If we could hear black holes, how loud would they be? I know that this is basically the same question as the question "How loud would the Sun be?", except this time there is a black hole. But since the black hole sucks in stuff like a passenger jet engine sucks in air, about how many decibels would it be? Also, would it lethal to humans if we were 1 AU away and what would it do to our planet?
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2$\begingroup$ -1. If light can’t escape a black hole, why do you think sound could? Also, black holes don’t “suck”. You might find it useful to read up on black holes in Wikipedia before asking about them. Or look through the questions and answers on Astronomy SE or Physics SE (search the tag black-hole). $\endgroup$– Chappo Hasn't ForgottenCommented Jun 14, 2018 at 4:46
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$\begingroup$ @Chappo I think the sound strength in the accretion disc could be calculated with (general) relativistic aerodynamics. Most probably it is not an easy calculation, and it hardly depends on the circumstances, but interpreting the question in this view makes it, in my opinion, on-topic. $\endgroup$– peterhCommented Jun 14, 2018 at 17:18
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$\begingroup$ @peterh yes, I’d expect some extreme acoustic effects within a BH accretion disk. The question is on-topic but borderline in quality: inexperienced OPs need to demonstrate some basic effort before posting their questions, to avoid us having to infer a related matter to make the question valid. $\endgroup$– Chappo Hasn't ForgottenCommented Jun 14, 2018 at 21:25
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$\begingroup$ @Chappo Btw, it is probably the only place in the Universe, where doppler effect of sound due to general relativity plays an experimentally verifiable role. But I don't know, this what the OP really asked for. But I think this is what he asked. If he wanted to know some different, then he may ask a new question :-) $\endgroup$– peterhCommented Jun 14, 2018 at 21:32
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$\begingroup$ Wait. But the point of no return is the event horizon, not the accretion disk. Also, what about the jets? $\endgroup$– Leo PanCommented Jun 15, 2018 at 2:00
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2 Answers
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Nothing comes out from the BH (well, with a single known, very small exception).
The accretion disk of the Black Holes most likely has various streams, pressure differences and so in it. Calculating it or measuring it is surely a very hard task, if you have luck, a professional explains the results in a better answer.
The accretion disc of the typical, star-sized black holes is much smaller as 1AU. And sound doesn't propagates in vacuum. Thus, from 1AU distance from the Black Hole we most likely wouldn't hear anything.
Other radiations (mainly gamma from the disc) may be deadly even from 1AU, but not its sound.
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1$\begingroup$ Also I think think that this question would deserve a professional answer. $\endgroup$– peterhCommented Jun 14, 2018 at 17:40
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1$\begingroup$ A professional answer can talk about phonons, and Sonic Black Holes. a SNB can envelop normal black holes if the speed of phonons is space is slower than light, which it theoretically is. So if phonons travel at 0.1 times the speed of light in a vacuum by phonon tunneling, the the sonic black hole would exist where matter falls in at that speed. $\endgroup$ Commented Jun 16, 2018 at 7:09
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$\begingroup$ BTW, you're going to get a lot of nitpicking for saying the Hawking radiation originates from "inside" the event horizon. :) $\endgroup$ Commented Jun 18, 2018 at 21:03
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Physicists talk about phonons to describe energy transmission in elastic media similar to sound. There hasn't been much research about phonon transmission through the void of space because it is a forbidden event in physics. There is no sound in space. Some scientists have studied phonon tunneling through few angstroms of vacuum. https://physicsworld.com/a/phonons-tunnel-across-the-vacuum/
To find out more, you can read about phonons and sonic black holes.
Ears and microphones require air pressure and displacement of mass onto the auditory diaphragm. The mass and pressure of the interstellar medium is too low to actively move sound-sensing diaphragms, unless you have a lot of ions/gas massive objects to do that. In space there is a vacuum. if an astronaut played drums beside you in space, they would not displace any air pressure. there would be no noise.
If you had a 1000Watt studio speaker beside you in space, it's quite possible that you would receive less than a single electron worth of sound pressure energy from that speaker per m2. something like 10^-20 watts. 100 dB is about 0.01W/m2 and is at the limit of hearing impairment.
Do you know the warning: "In Space, No one can hear you scream"... the film aliens...
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1$\begingroup$ There is sound in space, just with typical sound speeds of the order of tens of kilometers per second. Sound plays a very significant role e.g. in such diverse fields as collapse of protostellar clouds and in the "baryonic acoustic oscillations" in the early Universe. $\endgroup$– pelaCommented Jun 14, 2018 at 20:54
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1$\begingroup$ We would have to disagree about what sound is, and sound volume... What is the transmission medium and physical strength of the sound that you describe? The sound pressure level generated by a violin in space would be how much in terms of SPL? any microphone in space would register 0 dB unless a gas/ion mass moved the diaphragm. the mass of a vacuum is not sound-transmitting. $\endgroup$ Commented Jun 16, 2018 at 2:17
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2$\begingroup$ I agree that the sound is too… dim?…low?… to be heard by humans, but in physics we cannot disagree what sound is: it is the oscillating compressions/rarefactions of particles in a fluid. In space, that fluid is usually an extremely dilute atomic gas or plasma. But nevertheless, "sound in space" is term used frequently by astronomers. For instance, in the "warm neutral medium", you could have n = 1 atom per cm$^3$ at a temperature of T = 1e4 K, so the pressure would be $P = n k_B T$ ~ 1e-13 Pa, corresponding to an SPL of 20 log(P/20µPa) = –163 dB. $\endgroup$– pelaCommented Jun 16, 2018 at 5:06
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1$\begingroup$ Because the mean free path of particles in the interstellar and intergalactic is so long, sound frequencies are extremely small — wavelengths are typically measured in AU or lightyears. The baryon acoustic oscillations have wavelengths of roughly half a billion lightyears. $\endgroup$– pelaCommented Jun 16, 2018 at 8:47
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1$\begingroup$ I don't really know anything about phonons. And you're right, the BAOs are not sound waves anymore. But they were, when the gas/photon fluid was oscillating in the gravitational wells of dark matter halos. When free electrons disappeared and photons started free-streaming, the fluid was relieved of pressure and the sound waves were essentially frozen in space and stopped propagating (though their wavelength expanded with the Universe). I don't agree that it's just an analogy, though, like comparing galaxy filaments to a sponge. It really was sound waves. $\endgroup$– pelaCommented Jun 16, 2018 at 11:39