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The speed of light, gravity, strong force, weak force

  1. Jun 8, 2008 #1
    I could benefit from someone shedding some light on a couple of questions that I have regarding the speed of various entites:
    1) It seems that the speed of gravity, e.g, if the sun disappeared, is the speed of light WRT things I've read. What is it that would inherently cause gravity to travel at c, since presumably the fundamental nature of the causation is different? Could it be that gravity's speed would be infinite, except that it can't exceed 'c'?
    2) Has anyone been able to define a speed associated with the strong and weak nuclear forces? If no, could/should one assume that those forces also move at the speed of light?
  2. jcsd
  3. Jun 10, 2008 #2
    The following is my understanding of things. I do not claim to be a particle physicist, so if somebody sees that I am wrong I would be most grateful to be informed....

    The graviton is presumed to have a mass of zero, thus it should move at the speed of light. Similarly, the gluon, which mediates the strong force, is a massless particle thus it moves at the speed of light. The weak force, however, is mediated by W and Z bosons which are very massive, therefore they must have a finite mass. How then the weak and electromagnetic force is unified I do not know.
  4. Jun 15, 2008 #3
    The graviton is an assumed particle and thus has not been proven. Therefore you cannot calculate its mass or speed in any given reference frame.
  5. Jun 15, 2008 #4
    The "speed of gravity" is the speed at which small perturbations in the metric propagate in an otherwise static background (i.e., gravitational waves). To talk about the speed of large deformations in spacetime is a bit iffy, because you need some background to measure the speed with respect to, and all you have is spacetime. For example, in an expanding universe points can move away from eachother at faster than c, but that's because the space between them is expanding. Locally they are not exceeding c. This is all at the level of classical GR, of course.

    cmos hit the nail on the head, but I'd just like to point out that the weak and electromagnetic are only unified at high energy, and are for all intents and purposes, separate at our energy scales (there is a broken symmetry).

    regarding Daedalus' comment, the graviton is massless and moves at speed c, because that's what the graviton is. It is not so much a predicted particle as a name we give to quantum excitations of the weak perturbed metric. If we ever discovered a massive spin-2 field, we would probably not call it a graviton. Also, there are some mathematically proven theorems that tell you that we'd be in bad shape if the graviton were massive.
  6. Jun 15, 2008 #5
    my comment stands. The "graviton" is a "predicted" particle. We cannot confirm its existence until they use the new particle accelerator to prove its there. Now it is probably true that it is massless but then again... it might not be, causing a complete reworking of quantum physics. But this is probably not the case.
  7. Jun 15, 2008 #6
    I just took issue with "therefore you cannot calculate its mass or speed in any given reference frame. " In any case, I think we understand eachother's points of view.
  8. Sep 17, 2010 #7
    I think first question of longone is more about classical mechanics and answering it in terms of quantum mechanics would be wrong.
    Is there any reason that why speed of light is c in terms of classical mechanics?
    In my opinion same would be the answer for gravitaional speed.
  9. Sep 17, 2010 #8
    This may be answered perhaps with the same mechanisms of blackholes and light not being able to escape ^^
  10. Sep 18, 2010 #9
    The reason behind this is space-time shape. How can you link this with gravitational speed.
  11. Sep 18, 2010 #10
    The gravitational field IS the metric, i.e. it gives the distance between two points. What is this distance physically? It's it the speed of light c times the amount of time it takes for a beam of light to travel between the two points. Keep in mind that a light cone has a boundary, and this boundary represents the boundary between points which in principle can interact, and those points which in principle cannot interact.

    Therefore if we change the gravitational field in one part of space, then we can think about what will happen if we send a light signal out from that part of space before we make the change and afterwords. It's clear that the time it takes for the light beam to travel will depend on if we send it before or after the change in the gravitational field.

    We can trace out the light cone through space time until the two points we want to know the distance between are allowed to have interacted. If you think about it, this shows that the time it takes for the change in the gravitational field of one part of space to affect another part is determined by the speed of light.

    One way to see this is to try and figure out the following situation. You are sitting at point A with some space time four vector, somewhere else at point B which has some other 4-vector, the gravitational field changes in some way. The problem is to determine how an observer at point A measures distances to all the points nearby to A as a function of time. If you think about sending light signals back and forth in order to make these measurements, it should become clear why we say gravitational fields travel at the speed of light.
    Last edited: Sep 18, 2010
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