consequences this increase has so far as the gravitational field in the interior is concerned, over and above the aforementioned frame dragging effects. This will require fairly non-trivial maths to treat properly, but my guess is that the increase in total energy will manifest in an increase in frame dragging effects only. I might be wrong though.

    Yeah, rereading that I can see how convoluted it is. The density would be symmetric across the sphere, about an arbitrary line passing through the center of the sphere. The density would be at a minimum at the 'poles' of this line of symmetry, and at a maximum at the 'equator' of the shell about that line of symmetry, if that makes any sense.
    Ok, I understand it now, but to be honest this is too complicated to answer. My guess would be that we are getting frame dragging effects that vary with the angle above the equatorial plane, resulting in a highly non-trivial space-time geometry in the interior region. It's an interesting scenario, but I doubt that there is even an exact analytical solution to this. Sounds to me like a classic case for numerical GR
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    Quote Originally Posted by IamIan View Post
    #1> Einstine's multi-year debates on quantum uncertainty.
    #2> Hawkings more recent multi-year debate colloquially known as the "the black hole war".

    Both examples are well known historical cases ... as you asked for.

    Both examples are directly about weather some specific thing could be observed or not ... even if just in theory... as you asked for.
    These two examples directly contradict your claim. For example, as a result of his doubts about quantum mechanics, Einstein, with Podolsky and Rosen, suggested an experiment that would demonstrate that QM must be incorrect or incomplete. There was much continuing debate for many years because technology at the time did not permit suitable experiments. This led to further work by Bell, etc. Finally, in the '70s, 40 years after the proposal, experiments started to be performed.

    So, it was pretty obvious right from the start of the debate what observations to make to distinguish the two hypotheses.

    The same is true for black holes. Apart from the fact it might be centuries before we are in a position to make the necessary observations (even though we know exactly what they are).

    significance is relative ... the same results of 1fm ( for example) will NOT be significant in every context ... the same results that are significant in QM are not necessarily significant in SR , or biology , etc ... I HAVE NEVER ONCE , claimed that the determination of significance is just some casual personal opinion , where everyone is correct , no matter what the result is ... weather a particular result is or is not significant will be determined by the relevant scientific community ... individual scientist do not necessarily speak for the whole of that relevant scientific community ... The whole of that relevant scientific community finds in the past for what counted for being significant or not , form a historical pattern that scientists in that field are able to use as a basis to have a reasonably accurate idea of weather that relevant scientific community will find some other specific result significant or not , even before those specific results are given via peer review to that relevant scientific community..... = significance is relative
    This is almost completely incomprehensible, I'm afraid. I suspect your confused writing reflects your confused understanding. However, from the little sense I can make of it, it still sounds as if you haven't understood. If it were a personal opinion why would everyone agree if it was correct or not? Whether [note spelling, by the way] a result is correct or not is not determined by the whole scientific community.It can be determined by a single researcher calculating whether the result is significant or not.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by Arcane_Mathematician View Post
    In my mind, due to general relativity, the mass of the shell would increase the faster it rotated, but that increase in mass wouldn't be uniform across the shell, due to the different speeds.
    That depends how you define "mass"; personally I don't like the notion of relativistic mass since it leads to all sorts of misconceptions if not treated and understood properly. I thus like to stick with the concept of "rest mass", which is a relativistic invariant, i.e. it is the same for all observers. What happens then is that the mass doesn't increase, only the total energy of the system does.
    I often see replies similar to this to questions about the gravitation associated with a relativistically moving mass, and they're wrong with regards to the question. Gravitation is not about the rest mass of an object. Gravitation is about each component of the energy-momentum tensor. For a stationary mass, the is the primary component and gives rise to the ordinary gravity with which we are familiar. When a Lorentz transformation increases this component, the corresponding components of the gravtitation will also increase. One could also directly examine the effects of a Lorentz transformation on the components of the Weyl tensor that is the external gravitation.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by IamIan View Post
    My comment about significance was in reply to KJW ... who was not in his post using the term as you describe it there ... he specifically described a situation ... and claimed that atomic scale differences doesn't lead to significant interior curvature ... this use of the term ... is not about random chance ... in that comment he was defining a scale that he thought difference would not be significant.

    I agreed with KJW's post... and posted such.
    On the basis of the subsequent discussion, I have to say that I wasn't specifically using the term "significance" to mean statistical significance with regards to measurement error (though it could be interpreted that way). In fact, I was using the term to mean whether or not the effect is anywhere near large enough to account for the phenomenon being discussed (the expansion of the universe).
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by KJW View Post
    Gravitation is not about the rest mass of an object. Gravitation is about each component of the energy-momentum tensor.
    Ok, good point.

    For a stationary mass, the is the primary component and gives rise to the ordinary gravity with which we are familiar.
    is interpreted as mass-energy density; while rest mass in relativistically invariant, this quantity is of course not ​( but the tensor as a geometric object is ). So you are right, this is probably a better way to look at things; the problem is just - how do you explain this to the average forum member who is at most vaguely familiar with high school algebra, and possibly has never even heard of tensors ?

    When a Lorentz transformation increases this component, the corresponding components of the gravtitation will also increase.
    When the mass is no longer stationary in the observer's frame, then the components , being momentum density, will become important as well. The field equations then connect all of these to the metric tensor components in non-linear ways, making the outcome less than intuitive. Consider for example what happens when the mass is very large, and the relative speed between object and observer is of relativistic magnitude :

    Aichelburg

    Like I said, gravity sometimes sucks
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    P.S. Just to put my original reply into some perspective - it is the result of a certain mindset which one tends to develop after having spend considerable time on science forums. I cannot even count how many times over the years I have been confronted by cranks saying : "Relativity says that when you accelerate an object, it gains mass; accelerate it long enough, and it must turn into a black hole. This isn't what we observe, so relativity must be wrong !!". You see, trying to respond by explaining the ins and outs of tensor calculus on manifolds, and how it is all connected in the field equations, and why that resolves the apparent paradox, is much like trying to nail a pudding to a wall - a complete waste of time. What you'll get in return is a stupid remark such as "You are hiding behind incomprehensible maths !", thus I have stopped using that approach. Instead, I find it easier to simply state the following :

    1. Schwarzschild event horizons are functions of rest mass ( let's ignore the Kerr-Newman and Reissner-Nordstroem variants for now )
    2. Rest mass is invariant for all observers, it is only total energy that increases
    3. Hence no black hole will form, no matter how fast something goes

    I know that this is neither rigorous nor 100% correct, nor even particularly meaningful, but it does provide an answer that most people will be able to understand and follow, and thus far it has been able to shut down the cranks ( or forced them to start arguing along nonsensical lines, getting them banned eventually ).

    Hope this explains my perspective a bit better
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    Quote Originally Posted by Markus Hanke View Post
    Relativity says that when you accelerate an object, it gains mass; accelerate it long enough, and it must turn into a black hole.
    Yes... that old chestnut. Of course it stems from the mistaken belief that inertial motion actually changes the object, missing the point of relativity. Perhaps best to point out that all inertial frames are equivalent.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by Markus Hanke View Post
    When the mass is no longer stationary in the observer's frame, then the components , being momentum density, will become important as well. The field equations then connect all of these to the metric tensor components in non-linear ways, making the outcome less than intuitive.
    Not only that, but we are not dealing with a vector, but a second-order symmetric tensor which contains within it the scalar representation of the Lorentz group (i.e. the trace). In other words, if we have an Einstein tensor of the form , then this will be invariant to Lorentz transformations.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    No worries Markus, I'm not a crank

    In my mind, I'm not even thinking in terms of approaching the speed of light, just thinking of a sufficiently large shell around a region of space, rotating with a relatively large angular velocity, such that the speed at the equator would be measurable as a percent, albeit low percent, of the speed of light. I vaguely remember that, to an object in some reference frame, another object in the same reference frame will have a mass of x, but when it accelerates at a very high rate, it's apparent mass would increase. In my second year physics course I remember that, along with time dilation and length contraction, contributed to the inability to achieve light speed by any object, since at that speed it's mass would appear to be infinite. I must admit, I do not remember the actual mathematical models, but I wouldn't shun away from seeing them, and would at worst just ask to have certain terms explained.

    Honestly, I'm right now just seeing if what I remember is accurate, it's been a good 5 years since I was last in a physics classroom
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    Quote Originally Posted by Arcane_Mathematician View Post
    No worries Markus, I'm not a crank
    Ha ha, yeah I know It's just that the scenario of a non-uniform shell rotating relativistically isn't trivial at all, it's actually pretty involved !
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by Arcane_Mathematician View Post
    No worries Markus, I'm not a crank
    Ha ha, yeah I know It's just that the scenario of a non-uniform shell rotating relativistically isn't trivial at all, it's actually pretty involved !
    Ooh, miscommunication, the non uniform shell would not be rotating, sorry about that! Having it rotate compounds the problem unnecessarily for what I was thinking
    Wise men speak because they have something to say; Fools, because they have to say something.
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    Quote Originally Posted by Arcane_Mathematician View Post
    No worries Markus, I'm not a crank

    In my mind, I'm not even thinking in terms of approaching the speed of light, just thinking of a sufficiently large shell around a region of space, rotating with a relatively large angular velocity, such that the speed at the equator would be measurable as a percent, albeit low percent, of the speed of light. I vaguely remember that, to an object in some reference frame, another object in the same reference frame will have a mass of x, but when it accelerates at a very high rate, it's apparent mass would increase. In my second year physics course I remember that, along with time dilation and length contraction, contributed to the inability to achieve light speed by any object, since at that speed it's mass would appear to be infinite. I must admit, I do not remember the actual mathematical models, but I wouldn't shun away from seeing them, and would at worst just ask to have certain terms explained.

    Honestly, I'm right now just seeing if what I remember is accurate, it's been a good 5 years since I was last in a physics classroom
    The subject of mass approaching the speed of light is quite interesting to me. I may be wrong here but I believe mass at the equator of some very fast spinning neutron stars comes the closest to the speed of light of any ever recorded. If this is wrong please show me.
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    Quote Originally Posted by KJW View Post
    Quote Originally Posted by IamIan View Post
    I agreed with KJW's post... and posted such.
    On the basis of the subsequent discussion, I have to say that I wasn't specifically using the term "significance" to mean statistical significance with regards to measurement error (though it could be interpreted that way). In fact, I was using the term to mean whether or not the effect is anywhere near large enough to account for the phenomenon being discussed (the expansion of the universe).
    Thank for clarifying.

    I would still agree.

    - - - - - - - -

    Quote Originally Posted by MagiMaster View Post
    Also, the arguments Einstein had
    The existence of the argument is direct evidence of my claim ... that it can be hard to know ... If your claim had been correct there would not have been any argument at all among the experts in the field.

    Again I remind you ... you may want to reread what you wrote:
    Quote Originally Posted by MagiMaster
    Basically, by the time we know enough about something to ask whether or not it would be theoretically possible to observe it, we know enough to answer that (specific) question. (Feel free to point me at some historical instances where this hasn't been the case though.)
    Quote Originally Posted by Strange View Post
    There was much continuing debate for many years
    I claim it can be hard to know. ... The existence of experts debating at all is evidence of that.

    Einstein would propose something ... it would get countered ... etc ... there was a back and forth for years, just in theoretical analogies of could we observe this or could we observe that.

    - - - - - -

    Quote Originally Posted by Strange
    Quote Originally Posted by IamIan
    I HAVE NEVER ONCE , claimed that the determination of significance is just some casual personal opinion
    If it were a personal opinion why would everyone agree if it was correct or not?
    I don't know ... it is not my claim.

    For not being something I claimed ... my best guess ... Maybe it is along similar lines as we do for standard deviation conventions in statistics ... numbers and mathematics don't care weather you go out 1 standard deviation or 2 million standard deviations ... but 2 million is not 'the norm' to do ... nothing in mathematics or numbers prevents you from going out to 2 million standard deviations ... or more... but that 2 million is not the agreed ( in that relevant community ) standard in statistics for the use of standard deviation.

    - - - - - - -

    Quote Originally Posted by MagiMaster View Post
    The differences you have been talking about, those of atomic variations in a shell
    As I already wrote previously ... That was about a less than perfect analogy was still useful to communicate a concept ... and I still think it is ... Newton's Shell does not have to be realistic , or a perfect analogy ... it is useful anyway ... despite it's flaws.
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    What you claim you're saying and what you're actually saying seem to be at odds with one another. The differences are insignificant in a technical sense and the arguments you mentioned were not about what you say they were about.

    And you have yet to show anything that even begins to contradict the shell theorem or show that there would be any way to determine the existence of a shell from gravitational effects. The theorem is not flawed, but your understanding of it, its implications and perhaps the concept of a domain of applicability seems to be.

    And in statistics, if you find something even 6 or 7 standard deviations off from the norm, that probably means you haven't accounted for something and either you've done something wrong or you've found something new. It's always safest to assume the former until you can prove otherwise.
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    Quote Originally Posted by Arcane_Mathematician View Post
    Ooh, miscommunication, the non uniform shell would not be rotating, sorry about that! Having it rotate compounds the problem unnecessarily for what I was thinking
    Oh, I see, sorry about that, I misunderstood you. Well, in that case what you get is curvature of space-time inside the shell that varies smoothly between the areas of high density and lower density; things would slowly drift towards the high density places. This is actually fairly straightforward.
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    Quote Originally Posted by IamIan View Post
    [The existence of the argument is direct evidence of my claim ... that it can be hard to know ... If your claim had been correct there would not have been any argument at all among the experts in the field.
    Hard to know what?

    You need to put a bit more effort into writing clearly. I can't tell if you have misunderstood the original point or are arguing against it.

    Just to be clear, the original point was about the difference between observable and observed and, specifically, that it is generally easy to determine whether something will be observable.

    In both the cases you cite the disagreements centred around what would be observed in a given situation. It is therefore an example that supports the original point: despite the disagreements and uncertainties it was clear what could observed in principle to test the theory.

    Now, it may be hard to make the observation. In the case of the EPR "paradox" it took about 40 years to develop the technology. In the case of Hawking radiation it is likely to take centuries; it may even be impossible in practice: but it is still easy to define what needs to be observed and how.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by Arcane_Mathematician View Post
    Ooh, miscommunication, the non uniform shell would not be rotating, sorry about that! Having it rotate compounds the problem unnecessarily for what I was thinking
    Oh, I see, sorry about that, I misunderstood you. Well, in that case what you get is curvature of space-time inside the shell that varies smoothly between the areas of high density and lower density; things would slowly drift towards the high density places. This is actually fairly straightforward.
    That's what I would envision, just wanted to be sure that was what would actually happen. And, so long as the shell is symmetric about some axis, I would assume that the curvature would tend objects towards the plane situated about the center of the shell, where the density about the shell is highest, and not so much towards any particular side.

    I'm no longer super familiar with the mathematics abound in this topic, so it's all a bit foreign to my mind, though it does make a bit of sense.
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  87. #86  
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    Quote Originally Posted by Strange View Post
    Hard to know what?
    If some specific thing could ever possibly be observable, or not.

    Could it ever be possible for us in principle to observe anything escaping a black hole?
    Could it ever be possible for us in principle to observe more certantiy than the uncertantiy principle allows?
    etc....

    Quote Originally Posted by Strange View Post
    Just to be clear, the original point was about the difference between observable and observed and, specifically, that it is generally easy to determine whether something will be observable.
    Well thanks for clarifying your point of view ... I don't think you are/were talking about the same thing I am ... which would help explain the confusion.

    For clarification:

    I was not talking about the difference between observable and observed.... so take that out entirely.

    The Original point about observability ... and the point I have been discussing.

    A > My Point = It can be difficult to ... know at one point in time ... that after any arbitrary level of scientific advancement in the future ... what could or could not ... ever possibly be observable.

    ( Note: Sometimes it can be easy. )

    B> I disagree with = "Basically, by the time we know enough about something to ask whether or not it would be theoretically possible to observe it, we know enough to answer that (specific) question."

    Quote Originally Posted by Strange View Post
    In both the cases you cite the disagreements centred around what would be observed in a given situation. It is therefore an example that supports the original point: despite the disagreements and uncertainties it was clear what could observed in principle to test the theory.
    For there to be a disagreement ... violates Claim B above.
    Both knew enough to ask the question ... so if B is true ... both would know what would be possible to observe in principle.

    Einstein ( expert in field ) proposed with his slit experiment , and claimed that any arbitrary amount of certainty in both aspects of light could be observed in principle... at that time , he disagreed with the uncertainty principle.

    Without doing any actual experiments at all... Bohr ( expert in field ) , disagreed with Einstein's claim , and claimed that in principle both could not be observed with certainty ... he agreed with the uncertainty principle.

    They were disagreeing about what would in principle be observable ... sense both knew enough to ask the question ... having opposing views , violates the claim made in B.

    If Experts in the field disagree and argue for years ... that seems like a strong case that A is possible ... that it is possible that it can be hard to know.

    So hard that even experts in the field disagreed and debated about it for years... despite knowing enough to ask the question.

    - - - - - -

    If you prefer a different examples:

    Example C:
    Einstein asking if anything could escape a black hole ... Einstein knew enough to ask the question ... but he said the answer is no ... nothing can escape , not even light... it would never be observable.

    Fast forward with advancements in science ... Hawking ... also knew enough to ask the question ... but Hawking came up with a different answer ... Hawking says yes ... Hawking Radiation can escape ... and eventually cause a black hole to 'evaporate'... and would in principle be something that could be be observable as escaping from a black hole.

    Again experts in the field ... who both know enough in their time to ask the question ... if B were true ... they would both have known the correct answer ... sense they disagree ... there is a flaw in B.

    Example D:
    If we look at the 'black hole war' ... we see ... Hawking proposed that information is lost in black holes, and not preserved in hawking radiation ... thus it would never in principle be observable ... but Susskind disagreed... and claimed it was in principle observable.

    Again , experts in the field .. both know enough to ask the question ... if B were true ... they would both have known the correct answer ... sense they disagree ... there is a flaw in B.

    - - - - - - - - - - - - -

    Quote Originally Posted by MagiMaster
    the arguments you mentioned were not about what you say they were about.
    Can you be more specific?

    Quote Originally Posted by MagiMaster
    And you have yet to show anything that even begins to contradict the shell theorem or show that there would be any way to determine the existence of a shell from gravitational effects.
    That should be no surprise ... sense , I haven't been trying to contradict the shell theorem ... which Newton using Newtonian physics showed was no net force.

    The second part is not true.

    I listed 2 ways ... weather they are ever possible to carry out is another question.

    Also Markus Hanke in post # 56 pointed out ways it would be possible to determine the existence of a shell from gravitational effects ... the only thing needed , was significant movement of the shell itself ... Which AFAIK a stationary shell was not included in Newton's Shell Theorem... in Newtonian Physics of gravity it wouldn't have mattered ... but in better modern physics it does matter... hint , that's one of the flaws , that exist , but does not prevent Newton's shell theorem from being useful.

    Quote Originally Posted by MagiMaster
    The theorem is not flawed,
    There is a difference between ... the theorem is flawed ... and the theorem contains flaws.

    I'm not trying to contradict the shell theorem. ... The shell theorem is useful to us not because it is perfect ... but despite its flaws it is still useful ... Using Newtonian physics it looked at gravity as a force , and shows no net force... that's what it did... even if the Newtonian Physics it is based on we've learned has flaws ... even if the assumptions made in it are not possible ... it is still useful , despite all that , not because those things don't exist.

    Quote Originally Posted by MagiMaster
    And in statistics, if you find something even 6 or 7 standard deviations off from the norm, that probably means you haven't accounted for something and either you've done something wrong or you've found something new. It's always safest to assume the former until you can prove otherwise.
    What is your criteria for determining that you want to draw that line there ... at that point between 6 and 7 ... and not at 1 or 2 standard deviations ... or 10 or 11?... what is your mathematical basis for choosing that specific location between 6 or 7 standard deviations?
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    Quote Originally Posted by IamIan View Post
    Could it ever be possible for us in principle to observe anything escaping a black hole?
    Yes - QM particles tunnelling through the event horizon from the interior region.

    Could it ever be possible for us in principle to observe more certantiy than the uncertantiy principle allows?
    The HUP is about pairs of observables, not single observables. You can observe anything with arbitrary precision - but of course there is a price to pay

    Hawking Radiation can escape
    No. Hawking radiation originates outside the horizon, so it doesn't really "escape".

    Again experts in the field ... who both know enough in their time to ask the question ... if B were true ... they would both have known the correct answer ... sense they disagree ... there is a flaw in B.
    Both Hawking and Einstein agree - not sure what you mean here.

    the only thing needed , was significant movement of the shell itself ... Which AFAIK a stationary shell was not included in Newton's Shell Theorem
    It's the other way around - the shell theorem applies only to stationary shells; in Newtonian physics that wouldn't make a difference, but in GR it does.

    hint , that's one of the flaws , that exist
    It's not a flaw, it's just a mathematical fact. The shell theorem ( and its GR counterpart, the Birkhoff theorem ) rely on certain symmetries; if you take away those symmetries the original theorem is no longer valid. Their domains of applicability are limited.
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    Quote Originally Posted by IamIan View Post
    [Hard to know if] some specific thing could ever possibly be observable, or not.
    OK. That wasn't the point I thought we were discussing. That is just a debate over future technology. Nothing to do with the underlying science.

    I was not talking about the difference between observable and observed.... so take that out entirely.

    The Original point about observability ... and the point I have been discussing.

    A > My Point = It can be difficult to ... know at one point in time ... that after any arbitrary level of scientific advancement in the future ... what could or could not ... ever possibly be observable.

    ( Note: Sometimes it can be easy. )

    B> I disagree with = "Basically, by the time we know enough about something to ask whether or not it would be theoretically possible to observe it, we know enough to answer that (specific) question."

    Quote Originally Posted by Strange View Post
    In both the cases you cite the disagreements centred around what would be observed in a given situation. It is therefore an example that supports the original point: despite the disagreements and uncertainties it was clear what could observed in principle to test the theory.
    For there to be a disagreement ... violates Claim B above.
    Both knew enough to ask the question ... so if B is true ... both would know what would be possible to observe in principle.
    It is all about the difference between what can be observed (in principle) and what will actually be observed when that measurement is made.

    The claim is that, in general, we know what could be observable, i.e. what measurements could be made. From that, it is possible to predict what would be observed if theory is correct. There may then be disagreement about whether the theory is correct or not; in other words to disagree what the observation would show.

    So, both might agree on what is observable; they might disagree on what would be observed because they disagree on which theory is correct. (The Einstein case is bit subtler: he agreed on what would be observed but claimed that was evidence the theory must be incomplete.)

    From the beginning it was easy to describe what would need to be measured to test the EPR paradox. And it is easy to describe what would need to be observed to test Hawking radiation.

    Currently, pretty much all physicists agree what would be observed for any proposed quantum theory experiment.

    Some physicists disagree on what would be found if we were able to measure Hawking radiation: they agree on observable but disagree on what would be observed.

    Again , experts in the field .. both know enough to ask the question ... if B were true ... they would both have known the correct answer ... sense they disagree ... there is a flaw in B.
    Again, they can agree on what needs to be measured (observed) and how; they disagree on what such an observation would show. That is normal.
    Does that make more sense?
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    How can the dark matter, or empty parts of space, be pushing the universe outwards if it's a vacuum? Wouldn't this be pulling on everything? I read that there is like one hydrogen atom per meter in space, that's one hell of a vacuum.
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    @IamIan, I'm starting to think we may not be using the word observable in the same way. If I understood what you're trying to say (the fragmented sentences don't help) your Point A does not contradict my Point B.

    I'm not saying we can, now, easily say what all could theoretically be observed in the future. I'm saying we can now fairly easily tell what cannot even theoretically be observed (such as the differences between the various interpretations of QM) and for those things we have any kind of understanding of we can predict what kind of observations would be needed (even if we don't actually know how build the devices to make those observations).

    Those two do not cover every case though and what's left are mainly those things we don't even know exists, so we can't ask any questions about them at all.
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    Quote Originally Posted by The Huntsman View Post
    How can the dark matter, or empty parts of space, be pushing the universe outwards if it's a vacuum? Wouldn't this be pulling on everything? I read that there is like one hydrogen atom per meter in space, that's one hell of a vacuum.
    It is not dark matter but "Dark Energy" that accelerates the expansion. Dark matter is "normal" matter in the sense that it provides a gravitational "pull", as you rightly point out. Dark Energy on the other hand is a property of space-time itself, which results in a "negative pressure" effect, thereby "pushing" space-time apart. At the moment it looks like the Dark Energy effect wins out over gravitational pull, which is why overall the expansion is accelerating.

    Note that in the above I use "push" and "pull" simply to clarify the concept - in reality there are of course no forces involved as such, it is all just space-time geometry.
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  93. #92  
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    Quote Originally Posted by The Huntsman View Post
    How can the dark matter, or empty parts of space, be pushing the universe outwards if it's a vacuum? Wouldn't this be pulling on everything? I read that there is like one hydrogen atom per meter in space, that's one hell of a vacuum.
    Dark matter doesn't push anything. Dark matter is the stuff the accounts for what's holding galaxies together. (They're spinning too fast to hold themselves together with just the stars and dust we can see.)

    Dark energy is the stuff pushing the universe apart and from what I understand, it's simply a property of space itself regardless of what is or isn't in that space. (This isn't my area of expertise though.)

    Also, a vacuum doesn't pull on anything. It's the higher pressure air (or whatever) on the other side pushing dust (for example) into the vacuum.

    Edit: Ninja'd.
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  94. #93  
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    Quote Originally Posted by MagiMaster View Post
    Quote Originally Posted by The Huntsman View Post
    How can the dark matter, or empty parts of space, be pushing the universe outwards if it's a vacuum? Wouldn't this be pulling on everything? I read that there is like one hydrogen atom per meter in space, that's one hell of a vacuum.
    Dark matter doesn't push anything. Dark matter is the stuff the accounts for what's holding galaxies together. (They're spinning too fast to hold themselves together with just the stars and dust we can see.)

    Dark energy is the stuff pushing the universe apart and from what I understand, it's simply a property of space itself regardless of what is or isn't in that space. (This isn't my area of expertise though.)

    Also, a vacuum doesn't pull on anything. It's the higher pressure air (or whatever) on the other side pushing dust (for example) into the vacuum.

    Edit: Ninja'd.
    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by The Huntsman View Post
    How can the dark matter, or empty parts of space, be pushing the universe outwards if it's a vacuum? Wouldn't this be pulling on everything? I read that there is like one hydrogen atom per meter in space, that's one hell of a vacuum.
    It is not dark matter but "Dark Energy" that accelerates the expansion. Dark matter is "normal" matter in the sense that it provides a gravitational "pull", as you rightly point out. Dark Energy on the other hand is a property of space-time itself, which results in a "negative pressure" effect, thereby "pushing" space-time apart. At the moment it looks like the Dark Energy effect wins out over gravitational pull, which is why overall the expansion is accelerating.

    Note that in the above I use "push" and "pull" simply to clarify the concept - in reality there are of course no forces involved as such, it is all just space-time geometry.

    So because there is more 'negative pressure' in the universe, there is not enough 'positive pressure' to fill that void, and this in itself acts as a 'push'?
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    Quote Originally Posted by The Huntsman View Post
    So because there is more 'negative pressure' in the universe, there is not enough 'positive pressure' to fill that void, and this in itself acts as a 'push'?
    Yes, that is pretty much the basic idea. The net effect is a "push" rather than gravity overcoming it, resulting in a "pull". Hence the accelerating expansion.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by The Huntsman View Post
    So because there is more 'negative pressure' in the universe, there is not enough 'positive pressure' to fill that void, and this in itself acts as a 'push'?
    Yes, that is pretty much the basic idea. The net effect is a "push" rather than gravity overcoming it, resulting in a "pull". Hence the accelerating expansion.
    That is truly fascinating.
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    Quote Originally Posted by Strange View Post
    Again, they can agree on what needs to be measured (observed) and how; they disagree on what such an observation would show. That is normal.
    Does that make more sense?
    Yes it does ... and unless I'm missing something ... that seems to be like around ~90% similar to the point I was trying to make / express.

    I'm sorry , if I did not phrase it correctly ... or if my method of writing was not more clear... and made gave the wrong impression some how.

    Quote Originally Posted by MagiMaster View Post
    @IamIan, I'm starting to think we may not be using the word observable in the same way. If I understood what you're trying to say (the fragmented sentences don't help) your Point A does not contradict my Point B.

    I'm not saying we can, now, easily say what all could theoretically be observed in the future.
    Agreed ... the communication process broke down.

    It had seemed to me you were claiming the exact opposite of the bold part... especially after I had asked about the future and any future progress was dismissed as not having any impact at all.

    Successfully communicating a concept from my head to others ... is not one of my strengths ... and that weakness/failing is on me ... it's another project with on going work/effort ... which is one of the reasons I asked for a focus on the concept I was trying to describe ... see the forest past the trees as they say.

    Quote Originally Posted by Markus Hanke View Post
    It is not dark matter but "Dark Energy" that accelerates the expansion. Dark matter is "normal" matter in the sense that it provides a gravitational "pull", as you rightly point out. Dark Energy on the other hand is a property of space-time itself, which results in a "negative pressure" effect, thereby "pushing" space-time apart. At the moment it looks like the Dark Energy effect wins out over gravitational pull, which is why overall the expansion is accelerating.

    Note that in the above I use "push" and "pull" simply to clarify the concept - in reality there are of course no forces involved as such, it is all just space-time geometry.
    Curious about 2 things.

    #1>
    Wouldn't the Dark Energy as Energy ... also have a relativistic mass component?
    Even if it doesn't change the net effect ... wouldn't it still be there?

    #2>
    If Dark Energy makes up ~68% of the mass-energy of the universe ... and we have conservation of mass-energy.
    Doesn't that lead to , meaning that the Dark Energy has a finite upper limit? ... ie the expanded space has less dark energy density per unit volume ... than it did prior to expanding... if it contained the same density of dark energy per unit volume after the volume expanded ... that would seem to be violating conservation of mass-energy?
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    Quote Originally Posted by IamIan View Post
    Wouldn't the Dark Energy as Energy ... also have a relativistic mass component?
    No. Dark Energy is better thought of as a property of space-time itself; it enters the field equations on the left hand side, where it forms a linear combination with the Einstein curvature tensor. It does not enter the energy-momentum tensor, and thus does not act as a source of gravity.

    If Dark Energy makes up ~68% of the mass-energy of the universe
    Dark Energy is not the same as mass-energy - see above.

    Doesn't that lead to , meaning that the Dark Energy has a finite upper limit? ... ie the expanded space has less dark energy density per unit volume ... than it did prior to expanding... if it contained the same density of dark energy per unit volume after the volume expanded ... that would seem to be violating conservation of mass-energy?
    The density of Dark Energy remains constant over time; as I said above, it is best thought of as a property of space-time itself. As space expands, new Dark Energy is created; it is important to remember now that while Dark Energy itself has positive density, it does negative work on its surroundings. Therefore, as total Dark Energy increases during expansion, so does the negative work it does on the rest of the universe. The net balance remains always exactly zero, so the principle of energy conservation is not violated.

    This may sound a bit hand-wavey, but is all mathematically rigorous.
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    @IamIan, just to further clarify, I was trying to differentiate the two cases when I said that by the time we know enough to ask the question we know enough to answer it. For those things too far in the future, we don't even know enough to ask the question.

    Take a hypothetical grand unified theory. What would we need to observe to confirm or refute that theory? Well, we don't have the theory, so we don't know. By the time someone comes up with such a theory, figuring out what observations to make should be relatively easy.
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  100. #99  
    Forum Freshman IamIan's Avatar
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    Quote Originally Posted by Markus Hanke View Post
    If Dark Energy makes up ~68% of the mass-energy of the universe
    Dark Energy is not the same as mass-energy - see above.
    If it isn't mass-energy ... Why then , is it often referred to as ~68% of the mass-energy of the universe?

    Quote Originally Posted by Markus Hanke View Post
    Doesn't that lead to , meaning that the Dark Energy has a finite upper limit? ... ie the expanded space has less dark energy density per unit volume ... than it did prior to expanding... if it contained the same density of dark energy per unit volume after the volume expanded ... that would seem to be violating conservation of mass-energy?
    The density of Dark Energy remains constant over time; as I said above, it is best thought of as a property of space-time itself. As space expands, new Dark Energy is created; it is important to remember now that while Dark Energy itself has positive density, it does negative work on its surroundings. Therefore, as total Dark Energy increases during expansion, so does the negative work it does on the rest of the universe. The net balance remains always exactly zero, so the principle of energy conservation is not violated.

    This may sound a bit hand-wavey, but is all mathematically rigorous.
    So does that mean the ~68% ( often quoted today ) Dark Energy part of the universe ... will eventually be 80% Dark Energy ... then 90% dark energy ... then 99.9999999999999999999999% Dark Energy? ... etc.?

    - - - - - - - - - -

    Quote Originally Posted by MagiMaster View Post
    @IamIan, just to further clarify, I was trying to differentiate the two cases when I said that by the time we know enough to ask the question we know enough to answer it. For those things too far in the future, we don't even know enough to ask the question.

    Take a hypothetical grand unified theory. What would we need to observe to confirm or refute that theory? Well, we don't have the theory, so we don't know. By the time someone comes up with such a theory, figuring out what observations to make should be relatively easy.
    I think in the specific example you gave of asking about the grand unified theory ... that seems fine ... I see how that specific question would fall in line with that concept you propose.

    The issue I saw before ... and I still see here again ... I will try and explain bellow ... let me know if you can see the concept I am trying to describe?

    That one you listed ... isn't the only kind of question we can ask ... And I think the singular criteria of 'know enough to ask' ... is just , too thin by itself ... considering the wide variety of types of questions that can be asked ... or ever could be asked... about anything ... or any kind of observation ... there is no restrictions listed to sufficiently limit the kind of question anyone could ask... about anything.

    I think as it is , the scope is far too broad , considering the extremely small simple singular criteria... left as it is ... it just seems to be pulling you back into the same kind of claims about the future that you had to correct in post #90 with "I'm not saying we can, now, easily say what all could theoretically be observed in the future".

    With 'know enough to ask' ... being the only criteria ... than someone can ask a question about ... 'what could theoretically be observed in the future' ... that kind of question is not off the table as long as ( able to ask) is the only criteria being listed.
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  101. #100  
    Forum Radioactive Isotope MagiMaster's Avatar
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    No, not "able to ask," but "know enough to ask." That is, know enough to actually properly pose the question. And also, I'm limiting my claims to a fairly specific question (whether or not something is theoretically observable), not just any question. So I stand by what I've said, even if I may not be communicating that completely clearly.

    Now, I'm sure there are probably a rare few cases at any one time that lie right on the edge of our knowledge and we can nearly describe some object, but with enough uncertainty that we can't say for sure whether it is really observable or not, but I can't think of any specific examples either current or historical. Plus, as our knowledge increases such cases should quickly fall to one side of that edge.

    It's really hard to give a good example as I can't exactly describe an object I don't know about without just making stuff up, but one object that might be on that edge would be a strangelet or strange matter. (Someone correct me if I'm wrong here. It's not really an area I'm familiar with.)

    Look at it from the other way. Ask whether a specific thing is theoretically observable. If we already have any kind of working model that covers that case (the question is within the theory's domain of applicability), then we can plug the numbers in and answer the question. Then, by the correspondence principle, any future theory should give nearly the same answer, which shouldn't change the basic observability of something. On the other hand, if we have no model that'd cover that case, then we don't even know enough to properly describe that object so we can't really answer many questions about it.

    All that said, the shell theorem and the lack of differences between no gravitational force and no net gravitational force is well within the domain of applicability of GR. So, the correspondence principle says any future theories must give the same answers, within a known degree of error, to current theories when the domains of applicability overlap. (That is, we know how accurate our current theories are. For GR, very.)
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