# Can gravitational waves be used to investigate the inside of black holes?

I do understand that nothing, no particles and no information can escape a BH.

Gravitational waves are real, they have been observed.

Gravitational waves are disturbances in the curvature of spacetime, generated by accelerated masses, that propagate as waves outward from their source at the speed of light.On 11 February 2016, the LIGO and Virgo Scientific Collaboration announced they had made the first direct observation of gravitational waves. The observation was made five months earlier, on 14 September 2015, using the Advanced LIGO detectors. The gravitational waves originated from a pair of merging black holes.[8][9][10] After the initial announcement the LIGO instruments detected two more confirmed, and one potential, gravitational wave events.[11][12] In August 2017, the two LIGO instruments and the Virgo instrument observed a fourth gravitational wave from merging black holes,[13] and a fifth gravitational wave from a binary neutron star merger.[14] Several other gravitational wave detectors are planned or under construction.

https://en.wikipedia.org/wiki/Gravitational_wave

Now GWs are said to couple to matter much much weaker then EM waves, thus GWs experience much less scattering or absorption, GWs should be unaffected by the opacity of the early universe.

Due to the weakness of the coupling of gravity to matter, gravitational waves experience very little absorption or scattering, even as they travel over astronomical distances. In particular, gravitational waves are expected to be unaffected by the opacity of the very early universe. In these early phases, space had not yet become "transparent," so observations based upon light, radio waves, and other electromagnetic radiation that far back into time are limited or unavailable. Therefore, gravitational waves are expected in principle to have the potential to provide a wealth of observational data about the very early universe.

Now GWs are said to have a property that even EM do not have, that is GWs can pass through any intervening matter without being scattered.

Second, gravitational waves can pass through any intervening matter without being scattered significantly. Whereas light from distant stars may be blocked out by interstellar dust, for example, gravitational waves will pass through essentially unimpeded.

I do understand that nothing, no particle, no information can escape a BH. There is still no consensus about whether GWs are made of gravitons (hypothetical) or not. GWs are basically disturbances in the curvature of spacetime.

In general relativity, there is no dispersion in gravitational waves, while some modified gravity theories predict dispersion phenomena in the propagation of gravitational waves. In this Letter, we demonstrate that this dispersion will induce an observable deviation of waveforms if the orbits have large eccentricities. The mechanism is that the waveform modes with different frequencies will be emitted at the same time due to the existence of eccentricity. During the propagation, because of the dispersion, the arrival time of different modes will be different, then produce the deviation and dephasing of waveforms compared with general relativity.

http://inspirehep.net/record/1708562/

The fact that GWs can pass through any matter without being scattered significantly could mean that GWs (being just distortions of the curvature of spacetime itself) could pass through BHs, and carry information on the inside of the BH.

A BH itself represents an extreme distortion of curvature at the EH of the BH, and GWs are themselves distortions of the same curvature of spacetime.

Question:

1. Can we use GWs to investigate the inside of BHs?
• That paper has nothing to do with gravitational waves passing through black holes. Saying “this could mean...” is not correct. – G. Smith Jul 11 at 16:54
• @G.Smith correct, thank you I edited. – Árpád Szendrei Jul 12 at 9:22
• For an external observer, the volume of a black hole is zero. This implies that the region "inside" a black hole does not exist. - arxiv.org/pdf/0801.1734v1.pdf – safesphere Jul 16 at 7:59

This will extend the answer of Brick a little.

It is possible to disturb the region of spacetime beyond the horizon of a black hole, either by sending gravitational waves into it or simply by dropping matter into it. One will then have gravitational waves propagating around in the region of spacetime beyond the horizon. However, that is where they stay. All the null geodesics wrap around and fall back, none escape out through the horizon. Gravitational waves have null wave-vectors (a fancy way of saying they propagate at the speed of light) so the disturbance they make will not come back out of the black hole.

Of course one can also imagine gravitational waves being produced by processes such as collisions of heavy objects orbiting in the region outside the horizon, and these waves can escape.

There is one little added detail perhaps worth a mention. The horizon can itself shrink (eventually) owing to Hawking radiation. This is a process which is vanishingly slow for any ordinary black hole, but for physics we want to consider all possibilities. This leads to some complete unknowns. We don't yet have a robust understanding of the implications of black hole evaporation, concerning the information which went into the black hole. However this is a marginal point for the question which was asked. The main point in answer to the question is that gravitational waves have the same causality limits (commonly called speed limits) as light waves.

• @safesphere Disagree. (1) There are plenty of reference frames inside horizons, for example LIFs. (2) crossing horizon the wrong way means passing out through a null surface; this is exactly the same as speed $v > c$. (3) Hawking rad true but so what; for Unruh do you know about the Sokolov-Ternov effect? – Andrew Steane Jul 16 at 8:24
• (1) Since the horizon is smooth I think it is fair and reasonable to extend our reasoning faculties through the horizon and discuss what goes on there on the assumption that the field equation does not immediately fail beyond a horizon. (2) Exceeding the speed of light is not quite the same as moving back in time; $v>c$ is neither forward nor backward in time but simply is not timelike at all. This is relevant to quantum field theory where terms involving spacelike intervals contribute to the propagator. – Andrew Steane Jul 16 at 10:11
• @safesphere I don't understand your argument about the geodesic being undefined. If we write down a metric in, say Eddington-Finkelstein coords then it satisfies the field eqn and there's no need to ever bother with Schwarzschild coords, so no coordinate singularity ever arises in the first place (at the horizon). Isn't a timelike geodesic here doing the same as it would in any other region of spacetime? Any timelike line is continually crossing null surfaces (open ones) because spacetime is full of them. I would be happy to know why this argument is wrong if it is. – Andrew Steane Jul 16 at 22:04

The answer is no. Gravitational waves travel at the speed of light and are therefore also unable to escape the black hole for basically the same reasons that the EM wave cannot. It has nothing to do with the degree to which they do or do not interact with matter. It has nothing to do with scattering. In order to "escape" the wave would have to propagate with a non-physical speed.

• I am trying to understand how black holes have a high gravitational pull if no gravity can escape – Adrian Howard Jul 11 at 18:22
• Gravitational waives are not "gravity escaping". That terminology has no physical meaning as far as I can tell. Gravity is the effect of the local curvature of spacetime, whatever it happens to be wherever you are. That's true in a black hole, far from a black hole, near the sun, …. If you're worried about your graviton as a mediating particle of the force, I think you're stuck with the answer that there's no complete theory of quantum gravity. @AdrianHoward – Brick Jul 11 at 18:25
• perhaps I should reword it, if gravitational waves are "unable to escape the black hole" then how is the gravity of the black hole detected outside the event horizon, I am not saying you are wrong, I am only trying to get a better understanding of black hole physics – Adrian Howard Jul 11 at 19:29
• Gravitational waves have nothing to do with detecting gravity itself. Your premise seems to be wrong. This is like asking how you know there's a lake there if you cannot see ripples on its surface. You know the lake is there because it's a big location of water. The waves are a (usually small) variation of the surface of the lake, while the existence of the lake itself is a bigger feature. @AdrianHoward – Brick Jul 11 at 19:33
• @AdrianHoward Gravity doesn't escape a black hole, because all matter is at the horizon, not inside. See the correct answer accepted here: physics.stackexchange.com/questions/937/… – safesphere Jul 16 at 6:55

Gravitational waves occurs due to the rotation of BH non symmetrically ... GW are created by the event horizon of the BH because inside the event horizon nothing is there at singularity.. BH are creating a tunnel into space and time simultaneously.. all the inside information is erased from classical point of view but all the information in the BH are converted into 2 dimensional at event horizon... So gravitational waves doesn't carry information about source or information..