Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Where did the gravity come from?

  1. Jun 26, 2011 #1
    Gravity gravity gravity gravity......is that true that gravity travels at the same speed as the speed of light? or it travels faster? :uhh:
    Where did it come from ?
     
  2. jcsd
  3. Jun 26, 2011 #2

    Dale

    Staff: Mentor

    Gravity comes from the stress energy tensor which includes energy, momentum, pressure, and stress components.
     
  4. Jun 26, 2011 #3
    What about it's speed, sir? :smile:
     
  5. Jun 26, 2011 #4

    Dale

    Staff: Mentor

  6. Jun 26, 2011 #5

    Pengwuino

    User Avatar
    Gold Member

    It's assumed gravity travels at the speed of light, although this has not been definitively shown.
     
  7. Jun 26, 2011 #6

    bcrowell

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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
  8. Jun 26, 2011 #7
    Thank you sir for answering me.
     
    Last edited by a moderator: May 5, 2017
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Where did the gravity come from?
Loading...