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I'm struggling for a while with this question and I'm not knowledgeable enough in SRT/GRT, so I'll ask here:

Suppose some big cosmic event like a Black Hole Merger that generates significant Gravitational Waves (GWs). My first assumption would be that, since GWs and massless particles, say photons, travel with c, any spontaneously emitted particle from anything close to the above event (directed at me, the observer, far, far away) is bound to the relative phase of the GW forever - i.e. if it happened to be emitted into a GW "trough", it always stays there - same for "crests"! Furthermore: GWs distort spacetime. So, my questions are:

  • Would that mean that any such photon trapped in a GW trough arrives slightly later as one would expect in an undistorted journey? Or, vice versa, arrive "earlier"? This seems hard to believe since this would constitute FTL travel...
  • Would that even matter for an outside observer?
  • If yes, could one build some new GW detector out of this? For example, you could digitally filter out photons "undisturbed" by GWs (if that's even possible...), then check for "early"/"late" photons - which could theoretically be used to reconstruct the wave event(s)
  • It's Spacetime, after all. So, the (local) expansion/contraction of space should play a part, as well as the distortion of the time dimension. And I'm not good enough to give this the full 4-vector spacetime treatment, but could it be that the two "effects" just cancel out?

Edit: An answer pointed out that it is relevant that gravitational waves are longitudinal, not transversal waves. However, I'm not so sure about that. After all, it's exactly this what LIGO et al. are measuring - expansions and contractions of space. The question is if this is temporally relevant, too.

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In multi-messenger astronomy it has experimentally be observed that gravitational waves and light originating from a far-away source (for example the 2017 neutron stars merger) are detected on Earth with a temporal dispersion suggesting that both travel at the same speed.

But that you already know.

Now note that gravitational waves are transverse, so there can not be a "trough/crest" effect like the one you have in mind.

Moreover, it is dangerous to imagine a photon as a tiny point-like object (here possibly riding a wave) - this will never provide a correct intuition. As far as we know a photon is only localized upon detection.

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  • $\begingroup$ Well, that's 2 points (longitudinal vs transversal and wave-like nature of particles) I've never considered. Thank you for that! $\endgroup$
    – user58973
    Commented May 26, 2021 at 9:06
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Gravity can shift the frequency of light we see but we will always see light traveling at c in a vacuum. Gravitational waves also propagate at c so the light and gravitational wave would travel together.

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