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Speed of Gravity? Questions about the Kopeikin Experiment of Sept 8 2002

  1. Aug 24, 2009 #1
    During my contemplation of gravity, I recently came across the Experiment performed on Sept 8 2002 using the VLBA to measure the speed of the lensing effect caused by Jupiters gravitational influence upon a quasar stream.
    The accuracy achieved by using the VLBA was immensly impressive, and they came to the result that gravity has a speed of 1.06 the speed of c, with a + or - of 0.21.
    Now this result is exactly what i would have expected them to discover, i have no qualms about their results.
    Where I'm confused, is as to why the experiment was performed in the first place. For surely, if an object of mass is limited to the speed of light as its maximum velocity, Then it would not matter how fast gravity/gravaton moves. Even if the gravity could cause its influence at at infinate velocity, it still could never move the quasars faster than the speed quasars themselves are limited to : i.e. the speed of c.
    Is there some obsqure piece of logic or information that I've missed, regarding this experiment? If anyone could shed some light on this matter for me, it would be much apreciated, because at the moment it appears to me to prove nothing except that we can measure things very accurately.
  2. jcsd
  3. Aug 25, 2009 #2
    Waou!!! You are mixing together gravity (acceleration) and speed, mass and speed of light. No wonder you're confused. You're also confusing me.

  4. Aug 25, 2009 #3
    I am not very familiar with the experiment, but here's my understanding/interpretation. Normally light waves travel in a straight line. When light waves from the quasar pass through Jupiter's gravitational field, they are bent, and it takes longer to travel along the curved trajectory, ergo there is a measurable time delay. Since Jupiter is non-stationary, speed of gravity affects the exact shape of the trajectory (light "sees" Jupiter where it was when gravitons left it, rather than where it really is) and so there's a measurable effect somewhere.
  5. Aug 25, 2009 #4
    Your understanding of the experiment is quite correct. I just had a bit of difficulty getting the sense of it when all the values are mixed together.

    To come back to your initial question. There are many things that seem useless at first. Just bear in mind that it brings scientific knowledge a tad further.

    A small anecdote: A little while ago, Faraday was old a conference on electricity. The theory of electricity was at its beginning at the time. At the end of the speech, an old lady stood in the room and ask the imminent scientist what good would this electricity brings to the real people. Just to say that some discoveries have no initial use, but the knowledge is always good to have.

  6. Aug 25, 2009 #5
    Thanks for your replies. But I think prehaps i didn't make the exactities of my question very clear. What I'm trying to get at, is that if you type in the question: what is the speed of gravity, the answer will come back as the speed of c, because this has supposedly been proven by the Kopeikin experiment that I mentioned.The door has been closed upon an important discussion, due to what appears to me as wrong information. This would be a terrible shame for physics, I am hoping someone can correct either an error on my behalf, or to affirm my logical suspisions.
    Now I understand about lensing effect and the method used to measure it using the VLBA.
    What I'm concerned about is that: the lensing effect could never take place, or appear to take place at a speed greater than c, simply because an object of mass, i.e. a photon or quasar cannot move faster than c, and so even if Gravity/gravaton were moving/attracting at an infinate speed, this stiil could not cause lensing effect at a faster rate than the quasars maximum capacity which is equal to c. So to me, this experiment doesn't prove that gravity has a speed equal to c, all it proves is that a quasar can't be moved faster than c, which we already know. I dont want to see gravity classified wrongly because of false information. I hope Ive made the aim of my question a little clearer.
    If you get what I'm on about, or know the real answer please let me know. Thankyou.
  7. Aug 26, 2009 #6
    Hi there,

    I believe you are mixing things up, again!!!

    To make things perfectly clear, gravity is an acceleration, which means a variation in the speed. You cannot talk about the speed of gravity, since they are two different entities.

    Like you said, light travels in straight lines (anyways from the classical point of view). The lensing effect explains why light rays are bend by a very massive object (Jupiter in this case). The only explanation scientist came with is that light, following the duality principle, is affected by gravity (call it gravitons if you like it). Therefore, under an intense gravitational field, light rays seem to be deviated from their original path. But, they keep their speed of c (not a bit more, not a bit less). Gravity only deviates the trajectory of the photons, but does not change their speed.

    Hope this clears it up. Cheers
  8. Aug 26, 2009 #7


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    No, gravity is a force, not an acceleration. And you certainly can talk about the "speed of gravity", referring to the speed of the "gravitons" (if you accept their existance) or the speed with which changes in mass of an object are felt by a distant object.

  9. Aug 26, 2009 #8


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    You seem to be under the impression that they were measuring the motion of the quasars which is certainly not true. They were measuring the effect of Jupiter's mass on light from quasars. And they were not measuring how fast the photons were traveling, they were, of course, traveling at "c". They were measuring how fast changes in the distance from Jupiter to the photons (due to their motion) were "felt" by the photons.
  10. Aug 26, 2009 #9
    A question such as : "if some giant would start to shake the Sun, how would it take the Earth to shake as well ?" is a suitable motivation for the experiment.
  11. Aug 26, 2009 #10

    George Jones

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  12. Aug 26, 2009 #11


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    "if some giant would start to shake the Sun, how long would it take the Earth to shake as well ?"
  13. Aug 26, 2009 #12
    Thank you for the correction.
  14. Aug 27, 2009 #13
    Hey I wish you guys would make up your minds. Just when I was getting comforable without my old Newtonian conceptions of force , thought I got the equivalence principle (g equivalent to accel) and inertial motion in curved spacetime etc etc. you hit me with this.?????? :eek:

    So is there a defintive answer. Is gravity in GR considered a force or not???

  15. Aug 27, 2009 #14
    Are you sure you don't want to rethink your post. Weight is a force that is associated with the gravitational acceleration (gravity). Gravtiy is a variation of speed, which near our planet is roughly equal to 10m/s^2. Our planet pulls on the masses nearby by developing a weight (in Newton).

    To come back to the speed of gravitons. Firstly, the gravitons are still undetected experimentally. Therefore, gravitons is part of a theory that explains the gravitational pull, without any experimental evidence.
  16. Aug 27, 2009 #15
    You didn't understand what he said. 10m/s^2 is gravitational acc, but gravity must be defined as a force.The speed of gravity is the speed at which the effects of changes in mass at a point in space is propogated, that is the rate of travel of inf
    Gravitons are hypothetical particles, true.But there must be a carrier of inf You could similarly say photons are hypothetcal, you have a photoelectric theory to prove that.
    Gravitons also do not exist at rest, so there is no problem with the theory
    Last edited: Aug 27, 2009
  17. Aug 27, 2009 #16
    I stand by what I said, the weight is the force developed by gravitational attraction between bodies. The weight of a person on Earth is measured according to the gravity multiply by the mass of the person. Gravity is not a force but an speed variation, coming from a free fall of an object.

  18. Aug 27, 2009 #17
    The structure of the field is spacetime, so gravity is a force. Gravitons carry the force
    Gravitons are massless because there is no probable upper limit on how stong a field can be, so allows them to carry as much energy as observed
    There is an issue with renormalization though
    Last edited: Aug 27, 2009
  19. Aug 27, 2009 #18
    General relativity predicts that gravitational waves propagates at c. Gravitons are merely the concept of a quantum of gravitational wave. We usually write the same Einstein-Hilbert lagrangian in quantum field theory and we know that at low energy (long distances) the theory we obtain is the same as Einstein's GR. At It is merely another formulation of GR !
  20. Aug 27, 2009 #19
    That is a misuse of the term "gravity"

    And that is an entirely new definition
  21. Aug 27, 2009 #20
    The speed at which the effect of existence of an object reaches you is the speed of gravity
  22. Aug 27, 2009 #21
    Hi guys thanks for some of your posts about the Fomalont, Kopeikin experiment. I looked into the criticisms by Clifford Will, as George Jones suggested. It said that he carried out detailed calculations, in these he put gravity's speed at any value, and yet the results for the delay of light was independent of gravity's speed.That it depended only on the speed of light, so its not possible to determine the speed of gravity from these light delay observations.
    Unfortunately I'm not a mathmatition, and so can't delve into the reasoning for his claimed outcome. Did he come to this conclusion because the experimental results obtained did not possess the neccesary accuracy?
    He also said that the experiment is capable of measuring the speed of gravity.
    Now I'm really hoping that ive not been told about some fundamental part of the physical experiment, because i just can't get past the logic of one small part of it.
    That is: The VLBA was used to measure the distance, that the light from the quasar source, was deviated from its natural course by Jupiters Gravitational field.
    Now that distance that the light's path was altered by, was used to determine the speed of gravity. As the further the distance the light deviated from its natural path, the faster gravity was 'pulling' it.
    Is what I've written as my understanding of the experiment correct?
    Because if it is, then the results would always come back as gravity having a speed of influence equal to c. This is because the distance the light's path deviated by, caused by the 'lensing effect', could never be a greater distance, than the distance light CAN travel, during the time taken to pass through Jupiters gravitational field. The lensing effect could never cause a distance of deviation greater than the speed of c, multiplied by the time taken to pass through jupiters gravitational influence. Because; even if gravity is felt at an infinate speed, it still can't cause an object of mass to move/deviate at a speed greater than c.
    I know Im repeating myself, but I still don't feel that I've come across an adequate explanation for the point that I've stated.
    If its that ive got something wrong in my understanding of the process of the experiment, i would really appreciate the correction. Thank you.
    Last edited: Aug 27, 2009
  23. Aug 27, 2009 #22


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    Well, actually it depends which frame you measure it in!

    Relative to a free-falling frame, there is no force due to gravity, at least not locally. There are tidal forces in a large free-falling "rigid" object, which try to distort the object's shape.

    Relative to any other frame, there is a force of gravity on any massive particle. You could also describe it as an "acceleration due to gravity".

    I'm not sure I would say "gravity is a force", though. In terms of relativity, gravitation is a curvature of spacetime.
  24. Aug 27, 2009 #23
    Would you probe a region of high curvature variations with your body then ? :tongue2:
  25. Aug 27, 2009 #24


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    No, because my body doesn't like being curved too much!

    I did say
    I think it might better to say gravity causes forces in various circumstances, rather than gravity is a force.
  26. Sep 2, 2009 #25
    In all I've read, in GR gravity is not a force, per se. Particles, in the absence of anything except mass, simply move along geodesics which are determined by the curvature of the space. Read Wheeler's famous quote , paraphrased; "The curvature of space tells mass how to move, mass determines how space curves...or something vaguely like that".

    Asking about the speed of the change in curvature a long way away from the object causing the curvature when that object moves is a totally legitimate question, with many papers written on it, ranging from a purely Newtonian point of view to GR. "How fast the Graviton?"
    A good, if somewhat random walk, discussion :smile:
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