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

Doppler Effects on Gravity

  1. Apr 26, 2008 #1


    User Avatar

    Assume 2 massive bodies are moving away from each other at high speed.
    Since the gravitons which cannot travel at faster than the speed of light,
    are having a hard time being exchanged by these two bodies,
    it must be plausible to assume that the gravitational pull between these two bodies would be diminished.

    Relativity mechanics should be able to calculate the new gravitational forces between the 2 moving bodies, which will reduce to zero gravitational forces when the relative velocity of both bodies reaches a certain value.

    Assuming that for a graviton to be exchanged between 2 massive bodies, that graviton has to make a round trip, ie. forward and back, and assuming that gravitons travel at the speed of light, this will lead us to deduce that the effective velocity of gravitons are half of the speed of light. (c/2).

    This leads me to the prediction that at speeds close to c/2, the gravitational pull between 2 bodies would reduce to zero newtons.

    Now in the case where both bodies are moving away from each other faster than half the speed of light, this might result in repulsive gravitational forces.

    The accelerating, expanding universe observations is an indisputable and repeatable experimental proof of repulsive gravity and which may be modeled based on my hypothesis above.

    Any comments?
  2. jcsd
  3. Apr 27, 2008 #2
    There are lots of things wrong with what you're suggesting, but i applaud your efforts.
    I wouldn't call it "plausible to assume" that the gravitational force would be reduced for distancing objects...
    Gravity does travel at the speed of light, in all reference frames. Also, mass increases at relativistic speeds, (increasing the force of gravity).
    The graviton is hypothesized, and in no way confirmed, observed, or even generally agreed upon. Therefore its hard to say that it would require the graviton to go back and forth (unlike all other gauge bosons, mind you); and if it did go back and forth that would imply that they go at twice the speed of light, (i don't know how that implied from the assumption that "gravitons go the speed of light.").
    Then its quite a stretch to say that faster than this limiting speed would result in... "repulsive gravity". Instead of zero gravity. Note that the repulsive gravity would presumably be conveyed by the same particle, with the same speed limitations.
    In no way does the expansion of the universe imply that there exists negative gravity. It does however suggest a half dozen other theories that you should look into; for instance the buzz word of the decade "dark matter."
  4. Apr 27, 2008 #3


    User Avatar
    Gold Member


    I'd like to hear more about this -


    Why do gravitons have to make a round trip as opposed to other gauge bosons ?
  5. Apr 27, 2008 #4
    What i meant was, i don't think they do go back and forth... this is a topic on the forefront of modern physics -> far from agreed upon or completely understood. I'd be happy to explain anything i know.
  6. Apr 27, 2008 #5


    User Avatar


    Has there been any experimental confirmations that changes in Gravity travels at the speed of light? If there are, it would immediately debunk my hypothesis in it's current form.

    About Gravitons moving back and forth,

    A lot of "force" particles are also known to move back and forth, like in the strong nuclear force. That's why the force can change from attraction to repulsion when distance increases.

    The action of Gravitons can be visualized this way,
    If Gravitons move back and forth, it would be like a person grabbing a ball from another person, resulting in attraction. If Gravitons only move one way, it would be like the ball being thrown by one person to another, resulting in Repulsive Gravity.

    Personally, I fail to see why Repulsive Gravity cannot be used to model our accelerating, expanding universe. We don't have to follow the dark matter crowd.
  7. Apr 28, 2008 #6
    There has been such experimental confirmation, unfortunately i cannot site any in particular (shouldn't be hard to find in a google scholar search).

    Its really not fair to say that "a lot of 'force' particles are also known to move back and forth." First of all, W and Z bosons most certainly don't, and photons can but don't need to. So already 3 out of the 5 do not need to move back and forth to mediate their respective forces, and we're talking about 1 of the remaining 5, so only 1 even has the chance of moving back and forth (the gluon). I most certainly can't state it as a fact, but i'm pretty sure the strong force is attractive, except for a few situations when it can be repulsive (at smaller distance scales than when it is attractive).

    Now besides such trivial issues, there is nothing about a back-and-forth (BF) exchange, as apposed to a one directional exchange, that explains why a force would change from attractive to repulsive, so again, "That's why the force can change from attraction to repulsion" isn't a very fair statement to make.

    Why does a person grabbing a ball from another person mean attraction? and wouldn't they have to grab it, and then give it back - to fit your model. And the ball being thrown from one person to another, is a classic qualitative introduction to advances physics way of introducing gauge bosons; and is used as a way of explaining attractive forces - so its not especially legit for you to reuse it in the opposite way.

    In one respect you are completely right, the accelerating expanding universe could definitely be modeled by some sort of incredibly elaborate, suddenly reversing, non-empirical new gravity. But our accelerating expanding universe can also be modeled by things that make sense / are necessarily true.
  8. Apr 28, 2008 #7
    It is hard in GR to make definitive statements about a particular local situation.

    If we take the system as you described as the whole universe then the only thing that matters is if the combined mass is larger than the critical mass. In the case it is larger the two objects will come together regardless of the speed of separation.

    If we describe GR in terms of graviton interactions then we have to realize that not only do gravitons couple to mass-energy but also to each other. In the case of two masses we have the simplest case of a non stationary universe, which effectively means that if we were to attempt to describe this system in terms of graviton interactions we cannot ignore the non-linear effects from as far away as infinity.
  9. Apr 28, 2008 #8


    User Avatar

    Let me explain the ball passing analogy in a clearer way.

    If a person A grabs a ball from the hand of another person B, naturally the friction between the ball the B's hands could cause B to move towards A ever so slightly.
    This movement can be seen as an attractive force.
    After that B grabs the ball from A's hands, by doing so enhances the attractive force.

    If A throws the ball at B, naturally the momentum of the ball would cause B to move away from A. This movement can be seen as due to a repulsive force.


    The principle of wave-particle duality allows us to make predictions using either face without considering the other face and still get some results.
    For example, Maxwell didn't know anything about photons but he managed to calculate the value of c from just wave equations.
    Similarly, we can use photons to explain the photoelectric effect. If we try to use waves to explain the photoelectric effect, we get nowhere.
  10. Apr 28, 2008 #9
    The analogy simply does not run so deeply.
    And by the analogy, as soon as the 2 bodies move any distance away from each other, the force would switch; and if the force switches instantaneously anywhere, you would have some serious tidal effects.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Doppler Effects on Gravity
  1. Doppler Effect (Replies: 25)

  2. Doppler effect (Replies: 2)