Since gravity propagates at the speed of light, what happens to an object's field of gravity if the object travels very near to the speed of light (like 99.99999%)? Does it stop propagating forward, and only propagates to the sides and behind? And is this also what happens to the tiny gravity of a photon? And wouldn't this mean that the gravity of near-lightspeed objects and photons does not fully (but only partially) join the cumulative and continuous effect of gravitation on a cosmic scale, since no gravity is projected ahead of them...? And finally, are there any popular science books that go into these things: fundamental properties of objects travelling very close to lightspeed, and the specific gravitational properties of photons? Thank you.

  • $\begingroup$ It may be a good idea to separate out the last part of your question (popscience books) into a separate thread. Check out our policy for resource recommendations. If you do that, you'll need to add the resource-recommendation and make it a community wiki. $\endgroup$
    – user191954
    Jul 25 '18 at 3:43
  • $\begingroup$ And this isn't to deter you or anything, but sometimes we tend to be a bit hostile about the quality and reliability of popsci books. You'll frequently find that the math in real textbooks isn't as excruciating as it looks (though it isn't easy, or at least for me it isn't). $\endgroup$
    – user191954
    Jul 25 '18 at 3:44

to your question, let's take the case of particles with rest mass first.

Let's take a neutrino, and see what happens to the gravitational field it (its stress-energy) creates and how it propagates to its sides, behind it and forward.

Well, actually to the sides and behind was not your question, but as per SR, massless particles (photons) seem to travel at c from all inertial reference frames, regardless of the speed of the frame.

In this case, thought the neutrino travels at almost light speed, it still creates a gravitational field around it, and the gravitational field as you say propagates with speed c.

From the inertial frame of reference of the neutrino, it will seem as the gravitons (or the gravitational field propagates with speed c) forward.

So although the neutrino itself travels near speed c, it will see the gravitational field in front of it propagate with speed c.

This is like to say that since gravitons are massless, and travel at speed c, SR rules apply to them the same way as to photons.

So gravitons will seem (or the gravitation effects) to propagate at speed c from all inertial reference frames.

  • $\begingroup$ Please provide a reference-- $\endgroup$
    – S. McGrew
    Jul 25 '18 at 2:13
  • $\begingroup$ arxiv.org/pdf/gr-qc/0607045.pdf $\endgroup$ Jul 25 '18 at 5:00
  • $\begingroup$ researchgate.net/publication/… $\endgroup$ Jul 25 '18 at 5:00
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    $\begingroup$ Unless I've overlooked something, those references only discuss gravitons in a rest frame. I think the question is really directed at the apparent shape of the gravitational field of a particle moving at relativistic speed relative to an observer: whether or not the gravitational field appears radially symmetric. It's a good question: does the gravitational field appear distorted due to Lorentz contraction, same as the electric field around a charged particle? I assume it does, but don't know. $\endgroup$
    – S. McGrew
    Jul 25 '18 at 5:26

It is important to note that in special relativity, all objects that travel at the speed do so in every inertial reference frame, including other inertial frames that are themselves traveling at the speed of light. The photon "observes" gravitons radiating away from it (if indeed that picture is accurate) at the speed of light, because in it's reference frame it is at rest. Likewise the gravitons that affects it coming from other objects, are "seen" to approach it at the speed of light. And so it's response to gravity in its reference frame is identical to an object experience gravity at rest. It is only when observing the photon interacting with the graviton from an outside reference frame that any peculiarity is noticed.


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