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Thread: Does gravity cast a shadow?

  1. #1 Does gravity cast a shadow? 
    Forum Freshman 6nqpnw's Avatar
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    I'm too ignorant to intelligently rephrase the question.

    What happens (if anything) to the sun's & earth's gravitational influences on our moon during a lunar eclipse? I'm inclined to say 'nothing happens' because the moon has yet to be flung off its orbit; but wouldn't that imply that gravity influences spacetime unabated by matter? Which would make sense since all atoms are afterall 99.9999% empty space.


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  3. #2  
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    Actually, I think that is a great way to ask the question!

    The answer is no, the Earth does not shelter the moon from the gravity of the Sun during a lunar eclipse.


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  4. #3  
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    Every body in the Universe continually exerts it's effect on every other one. The alignment makes no difference in the big picture.
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  5. #4  
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    So it's safe to also say the Earth doesn't shelter the moon from the gravity of the Sun during...EVER (lunar eclipse was for analogous effect).

    Also, could u help me understand whether gravity effects spacetime (specifically a geodesic of light) or the photons themselves?
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  6. #5  
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    Yes, that's relativity (the verified explanation of gravity). It "bends" spacetime, so in fact the photons or the planets are following straight lines in spacetime.
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  7. #6  
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    I don't want to come-off as crass, but which is it? Gravity effects waves&particles or spacetime?
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    I think it's almost effective to say that gravity changes the laws of inertia. Bending space time is the official version, though. Harder to understand, but certainly the most accurate.

    If you think of it in terms of changing the laws of inertia, then it's easier to imagine. Newton's laws state that a body in motion tends to stay in motion (maintaining the same velocity and direction) unless a force acts upon it to change its course. In curved space time that law changes, so a body in motion tends to change velocity and/or heading even though no forces are acting on it. (In GR, there is no "force of gravity". Resisting gravity is what requires a force, because you're working against the changed laws of inertia.)

    At least that rationale is what works for me. I'm not absolutely sure it's correct, but it seems to work.
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  9. #8  
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    Quote Originally Posted by 6nqpnw View Post
    I don't want to come-off as crass, but which is it? Gravity effects waves&particles or spacetime?
    So in other words, you have no idea what you are talking about.

    Do you understand relativity? Or are you just saying it's wrong without taking 10 minutes of effort to understand what it says, and how accurately it describes the universe we live in?
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    Quote Originally Posted by MeteorWayne View Post
    Quote Originally Posted by 6nqpnw View Post
    I don't want to come-off as crass, but which is it? Gravity effects waves&particles or spacetime?
    So in other words, you have no idea what you are talking about.

    Do you understand relativity? Or are you just saying it's wrong without taking 10 minutes of effort to understand what it says, and how accurately it describes the universe we live in?
    To be fair, I don't think he was denying the existence of relativity.


    Gravity is bent spacetime. The light always follows a straight path through space, so if space is bent, light will still follow a straight path from its own perspective, but a curved one from ours. If you aimed a LASER at the near edge of a planet or so, the light will follow a curved path when looked at from above, but a straight line from its own perspective.
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  11. #10  
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    Quote Originally Posted by kojax View Post
    I think it's almost effective to say that gravity changes the laws of inertia. Bending space time is the official version, though. Harder to understand, but certainly the most accurate. If you think of it in terms of changing the laws of inertia, then it's easier to imagine. Newton's laws state that a body in motion tends to stay in motion (maintaining the same velocity and direction) unless a force acts upon it to change its course. In curved space time that law changes, so a body in motion tends to change velocity and/or heading even though no forces are acting on it. (In GR, there is no "force of gravity". Resisting gravity is what requires a force, because you're working against the changed laws of inertia.) At least that rationale is what works for me. I'm not absolutely sure it's correct, but it seems to work.
    This change to inertia is something that people initially jumped upon as a difference between Newtonian Universal Gravity and GR. However, we can also change inertia in the Newtonian case so that bodies in free (fall) motion also take paths around massive objects. This was worked out by Élie Cartan in the early 1920s. The reworking is often referred to as "Newton-Cartan theory," but it is really Newtonian Universal Gravity rewritten--not that Cartan didn't do excellent work in figuring out the details.
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  12. #11  
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    I'm not saying anything is right or wrong and certainly didn't mean to imply such. I want help understanding why GR requires the bending of spacetime rather than being limited to the interactions of matter&energy.

    I'm asking with the understanding that spacetime is a 4D model - an invented construct with no true physical presence; a grid for referencing specific points of time relative to other points. Gravity (according to GR) distorts this imaginary grid; so what's the point in having the grid in the first place? Wouldn't it be just as accurate to state that gravity attracts any and all matter&energy? Or does this somehow violate GR?

    @Kojax ::: That does help...sorta, thx. I'm struggling with the concept of removing gravity as a force. I realize the convenience of curved spacetime for visualization, but to declare gravity as not being a force?!

    And there's no need to take offense ::: when asked if I understand my own questions, my response is "No." When I feel I've reached an understanding about anything related to cosmology or quantum mechanics, I realize I haven't given it enough thought ::: then I come here.
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  13. #12  
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    Quote Originally Posted by 6nqpnw View Post
    I'm asking with the understanding that spacetime is a 4D model - an invented construct with no true physical presence; a grid for referencing specific points of time relative to other points. Gravity (according to GR) distorts this imaginary grid; so what's the point in having the grid in the first place? Wouldn't it be just as accurate to state that gravity attracts any and all matter&energy? Or does this somehow violate GR?
    I think it is largely a mathematical convenience; it happens that the behavior of matter affected by other matter/energy is easily described in terms of 4D geometry. (Note that it is actually a continuous "surface" rather than a grid.)

    I haven't seen one but I'm sure a fairly simple explanation could be given as to why it has to be this way. I don't know if I am up to it (especially off the top of my head).

    If we start with a simple model: a flat surface and we want to calculate how light will reflect off it. First we need the surface "normal" (the vector perpendicular to the surface - just in case). One a flat surface, this will be the same everywhere so we only need a single vector to describe it. Now if we consider that the surface is curved then the will need a vector at every point to represent the normal. If we have more complicated geometry (twists, etc) then the math to describe the relationship between the normal vectors at every point gets a little more complicated. Now change that 2D surface into a 4D one and you can see that representing the relationship between the normal at every point becomes quite complex.

    In practice, GR needs much more than just the normal at every point and so we end up with tensors and the whole differential geometry thing.

    I don't know if that helps (and I'm sure someone more familiar with the math could explain it better).

    Oh, this might help: Introduction to mathematics of general relativity - Wikipedia, the free encyclopedia
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    I know relativity is universally accepted and proved. But are there no research into how the mathematics translates into physicality? To simply say, well we have a large mass over there and a smaller mass over here and oh, the spacetime is curved in between and that is what gravity is and it's all simply a matter of perspective. It seems to me like a doctor describing the symptoms without understanding the events at a cellular level. If the path of the photon was always straight then either other masses must be moving nearer or there is some physicality to spacetime.
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  15. #14  
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    Quote Originally Posted by ballyhoo View Post
    I know relativity is universally accepted and proved. But are there no research into how the mathematics translates into physicality? To simply say, well we have a large mass over there and a smaller mass over here and oh, the spacetime is curved in between and that is what gravity is and it's all simply a matter of perspective. It seems to me like a doctor describing the symptoms without understanding the events at a cellular level. If the path of the photon was always straight then either other masses must be moving nearer or there is some physicality to spacetime.
    I agree it is difficult to imagine and that is we have not evolved at a scale where these things start to become directly observable, i.e. we never travel at speeds fast enough to notice the tiny effects of relativity at that scale. What we have is the counter-intuitive results of experiments and the math that describes and predicts what we see.

    Think of space as a grid. The grid lines are bent around masses. As light always travels in a geodesic, it will follow the grid lines, even though they are curved from an outside perspective. From the perspective of the light though, it is moving in a straight line.

    If you were living as a 2D being on the surface of a ball, you will always move in a straight line from your 2D perspective. Yet, the maths exist that can describe the curvature of the ball in only 2D (counter-intuitive, I know). Similarly, light in our universe follow a 3D curvature that is straight from it's perspective. The inertia of objects with mass means that the bent grid line merely nudges it the direction of the other mass, the magnitude of which depending on the degree of curvature of the first mass (not on the mass of the travelling object, which is why light and heavy objects fall at the same rate on earth).
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    But isn't a theory being developed called loop quantum gravity that might offer an alternative explanation while reconciling with general relativity? https://en.wikipedia.org/wiki/Loop_quantum_gravity or is this what you are trying to describe bu the grid of nodes and links?
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  17. #16  
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    Quote Originally Posted by 6nqpnw View Post
    Gravity (according to GR) distorts this imaginary grid; so what's the point in having the grid in the first place? Wouldn't it be just as accurate to state that gravity attracts any and all matter&energy?
    I'm afraid things aren't quite that simple. There are many effects in GR which cannot be explained by a simple attraction force. I give you some examples :

    1. Gravitational time dilation. Clocks run slower in the presence of a strong gravitational field; this is due to the curvature of spacetime, and not explainable in terms of forces
    2. Frame dragging. This is a distortion of spacetime caused by the rotation of a massive body, and leads to a precession in the orbit of a test particle around that body.
    3. The Reissner-Nordstroem black hole. This is a charged, rotating black hole. The singularity of such a black hole would actually be gravitational repulsive rather than attractive.

    Again, none of these are explainable in terms of simple force, and there are many other examples. Thus the picture of geometric distortion has merit, because GR needs to be able to explain all of these phenomena.
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  18. #17  
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    Let me reiterate that I am certainly a simple-minded soul & tend to reduce complexification to asinine analogies of common sense & intuition - please forgive unintentional offenses.

    I had forgotten all about time dilation, but couldn't the interaction of gravity with matter/energy create some sort of quantum resistance to account for time dilation & frame-dragging? I'm not familiar with frame-dragging, nor a gravitationally repulsive BH, so please respond accordingly - small words & no numbers And I ask this with the certainty that the examples not on your list would have thwarted my inquiry.

    Allow me to illuminate my ignorance. We all know the "hammer & feather" experiment conducted on the moon. Well, replace the hammer with a 5-inch neutron star & the feather with a single atom - they still hit the ground @ the same time! (or so I was told). HOW is that possible?! What's going on? Understanding this may help wrap my mind around curved spacetime. At any rate, thanks for giving me something to look into, Markus.
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    Allow me to illuminate my ignorance. We all know the "hammer & feather" experiment conducted on the moon. Well, replace the hammer with a 5-inch neutron star & the feather with a single atom - they still hit the ground @ the same time! (or so I was told). HOW is that possible?! What's going on? Understanding this may help wrap my mind around curved spacetime. At any rate, thanks for giving me something to look into, Markus.
    That is worked out with good old Newtonian gravity. In fact, the feather and hammer fall at the same rate relative to their starting points, but if the moon were 100% rigid, it would fall towards each of them as well, although imperceptably slowly. Basically, the acceleration with which something falls towards a mass relative to their starting points is determined by the mass of the second body only. With two masses, each would fall towards a common centre of gravity at a rate determined by the size of the other mass, but the closing acceleration between the bodies would be the sum of both. Two equal masses would fall at acceleration X relative to their starting points, but total closing acceleration would be 2X.
    Last edited by KALSTER; February 16th, 2012 at 06:43 PM.
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  20. #19  
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    Quote Originally Posted by 6nqpnw View Post
    I had forgotten all about time dilation, but couldn't the interaction of gravity with matter/energy create some sort of quantum resistance to account for time dilation & frame-dragging? .
    No such effect is currently known to exist. It is also not needed, since both of these effects ( and all others which I haven't mentioned yet ) are perfectly explained by GR.

    We all know the "hammer & feather" experiment conducted on the moon. Well, replace the hammer with a 5-inch neutron star & the feather with a single atom
    Ok, here is where you run into difficulties - in the original experiment both hammer and feather have a much smaller mass than the moon, so they fall "normally" down on the surface. A neutron star on the other hand has a very large mass and thus a very strong gravitational field. What would happen in your example is that both feather and moon will actually fall into the neutron star
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  21. #20  
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    Quote Originally Posted by KALSTER View Post
    ...if the moon were 100% rigid, it would fall towards each of them as well, although imperceptably slowly.
    That certainly fills-in-the-blank for me. Thank you.

    Quote Originally Posted by Markus Hanke View Post
    ...both feather and moon will actually fall into the neutron star
    LOVE IT!! So fun to think it through. Thanks for your insight.

    Quote Originally Posted by Markus Hanke View Post
    No such effect is currently known to exist. It is also not needed, since both of these effects ( and all others which I haven't mentioned yet ) are perfectly explained by GR.
    I disagree. Though GR is what I consider to be truly beautiful in all its artistically scientific splendor; it however does not explain it all. I agree GR does explain & predict astrological phenomena; there still is a disconnect from QM.

    I'll try to clarify: earlier discussion in this thread suggested spacetime within an atom is affected by gravity - no one disputed. With that in mind (& assuming it's accurate), an atom in a significantly stronger gravity field would produce a warped electron cloud, would it not? Since electrons are zooming through curved spacetime around a nucleus, change the curvature - change the paths/shape.

    This, of course, would all fall apart if the electron were exempt from GR.
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  22. #21  
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    Quote Originally Posted by 6nqpnw View Post
    I disagree. Though GR is what I consider to be truly beautiful in all its artistically scientific splendor; it however does not explain it all. I agree GR does explain & predict astrological phenomena; there still is a disconnect from QM.
    So what? Astrology is pseudoscientific crap.

    I'll try to clarify: earlier discussion in this thread suggested spacetime within an atom is affected by gravity - no one disputed. With that in mind (& assuming it's accurate), an atom in a significantly stronger gravity field would produce a warped electron cloud, would it not? Since electrons are zooming through curved spacetime around a nucleus, change the curvature - change the paths/shape.

    This, of course, would all fall apart if the electron were exempt from GR.
    Which it isn't, so again, so what?
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  23. #22  
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    I'm a schmuck! Astrological was a typo - astronomical ::: sry for the confusion; big no-no here. And thx for highlighting my embarrassment.

    Moving on. Are you saying electron orbitals don't traverse through spacetime?
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    No, you're the one that suggested electrons are exempt despite voluminous evidence to the contrary. Got anything to defend your assertion?
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  25. #24  
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    I'm not completely certain of exactly what it is I'm trying to suggest , Mr. Wayne. But it seems to me that since atoms are 99.9999% empty space (also known as spacetime), that this empty space would be affected (curved) by gravity. And since it's proven that photons (pure energy / waves) follow this curvature, then wouldn't also electrons (regardless whether they're waves or particles).If we were to develop a method of testing electron orbitals in various gravitational fields, would an altered/warped electron cloud not be anticipated? As GR would predict such a phenomenon, would it not?
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  26. #25  
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    Quote Originally Posted by 6nqpnw View Post
    I'm not completely certain of exactly what it is I'm trying to suggest , Mr. Wayne. But it seems to me that since atoms are 99.9999% empty space (also known as spacetime), that this empty space would be affected (curved) by gravity. And since it's proven that photons (pure energy / waves) follow this curvature, then wouldn't also electrons (regardless whether they're waves or particles).If we were to develop a method of testing electron orbitals in various gravitational fields, would an altered/warped electron cloud not be anticipated? As GR would predict such a phenomenon, would it not?
    While you are technically correct in saying that spacetime is curved even within an atom in the presence of a gravitational field, it should be noted that this effect would be so tiny as to be entirely negligible in most circumstances. You would need a very strong gravitational field to get any kind of measurable effect out of this.
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    "Entirely negligible"? When focused on the effects of a single atom, I agree; scaled to billions & considering its cumulative effects, then I'm not so certain.

    Look at our earthly effects of gravitational time dilation: cosmonauts' aboard the ISS age more slowly than their terrestrial constituents. The farther from Earth, the less compressed spacetime becomes; thusly, electron orbitals are expanded & consequently have longer paths. The overall effect on our cosmonaut would be less aging - not because time is slower, but rather there's less electron orbital activity (billions of times over & over) in the same amount of time as those in a more compressed area of spacetime (on Earth).
    Last edited by 6nqpnw; February 27th, 2012 at 03:35 PM. Reason: for clarity
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    Whoa there!!!

    The cosmonauts were in Low Earth Orbit, where the difference in gravitational potential is slight and the difference in relative speed is larger, so the time-dilation of Special Relativity wins out and they aged a little slower than people on the surface of the Earth.

    But further out, where the GPS satellites orbit, the difference in gravitational potential outweighs the difference in relative speed, so time on a GPS satellite runs faster than on the surface of the Earth. And the further away from Earth you get, the greater the difference... in flat space is where time passes at its fastest.

    You need to move to a place with higher gravity (more curvature) than around the Earth in order to find time passing slower than on Earth due to curvature.
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  29. #28  
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    Quote Originally Posted by 6nqpnw View Post
    "Entirely negligible"? When focused on the effects of a single atom, I agree; scaled to billions & considering its cumulative effects, then I'm not so certain.

    Look at our earthly effects of gravitational time dilation: cosmonauts' aboard the ISS age more slowly than their terrestrial constituents. The farther from Earth, the less compressed spacetime becomes; thusly, electron orbitals are expanded & consequently have longer paths. The overall effect on our cosmonaut would be less aging - not because time is slower, but rather there's less electron orbital activity (billions of times over & over) in the same amount of time as those in a more compressed area of spacetime (on Earth).
    1. You were talking about individual atoms
    2. Time dilation has nothing to do with electron orbits
    3. Electron orbits are not appreciably effected by a gravitational field as weak the Earth's
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  30. #29  
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    Quote Originally Posted by 6nqpnw View Post
    "Entirely negligible"?
    Yes. Entirely negligible.
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  31. #30  
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    Thanks Mr. Markus, but SpeedFreek has already set me straight.

    And your thoughtful feedback is always appreciated.
    Last edited by 6nqpnw; February 29th, 2012 at 05:55 PM. Reason: typo
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