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Each object forms a gravity well in space-time. Okay, two objects are approaching each other at nearly the speed of light. How do they pick up each other's "gravity radar" signals in order to form a common unified gravity well and be attracted to each other?

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It works in the same way that light travels. If you're familiar with special relativity, then you'll know that when two observers are moving towards each other at close to the speed of light it will still be 'c' in their reference frame.

The same is true for gravitational waves. From each persons perspective, the gravitational waves will propagate at the speed of light.

If you're interested in the mechanism on how that works, read up on special relativity.

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  • $\begingroup$ Doesn't that suggest that whatever transmits gravity IS itself light? Why else should gravity be limited to the speed of light just because photons are limited to the speed of light? $\endgroup$
    – user165197
    Commented Aug 7, 2017 at 3:29
  • $\begingroup$ That's actually a good point. This video is exactly on that topic and explains the idea very well. youtube.com/watch?v=msVuCEs8Ydo $\endgroup$ Commented Aug 7, 2017 at 4:26
  • $\begingroup$ @DaneJoe so how would a black hole pull you in? I am serious since I don't know enough to be snarky, but I do know enough to be curious. $\endgroup$
    – user163104
    Commented Aug 7, 2017 at 10:27
  • $\begingroup$ @Countto Because the light particles aren't actually escaping from the inside of black hole, they are an extension of preexisting conditions infinitesimally prior to the formation of the black hole. Furthermore, they are virtual. $\endgroup$
    – user165197
    Commented Aug 9, 2017 at 22:41
  • $\begingroup$ @Danejoe yes, I kinda figured that part out yesterday, no offense meant, thank you very much for answering. But I am definitely misunderstanding you somewhere, and I just need clarification on this. Are you saying that photons are, in effect gravitons, in which case, would not gravity be weaker nearer the event horizon, due to gravitational redshift, which I don't follow the logic of? Or are you saying what the OP originally asked about, that there is some sort of signalling system involving photons? Best yet, is there a paper on this, rather than taking up your time with my guesses, ta $\endgroup$
    – user163104
    Commented Aug 9, 2017 at 23:00
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They don't need to sense each other by radar like methods. They don't need to send out signals, it's a one way system, they get pulled together when gravity is strong enough to overcome any other velocity they might have.

They meet up in somewhat the same way as the Earth goes around the sun. Spacetime is curved (in a way that every picture you have seen does not represent accurately, because we cannot visualise four dimensions, so "gravity well" might give you a wrong idea, only math can really describe it) .

They follow geodesics, which are the paths of shortest distance in spacetime, but because spacetime is curved, they are not always straight lines.

For the situation you describe, they will be pulled together in a way that makes the distance of travel in the 4D spacetime between them as short as possible.

Luckily in the case of the Earth, our path is curved around the sun, rather than straight at it, because of the way our velocity vector is pointed.

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