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I already asked a question but it seemed that it was not precise enough. My problem is that I cannot see how gravitational waves have anything to do with spacetime. An EM wave or a phonon can put matter in motion, but nobody say this is because spacetime itself is altered. Where is the proof that gravitational waves are a "propagation of spacetime ripple" and not just the propagation of an energy able to induce motion just like EM waves would do upon charged particles, except it is not about charge here?

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    $\begingroup$ Gravitational waves do propagate energy and do induce relative motion, but they do so by slightly modifying the geometry of spacetime. The “proof” is that the LIGO observations are consistent with predictions based on General Relativity, which is a theory of gravity as spacetime geometry. $\endgroup$ – G. Smith Apr 25 at 13:06
  • $\begingroup$ I am not questioning the existence of gravitational waves. What I want to understand is what is really meant by "modifying the geometry of spacetime" and not just be refered to GR because I suspect one could produce a theory about EM that involves a modification of a specific spacetime curved by electric charge. In other terms, I am wondering if the notion that spacerime itself is changed by gravitational waves is an artefact of the theory of GR, because it does not make sense to me that any alteration of spacetime could be observed. $\endgroup$ – Exocytosis Apr 26 at 12:40
  • $\begingroup$ There is no accepted geometric theory of electromagnetism, although there have been attempts at one. If you can produce such a theory, you should publish it. Your question would make more sense if you explained why you don’t think alterations of spacetime can be observed. On the other hand, it would then be more likely to get closed as a rejection of mainstream physics. $\endgroup$ – G. Smith Apr 26 at 13:20
  • $\begingroup$ I am not offering a counter theory, and I dont think asking for explanations and justifications about a mainstream one is subject to censorship unless we are in a theology seminar. I did explain why I have a hard time accepting the idea gravitational waves are timespace oscillations. This is because there is no measurement done by gravitational waves detector that I know of that are different in nature than other energy waves. Thus my question asking for an explanation. $\endgroup$ – Exocytosis Apr 27 at 4:33
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    $\begingroup$ Do you know of another kind of wave that makes objects move farther apart in one direction while moving closer together in a perpendicular direction, and then vice versa? Electromagnetic waves don’t do this. Nor do sound waves, as far as I know. But waves in spacetime geometry do. $\endgroup$ – G. Smith Apr 27 at 9:38

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