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

The speed of gravity?

  1. Dec 8, 2004 #1
    I have just learned that gravitaional energy propagates as particles called gravitons and also as waves. I was wondering if gravitons are to gravity as photons are to light, and therefore, what is the speed of gravity?

    Does anyone know if this actually exists?
  2. jcsd
  3. Dec 8, 2004 #2
    The graviton has not yet been experimentally detected, but it is predicted to be a spin-2 boson, with ZERO MASS. The zero mass implies that it would travel at the speed of light.
  4. Dec 8, 2004 #3


    User Avatar
    Science Advisor
    Homework Helper

    1.Energy (energy-momentum) of the gravitational field is a tricky subject in GR,as it is described by a frame dependent quantity,called the "energy-mometum pseudotensor".
    2.The theory of gravitational field propagation through waves is a subject covered in the chapter "weak field limit" of GR (i quantized gravity in its weak limit last spring,so i should be aware of what i'm talking about).The equations of Einstein in the absence of matter in the weak field limit and with an apropriate gauge take the familiar form of the ones for the EM potential for EM flield in flat Minkowski space.That's why we conclude that gravity in the weak field approximation propagates through space-time under the form of waves travelling at "c".
    3.The notion of "graviton" is strange to the realm of GR,as it's seen as the quanta of gravitational field.
    a) No such particles (of spin 2,BTW) have ever been observed.
    b) Since,till now,there's no ultimate theory of quantum gravity,they're just a theoretical abstraction aimed of fiding a spin 2 particle (partner) in the same spin supermultiplet with the gravitino.

    Gravitons are to gravity what photons are to light.It's just that we know for sure that photons exist (though our senses cannot feel them,they feel only the "wave" behavior of light),while gravitons...Well,who knows...???

    PS.Interpretation of full gravity as a field theory just like the ones in the SM,and quantizing it,fail,as the theory obtained is not renormalizabile, (i.e.the values of its observables (like the Green functions) are infinite).
    Last edited: Dec 8, 2004
  5. Dec 8, 2004 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    To take an 'experimental' look at this. First, gravitational waves have yet to be unambiguously detected; several detectors have now been built (e.g. LIGO), and hopes are high that an 'inspiral event' (e.g. two neutron stars orbiting every closer until they merge/collide), or an asymmetric supernova will be 'seen' in the next few years. If either event also generates 'observable' EM (e.g. light, radio, gammas), or even neutrinos, we will also have some solid results on the 'speed' of gravity (there was one well-publicised observation recently purporting to show that gravity travels at c; unfortunately the analysis seems to have missed a few key points).

    Second, observations of some neutron star binaries (with at least one member a pulsar) show decaying orbits. The rates of decay very nicely match the hypothesis that energy is being lost from the systems in the form of gravitational radiation, as predicted by GR (some hard working scientists got Nobels for this). Since gravity travels at c in GR, these results provide indirect support for GR and the speed of gravity being c.

    Third, and most generally, GR has passed all its obsevational and experimental tests to date - including one which would have shown a deviation if GR were out by as little as 1 part in 20,000. While these tests have only probed GR in a limited range of domains (astonishing as that might seem - the tests go right up to the whole universe, size-wise), one can say that there is no experimental or observational evidence, direct or indirect, that even hints at the speed of gravity being anything other than c.
  6. Dec 8, 2004 #5


    User Avatar
    Gold Member

    If you "learned" this, you have been misinstructed, as there are no consensus in the physics community on the accuracy of this point, and even the proponents of the graviton acknowledge that while it makes a certain amount of sense as a theoretical concept, that it has not been established.

    A graviton would be a force carrying particle, like a photon, gluon, W+, W- or Z in current theories. Those are all spin 1 particles, however. A graviton would have to be spin 2. Why? A spin zero would not interact with photons, yet gravity affects light, so this cannot be true. A spin one-half would behave like quarks and electrons and neutrons, which gravity does not. A spin 2 naturally yields a Rank 2 tensor to describe its behavior which is what we observe in GR to describe relativity.

    The assumption of zero mass flow from the assumption that gravity is an infinite distance force. Zero mass particles would travel a c. Models that show gravity, either by particle or other means propogating at c seem to work better than those that assume a lower speed (although it day to day, this would be unnoticable except for large masses moving a great speeds).
  7. Dec 8, 2004 #6
    I found this article that postulates a massive graviton as candidate to dark matter. I wonder what would be the velocity of this massive graviton if its existence would be confirmed
    Massive graviton as a testable cold dark matter candidate
    Last edited: Dec 8, 2004
  8. Dec 9, 2004 #7
    just wondering, and please shout at me if I'm way out line, but....

    If gravity propagates at c, the aren't all the arguments about the total mass of the universe and it collapsing irrelevant, because the universe is expanding so fast that gravity couldn't keep up and make it collapse.
  9. Dec 9, 2004 #8
    The speed of gravity can be thought as faster than c. Although faster-than-light force propagation speeds do violate Einstein special relativity (SR), they are in accord with Lorentzian relativity, which has never been experimentally distinguished from SR -- at least, not in favor of SR. Indeed, far from upsetting much of current physics, the main changes induced by this new perspective are beneficial to areas where physics has been struggling, such as explaining experimental evidence for non-locality in quantum physics, the dark matter issue in cosmology, and the possible unification of forces
  10. Dec 9, 2004 #9


    User Avatar
    Staff Emeritus
    Science Advisor

    This is just wrong.

    I'd suggest the sci.physics.faq

    Does gravity travel at the speed of light

    for anyone interested in a non-technical summary of mainstream non-crank current information about the speed of gravity.
  11. Dec 9, 2004 #10
    if gravity travels at speed of light. is it speed constant from every frame of reference just like light??
  12. Dec 9, 2004 #11
  13. Dec 14, 2004 #12
    I personally think gravity is a natural by product of mass and has no force carrier, but if it does exist the LHC should find it.
  14. Dec 14, 2004 #13
    Louis Cypher

    Has anyone seen the Nyman theory of gravity having no force carrier; it's interesting, whether it's true or not though, who knows?
  15. Dec 14, 2004 #14


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Welcome to Physics Forums Louis Cypher!

    Do you have a reference (preferably a peer-reviewed paper) to the Nyman theory of gravity? To what extent is it consistent with GR? How does it stack up against good experimental and observational results (e.g. those cited as tests of GR - which GR passed with flying colours)?
  16. Dec 14, 2004 #15
    I remember reading an article about a year ago about some scientists that due to some rare phenomenon were able to measure the speed of gravity and found that it was the same as the speed of light. I however cannot find that article at this moment therefore I'm not sure I remember corrrectly
  17. Dec 14, 2004 #16


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Welcome to Physics Forums SAV}{VAS!

    You may be referring to the work of Kopeikin. There's another thread in PF with an extensive discussion of this, and other aspects of 'the speed of gravity' - here.

    (BTW, Kopeikin's idea was a good one, and the observational data OK; however, it seems he 'did his sums wrong'; GR predicts an effect in the quasar-Jupiter alignment, but it would be far too weak to be detected with Kopeikin's setup :cry:).
  18. Dec 14, 2004 #17
    You might be talking about this experiment, which failed to actually show that gravity speed = c, on many levels. This site also backs-up my post earlier about the probability of superluminal gravity.

    http://www.metaresearch.org/media%20and%20links/press/SOG-Kopeikin.asp [Broken]
    Last edited by a moderator: May 1, 2017
  19. Dec 14, 2004 #18


    User Avatar
    Science Advisor
    Gold Member

    Don't believe everything you see on the net, or much of anything you see on that particular site.
    Last edited by a moderator: May 1, 2017
  20. Dec 16, 2004 #19
    I have a quick question about gravity. Could someone explain where I'm going wrong here.
    If the graviton travels at c, it suggests to me that it traverses through space-time in the same manner as the photon, because if it doesn't, it would not be bound by SR.
    Well, since space-time is warped by gravity, doesn't that mean that the graviton must indirectly affect itself, as it must travel though space-time.
    Now, if this is the case, then doesn't this mean that the graviton must also be 'redshifted' in the same way as the photon?

    This 'redshifting' would lead to a kind of chain reaction, maybe having a similar effect to dark energy, no?

    Anyways.. I know I'm wrong, I just don't know why:/
    Last edited: Dec 16, 2004
  21. Dec 16, 2004 #20


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Welcome to Physics Forums |2eason!

    Yes indeed, both photons and gravitons (if they really exist) do themselves cause space-time to warp - the 'correct' description of (for?) both is GR, not SR.

    Gravitational waves would also be 'red-shifted' too, just like photons.

    However, the energy of even powerful gravitational waves is incredibly tiny - that's why it's so hard to detect them!

    What does '... lead to a kind of chain reaction, maybe having a similar effect to dark energy' mean?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook