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What tests have been done to confirm the speed of gravity?

  1. Jun 24, 2011 #1
    How fast does gravity travel, and how do we know this? I know it can't travel faster than light, but it seems it's only been assumed it travels at the same speed of light?
     
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  3. Jun 24, 2011 #2

    PAllen

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    The majority view is that no significant tests of this have been done. It is very hard to do this either experimentally or via astronomic observations.

    There is one measurement which its authors believe measured this, but the majority of GR experts disagree. Here is a good summary of this debate:

    http://physics.wustl.edu/cmw/SpeedofGravity.html
     
  4. Jun 24, 2011 #3
    See thehttp://en.wikipedia.org/wiki/Speed_of_gravity" [Broken] article.

    Gravity travels at the speed of light. PAllen is correct in that there haven't been any solid, direct tests of this. There are phenomenon which provide strong, indirect evidence however. The biggest example is the http://en.wikipedia.org/wiki/Hulse-Taylor_binary" [Broken]. While none of the measurements directly relate to the 'speed of gravity' per se, the agreement between observations and theory is so incredibly accurate that it strongly suggests the underlying principles behind the theory are correct---including gravity traveling at c.

    There are numerous other similar examples, like the precession of Mercury's orbit, etc.

    Another thing to consider is that many many different aspects of theoretical physics suggest that gravity should travel at the speed of light---not only General relativity, but particle physics, quantum field theory, string theories, etc.
     
    Last edited by a moderator: May 5, 2017
  5. Jun 24, 2011 #4

    bcrowell

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    FAQ: How fast do changes in the gravitational field propagate?

    General relativity predicts that disturbances in the gravitational field propagate as gravitational waves, and that low-amplitude gravitational waves travel at the speed of light. Gravitational waves have never been detected directly, but the loss of energy from the Hulse-Taylor binary pulsar has been checked to high precision against GR's predictions of the power emitted in the form of gravitational waves. Therefore it is extremely unlikely that there is anything seriously wrong with general relativity's description of gravitational waves.

    Why does it make sense that low-amplitude waves propagate at c? In Newtonian gravity, gravitational effects are assumed to propagate at infinite speed, so that for example the lunar tides correspond at any time to the position of the moon at the same instant. This clearly can't be true in relativity, since simultaneity isn't something that different observers even agree on. Not only should the "speed of gravity" be finite, but it seems implausible that that it would be greater than c; based on symmetry properties of spacetime, one can prove that there must be a maximum speed of cause and effect.[Rindler 1979] Although the argument is only applicable to special relativity, i.e., to a flat spacetime, it seems likely to apply to general relativity as well, at least for low-amplitude waves on a flat background. As early as 1913, before Einstein had even developed the full theory of general relativity, he had carried out calculations in the weak-field limit that showed that gravitational effects should propagate at c. This seems eminently reasonable, since (a) it is likely to be consistent with causality, and (b) G and c are the only constants with units that appear in the field equations, and the only velocity-scale that can be constructed from these two constants is c itself.

    High-amplitude gravitational waves need *not* propagate at c. For example, GR predicts that a gravitational-wave pulse propagating on a background of curved spacetime develops a trailing edge that propagates at less than c.[MTW, p. 957] This effect is weak when the amplitude is small or the wavelength is short compared to the scale of the background curvature.

    It is difficult to design empirical tests that specifically check propagation at c, independently of the other features of general relativity. The trouble is that although there are other theories of gravity (e.g., Brans-Dicke gravity) that are consistent with all the currently available experimental data, none of them predict that gravitational disturbances propagate at any other speed than c. Without a test theory that predicts a different speed, it becomes essentially impossible to interpret observations so as to extract the speed. In 2003, Fomalont published the results of an exquisitely sensitive test of general relativity using radar astronomy, and these results were consistent with general relativity. Fomalont's co-author, the theorist Kopeikin, interpreted the results as verifying general relativity's prediction of propagation of gravitational disturbances at c. Samuel and Will published refutations showing that Kopeikin's interpretation was mistaken, and that what the experiment really verified was the speed of light, not the speed of gravity.

    A kook paper by Van Flandern claiming propagation of gravitational effects at >c has been debunked by Carlip. Van Flandern's analysis also applies to propagation of electromagnetic disturbances, leading to the result that light propagates at >c --- a conclusion that Van Flandern apparently believed until his death in 2010.

    Rindler - Essential Relativity: Special, General, and Cosmological, 1979, p. 51

    MTW - Misner, Thorne, and Wheeler, Gravitation

    Fomalont and Kopeikin - http://arxiv.org/abs/astro-ph/0302294

    Samuel - http://arxiv.org/abs/astro-ph/0304006

    Will - http://arxiv.org/abs/astro-ph/0301145

    Van Flandern - http://www.metaresearch.org/cosmology/speed_of_gravity.asp [Broken]

    Carlip - Physics Letters A 267 (2000) 81, http://xxx.lanl.gov/abs/gr-qc/9909087v2
     
    Last edited by a moderator: May 5, 2017
  6. Jun 24, 2011 #5
    I remember that van Flandern paper, and at the time was amazed that he confounded the nature of orbital mechanics within a scalar field with the propagation of gravity waves. Search on 'earth' and see if I've misunderstood his blunder or what.

    Speed of a field has no meaning.
     
  7. Jun 24, 2011 #6

    PAllen

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    This is a bit off topic, but I followed the Carlip - van Flandern debate in detail. I believe that van Flandern actually believed as follows:

    1) Light travels at c, but changes in the 'coulomb field' propagate much faster than c.

    2) Gravitiational waves may travel at c, but the gravitational attraction between two bodies reflects their current position to a speed enormously greater than c, possibly infinite.

    In response to this are some comments by Carlip that it might conceivably be possible to build theories with this structure, but only at the cost of dis-unifying what Maxwell's theory and GR succeeded in unifying.
     
  8. Jun 24, 2011 #7
    I've looked at his orbital mechanics and his diagram again. I feel that he was stuck in a Newtonian world of 'forces', 'attraction', rule of thumb approximations to get everyday orbits right, and the earth's motion being somehow heliocentric, and then looking through this mess and finding fault with GR.

    My understanding of GR is that the earth's movement is responsible only to the immediate field in which it moves.
     
  9. Jun 24, 2011 #8

    PAllen

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    I don't disagree with what you say here. What I recall van Flandern saying is the light and gravitational waves were a separate phenomenon from force propagation. I find this bizarre, but accuracy requires stateing that van Flandern did believe light traveled at c, and gravitational waves may exist, and would likely travel at c.
     
  10. Jun 24, 2011 #9

    atyy

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    Here's a really interesting review by Hinterbichler about whether gravity can, at least theoretically, have a mass. http://arxiv.org/abs/1105.3735
     
  11. Jun 26, 2011 #10
    The issue raised by inquisitors like Val Flandern is whether static gravitaional and electrostatic sources can be treated differently from wave propagation - light for example requires an event to produce a photon which must travel to the point where it is observed - whereas there is no known particle that travels between masses or charges that can be measured (to date)- the static force producing field requires a different physical concept - so there is good reason to question why the physics of light propagation apply to these fields that are not structured from waves. Also, if I recall correctly, there is only one binary experiment that closely corresponds to the predicted energy loss via gravity waves
     
  12. Jun 26, 2011 #11
    I don't even understand the meaning of force 'propagation'. A body stands in relationship to a scalar field: magnetic, electric, or gravitational, and requires no reference to the field generator: it is directly responsible only to its local field for its behaviour.

    Conceptually (although calculation assumptions make a centre of mass convenient) it is easier, and more real, to think of binary stars revolving in a field that they happen to have provided jointly. What does 'the speed of gravity' mean under such circumstances? Speed of gravity from where? There is no 'where'.
     
  13. Jun 26, 2011 #12
    As for the speed of gravity waves, it's my understanding that from the early days of testing Einstein, the orbit of Mercury to the measurements of binary systems are indirectly consistent with a propagation speed of c.
     
  14. Jun 26, 2011 #13

    PAllen

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    No, this experiment provides no such measurement. See the reference provided in my first post in this thread. The top GR phenomenologists believe no measurement so far says anything directly about the speed propagation of gravity waves or gravitational (metric) disturbance. Indirectly, you can argue that all experimental confirmations of GR add to confidence in not yet tested prediction, but that is not the same as direct confirmation.
     
  15. Jun 26, 2011 #14

    PAllen

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    Maxwell's equations explain charged particle interactions, magnetism, and EM radiation in one coherent frame. To separate these things, you would need a new theory that dis-unifies them (as Carlip commented). To my knowledge, van Flandern et. al. never bothered to propose such a theory.

    Similarly for GR.

    This must have the status of nonsense until there is a theory that explains known results as well as accepted theories.
     
  16. Jun 26, 2011 #15

    bcrowell

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    Dan said "indirectly," not "directly," and I think it's valid to consider the Hulse-Taylor system (but not Mercury's orbit) as an indirect test of propagation at c.
     
  17. Jun 26, 2011 #16
    That's correct. I chose the word carefully. I guess I chose Mercury's orbit not carefully.
     
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