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So we can't hear sound in a vacuum because there's no air to propagate the vibration. But from what I understand, black holes make vibrations in space-time. So much so that scientists can detect black holes by measuring these vibrations using equipment that will actually, physically vibrate because of black holes. Right?

Does that mean that if you stand close enough to a black hole... Not close enough to get sucked and die. Wearing only some space suit to keep you alive. Would you actually hear a sound, coming from the black hole, through the vacuum, vibrating your suit and your ear drums? Or do we need specialized equipment to detect those vibrations that our ears drums do not have?

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  • $\begingroup$ I suspect that you've misread something. A pair of black holes in orbit around each other emit gravitational waves, and the amount of energy emitted gets quite large as they collide. $\endgroup$
    – PM 2Ring
    Dec 1, 2021 at 13:57
  • $\begingroup$ safesphere, while observation of the system by an external observer, the tidal forces would have stretched the individual around the event horizon long before the merge happened. This is a thought experiment of what the local observer would understand. From that and the understanding that space time is locally flat the observer in the gravity well has a much different experience. And in that experience since you are part of the spacetime, the distance between you ears contract at the same rate as everything else in space time. It would thereby do unnoticed. $\endgroup$
    – Michael
    Dec 9, 2021 at 20:40
  • $\begingroup$ When you suit compresses due to the gravitational wave, everything does it. This includes all of the atoms in the body. There is a good deal of calculations that prove out my answer. $\endgroup$
    – Michael
    Dec 9, 2021 at 22:26
  • $\begingroup$ The last statement is evident that your knowledge of GR is lacking. To the outside observer the atoms do become smaller when a space time contracts. But for the local observer they do not. This is part of the paradoxical results in GR. $\endgroup$
    – Michael
    Dec 10, 2021 at 14:53

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Gravitational waves are measured by using a laser that is split and reflected off of two perpendicular lines with reflective targets at the ends. The 2 lasers beams are then recombined to produce an interference pattern. If everything is stationary, there will be no change to that pattern. If one leg is moving relative to the other then the pattern will shift. It is that measurement in that shift that tells us if there is a gravitational wave. Towards the end of two orbiting massive objects that speed increases and subsequently the frequency of the wave. You can then use those phase changes to create a sound wave which is known as a chirp. Individual black holes are not going to have this frequency change in general. This all works because the speed of light is constant in all frames of reference.

The questions around someone sitting in space and hearing the gravitational wave is not possible. This is because your entire self is in the system. As your space suit expand and contract due to the gravitational wave so does everything else inside the suit including the individual and the air. So there will no imposed compression on the air or space suite relative to anything else that would cause sound. So, in the end, you do not realize anything is actually happening.

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  • $\begingroup$ I think you are minimizing the energy coming out of the merger by comparing it to some nuclear bombs. For example the Hiroshima bomb had less than 1 gram of mass turned into energy. A black hole merger is moving multiple solar masses across the event horizon. The computation (that's my side) shows and the observation bears it out that these are the largest energetic events in the Universe. Also don't forget that the observation for the distant observer is very different from the local observer. Oh the joy of solving the Einstein equations. $\endgroup$
    – Michael
    Dec 9, 2021 at 20:46
  • $\begingroup$ When you are talking about tidal forces pulling bodies apart, you are speaking of the external observer. The local observer has a different experience. The calculations confirm this. $\endgroup$
    – Michael
    Dec 9, 2021 at 22:24
  • $\begingroup$ How about we go with some basic texts on GR. Wald's General Relativity has some good expression around the contraction of space time. For a more basic text, check out Introduction of Einstein's Relativity by Ray D'Inverno or Introduction to the Theory of Relativity by Peter Bergmann. They have all of the math and explanation that one would need to fill out your understanding. $\endgroup$
    – Michael
    Dec 10, 2021 at 14:51
  • $\begingroup$ As are your statements. Where are the published papers that show this result is incorrect? The reality is that you will not find either in the literature. Why? Because this question and answer do not raise to the level of writing a paper. These questions are reserved as thought experiments that we give to first year graduate students. And incorrect responses are not constructive. They are just wrong. $\endgroup$
    – Michael
    Dec 10, 2021 at 21:41
  • $\begingroup$ We calculate things all the time. That does not make them publishable. And I do not care if it is down voted. My correct answer does not change because of your incorrect opinion and understanding. $\endgroup$
    – Michael
    Dec 11, 2021 at 13:07

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