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

Gravity and the speed of light

  1. Oct 29, 2009 #1
    Hi folks,
    Just a quick question:-

    Relativity says that nothing, even the gravitational impulse can travel faster than the speed of light. Is there any experimental evidence available online that proves that gravity's 'pull' or 'force' acts thru space at the speed of light?
    I believe it's faster, that gravity is an instantaneous interaction. But I'm just a lay nut so I might be imagining things! I believe that if 2 supermassive objects at great distance from each other start to interact gravitationally, that it's an instantaneous reaction, it doesn't take the time it would take for light or other radiation to cross the distance, it's instant.

    How wrong am I? Am I seriously misunderstanding relativity/spacetime/gravitation?

    Thanks in advance,
  2. jcsd
  3. Oct 29, 2009 #2
  4. Oct 29, 2009 #3
    The basic problem is that if you can send information faster than light, then you can send information back in time, and this is bad. This is not to say that it's impossible, but a) we don't know of any cases in which you can send information back in time and b) if we could then there would be a lot of explaining.

    If you assume that gravity is purely Newtonian with an infinite velocity then this is clearly wrong since you end up with spacecraft motions that are clearly wrong. You can change your theory to fit current observations, but that does take a bit of doing.

    Science works by process of elimination. I'm pretty sure that if you have a theory of gravity in which the gravity moves instantly, that there are experiments that you can use to disprove that. I'm also pretty sure that if you have a theory in which gravity moves 0.0001 m/s faster than light, that our measurements aren't good enough to exclude that. Whether there are experiments that rule out that gravity moves say 10% faster than the speed of light, I don't know, but I'm sure there is some review article out there.

    I don't think it's possible to *prove* that gravity doesn't move faster than light. It is possible to that if gravity behaved in X manner, that it would be inconsistent with experiment.
  5. Oct 29, 2009 #4

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

  6. Oct 30, 2009 #5
    Ok, there's some material here to get me learning, thanks!

    What if gravity were a fundamental property of ALL particles?
  7. Oct 31, 2009 #6


    User Avatar
    Science Advisor

    What do you mean? As far as I know, gravity influences all particles. You can't "escape" from a gravitational field like you could "escape" from an electromagnetic field by putting your charge to 0. Everything which has energy or mass is influenced by gravity and exerts gravity. And that means [itex]\emph{everything}[\itex]: neutrino's, light, electromagnetic fields, elektrons, stars, etc. etc.
  8. Nov 1, 2009 #7


    User Avatar
    Science Advisor
    Gold Member

    What experimental evidence do you suggest that supports this hypothesis? GR predicts weird effects like frame dragging. Gravity Probe B tested and confirmed these effects with great precision. What is left to be tested?
  9. Nov 2, 2009 #8
    Ok, well what if just one particle was responsible for gravity? Say for argument's sake this particle is the neutrino but it could be any particle in the standard model or some particle yet to be discovered. It exerts gravity over all other particles and therefore is the cause of all gravitational fields. How would the math work then?

    BTW, I'm just a lay nut trying to learn by throwing stuff out there!
    I don't believe in the graviton. Show me one. I've no problem with broken symmetry and the demonstrable parts of the standard model but I think gravity is something else entirely and while the math of relativity seems to work perfectly with respect to gravity, I believe it's not understood at all on a quantum level, at least by me in any case!
    Thanks in advance for any replies,
  10. Nov 3, 2009 #9
    reasons? Have you ever seen a photon? How about a quark?

    Skepticism is always reasonable, but unless you have some facts to support you, it's unlikely anything here will be specific enough to satisfy you.
  11. Nov 3, 2009 #10
    Never seen a photon but the mathematical theory behind them owes much to observation, the theory behind gravitons is pretty hypothetical in comparison is it not? It would be more of a mathematical extrapolation than anything approaching a law, no?

    Have quarks not been detected in particle accelerators by now? I thought they had...

    My bad, maybe
  12. Nov 4, 2009 #11
    It works very badly, and you've hit at the reason why it's been so difficult to get a quantum theory of gravity.

    The quantum theory of electromagnetism involves electrons tossing photons back and forth, and those photons are mathematically treated as a "small correction" to the vacuum.

    The problem with doing the same trick to gravity is that you assume that gravity occurs by exchanging "gravitons" you run into the problem that since everything produces gravity, then gravitons produce gravitons which produce more gravitons, and when you add everything up, you get infinities all over the place.

    One reason that people are interested in string theory is that when you get to high enough energies, the gravitons become strings that presumably don't produce more gravitons, and so by cutting things off, you get finite answers. However, you run into a problem in that you can cut things off any which way, so instead of getting finite answers, you get any answer that you want, which is also bad.

    Nature doesn't care what you believe. :-) :-)

    The trouble is that you really can't have two completely separate theories of the universe, because you run into situations in which both theories are active.
  13. Nov 4, 2009 #12
    Well.... No one has ever gotten a theory involving gravitons to actually work. Now you could say at that point "why bother?" and just say that things are run by GR. The trouble is that in some situations that we can imagine, GR just stops working. Unfortunately, those situations are extreme enough so that we just can do something like make a black hole and see what happens.

    We have seen things for which quarks is the most straightforward explanation that anyone has come up with.
  14. Nov 6, 2009 #13
    research the higgs mechanism, gravity is not a force with infinite speed
  15. Nov 6, 2009 #14
    The higgs mechanism might be totally wrong.

    The current situation is

    1) we have no observations that indicate that gravity travels at faster than light (and people have tried to explain that to Tom Van Flattern without success, but that's his problem)

    2) if gravity did travel faster than light, then there would be a lot of theoretical problems
  16. Nov 20, 2009 #15
    OK, thanks. Quarks would still have a firmer basis in observable reality than the graviton, then, no?
  17. Nov 21, 2009 #16


    User Avatar
    Science Advisor
    Gold Member

    Planetary motion studies rule out superluminal gravity. See also:

    Aberration and the Speed of Gravity
    "The observed absence of gravitational aberration requires that ``Newtonian'' gravity propagate at a speed $c_g>2\times10^{10}c$. By evaluating the gravitational effect of an accelerating mass, I show that aberration in general relativity is almost exactly canceled by velocity-dependent interactions, permitting $c_g=c$. This cancellation is dictated by conservation laws and the quadrupole nature of gravitational radiation."
    Last edited: Nov 21, 2009
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Gravity and the speed of light
  1. Speed of Gravity (Replies: 2)

  2. Speed of light? (Replies: 2)

  3. Speed of light (Replies: 3)