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?
  22. Dec 16, 2004 #21

    Hans de Vries

    User Avatar
    Science Advisor

    The word "graviton" gives 212,000 google entries while "general
    theory of relativity" gives only 106,000. So it has become a popular
    word. However, the graviton is not part of the "general theory of
    relativity" at all.

    The latter theory does use c as the speed of propagation. But it
    does not describe propagation in terms of particles because it's
    not a Quantum Theory.

    General Relativity still is the standard theory. Well tested and with
    practical applications like in the Global Positioning System were
    accuracy is significantly improved by compensating for GR effects.

    Although the word graviton is now part of the common language,
    it's not that simple to establish the theories based on it. Further
    more, The space time warping that you mention is often not a
    part of these theories. So mentioning the two together only make
    sense in some of the theories.

    What is believed to be true in general in most theories is that
    anything that has energy does indeed also contribute to gravity.

    gravitons would be redshifted but it would probably be harder to
    tell in comparison with photons which are emitted at characteristic

    There is no chance for the chain reaction you're mentioning
    because gravity is such a weak force. Each iteration does diminish
    by a factor of ten to the order forty or so.

    Regards, Hans.
  23. Dec 17, 2004 #22
    Well, redshifting in light cases a reduction in the energy of light via the down-shifting wavelength. If the same thing applies to the graviton, then the effect of gravity would lessen as the energy got redshifted.
    Afaik, the two types of reshifting are gravitional cased by some 'uphill' gradient and doppler caused by the expansion of the universe.

    So if the expansion of the universe caused the reduction of energy of gravitons because of redshifting, this would contribute to any overall reduction in the power of gravity at cosmic scales, further increasing the rate of expansion surely?

    edit: I don't want to sound like some crank with a theory here, so the statement should be read as a question.
    Last edited: Dec 17, 2004
  24. Dec 23, 2004 #23
    |2eason, you bring up an interesting idea of this. This may be a possibility, as it would fit some of the facts (increasing acceleration of the expansion or "stretching" of the universe). Since gravity is a particularly weak force compared to the other three (known) forces, it would be logical to consider the possibility that while being redshifted, the effects of gravity would diminish. Gravitational influence is weaker over larger distances, as we all see by the equation, F = (Gm1m2)/r^2, an equation introduced to many of us in our first encounter with physics. Electromagnetic force, though far more powerful than gravity, is also affected by the distance between objects/charges. Nuclear weak and strong forces have their ranges as well, making distance one of the major contributions to force strength. |2eason, it is a good point to bring up that, if redshifting in the energy of light (EM - photon) results in reduction of energy, then it is possible that the effect of gravity (through the graviton) would also diminish as the energy is redshifted. Quite recently, the nuclear weak and nuclear strong forces, along with EM force, have almost been united; however, the absence of gravity in this unification leaves a lot out - so the unification won't be completed until gravity can be accounted for in unification. The route to take in uniting these forces may be to connect gravity and EM forces first, then test if it can fit in with the nuclear forces, which will prove to be more difficult. The reason I mention unification is because the gravitational force and electromagnetic force both may involve particles (and it is logical to consider it) - one that is known (the photon) and one that hasn't been proven yet (the graviton). With the proving of the graviton, it may be possible to unify both EM and Gravitational force - we would all hope! String theory will have a lot to do with unification, as quantum physics along with GR is required together to achieve some kind of unification.

    Also, this is my first post here - I came across this site two days ago and I am enjoying the many diverse and varied comments about physics - every response gives me something to think about! I enjoy seeing questions being answered, but the most exciting part of physics is when there is a new question to consider - the more questions there are, the further we come along in understanding the physics of nature. I consider myself a theoretical scientist, though all of my theories are based on other facts and theories. I lose track of time while I am busy with anything having to do with physics - it is most enjoyable to apply mathematics to the physical universe we live in! That's why I majored in physics - and continue taking courses in it - because each new thing learned and each new challenging concept is a significant motivation to me! Even with the world in a state of conflict, I think that the many hours per day I spend thinking about physics keeps me going! Thank you whoever created Physics Forums!!! Oh, and Happy Holidays! :smile:
  25. Dec 24, 2004 #24
    Welcome to PF, Elrond! You're not Elrond from the OwP are you?
    Last edited: Dec 24, 2004
  26. Dec 25, 2004 #25
    OwP? Nope - that's also my screenname on a site called armada2files - I am into Star Trek games as well - lol. But physics is my paramount of interests. Thank you for welcoming me to this amazing site! I love to learn and it is exciting to be challenged in physics - the challenge of unifying the forces is most exciting. All of the fun in physics is not in finding the answer - it's in the process of using the scientific method. I think I learn something new here when I read any post - or at least consider a new view on physics. I can say that it's a great time to be living with the science of the modern day - I can only hope that I get to see a great century in science unfold, as long as that progress isn't hampered by the crisis in the world of these days! I watched the Nova presentation on PBS a few nights ago (about string theory) and I can't wait for the continuation of that series! I know that it is most difficult to distinguish opinion from fact from theory from possibility, especially on the web - but all of those categories of information have one thing in common - the ability to be considered. And using the right combination of facts, opinions, theories, etc., scientists can come up with more facts that can lead science to the goal of unification of forces and particles! What a great age in Physics!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook