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Why does light and gravity travel at the same speed?

  1. Feb 6, 2012 #1
    Why does gravity effect objects as fast as light. For instance, if the sun suddenly disappeared, earth would continue orbiting the place the sun left for 8 minutes, which is the same time it takes light from the sun to reach us.. i think this might be some sort of standard speed in the universe in which information can travel.. but also why when exceeding that speed would you go back in time, why specifically THAT speed (299m/s).. it has to be a result of something else, doesn't it?
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  3. Feb 6, 2012 #2


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    Essentially, that is the speed of waves in the "fabric" of space. And your statement that "when exceeding that speed would you go back in time" is not correct- you cannot go faster than the speed of light. It happens that, in the formula for time dilation, if you simply put in a speed greater than the speed of light, you get an imaginary number solution. Some people interpret that as "going back in time" but it really just means you have done a meaningless operation.
  4. Feb 6, 2012 #3
  5. Feb 6, 2012 #4


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  6. Feb 6, 2012 #5
    The most direct answer to your question from a quantum mechanics viewpoint is that the force carrier for gravity, the as yet undiscovered 'graviton', would be massless and is therefore able to travel at the maximum possible speed for a massless object. I'm not sure what the explanation is in terms of General Relativity because gravity isn't a force in that model, and I've no idea by what maths and reasoning its propagation are constrained to c.

    The notion of exceeding the speed of light and travelling back in time is derived from the idea that any object at the speed of light experiences no movement in time, therefore going beyond that speed implies time reversing. As has been pointed out, exceeding that speed is currently considered impossible - particularly if the object has any actual mass.

    As for "why 299m/s?"... Well, I'd like to see what others say. The speed can be derived from (I think) Maxwell's equations using the eletric and magnetic constant, but when I checked into that it seemed at least one of them was itself defined using c, which struck me as circular.
  7. Feb 6, 2012 #6


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    Isn't it just by considering small deviations away from a flat spacetime, so your metric takes the form [itex]g_{ab}=\eta_{ab}+h_{ab}[/itex]. Then, in a vacuum, the field equations imply that the perturbations propagate according to a wave equation,

    \Bigg(-\frac{\partial^2}{\partial t^2}+c^2\nabla^2\Bigg)h_{ab}=0

    with speed of propagation, c.
  8. Feb 10, 2012 #7
    In my view anything which propagates through spacetime does so at the speed of light. Gravitational waves propagate like radiation but with zero frequency. That is only the amplitude of the wave propagates, leaving spacetime in a state of curvature.
  9. Feb 10, 2012 #8
    Would you mind going into a little more detail here on what causes a lack of frequency?

    I guess I've just never imagined gravity to "propagate in waves" since it doesn't display any of the classical properties like EM radiation does.
  10. Feb 10, 2012 #9
    Yes I certainly wouldn't say propagates in waves. More like a disturbance (Einstein would say curvature) which spreads out throughout spacetime. My take on it is that particles are forms of compressed spacetime causing a stretching effect on neighboring elements of spacetime, like a quantum theory of gravity. The stretching effect of course diminishes farther from objects as the effect is shared by more elements of spacetime. I like to think spacetime has memory: it tries to restore itself to its original configuration. This memory might also be the source of other forces. From this viewpoint the universe is really a re-configuration of spacetime. The internal energy of matter is balanced by negative gravitational potential energy. Of course eventually spacetime overcomes this imbalance as quark matter decays or evaporates through black holes. The leptons, however, seem to be subject only to annihilation, as I understand.
  11. Feb 10, 2012 #10
    This has always been something that bothers me that I have never fully understood.
    If gravity is bound by the same rules as light and cannot travel faster then light, that suggests that gravity should be effected by the curvature of space. If that is so, then how can gravity escape a black hole? The two seem contradictory to me.
  12. Feb 10, 2012 #11
    Except for Hawing radiation I don't believe gravity does escape from a black hole. Note if too much gravity escapes the black hole it seems it wouldn't be a black hole anymore.
    Last edited: Feb 10, 2012
  13. Feb 11, 2012 #12


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    Gravity does not have to "escape" a black hole. Gravity is a field. That means it has a value everywhere at all times. Before the black hole formed, the mass that curved space was there. After the black hole formed, the value of the field does not change.
  14. Feb 11, 2012 #13
    Yeap I'm still confused. I'm afraid you may not be able to dumb this down enough for me. Lets try this example in reverse and maybe then you can point out where my thinking goes wrong.
    If a black hole losses mass, you would expect that it would have less gravitational effect on a star orbiting outside of its event horizon. This effect should travel at the speed of light to the areas surrounding the black hole. I still just don't see how it can relay the information out of the black hole. Maybe my confusion is because I think of it as a wave or particle (graviton), and I don't see how a graviton can escape when other particles cannot.
  15. Feb 11, 2012 #14


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    Yep. That's the problem. We do not yet have a quantum mechanical description of gravity. Currently, relativity is the going theory. And it describes gravity as a field.
  16. Feb 11, 2012 #15
    This one confuses me too, but I was wondering if the answer has something to do with things that fall into a black hole appearing as frozen on the horizon, and redshifted so far they are invisible, but maybe a hypothetical graviton particle or similar could still be observed.

    Same for the event horizon of the universe.

    I don't know if this makes any sense or not though, but maybe it is one way to look at it.
  17. Feb 11, 2012 #16
    Good points.
    Last edited: Feb 11, 2012
  18. Feb 11, 2012 #17
    Originally Posted by JonDE
    Maybe my confusion is because I think of it as a wave or particle (graviton), and I don't see how a graviton can escape when other particles cannot.

    Yep. That's the problem. We do not yet have a quantum mechanical description of gravity. Currently, relativity is the going theory. And it describes gravity as a field.

    These statements are very profound. Seems we need new Physics? Electromagnetic waves / particles cannot escape an extremely high gravitational field, yet hypothetical gravitons can?
  19. Feb 11, 2012 #18
    Gravitons are the gravitational equivalent to photons for the electromagnetic fields. Gravitons spread the gravitational field of a particle at its creation. The gravitational field of the particle then becomes attached to the particle as long as it's not acceerated. If that happens then the gravitons come into play again to update the gravitational field. But gravitons play no part in a static gravitational field for a particle moving with no acceleration.

    Radiation escapes black holes when its wavelength is greater than the diameter of the event horizon. I don't think gravitons are part of a black hole. They only update the gravitational field, as when the black holes lose mass by Hawing radiation.
    Last edited: Feb 11, 2012
  20. Feb 11, 2012 #19
    Per Einstein gravity is curvature of spacetime. The gravitational field of the black hole is the sum of the individual contributions from each particle in the black hole. If I understand Hawking radiation it annihilates particles in the black hole, and so the contribution of their gravitational field to that of the black hole will be lost, reducing the gravitational field of the black hole itself and reducing the curvature of spacetime.
  21. Feb 12, 2012 #20


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