Notices
Page 1 of 2 12 LastLast
Results 1 to 100 of 101
Like Tree14Likes

Thread: Does Gravity do any work

  1. #1 Does Gravity do any work 
    Forum Freshman
    Join Date
    Nov 2009
    Posts
    50
    Does Gravity do any work? I think there is a school of thought that believes that Gravity does no work. I think that the concept may be in error. Thanks for comments. Joe L. Ogan


    Reply With Quote  
     

  2.  
     

  3. #2  
    Forum Freshman
    Join Date
    Nov 2009
    Posts
    59
    Another inspired question!

    Yes, gravity works very hard most of the time but it has a good rest at weekends.

    Gravity is a force. It operates between bodies with mass. Gravity is a passive force which depends on the mass of objects.


    Reply With Quote  
     

  4. #3  
    Time Lord
    Join Date
    Mar 2007
    Posts
    8,046
    Gravity can do work in the same sense that a battery can do work. You have to charge it first. Basically, it's a storage device. If you do work now, by lifting something up in the air, then gravity will do work later, when you allow the object to fall again.
    Reply With Quote  
     

  5. #4  
    The Doctor Quantime's Avatar
    Join Date
    Jun 2007
    Location
    United Kingdom
    Posts
    4,546
    The more mass, the more work.

    A lack of capacity of mass means that it will obviously have a little effect on other mass, essentially doing bugger all. Thats why gravity is the weakest, its on the universal job seekers allowance.

    I suppose you could link that its mass equivalent to its energy links to its potential work, I'm sure you can gather equations if you want.

    Is this just a curious question or are you intending it to be more?
    "If you wish to make an apple pie from scratch, you must first invent the universe". - Carl Sagan
    Reply With Quote  
     

  6. #5 Does Gravity do any work? 
    Forum Freshman
    Join Date
    Nov 2009
    Posts
    50
    This is something that puzzles me. It puzzles people who are a lot better educated in Science than I. About 50% of Scientists appear to think that it does work and 50% do not. One compared it to a battery and that we had to charge for it to work. I do not believe that. If there is a charge there it is always there. I am also intrugued by some of the similarity to Magnetism. I am just interested from an intellectual curiosity. Thanks for any help you can give me. Joe L. Ogan
    Reply With Quote  
     

  7. #6  
    The Doctor Quantime's Avatar
    Join Date
    Jun 2007
    Location
    United Kingdom
    Posts
    4,546
    Gravity and magnetism are the same force. we have no evidence or experimental empirical evidence to suggest this however and my theorising on the quantum uncertainty and behaviour of particles & nature of the universe has linked charge and mass.

    We, rather I, do not have the maths knowledge to emphasise on these theories however.

    Agreement with your statement we are. I hope you find satisfaction.
    "If you wish to make an apple pie from scratch, you must first invent the universe". - Carl Sagan
    Reply With Quote  
     

  8. #7 Re: Does Gravity do any work? 
    Suspended
    Join Date
    Apr 2007
    Location
    Pennsylvania
    Posts
    8,822
    Quote Originally Posted by Joe L. Ogan
    This is something that puzzles me. It puzzles people who are a lot better educated in Science than I. About 50% of Scientists appear to think that it does work and 50% do not. One compared it to a battery and that we had to charge for it to work. I do not believe that. If there is a charge there it is always there. I am also intrugued by some of the similarity to Magnetism. I am just interested from an intellectual curiosity. Thanks for any help you can give me. Joe L. Ogan
    It's not really a puzzle, Joe. The only disagreement is semantic. Gravity is a force. If you lift something up, the object you lift acquires potential energy. As gravity pulls it back down the energy is released. Did you do the work, or did gravity do the work? It's just in how you look at it. But gravity is not a source of free energy if that's what you're thinking.

    There are lots of ways to store potential energy. Batteries, compressed springs, magnets, etc.
    Reply With Quote  
     

  9. #8  
    Forum Freshman
    Join Date
    Nov 2009
    Posts
    31
    Just a question in the question....

    During the last months I asked to many scientist this question: what's the origin of the Gravity?

    Someone tell me that it starts from the center of the earth... but it is not enough...

    Someone tell me that it's an energy that pulls a mass from an other one that actually has a major mass...

    A physicist tell me that it comes from the magnetic force... and in the earth we supose the the nucleus consists in iron liquide... and the gravity is a kind of force that comes from this other force... (I'm not really convinced in this idea)

    Actually I know because of my study that gravity force is produced when there is a different mass or two object with different mass so that it can start a field of energy (gravity energy) with forces' lines which are the cause of the flow of the gravity force from the mass with less mass to the other one... and of course we can say that the earth, by having a distribution of mass from the core to the surface, is controlled by the same force... which is terrestrian gravity...

    BUT: WHERE IT COMES FROM THIS FORCE?

    I mean:

    the magnetic energy comes from an electron that is moves...

    the electric energy comes from al electron itself

    the electromagnetic energy comes from electric and magnetic fields that are moving at the same times...

    What is the orgin of the gravity field
    Reply With Quote  
     

  10. #9  
    The Doctor Quantime's Avatar
    Join Date
    Jun 2007
    Location
    United Kingdom
    Posts
    4,546
    Thats the same as asking where charge comes from. It just is.
    "If you wish to make an apple pie from scratch, you must first invent the universe". - Carl Sagan
    Reply With Quote  
     

  11. #10  
    Moderator Moderator Dishmaster's Avatar
    Join Date
    Apr 2008
    Location
    Heidelberg, Germany
    Posts
    1,624
    Quote Originally Posted by Quantime
    Gravity and magnetism are the same force. we have no evidence or experimental empirical evidence to suggest this however and my theorising on the quantum uncertainty and behaviour of particles & nature of the universe has linked charge and mass.

    We, rather I, do not have the maths knowledge to emphasise on these theories however.

    Agreement with your statement we are. I hope you find satisfaction.

    I suggest then that you leave this thread and open a new one in the "New Theories" section.

    Your views are contradicting the current understanding of Physics. So, unless you can prove your ideas, you should not put them forward as fact and draw conclusions from them.

    Dishmaster
    (Moderator)
    Reply With Quote  
     

  12. #11 Re: Does Gravity do any work? 
    Suspended
    Join Date
    May 2008
    Posts
    1,096
    Quote Originally Posted by Joe L. Ogan
    This is something that puzzles me. It puzzles people who are a lot better educated in Science than I. About 50% of Scientists appear to think that it does work and 50% do not. One compared it to a battery and that we had to charge for it to work. I do not believe that. If there is a charge there it is always there. I am also intrugued by some of the similarity to Magnetism. I am just interested from an intellectual curiosity. Thanks for any help you can give me. Joe L. Ogan
    As gravity is a property of all matter, it is simply there and does no work in the same way a star burns, etc. It just is. But as others have said, if you want gravity to work for you, you have to put in something first.

    It is just an idea I have but I think gravity may be an object falling in an unknown direction. As we are firmly anchored in the third dimension, we cannot collapse in on ourselves as gravity would want but in space where it is allowed independence, I believe this "unknown direction" may be expressed by rotation of all large bodies, in that they are attempting to forever fall inwards on themselves.

    One possible way of checking this out if we sufficiently technologically advanced (we aren't) would be to follow the collapse of a supernova into a black hole in that it would not collapse in on it self like a balloon almost instantly deflating but twist in on itself, following the unknown direction which causes rotation.
    Reply With Quote  
     

  13. #12  
    Forum Bachelors Degree Waveman28's Avatar
    Join Date
    Mar 2009
    Location
    Australia
    Posts
    417
    Quote Originally Posted by Quantime
    Gravity and magnetism are the same force. we have no evidence or experimental empirical evidence to suggest this however and my theorising on the quantum uncertainty and behaviour of particles & nature of the universe has linked charge and mass.

    We, rather I, do not have the maths knowledge to emphasise on these theories however.

    Agreement with your statement we are. I hope you find satisfaction.
    Gravity and magnetism are definitely not the same force. However, you are right in one sense, which comes from the fact that all forces are just different manifestations of one existing force. And yes, gravity does do work.
    Reply With Quote  
     

  14. #13  
    Administrator KALSTER's Avatar
    Join Date
    Sep 2007
    Location
    South Africa
    Posts
    8,232
    Your views are contradicting the current understanding of Physics.
    I think he was talking about the proposed gravitomagnetism? It has not been rigorously shown to be true, but AFAIK it is a proposal that is being looked at in serious circles, it does provide testable predictions and is being tested for by the gravity probes.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
    Reply With Quote  
     

  15. #14 Re: Does Gravity do any work 
    . DrRocket's Avatar
    Join Date
    Aug 2008
    Posts
    5,486
    Quote Originally Posted by Joe L. Ogan
    Does Gravity do any work? I think there is a school of thought that believes that Gravity does no work. I think that the concept may be in error. Thanks for comments. Joe L. Ogan
    Force applied over distance is work.

    Gravity can certainly result in the application of force over a distance.
    JoshuaL likes this.
    Reply With Quote  
     

  16. #15  
    Time Lord
    Join Date
    Mar 2007
    Posts
    8,046
    I think a lot of this centers on a problem familiar to businesspeople : are we talking gross or net?

    The only way gravity does any net work is if an unfamiliar object enters the area and falls into the gravity well. Otherwise, the work done by gravity pulling something downward is always equal to the work that has already been done by someone or something having lifted it upward first.
    Reply With Quote  
     

  17. #16 Gravitational Work? wahhh?? 
    Suspended
    Join Date
    Dec 2009
    Posts
    3
    The Physics of work is defined as
    W=F(cos@)X

    where W=force, @ if the angle at which the force is acting in, and X is the displacement.

    With that said, it become clear that gravity ITSELF does not do any work due to the fact that gravity ITSELF is not moving.

    But when it comes to another object, then yes, gravity acting on another object may do work, as long as the object is being displaced, or moved.

    A fine questions this is! :D
    Reply With Quote  
     

  18. #17  
    Forum Freshman
    Join Date
    Jan 2010
    Posts
    79
    when you drop a stone from the tenth floor of the building what does gravity do?
    It pulls it down by force causing it to be displaced.
    Therefore:
    Work = F . D
    So gravity can do work!!
    Reply With Quote  
     

  19. #18  
    Time Lord
    Join Date
    Mar 2007
    Posts
    8,046
    Quote Originally Posted by kakarot
    when you drop a stone from the tenth floor of the building what does gravity do?
    It pulls it down by force causing it to be displaced.
    Therefore:
    Work = F . D
    So gravity can do work!!
    That's just like how a battery does work. Sometimes when you find the battery, it is already fully charged. If you use it, you uncharge it. If the stone began its existence at the top of a building, then the battery was already charged. If you had to carry the stone to the top of the building, then that act was the act of charging the battery.

    So, yes: batteries do work, and so does gravity. They're not original sources of energy, so nobody's going to be building a power plant that runs by consuming gravitational force any time soon. However, both are good ways to store energy. Ever heard of Pumped Storage?


    http://en.wikipedia.org/wiki/Pumped-...droelectricity
    Reply With Quote  
     

  20. #19  
    Forum Sophomore
    Join Date
    Aug 2012
    Posts
    121
    I know this is an old thread but I want to bring it back to ask a related question. If gravity does work, then wouldn't that mean that gravity has mass?

    W= F . D
    F= M . A
    W= [M . A] D
    Reply With Quote  
     

  21. #20  
    Universal Mind John Galt's Avatar
    Join Date
    Jul 2005
    Posts
    14,169
    How do you form that conclusion from those three equations?
    Reply With Quote  
     

  22. #21  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Gravity doesn't do work in the usual sense. You can use gravity to drive a waterwheel and get work done, but think it through. If you lift a brick, you do work on it. When you let it go it falls down and gravity converts the work you did into kinetic energy. Conservation of energy applies here. Gravity didn't add any energy to the brick. The kinetic energy of the falling brick came out of the brick. Gravity merely liberated it.

    Gravity is a pseudoforce, and in itself doesn't have mass. However in the Foundation of the General Theory of Relativity you can read Einstein saying this: "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy". In essence the space where a gravitational field is, has greater "energy density" than free space, and this extra energy has a mass-equivalence and a gravitational effect.
    Reply With Quote  
     

  23. #22  
    Forum Sophomore
    Join Date
    Aug 2012
    Posts
    121
    Quote Originally Posted by John Galt View Post
    How do you form that conclusion from those three equations?

    I am not forming a conclusion, I am asking a question. Since gravity is a force and force equals mass times acceleration, then does that mean gravity has mass?
    Reply With Quote  
     

  24. #23  
    Forum Sophomore
    Join Date
    Aug 2012
    Posts
    121
    Quote Originally Posted by Farsight View Post
    Gravity doesn't do work in the usual sense. You can use gravity to drive a waterwheel and get work done, but think it through. If you lift a brick, you do work on it. When you let it go it falls down and gravity converts the work you did into kinetic energy. Conservation of energy applies here. Gravity didn't add any energy to the brick. The kinetic energy of the falling brick came out of the brick. Gravity merely liberated it.

    Gravity is a pseudoforce, and in itself doesn't have mass. However in the Foundation of the General Theory of Relativity you can read Einstein saying this: "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy". In essence the space where a gravitational field is, has greater "energy density" than free space, and this extra energy has a mass-equivalence and a gravitational effect.
    Thank you. That makes sense, kind of. So, a gravitational field has mass, right?
    Reply With Quote  
     

  25. #24  
    Forum Sophomore
    Join Date
    Aug 2012
    Posts
    121
    Do black holes have a gravitational field?
    Reply With Quote  
     

  26. #25  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Not quite, because mass is all to do with pushing something and making it move. You can't really do that with a gravitational field. Hence its better to say that the energy of a gravitational field has a mass equivalence.
    Reply With Quote  
     

  27. #26  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,601
    Quote Originally Posted by Out of the box View Post
    Do black holes have a gravitational field?
    Yes. They have mass and, effectively, it is their gravitational field that makes them black holes.
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
    Reply With Quote  
     

  28. #27  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,601
    Quote Originally Posted by Out of the box View Post
    So, a gravitational field has mass, right?
    Mass and all forms of energy (and a few other things) cause a gravitational effect. A gravitational field has energy and so causes gravitation.
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
    Reply With Quote  
     

  29. #28  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Quote Originally Posted by Out of the box
    Do black holes have a gravitational field?
    Yes. And in anticipation of your next question: the mass-equivalence of the energy of that gravitational field doesn't fall into the black hole. A gravitational field alters motion through space, it doesn't suck space in.
    Reply With Quote  
     

  30. #29  
    Forum Sophomore
    Join Date
    Aug 2012
    Posts
    121
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Out of the box View Post
    Do black holes have a gravitational field?
    Yes. They have mass and, effectively, it is their gravitational field that makes them black holes.
    Why wouldn't the mass from the gravitational field be pulled into the black hole?
    Reply With Quote  
     

  31. #30  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,601
    Quote Originally Posted by Out of the box View Post
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Out of the box View Post
    Do black holes have a gravitational field?
    Yes. They have mass and, effectively, it is their gravitational field that makes them black holes.
    Why wouldn't the mass from the gravitational field be pulled into the black hole?
    The gravitational field doesn't have mass it has energy. And it is change in the geometry of spacetime around the (mass of) the black hole. It is not something that can be "pulled in".

    And kudos to Farsight for correctly guessing you next question.
    Last edited by Strange; October 4th, 2012 at 12:48 PM.
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
    Reply With Quote  
     

  32. #31  
    Forum Sophomore
    Join Date
    Aug 2012
    Posts
    121
    Quote Originally Posted by Farsight View Post
    Quote Originally Posted by Out of the box
    Do black holes have a gravitational field?
    Yes. And in anticipation of your next question: the mass-equivalence of the energy of that gravitational field doesn't fall into the black hole. A gravitational field alters motion through space, it doesn't suck space in.
    Thanks again Farsight, and in agreement with Strange, kudos on your telepathic (intuitive) abilities.
    Reply With Quote  
     

  33. #32  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    I agree with Farsight in that gravity does not perform work. My explanation of this is simply that gravity is a result of space-time geometry, and not a "real" force in the mechanical sense.
    Reply With Quote  
     

  34. #33  
    Forum Masters Degree
    Join Date
    Aug 2011
    Posts
    703
    Another name for gravity: "conservative force field".

    It mean: every work that gravity do, can be retrieved back (like battery). ie: if we release a ball from a height and it freefall, when it bounces with no energy loss thru friction then the ball will return to the same position like no changes has ever occured.

    Spring and rubber band also feature a "conservative force field". So gravity behave like spring.
    Reply With Quote  
     

  35. #34  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    LOL, thanks guys.

    While I'm here, can I say that people sometimes think of curved spacetime as curved space, which causes them problems. Check out the wikipedia spacetime page and see the bit of the caption under the picture that says "The grid lines do not represent the curvature of space but instead the coordinate system". Also have a look at John Baez's website, where you can read this: "Similarly, in general relativity gravity is not really a 'force', but just a manifestation of the curvature of spacetime. Note: not the curvature of space, but of spacetime. The distinction is crucial." IMHO the best way to understand this is to go back to basics and imagine you've used light beams and a light clock to measure distance and time at various locations through an equatorial slice through the Earth and the surrounding space. Then you plot all your measurements on a "spacetime chart" or coordinate system, which ends up looking like the depiction of Newtonian gravitational potential on wiki.


    CCASA image by AllenMcC, see File:GravityPotential.jpg - Wikipedia, the free encyclopedia

    It's similar to the depiction on the wiki spacetime page, and all those pictures of a bowling ball on a rubber sheet. The slope at some location indicates the gravitational force at that location, in that the steeper the slope the faster you start to fall. The curviness of the slope indicates tidal force, which relates to what's called the Riemann curvature tensor. Basically, that curviness is spacetime curvature. It's the curvature of your spacetime chart aka coordinate system rather than the curvature of space.
    Reply With Quote  
     

  36. #35  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Note: not the curvature of space, but of spacetime. The distinction is crucial."
    Indeed. Under General Relativity time and space are both part of the same manifold - they cannot be separated. Curvature in space always goes hand-in-hand with curvature in time, and vice versa.
    Reply With Quote  
     

  37. #36  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    It isn't curved space though Markus, so much as "curvature in a plot of your measurements of distance and time". See the next bit of the Baez article: "If you toss a ball, it follows a parabolic path. This is far from being a geodesic in space". If space was curved in the room you're in, then regardless of how fast you threw a ball, it would follow the same arc. Remember what we were talking about the other day? You might find this of interest. Look at the paragraph just before the end note.
    Reply With Quote  
     

  38. #37  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    If space was curved in the room you're in, then regardless of how fast you threw a ball, it would follow the same arc.
    No, it wouldn't - remember that, based on the Schwarzschild Metric, the curvature is only along the radial coordinate, and along time :



    That means that, while the ball gets accelerated down towards the earth at the same rate no matter how hard you throw the ball, you still get different arcs according to the kinetic energy you impart on it. In other words, it always follows the same geodesic along the r-coordinate only.

    EDIT : Last sentence was wrong.
    Last edited by Markus Hanke; October 5th, 2012 at 06:28 AM.
    Reply With Quote  
     

  39. #38  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Yes, the curvature is along the radial coordinate, it's a curvature of your coordinate system, it's a curvature in a plot of your measurements of distance and time. That isn't the same thing as curved space. Shine a lightbeam directly between two massive stars. You will measure something akin to the Shapiro delay, which we associate with curved spacetime. And that lightbeam goes straight as a die. Through curved space? Read the article I linked to. It's by Percy Hammond, co-author of Geometry of Electromagnetic Systems.
    Reply With Quote  
     

  40. #39  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Quote Originally Posted by Farsight View Post
    Yes, the curvature is along the radial coordinate, it's a curvature of your coordinate system, it's a curvature in a plot of your measurements of distance and time. That isn't the same thing as curved space.
    Look, I know you have stated that many times across several threads. I still do not agree with it, because the coordinate systems you use have no physical reality, only the underlying space-time itself does. The coordinates you use are nothing more than an artificial grid you lay down to be able to perform measurements; the point is, no matter how you arrange that grid, I triangle on it will never have a angle sum of 180 degrees. It's like the earth's surface - no matter how you unfold it, you can never make a flat sheet from it. Why ? Because the underlying manifold is not flat.

    You are of course free to deny that space-time is in fact curved; the thing is just that I ( and probably most other mainstream proponents ) will continue to disagree with this. For me the simple fact that the Einstein equations are invariant under arbitrary coordinate transformations already show clearly that the curvature is not just an artifact of measurements, but a fundamental property of space-time itself. To me space-time and the gravitational field are not two separate but dependent entities, but one and the same thing. If space-time wasn't curved than there would exist a coordinate mapping on that manifold with a vanishing curvature tensor; that is obviously not the case.
    Reply With Quote  
     

  41. #40  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Quote Originally Posted by Farsight View Post
    You will measure something akin to the Shapiro delay, which we associate with curved spacetime. And that lightbeam goes straight as a die.
    Why do you think you get a Shapiro-delay ? Because of space-time curvature. So yes, the curvature is quite real, even if the beam goes "straight" with respect to a stationary observer. Without curvature there would be no such delay.
    Reply With Quote  
     

  42. #41  
    Administrator KALSTER's Avatar
    Join Date
    Sep 2007
    Location
    South Africa
    Posts
    8,232
    And that curvature is the only explanation for why gravity has any effect on a massless particle in the first place, no?
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
    Reply With Quote  
     

  43. #42  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    No. See what I said earlier. The curvature is a curvature in your plot made using light clocks. It's the effect, not the cause.
    Reply With Quote  
     

  44. #43  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Kalster: think it through. Like Markus said, your coordinate system is nothing more than an abstract "grid" or plot. You lay it down using say light clocks, see post #34 above. A light clock near the surface of the Earth doesn't show a lower reading than one up in space because your plot of light clock readings is curved.

    Markus, sorry, I have to go. But for now in in brief: a manifold is an abstract thing too. It's a topological space, a "mathematical structure". Trust me on all this, I'm not making it up.
    Reply With Quote  
     

  45. #44  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Quote Originally Posted by Farsight View Post
    Like Markus said, your coordinate system is nothing more than an abstract "grid" or plot.
    Yes, that is why it is not fundamental. However, the geometry that underlies the coordinate system is. In other words - it doesn't matter what system of coordinates you choose, they will always be curved, because the underlying space-time which is measured by them is curved.

    a manifold is an abstract thing too. It's a topological space, a "mathematical structure".
    Of course, just like all other mathematical entities like scalars, vectors etc etc are mathematical structures. However, those mathematical structures are a representation of reality, that is why they are used to express physical laws.
    Reply With Quote  
     

  46. #45  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Sorry Markus, but spacetime is a "mathematical structure" too. It's a representation of reality, but note that it displays all times in one hit. There is no motion in it. There's a myth that light curves because it moves through curved spacetime, but I'm afraid that's a schoolboy error. Sadly some of those schoolboys have grown up and taught it to other schoolboys, by rote. They have this unicorn so drummed in that when they read Einstein talking about the equations of motion and inhomogeneous space, it goes totally over their head, especially when they are mathematicians who mistake abstraction for reality. Sit down and think it through for yourself. We can use the motion of light in light clocks in an equatorial slice around the Earth to make a plot to visualize Riemann curvature. Our plot isn't flat because our measurements are not uniform. Our plot isn't conical because the variations in our measurements are not uniform. Instead our plot displays a curvature in line with the non-uniformity of our measurements. We're plotting a non-uniformity in our measurements as a "metric", but a metric relates to measurement, it isn't the underlying reality. The motion of light through space is the underlying reality. Our measurements of it are not uniform because space isn't uniform. And because of that, when light moves through it, it curves.
    Reply With Quote  
     

  47. #46  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Quote Originally Posted by Farsight View Post
    Sorry Markus, but spacetime is a "mathematical structure" too. It's a representation of reality, but note that it displays all times in one hit. There is no motion in it. There's a myth that light curves because it moves through curved spacetime, but I'm afraid that's a schoolboy error. Sadly some of those schoolboys have grown up and taught it to other schoolboys, by rote. They have this unicorn so drummed in that when they read Einstein talking about the equations of motion and inhomogeneous space, it goes totally over their head, especially when they are mathematicians who mistake abstraction for reality. Sit down and think it through for yourself. We can use the motion of light in light clocks in an equatorial slice around the Earth to make a plot to visualize Riemann curvature. Our plot isn't flat because our measurements are not uniform. Our plot isn't conical because the variations in our measurements are not uniform. Instead our plot displays a curvature in line with the non-uniformity of our measurements. We're plotting a non-uniformity in our measurements as a "metric", but a metric relates to measurement, it isn't the underlying reality. The motion of light through space is the underlying reality. Our measurements of it are not uniform because space isn't uniform. And because of that, when light moves through it, it curves.
    I do not agree with that at all. So far as I am concerned curvature of space-time is very real, and it gives us the correct results, mathematically.
    The way I see it is that you are very much stuck in that you appear to think your point of view is superior to that of established mainstream science - incidentally, that's the very same trap you accuse us of having fallen victim to. I freely admit that I do not know how to convince you otherwise, hence I deem it to be best to just leave it at this for the moment. I am sure you understand that you aren't going to be able to convince me of your point of view either.

    And yes, I have thought it through, for a very long time, and in detail. I have never been taught any of it by anyone, I had to figure it all out by myself, piece it together from a variety of sources. In the end I arrive at the same conclusions as mainstream science as to what the field equations mean; to me it is all clear and unambiguous. But if you think of me as a brainless "schoolboy" because of that, then fine, be my guest.
    Reply With Quote  
     

  48. #47  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    I don't think you're a brainless schoolboy Markus. If I did I wouldn't talk to you. But nor do I think you've thought this through. I'm with Einstein on this, and the hard scientific evidence is with me, not you. Now come on man, think. Use a gedankenexperiment. You're in a spaceship, and you travel between those two stars laying out a cable behind you. Every ten thousand kilometres there's a parallel-mirror light clock fixed to the cable. You end up with something like this:


    ........O
    ---------------
    ........O


    You wait a for a good long while to reduce the effects of motion, then you reel in the cable and plot the clock readings. There's a curvature on your plot, it looks a little like a slice through the gravitational potential depiction above. You call this curved spacetime and assign causative reality to it, such that when I shine a light beam between those two stars and we measure a Shapiro delay, you say it's because spacetime is curved. But the reality is that the curvature on your plot is only there because some parallel-mirror light clocks ran slower than others. And they're light clocks, all they did was clock up the number of times a light beam bounced back and forth. They didn't measure some abstract thing called the flow of proper time. A clock accumulates some regular motion and displays a result that you call the time. And when a clock goes slower it's because that motion goes slower, not because your plot of clock readings is curved. The reason why the Shapiro delay occurs is staring you in the face. Light goes slower where gravitational potential is lower. That's what Einstein said repeatedly. You're Austrian aren't you? You can read the original papers, you know what geschwindigkeit means. If you'd really thought this through for a very long time and in detail, that's something you could not have missed.

    Anyway, let's try to get back on topic. Gravity doesn't do any work. That means a photon descending vertically through a gravitational field doesn't gain any energy. Its frequency doesn't increase. It only appears to increase because measuring devices and all other things are going slower where gravitational potential is lower. Conservation of energy applies. If there's one "law" in physics you should pay attention to, that's it.
    Out of the box likes this.
    Reply With Quote  
     

  49. #48  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    So in other words you deny that space-time is curved ?
    Reply With Quote  
     

  50. #49  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Quote Originally Posted by Farsight View Post
    You're Austrian aren't you?
    No, but I am fluent in German.

    you know what geschwindigkeit means
    It means velocity. Be careful about the distinction between velocity and speed. It is crucial in this context.

    But the reality is that the curvature on your plot is only there because some parallel-mirror light clocks ran slower than others.
    Which is because space-time is curved.

    And they're light clocks, all they did was clock up the number of times a light beam bounced back and forth.
    You can substitute any other kind of clock - mechanical, biological, atomic. All measure the exact same dilation. Because space-time is curved.

    And when a clock goes slower it's because that motion goes slower
    ...which is because space-time is curved.

    The reason why the Shapiro delay occurs is staring you in the face.
    Yes, it's because space-time is curved.

    If you'd really thought this through for a very long time and in detail, that's something you could not have missed.
    I don't feel I have missed anything. All of GR revolves around just one fundamental object - the metric . I understand this to represent the causal structure of space-time, i.e. its intrinsic geometry. Period. As simple as that. Everything else follows from this quite naturally, no need to consider speeds, light clocks, velocities etc etc. To me it all comes down to simple geometry, and everything from geodesics, time dilation, frame dragging, precession, the Raychaudhuri equation and so on immediately become clear. To me it is incomprehensible how you cannot see the beauty and simplicity in this, and need to fall back on concepts of inhomogenous space with varying electromagnetic properties.

    Tell me, why is a physically real curved space-time such a problem, in your opinion ? To me it is a very natural concept, and most certainly not more a stretch than inhomogenous space.
    Reply With Quote  
     

  51. #50  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Quote Originally Posted by Markus Hanke View Post
    Tell me, why is a physically real curved space-time such a problem, in your opinion?
    It's a problem because there is no motion in spacetime. This is of necessity so because spacetime models space for all times. Light does not move through it. This kills curved spacetime as the causative agent for the curvilinear motion of light through space. As for geschwindigkeit meaning velocity, please note this: this, and this:

    "Eine Krümmung der Lichtstrahlen kann nämlich nur dann eintreten, wenn die Ausbreitungsgeschwindigkeit des Lichtes mit dem Orte variiert".

    I translate for you:

    "A bending of light rays can only happen if the speed of light varies with location."

    You translated that. You translated geschwindigkeit into speed. And now you're saying geschwindigkeit means velocity? I'm sorry Markus, but that's checkmate.

    Look, let's park our disagreement on that. Let's talk about some other interesting things, like the descending photon that doesn't gain any energy. Here's another interesting one: a descending brick loses rest mass. Because conservation of energy applies, and its kinetic energy comes from itself. Because gravity doesn't do any work. Only when you weigh that brick, it appears to have gained rest mass because light is deflected twice as much as matter!
    Reply With Quote  
     

  52. #51  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Quote Originally Posted by Farsight View Post
    You translated that. You translated geschwindigkeit into speed. And now you're saying geschwindigkeit means velocity? I'm sorry Markus, but that's checkmate.
    There is a difference between "Geschwindigkeit" and "Ausbreitungsgeschwindigkeit". But be it as it may, all I am saying to you is to be aware of the difference between velocity and speed. If you look up these two terms you will see that they don't mean the same thing. Unfortunately the German language has only one word for it, so the difference is implied by the context.

    It's a problem because there is no motion in spacetime.
    Of course not, because it's 4-dimensional, so the photon, just like every other particle, is represented by a world-line. Why is that a problem for the concept of curved space-time ? That world-line would have a different geometry in a flat space-time, so I really don't get your argumentation here.
    Reply With Quote  
     

  53. #52  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    And that context was that "according to the general theory of relativity, the law of the constancy of the speed of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity". The fundamental SR assumption was the constant speed of light, not a constant vector-quantity velocity. If you think it was, I have a little something that will disabuse you of that notion. It's called a mirror.

    Spacetime is a mathematical space Markus. Nothing moves through it. It is something that is abstract. It has no physical reality. Its curvature does not cause the light clocks to run slow between those two stars, and it does not cause the light moving between those two stars to go slower either.

    But like I said, park it. Let's talk about gravity does no work. See my post above. Yes, invariant mass varies.
    Reply With Quote  
     

  54. #53  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Anyway, if anybody's interested, what's I think is rather striking is the similarity between Einstein's inhomogeneous space and what Newton said about the underlying cause of gravity in Opticks queries 20 and 21:

    Quote Originally Posted by Newton
    "Doth not this aethereal medium in passing out of water, glass, crystal, and other compact and dense bodies in empty spaces, grow denser and denser by degrees, and by that means refract the rays of light not in a point, but by bending them gradually in curve lines? ...Is not this medium much rarer within the dense bodies of the Sun, stars, planets and comets, than in the empty celestial space between them? And in passing from them to great distances, doth it not grow denser and denser perpetually, and thereby cause the gravity of those great bodies towards one another, and of their parts towards the bodies; every body endeavouring to go from the denser parts of the medium towards the rarer?"
    The "density" is back to front as compared to stress-energy, but such words are never ideal, and considering this was published in 1704, I think it's pretty amazing myself. There's other bits and pieces you pick up on, like the similarity between his corpuscles and the quantum nature of light. And he knew you could convert matter into light in something as simple as a fire, see Opticks query 30: "Are not gross bodies and light convertible into one another?" That's what pair production and annihilation do. And given that we can turn lead into gold, (see nuclear transmutation), his "occult" interest in alchemy seems understandable. Whilst his creationism is uncomfortable reading in these modern times, overall I see parallels between his "Opticks" interest in light and the importance of light in relativity, and all in all I think he was one hell of a smart guy.
    Reply With Quote  
     

  55. #54  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,601
    Quote Originally Posted by Farsight View Post
    AThe "density" is back to front as compared to stress-energy, but such words are never ideal ...
    So maybe phlogiston was not so wrong after all. Just change the name, reverse the logic, introduce some completely new formulae and underlying mechanisms and bingo!
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
    Reply With Quote  
     

  56. #55  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Get lost, Strange. Einstein used the word aether, see this: "Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether.. Newton used the word too, see above, and it's all over the arXiv. I'm telling you how it is.

    How can you not know how it is? Even if you only got your physics from pop-science magazines you should know that professional physicists don't think aether is a dirty word, see this. Also see this article on the standard model which refers to a Higgs field which "pervades the vacuum and fills all of space". Do you even know that some bona-fide physicists refer to it as the Higgs substance? What does that make it? Yeah, got it in one. So please, don't go trotting out the sarcastic phlogiston nonsense, it makes you look like an ignorant naysayer who's led a sheltered life.
    Reply With Quote  
     

  57. #56  
    Moderator Moderator
    Join Date
    Jun 2005
    Posts
    1,612
    Moderator note The fact that you post nonsense here is not it itself a reason for suspension. But the following easily could be - so cool it
    Quote Originally Posted by Farsight View Post
    Get lost, Strange.
    Reply With Quote  
     

  58. #57  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,601
    Quote Originally Posted by Farsight View Post
    Get lost, Strange. Einstein used the word aether ...
    I know he did. And so have many other people to refer to different things.

    Yes, you can call space-time "the aether". (Although I think that is misleading as it implies spacetime is some sort of "stuff" rather than just the geometry in which things take place.)

    Yes, you can call the Higgs field "the aether".

    Yes, you can call the non-empty vacuum "the aether".

    You can call dark energy "the aether".

    You can call the neutrino sea "the aether".

    You can call the CMB "the aether".

    I have seen mention of an hypothesis to explain (some of) the effects of dark matter by means of a new field, which the authors referred to as "aether".

    And on and on.

    And of course, several people are still trying to detect the classical aether representing an absolute rest frame. None have succeeded yet. But hey, who knows.

    As a result, you have the word aether being used for many different and often unrelated things. So it becomes fairly meaningless except as a generic term for "something pervasive". If you think that is useful, then great. Carry on.

    My biggest concern is that these various uses of the word "aether" will be misinterpreted by a variety of pseudoscientists and crackpots to argue that relativity or QM is wrong, or whatever their particular fantasy is. We get plenty of these here, using quotes like this from Einstein and others as support for their "personal theory of everything".

    But if you don't care that your use of the word will inevitably be misused because of its historical associations, then carry on. I'm not going to stop you. I might feel the need to point out, occasionally, that this is not the classical "luminiferous aether" for which there is no evidence and no need.

    and it's all over the arXiv
    Amusingly, some of those search results are for the chemical, ether. I'm pretty sure that exists.
    tk421 likes this.
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
    Reply With Quote  
     

  59. #58  
    Forum Freshman
    Join Date
    Oct 2012
    Posts
    76
    The hydraulic power plants of the world demonstrate that gravity does indeed do work, very well.
    Reply With Quote  
     

  60. #59  
    Forum Isotope
    Join Date
    Feb 2012
    Location
    Western US
    Posts
    2,666
    Quote Originally Posted by Farsight View Post
    Whilst his creationism is uncomfortable reading in these modern times, overall I see parallels between his "Opticks" interest in light and the importance of light in relativity, and all in all I think he was one hell of a smart guy.
    He was indeed one hell of a smart guy. However, much of your post is an exercise in post hoc pattern matching. It's uncomfortably close to the sort of activity that followers of Nostradamus, e.g., engage in.
    Reply With Quote  
     

  61. #60  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Quote Originally Posted by Guitarist View Post
    Moderator note The fact that you post nonsense here is not it itself a reason for suspension. But the following easily could be - so cool it.
    Nonsense, Guitarist? If you take issue with something I've said, no problem, let's discuss it. It is after all a discussion forum, and there's always room for improvement. If you'd like to explain why something I've said here is nonsense I'd be only too happy to hear it. If you can't, then please as a moderator spare me abusive words like nonsense. That's far more insulting than my dish-it back response to Strange's phlogiston sarcasm, which wasn't his first time. Can I remind you that our previous exchange was when I said The mass of a body is a measure of its energy-content, and you said Huh? not realising that those words were direct from Einstein's E=mc² paper. What Einstein said wasn't nonsense. Ditto for Newton. Now, since Markus is not currently around, why don't you pick up where he left off? We were talking about what actually causes light to bend. Alternatively, if you're not up to that, you could respond to my comments re photon energy and invariant mass which more closely relate to the thread title Does gravity do work? It would be a pleasure to have a conversation about it.

    Quote Originally Posted by Strange View Post
    He was indeed one hell of a smart guy. However, much of your post is an exercise in post hoc pattern matching. It's uncomfortably close to the sort of activity that followers of Nostradamus, e.g., engage in.
    No it isn't, it's telling you something you didn't know. Now you do. You'll get used to it. Now, would you like to talk about gravity and work?

    Quote Originally Posted by Strange View Post
    My biggest concern is that these various uses of the word "aether" will be misinterpreted...
    Your concern is noted, Strange. Now let's talk gravity.
    Last edited by Farsight; October 9th, 2012 at 01:58 PM. Reason: merged multiple responses
    Out of the box likes this.
    Reply With Quote  
     

  62. #61  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Now, since Markus is not currently around, why don't you pick up where he left off?
    That would indeed be interesting - let's see what Guitarist ( a mathematician by trade ) has to say to your apparent assertion that a non-constant metric tensor in the context of GR does not imply a space-time with intrinsic curvature.

    As for me - I have already explained to you that to me space-time curvature is quite real. I base this on my understand not of Einstein's words, but of his maths - the reason being that words are ambiguous ( as seen on numerous occasions, including this very thread ), whereas the maths are clear and concise. I have read his works, yet when it comes down to it I would always choose his maths over his words. You plug in numbers into those maths, you get results which are in accordance to observation. At the end of the day that's all there really is to it. We all understand that Riemann geometry is a model, a representation of reality - but it is one that works, because it gives us the correct results. In this there is no question for me as to what the metric tensor really represents, and it sure is not inhomogeneous space; furthermore I see no reason or principle in nature that in any way makes space-time curvature not plausible.
    That is pretty much where I had "left off" - at this point in time I am not sure what else to add to this. I know that you think I either haven't understood GR, or that I just blindly follow textbooks - that is fine with me, so long as you realize that that is just your personal opinion. From my point of view I do not see how you possibly could have more of an insight into these matters than any of the textbooks I have studied, so currently I see no reason to abandon space-time curvature for "inhomogeneous space". But know this - I am not a blind follower of anything; should it ever turn out that GR was indeed fundamentally wrong, then I will be the first one to concede my mistake. At the moment though I don't see any such fundamental flaws in the theory.

    We were talking about what actually causes light to bend.
    Light doesn't bend at all - it always follows straight lines. Straight lines in curved space-time which, as it so happens, differ in geometry from straight lines in flat space-time. It is a very natural notion to me.
    KALSTER, tk421 and Out of the box like this.
    Reply With Quote  
     

  63. #62  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    since Markus is not currently around
    I am always around...I am your local worst-nightmare-mainstream-science guy
    KALSTER likes this.
    Reply With Quote  
     

  64. #63  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Quote Originally Posted by Markus Hanke View Post
    That would indeed be interesting - let's see what Guitarist ( a mathematician by trade ) has to say to your apparent assertion that a non-constant metric tensor in the context of GR does not imply a space-time with intrinsic curvature.
    I didn't say that, Markus. Please check what I said.

    Apologies, the wife is calling, I have to go. I'll get back to you properly later or tomorrow.
    Reply With Quote  
     

  65. #64  
    Forum Freshman
    Join Date
    Oct 2012
    Posts
    76
    To me space curvature makes perfect since and have yet to see evidence that will not work with it. Yet, still most are ignoring this when they start making claims about conservation of momentum in astronomy while ignoring the effect of curvature of space. For example, the confusion of the words orbit and a rotation. They are not the same thing.
    Reply With Quote  
     

  66. #65  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Quote Originally Posted by Farsight View Post
    I didn't say that, Markus. Please check what I said.
    Apologies, the wife is calling, I have to go. I'll get back to you properly later or tomorrow.
    Well, it is quite possible that I am misunderstanding your stance on this point.
    It would be very helpful if you could clarify for us exactly what your understanding of the metric tensor in the context of GR is, and how it relates to the universe; you surely do know that this tensor is the fundamental object which GR concerns itself with.
    Reply With Quote  
     

  67. #66  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    It's essentially a "measurement matrix". In our Shapiro-like example we saw the beginnings of one, wherein we used parallel-mirror light clocks strung out like beads on a string. This gave us a line of measurements like so:

    .........................................O

    100-----99-----97-----94-----90-----94-----97-----99-----100

    .........................................
    O


    This is an array of scalar measurements, and is therefore a metric tensor of sorts. Whilst this is a "mathematical object", it isn't a fundamental physical object. We cannot see a thing called a metric tensor between those stars, we merely used the motion of light between mirrors to provide readings of "the time", and a plot of these readings exhibits a form of curvature.

    Of course we also use the motion of light to measure distance, and we can derive a more complex metric tensor: Yes, "in general relativity, the metric tensor (or simply, the metric) is the fundamental object of study". But in similar vein it's isnt a fundamental physical object. Instead it's a "measurement matrix" derived from the motion of light through space. I have hinted at the fundamentals of this. Remember that repeated Compton scatter that dissipated the photon into the motion of electrons? Remember that pair production, where we converted what could have become the motion of electrons into the electron itself? When it comes to fundamentals, I would venture to suggest that motion in space is more fundamental than many people appreciate, and that this is why the equations of motion are so very applicable.
    Reply With Quote  
     

  68. #67  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Quote Originally Posted by Farsight View Post
    We cannot see a thing called a metric tensor between those stars, we merely used the motion of light between mirrors to provide readings of "the time", and a plot of these readings exhibits a form of curvature.
    So it really all comes down to this - you acknowledge that a metric tensor introduces curvature into geometry, but you deny that it has physical reality because (quote) "we cannot see a thing called a metric tensor between those stars".
    Well, if you follow that reasoning then your "inhomogeneous space" is out the windows as well, because we do not see any inhomogeneities in the space between the stars either. To be honest, I find it hard to follow your line of reasoning on this - on what grounds exactly can you reject space-time curvature, but at the same time embrace a concept such as "inhomogeneous space", and then on top of it all claim that the latter is what Einstein really meant when he formulated his theory ?

    I would venture to suggest that motion in space is more fundamental than many people appreciate, and that this is why the equations of motion are so very applicable.
    Yes, and those equations of motion depend explicitely on the metric tensor, i.e. the geometry of space-time.
    Reply With Quote  
     

  69. #68  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Just a side note : the metric tensor is a function which maps two tangent vectors at a given point on a ( smooth, differentiable ) manifold into a real number. As such it is conceptually related to the usual dot product, but of course not exactly the same thing. The point here is that the metric tensor it is not defined via measurements, it is rather the other way around - one needs to "equip" a manifold with a metric first before it is even possible to define physically meaningful measurements. Given a metric tensor one defines a generalized volume element of the form



    Given this one can then define measurements such length, angles, area, volume etc etc. Written as above this is entirely independent of the choice of coordinates, and the number of dimensions.
    Reply With Quote  
     

  70. #69  
    Suspended
    Join Date
    Oct 2012
    Posts
    40
    [QUOTE=Joe L. Ogan;204569]Does Gravity do any work? I think there is a school of thought that believes that Gravity does no work. I think that the concept may be in error. Thanks for comments. Joe L. Ogan[/QUOTE

    In Physics, if you move against a field, you do work. If you move along the field , there is no work.

    But that gets complicated for Electromagnetics. I'm still trying to figure it out.
    Reply With Quote  
     

  71. #70  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Quote Originally Posted by Markus Hanke View Post
    So it really all comes down to this - you acknowledge that a metric tensor introduces curvature into geometry, but you deny that it has physical reality because (quote) "we cannot see a thing called a metric tensor between those stars".
    Not quite. The curvature is in the geometry of a static mathematical space called spacetime. There's no curvature of the physical space where a gravitational field is located. If we were able to watch a light beam moving along our straight line between the two stars, we'd see it start quickly, then slow down, then go faster again. This is merely an extension of the light moving like this |--| between the various parallel-mirrors in our light-clocks.

    Quote Originally Posted by Markus Hanke View Post
    Well, if you follow that reasoning then your "inhomogeneous space" is out the windows as well, because we do not see any inhomogeneities in the space between the stars either.
    But we do see a Shapiro delay, and it is called a delay. When we shine a light beam past a single star we see it bend a little, but we also know that most of the Shapiro delay is the result of what you'd call "curvature of time". It's only a short step from there to my two-star gedankenexperiment, and we know that .

    Quote Originally Posted by Markus Hanke View Post
    To be honest, I find it hard to follow your line of reasoning on this - on what grounds exactly can you reject space-time curvature, "...
    I don't reject spacetime curvature per se. I reject it as the cause of the curvature of light in a gravitational field, because it's nothing more than a curvature of an mathematical space in which no motion occurs. It's merely the an abstract curvature in a "matrix of measurements" such as those on our line between our stars. The light moving along that line does not suffer a Shapiro delay because our plot derived from light moving back and forth along that line is curved. It suffers a delay for the same underlying reason that caused our plot to be curved.

    Quote Originally Posted by Markus Hanke View Post
    but at the same time embrace a concept such as "inhomogeneous space"
    There's certainly something that causes the light-clocks in the middle of our line to show a lower reading, and the selfsame thing resulted in our Shapiro delay. It certainly isn't a "curvature of space", our line is straight. And what we measured in our light clocks wasn't some abstract thing called the flow of proper time. All we did was count the number of times light moved back and forth like this |--| between parallel mirrors. That light moved through space. So when a clock at one location shows a lower reading than one at another location, we can assert that it's because the space there is different. Hence inhomogeneous space.

    [QUOTE=Markus Hanke;357731]and then on top of it all claim that the latter is what Einstein really meant when he formulated his theory[/url]I consider myself to be in line with Einstein because he referred to inhomogeneous space, and because he gave the equations of motion. And because said a curvature of light can only occur when the speed of light varies with position. What's not to like? Light "veers" like a car veers when it encounters mud at the side of the road. The road is not curved, the path of the car is curved.

    Quote Originally Posted by Markus Hanke View Post
    Yes, and those equations of motion depend explicitely on the metric tensor, i.e. the geometry of space-time.
    Which is a mathematical space, a static plot derived from the motion of light through space, which we use define the things we call the second and the metre. Yes, the world is painted in light, Markus. But if you look closely, you can see the canvas.
    Reply With Quote  
     

  72. #71  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    There's no curvature of the physical space where a gravitational field is located.
    The gravitational field is not located in space-time, it is space-time. The gravitational field and space-time and one and the same thing.
    And please do not forget that it is space-time, and not just space.

    I reject it as the cause of the curvature of light in a gravitational field,
    Again, see above - the gravitational field and space-time are one and the same thing, hence the world-line of light rays will trace out geodesics.

    It certainly isn't a "curvature of space", our line is straight.
    Again you are forgetting that we are dealing with space-time. The line is straight in space, but curved in space-time.

    and because he gave the equations of motion.
    ...which are geodesic equations in a 4-dimensional space-time.
    Reply With Quote  
     

  73. #72  
    Administrator KALSTER's Avatar
    Join Date
    Sep 2007
    Location
    South Africa
    Posts
    8,232
    Are we still busy with Farsight's idea that gravity is not curved space-time? This has been going on for ages and Farsight seems to have the non-standard stance. Might I suggest you start a thread in New Hypothesis and flesh out your idea of what is going on once and for all, Farsight? Because discussion of the same thing in multiple threads is not going to fly for much longer. You guys can have at it all you want in there.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
    Reply With Quote  
     

  74. #73  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Quote Originally Posted by Markus Hanke View Post
    The gravitational field is not located in space-time, it is space-time. The gravitational field and space-time and one and the same thing.
    I will assume you omitted the word curved. I said space, Markus. The gravitational field is located in space. We say there is a gravitational field in the space around the Sun. If light moves through that space, it curves. Well away from the sun, its curvature is undetectable.

    Quote Originally Posted by Markus Hanke View Post
    And please do not forget that it is space-time, and not just space.
    The Sun is surrounded by space through which light moves. It is not surrounded by the static mathematical space called spacetime in which light does not.

    Quote Originally Posted by Markus Hanke View Post
    Again, see above - the gravitational field and space-time are one and the same thing, hence the world-line of light rays will trace out geodesics.
    They aren't the same thing. Look at my post #34. Remember what we used to make our plot - the motion of light through space, within parallel-mirror light clocks. It's a relief model, Markus, that's all.

    Quote Originally Posted by Markus Hanke View Post
    Again you are forgetting that we are dealing with space-time. The line is straight in space, but curved in space-time.
    It isn't curved in the real world, it's only curved in the model.

    Quote Originally Posted by Markus Hanke View Post
    ...which are geodesic equations in a 4-dimensional space-time
    which is a static mathematical model. There are curved worldlines in this model, but light does not move along them. We use those equations to work out how light moves through space. We use the model to make predictions about the real world, then we make observations to confirm that we have correctly modelled the motion of light through space. I will reiterate my two-stars example: our observations do not confirm that our Shapiro delay occurs because we have an abstract curvature in our plot. That curvature is there because our measurements of light moving back and forth through space like this |--| are not uniform. The curvature is there because the light moved slower in the middle, and it is for this selfsame reason that our Shapiro delay occurs. Re your side note, step outside and turn your face to the clear night sky. Gaze up at space. Look at the light of the stars. See the motion of that meteorite. You cannot see two vectors and their dot product, you cannot see a metric tensor. The map is not the territory Markus, the map is not the territory.
    Reply With Quote  
     

  75. #74  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Quote Originally Posted by KALSTER View Post
    Are we still busy with Farsight's idea that gravity is not curved space-time? This has been going on for ages and Farsight seems to have the non-standard stance. Might I suggest you start a thread in New Hypothesis and flesh out your idea of what is going on once and for all, Farsight? Because discussion of the same thing in multiple threads is not going to fly for much longer. You guys can have at it all you want in there.
    Check out the thread from post #34 Kalster. That's where I tried to clear up the confusion concerning curved space and curved spacetime. In the very next post Markus said "curvature in space always goes hand-in-hand with curvature in time", and I gave the two-stars Shapiro example that showed him to be wrong. Later on ttown exhibited the confusion again, and we got on to the cause of gravity. If gravity is curved spacetime, gravity can't be caused by curved spacetime now can it?. I'm battling misunderstanding and telling it like it is, I'm supporting my case, and it stands up to scrutiny. This is a five-star thread, chuck it in some cesspit if you like, but it will come across as censorship because you guys just can't take it when somebody comes along who knows more than you.
    Reply With Quote  
     

  76. #75  
    Administrator KALSTER's Avatar
    Join Date
    Sep 2007
    Location
    South Africa
    Posts
    8,232
    Quote Originally Posted by Farsight View Post
    Quote Originally Posted by KALSTER View Post
    Are we still busy with Farsight's idea that gravity is not curved space-time? This has been going on for ages and Farsight seems to have the non-standard stance. Might I suggest you start a thread in New Hypothesis and flesh out your idea of what is going on once and for all, Farsight? Because discussion of the same thing in multiple threads is not going to fly for much longer. You guys can have at it all you want in there.
    Check out the thread from post #34 Kalster. That's where I tried to clear up the confusion concerning curved space and curved spacetime. In the very next post Markus said "curvature in space always goes hand-in-hand with curvature in time", and I gave the two-stars Shapiro example that showed him to be wrong. Later on ttown exhibited the confusion again, and we got on to the cause of gravity. If gravity is curved spacetime, gravity can't be caused by curved spacetime now can it?. I'm battling misunderstanding and telling it like it is, I'm supporting my case, and it stands up to scrutiny. This is a five-star thread, chuck it in some cesspit if you like, but it will come across as censorship because you guys just can't take it when somebody comes along who knows more than you.
    Where did I say I want to chuck into some cesspit? I said I have seen this exact topic being rehashed in multiple threads and think it would be better if it was done in it's own thread. New Hypothesis is not a cesspit. I then politely asked your opinion about it. If I was censoring you, you would have been banned a long time ago.

    And by the way, I'll be the first to agree that you guys know more than me. From what I see though, your position is somewhat removed from the mainstream, which is why it will be your thread and why I asked you. Again, the thread will be open and just as active as before, it will just be where it belongs, instead of all over the place in other people's threads. Am I being clear?
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
    Reply With Quote  
     

  77. #76  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    I take a dim view of Hypotheses or ideas that lack both solid disproof and substantial empirical support, Kalster. It's hard up against pseudoscience, a den of crackpots, it tarnishes by association, I will not be part of it. Read the thread, I'm the one offering the substantial empirical support here. I'm the one offering the old idea, Einstein's idea, and the careful explanation that makes it clear. What's Markus offering? Conviction? What's guitarist offering? Threats? Or will it be reams of mathematics that you don't understand but which impress you like Emperor's New Clothes?

    Do what you will Kalster, it's up to you. But if I were you, I'd ask Markus.
    Reply With Quote  
     

  78. #77  
    Administrator KALSTER's Avatar
    Join Date
    Sep 2007
    Location
    South Africa
    Posts
    8,232
    Fine, I'll make it thread in physics then for the title only where you guys can hash it out.

    Or will it be reams of mathematics that you don't understand but which impress you like Emperor's New Clothes?
    Will you calm down for a second? I am not trying to suppress you. I have told you what my reasons are, that I know I am not qualified to judge properly, but what my impression is. You have no reason to behave this way. I will make the move from around your post #34 tomorrow.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
    Reply With Quote  
     

  79. #78  
    ***** Participant Write4U's Avatar
    Join Date
    Apr 2012
    Posts
    1,242
    IMHO, gravity is a result. A massive body distorts spacetime by stretching it. This is why time moves slower in the vicinity of a massive body, the distances in the structure of spacetime have increased. In addition, a smaller massive body with a smaller spacetime distortion will "fall" towards the more massive body. This is how our solar system is able to function with the sun as the greatest mass "holding" the earth and other planets, each which may "hold" its own moons. What keeps it all from falling towards the sun is the speed of the orbiting bodies trying to 'escape" their orbits.
    Out of the box likes this.
    Reply With Quote  
     

  80. #79  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    I will assume you omitted the word curved. I said space, Markus. The gravitational field is located in space. We say there is a gravitational field in the space around the Sun. If light moves through that space, it curves. Well away from the sun, its curvature is undetectable.
    Separating space-time and the gravitational field is a misconception; they are one and the same thing. Trying to separate them is the underlying reason why it isn't possible to write down a consistent QFT of massless spin-2 particles; naively trying to do this leads to unphysical models, as we all know. The solution is to not assume a field in space-time and then trying to quantize it, but to quantize space-time itself. Why ? Because space-time is the gravitational field. This is precisely what LQG is trying to do.

    The Sun is surrounded by space through which light moves. It is not surrounded by the static mathematical space called spacetime in which light does not
    What do you mean by "surrounded" ? The universe and everything in it is a 4-dimensional space-time manifold. Light rays trace out geodesics within that space-time, as already explained to you several times.

    We use those equations to work out how light moves through space.
    You are still not getting it. There is not space, there is only space-time. The geodesic equations give us exactly what it says on the tin - geodesics within that space-time. Mathematically these are just parametrized curves, and you can parametrize them whichever way you like. Why is this so difficult to understand ?

    You cannot see two vectors and their dot product, you cannot see a metric tensor.
    And neither do I see inhomogeneous space. I see a meteorite which traces a trajectory in curved space-time.

    and I gave the two-stars Shapiro example that showed him to be wrong.
    If this is what you really believe then I see little point to continue on with this.
    I have to be honest with you anyway - I think you are pretty much stuck in your way of thinking, and aren't prepared to listen to any other point of view anyway. We could keep going around in circles for another 500 posts without moving an inch - I see little value in this. I have nothing against you personally Farsight, but I would rather use my time in furthering my own understanding about physics, and that's certainly not happening in this discussion.
    If you wish to believe you have proven me wrong - please, be my guest.
    Reply With Quote  
     

  81. #80  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Quote Originally Posted by Farsight View Post
    Read the thread, I'm the one offering the substantial empirical support here.
    Farsight, you have not provided anything apart from your own interpretation about what GR should be, in your opinion. You have not provided anything new, or anything which would in any way contradict the physical reality of curved space-time. All you have done is re-interpret already known facts.

    You know, when it comes to physics we all have a choice :
    1. We can think ourselves superior to everyone else, reject centuries worth of accumulated knowledge, and get stuck in our own versions of reality; as Adam Savage once said: "I reject your reality and substitute my own !"
    2. We can accept the fact that great men and women have come before us, and have left us a wealth of knowledge; and then get down and dirty and try to really understand that knowledge that has been left for us.

    The latter is the hard way, because it involves getting down on our knees and acknowledge to ourselves that we are imperfect, that our knowledge and understanding is imperfect, and that we have a long, lonely road to travel. It involves late nights over textbooks under lamplight; it involves incredible frustrations over little things we just don't get; it involves uplifting epiphanies when the breakthrough finally comes and we understand; but above all it involves realizing that we might be wrong, and that there is always someone else out there who understands things better then we do. It's a difficult journey, but with incredible rewards.

    My personal opinion about your claims Farsight ( I think you are mature enough to realize that this is not a personal attack ) ? I think it is ridiculous that you appear to think yourself superior to everyone else's understanding of GR. Einstein did not describe "inhomogeneous space", he described curved space-time. Period. You don't believe me, and don't understand the maths behind it ? Then I suggest you get down and dirty and learn it. I had to do the same, and it was not easy, and I am still in the middle of that journey. But I am progressing, thanks to people like Guitarist who take a substantial amount of their own time to respond to my questions and explain stuff on this forum until I and people like me get it. You would be well advised to take advantage of his incredible knowledge, instead of just dismissing him as "threatening". His frustration is purely because of your refusal to acknowledge existing mathematical and physical knowledge - that's not his fault, it's yours.

    And as an additional note - if you think you can really grasp GR without having mastered the maths behind it you are wrong. Talk about pop-sci !

    What's Markus offering?
    I offer you a chance to understand. But it seems you aren't interested.
    It's your choice, just know that I will not repeat myself over and over again forever.
    KALSTER, John Galt and tk421 like this.
    Reply With Quote  
     

  82. #81  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Let's leave it at that then Markus. It's been a pleasure talking to you.
    Reply With Quote  
     

  83. #82  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Quote Originally Posted by Farsight View Post
    Let's leave it at that then Markus. It's been a pleasure talking to you.
    Likewise.
    And once again - please understand that I have nothing against you personally, nothing I have said was intended as any sort of personal attack. It is merely a difference in opinion.
    Reply With Quote  
     

  84. #83  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Not an issue Markus, I don't feel that you've made any personal attacks. And no problem re differences of opinion either. If we all agreed on everything, it would be a dull old world. There wouldn't be any point in discussing anything.
    Markus Hanke likes this.
    Reply With Quote  
     

  85. #84  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Quote Originally Posted by Farsight View Post
    Not an issue Markus, I don't feel that you've made any personal attacks. And no problem re differences of opinion either. If we all agreed on everything, it would be a dull old world. There wouldn't be any point in discussing anything.
    Very true !
    Reply With Quote  
     

  86. #85  
    Suspended
    Join Date
    Oct 2012
    Posts
    10
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Out of the box View Post
    So, a gravitational field has mass, right?
    Mass and all forms of energy (and a few other things) cause a gravitational effect. A gravitational field has energy and so causes gravitation.
    What is this? Is this a joke. A gravitational field causes gravitation. Where are you from?

    A field defines the motion a body will undergo under the effect of a larger mass.

    Who are these people. Kafirs. Non-believers.
    Reply With Quote  
     

  87. #86  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,601
    Quote Originally Posted by Edmond View Post
    What is this? Is this a joke. A gravitational field causes gravitation.
    No joke.

    Where are you from?
    How is that relevant?

    A field defines the motion a body will undergo under the effect of a larger mass.
    And, of course, the larger mass will be affected by the smaller as well.

    Kafirs. Non-believers.
    How is that relevant?
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
    Reply With Quote  
     

  88. #87  
    Suspended
    Join Date
    Oct 2012
    Posts
    10
    Shut up you monkey. Just Shut Up. I've had enough.

    Larger mass..Oh ya..Larger mass will move...How Son..How will it move.

    Tell me how a smaller mass will move the larger one.

    Where do you see it.

    Does the moon move the earth?
    Does the earth move the sun?

    Bloody Chimps running this place.
    Reply With Quote  
     

  89. #88  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,601
    Quote Originally Posted by Edmond View Post
    Does the moon move the earth?
    Does the earth move the sun?
    Yes. In both cases they orbit around their common barycenter, not around the center of the larger body.
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
    Reply With Quote  
     

  90. #89  
    Suspended
    Join Date
    Oct 2012
    Posts
    10
    Yes, you are right, mathematically, but it not the usual way of studying physics.

    We usually study one body at a time, with a free body diagram and all the forces acting on the body.

    I am not confident about the Inverse Square Law, I have said this time and again, but I'll accept your BARYCENTER for now.

    Yours Faithfully
    The Crank
    Reply With Quote  
     

  91. #90  
    Suspended
    Join Date
    Oct 2012
    Posts
    10
    And please read the post on Einstein, and do not trouble me again.
    Reply With Quote  
     

  92. #91  
    Moderator Moderator
    Join Date
    Jun 2005
    Posts
    1,612
    Quote Originally Posted by Farsight View Post
    If gravity is curved spacetime, gravity can't be caused by curved spacetime now can it?.
    Welcome to the non-linear world Farsight. If your mathematical skills were a little more advanced than they apparently are, you could see by inspection the Einstein field equations are non-linear,.

    And there are 10 of them ( as far as is known), And solving multiple non-linear simultaneous equations is something of a nightmare.

    Some FACTS you ought to know, given that your knowledge of gravitation is seems to be gleaned from Einstein's pop-sci writings.

    1. Einstein acknowledged his debt to Minkowski's "fusion" of space and time to create spacetime - he said that without this insight, the General Theory would have died in its infancy.

    2. Einstein PROVED that the gravitational field was itself a gravitational source - that's what non-linearity means!

    3. If one accepts that the Special theory, and indeed the Newton-Poisson equation for gravitation is to be regarded as limiting cases of the General Theory. then (a) this is impossible to prove without a metric (i.e. "something" evaluated by the metric tensor) and (b) in your claimed curved space and presumably "flat" time, whatever that means. Again, if your mathematical skills were a little more advanced, you would spot this instantly.

    4. You may regard the metric tensor as a figment of mathematicians' imagination, but how is it less "real" than, say, the light year?

    you guys just can't take it when somebody comes along who knows more than you.
    You have shown absolutely no evidence that you know any more than most people here
    Reply With Quote  
     

  93. #92  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Quote Originally Posted by Edmond View Post
    What is this? Is this a joke. A gravitational field causes gravitation. Where are you from?
    Strange is right - all forms of energy are a source of the gravitational field, including the field itself. That is why the field equations are non-linear :

    Reply With Quote  
     

  94. #93  
    New Member
    Join Date
    Oct 2012
    Posts
    1
    Does Gravity do any work? This really is an interesting question - primarily because it appears to be well formulated but in fact it is not. I think there are two cases to consider and in both cases the answer is no.

    1. For an object tin free fall the answer is no. Recall that gravitational acceleration is not caused by a force. Yes, Newtonian mechanics treats gravitation motion as an acceleration caused by a force, but there is no force present. That is, there is absolutely no way possible to measure any force causing gravitational acceleration. So using the definition of Work = F dot D, since F is zero for free falling object, gravity does not do any work on free falling objects.

    2. Lifting an object in a gravitation field does require work. If I’m holding a mass steady at a certain height here on Earth, I can measure a force acting on the object. That is, there is an actual measurable force present. If I lift the object higher I am plying force over a distance resulting in Work = F dot D that does not equal zero. So in this example of lifting an object from one height to another height work is being done, but it is done by the person lifting the object, not by gravity.
    Last edited by KALSTER; October 11th, 2012 at 01:24 PM. Reason: Fixed your formatting. PLease use standard formatting
    Reply With Quote  
     

  95. #94  
    Suspended
    Join Date
    Oct 2006
    Posts
    769
    Quote Originally Posted by Guitarist View Post
    Welcome to the non-linear world Farsight. If your mathematical skills were a little more advanced than they apparently are, you could see by inspection the Einstein field equations are non-linear
    The non-linearity is a non-issue, Guitarist. It's an energy thing. And please resist the urge to snipe, I'm not a total mathematical dummy, and I might start sniping at your physics skills.

    Quote Originally Posted by Guitarist View Post
    And there are 10 of them ( as far as is known), And solving multiple non-linear simultaneous equations is something of a nightmare.
    Sure thing. Here's a little reference to them from a maths course kindly put online by A G Polnarev: http://www.maths.qmul.ac.uk/~agp/MTH...notes20_10.pdf. They concern energy density, flux, pressure, and shear stress.

    Quote Originally Posted by Guitarist View Post
    Some FACTS you ought to know, given that your knowledge of gravitation is seems to be gleaned from Einstein's pop-sci writings.

    1. Einstein acknowledged his debt to Minkowski's "fusion" of space and time to create spacetime - he said that without this insight, the General Theory would have died in its infancy.
    Yep, he acknowledges Minkowski and other mathematicians in the prelude to The Foundation of the Generalised Theory of Relativity.

    Quote Originally Posted by Guitarist View Post
    2. Einstein PROVED that the gravitational field was itself a gravitational source - that's what non-linearity means!
    Tell me something I don't know. I'm forever quoting "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy".

    Quote Originally Posted by Guitarist View Post
    3. If one accepts that the Special theory, and indeed the Newton-Poisson equation for gravitation is to be regarded as limiting cases of the General Theory. then (a) this is impossible to prove without a metric (i.e. "something" evaluated by the metric tensor) and (b) in your claimed curved space and presumably "flat" time, whatever that means. Again, if your mathematical skills were a little more advanced, you would spot this instantly.
    Straw man. I've never referred to gravity as curved space, or talked about "flat" time. The point I've been making isn't some "my theory" I've dreamt up, it goes back to Einstein talking about inhomogeneous space:

    "According to this theory the metrical qualities of the continuum of space-time differ in the environment of different points of space-time, and are partly conditioned by the matter existing outside of the territory under consideration. This space-time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that “empty space” in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gμν), has, I think, finally disposed of the view that space is physically empty".

    We describe the state of inhomogeneous space via ten functions on energy density, flux, pressure, and shear stress to develop a model generally known as curved spacetime. Note that flux is a "flow", and that spacetime features no motion because it's a "static all times model". This is a very important point IMHO, which is oft overlooked.

    Quote Originally Posted by Guitarist View Post
    4. You may regard the metric tensor as a figment of mathematicians' imagination, but how is it less "real" than, say, the light year?
    I didn't say it was. Note though that to define a light year, you shine a beam of light, wait for a while and then you say that's a light year. Then you can use this to define both distance and time. You did this using the motion of light through space. What I was saying above with the two-stars gedankenexperiment is that you can similarly use the motion of light through space to define a simple array of light-clock scalar measurements, which is very primitive "metric tensor". I'm also asserting that the GR metric tensor is the same kind of thing rather than something that has a fundamental physical existence in its own right.

    Quote Originally Posted by Guitarist View Post
    You have shown absolutely no evidence that you know any more than most people here
    If you were to read my posts and attempt to respond to the argument backed by patent scientific evidence, then when you found yourself unable to counter that argument, you will perforce have to change your view. It might not come easy, because mathematicians sometimes undervalue scientific evidence, and you'll be working against conviction. But if you're as smart as you think, you'll come to appreciate the point I'm making. By the way, do resist the urge to go into a defensive mode wherein you throw out reams of expressions without defining your terms. It may come over as something like a witchdoctor throwing out incantations because a pharmacologist has turned up.
    Out of the box likes this.
    Reply With Quote  
     

  96. #95  
    Forum Masters Degree
    Join Date
    Jan 2011
    Posts
    500
    Quote Originally Posted by Farsight View Post
    Quote Originally Posted by Guitarist View Post
    2. Einstein PROVED that the gravitational field was itself a gravitational source - that's what non-linearity means!
    Tell me something I don't know. I'm forever quoting "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy".
    Indeed, you are quite good at cherry-picking things that superficially look like they will support your point. However, you demonstrably do not understand the science as you have admitted to not being able to do the relevant mathematics (despite commenting on it for about a decade).
    "According to this theory the metrical qualities of the continuum of space-time differ in the environment of different points of space-time, and are partly conditioned by the matter existing outside of the territory under consideration. This space-time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that “empty space” in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gμν), has, I think, finally disposed of the view that space is physically empty".

    We describe the state of inhomogeneous space via ten functions on energy density, flux, pressure, and shear stress to develop a model generally known as curved spacetime. Note that flux is a "flow", and that spacetime features no motion because it's a "static all times model". This is a very important point IMHO, which is oft overlooked.
    Case in point. If you have repeatedly been asked to provide evidence of your claim that this supposed "inhomogeneity" has been overlooked. You have failed to ever provide such evidence. You blather on about dark matter and galaxy rotation curves, but you have never provided specifics and you constantly confuse galaxy dynamics and cosmological dynamics. Given that you have done this for about a decade, how can you be doing anything other than deceit?
    Reply With Quote  
     

  97. #96  
    ***** Participant Write4U's Avatar
    Join Date
    Apr 2012
    Posts
    1,242
    Question,

    We speak of gravity as if it were a universal constant because of spacetime curvature. But is it possible the distribution of masses within the universe causes this curvature? It seems very strange that gravity could exist without the influence of mass.
    Reply With Quote  
     

  98. #97  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Quote Originally Posted by Write4U View Post
    It seems very strange that gravity could exist without the influence of mass.
    How do you mean that exactly ?
    Reply With Quote  
     

  99. #98  
    ***** Participant Write4U's Avatar
    Join Date
    Apr 2012
    Posts
    1,242
    Perhaps I misunderstood, Markus,
    Separating space-time and the gravitational field is a misconception; they are one and the same thing. Trying to separate them is the underlying reason why it isn't possible to write down a consistent QFT of massless spin-2 particles; naively trying to do this leads to unphysical models, as we all know. The solution is to not assume a field in space-time and then trying to quantize it, but to quantize space-time itself. Why ? Because space-time is the gravitational field. This is precisely what LQG is trying to do.
    I cannot see how a geometric spatial structure can be gravitational without some sort of mass being associated with this structure. This would imply that spacetime itself has mass causing gravity. Moreover as spacetime is curved due to gravity, where does the uneven distribution of mass and it's resulting curving of gravitational fields come from in the first place? Does spacetime itself have massive lumps? And if that is the case, how can we speak of universal constants which imply a uniformity of spacetime properties and behavior?

    Would it not be more logical to assume that clumps of matter (galaxies, galactic clusters, etc) within spacetime are causing the curvatures in spacetime on a grand scale, as individual massive objects do locally?

    It also raises the question how spacetime gravity is directed inwards into itself. Obviously it would be unable to exhibit a gravitational field outward. Gravity can only exist and act inside spacetime.

    The notion that a gravitational field itself is causal to an even larger gravitational field, and in turn is causal to yet another greater gravitational field, ad infinitum, seems counter intuitive to me. This reminds me of a Russian nesting egg, but I have never heard of such a proposal.

    Spacetime and gravity were created during the BB which introduced mass and its accompanying gravitational fields into the equation. But what happens when entropy and the expansion of spacetime creates such vast distances that the spacetime gravitational influences become so weak (localized) that spacetime flattens out except for isolated "gravitational dimples" (caused by isolated massive objects) on the geometric structure of spacetime?

    Perhaps I am missing an important detail......
    Last edited by Write4U; October 14th, 2012 at 03:13 AM.
    Reply With Quote  
     

  100. #99  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,180
    Quote Originally Posted by Write4U View Post
    I cannot see how a geometric spatial structure can be gravitational without some sort of mass being associated with this structure. This would imply that spacetime itself has mass causing gravity.
    All forms of energy are a source of the gravitational field, and since the field itself contains energy, it is thus self-coupling, i.e. the field itself affects its own geometry. Mathematically this is evident in the fact that the field equations are non-linear.

    Moreover as spacetime is curved due to gravity, where does the uneven distribution of mass and it's resulting curving of gravitational fields come from in the first place?
    Mass has been present since shortly after the BB, when it "froze" out of the underlying fields through the Higgs mechanism.

    And if that is the case, how can we speak of universal constants which imply a uniformity of spacetime properties and behavior?
    It doesn't have lumps, and it appears uniform ( apart from curvature ) at least on a macroscopic scale. That description will likely break down at very high energies though.

    Would it not be more logical to assume that clumps of matter (galaxies, galactic clusters, etc) within spacetime are causing the curvatures in spacetime on a grand scale, as individual massive objects do locally?
    They do. As I stated earlier, all forms of energy are a source of the gravitational field.

    The notion that a gravitational field itself is causal to an even larger gravitational field, and in turn is causal to yet another greater gravitational field, ad infinitum, seems counter intuitive to me.
    That's not how it is - the field is self-coupling, but that does not mean that there are any infinities involved. It only means that the elements of the field potential ( the "metric tensor" ) depend on each other in a non-linear manner, so the result is not a neat inverse-square law. This is the reason why Newton's gravity gives wrong results in some cases ( Mercury orbit, light deflection etc etc ), whereas GR gets it correct.

    Spacetime and gravity were created during the BB
    I don't think they were "created" during the BB - but nonetheless, I know what you mean to say

    But what happens when entropy and the expansion of spacetime creates such vast distances that the spacetime gravitational influences become so weak (localized) that spacetime flattens out except for isolated "gravitational dimples" (caused by isolated massive objects) on the geometric structure of spacetime?
    Then you have a flat universe. It needs to be noted though that each of these "dimples" have gravitational fields which extend asymptotically into infinity; these fields don't just "disappear" after some distance, even if their effects are no longer measurable. Also bear in mind that the vacuum itself has a non-zero energy density to the best of our knowledge, so that alone would also constitute a source of a global field.
    I suppose what I am trying to point out is a very simple fact - there is no point anywhere in the universe from which gravitational fields are absent, simply because all gravitational fields extend into infinity. Bearing that in mind it is not such a stretch of the imagination to come to the realization that the gravitational field and space-time are really one and the same thing. The very existence of space and time is defined by gravity.

    Without wanting to open a can of worms now, this has of course far-reaching consequences, because space-time could not exist without energy, and vice versa. There ought to be a very fundamental connection between energy and space-time, apart from curvature; perhaps it is even wrong to think about them as separate entities, perhaps energy and space-time themselves are only two aspects of the same underlying "thing", and they can be unified in some way at very high energies. Since the quantum fields describing all other forces and interactions are quantized, then so would space-time itself need to be.
    I will stop here, but if you follow that direction of thought you get some really interesting ideas which challenge some of our most basic assumptions about what our universe is like.
    Reply With Quote  
     

  101. #100  
    ***** Participant Write4U's Avatar
    Join Date
    Apr 2012
    Posts
    1,242
    Thanks Markus,
    You have presented several aspects which I had not considered.

    Intuitively (obviously I'm not a scientist), I did feel that a geometric structure has an inherent tension, but I always interpreted that as a form of potential rather than gravity. This is a new perspective and gives me much grist for thought....
    Reply With Quote  
     

Page 1 of 2 12 LastLast
Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •