A physics question from a conversation I had

Talked with a player here, Joe Shmo, we got to talking about physics and stuff, and the speed of sound, he was suprised the speed of sound underwater was faster than in air. I told him it was some 15 times faster than in air because the speed of sound is greater when the density of the material is greater, so the speed of sound in an iron rail is greater yet than in water. So the subject of black holes came up. He then said, so what is the speed of sound in a black hole. Hmm, interesting question, no?
If the density of matter is close to infinite in a black hole, then could the speed of sound there be faster than the speed of light? I assume there is no reason why sound could not propagate inside a black hole.

Originally posted by sonhouse
Talked with a player here, Joe Shmo, we got to talking about physics and stuff, and the speed of sound, he was suprised the speed of sound underwater was faster than in air. I told him it was some 15 times faster than in air because the speed of sound is greater when the density of the material is greater, so the speed of sound in an iron rail is greater ye ...[text shortened]... e speed of light? I assume there is no reason why sound could not propagate inside a black hole.

While sound is my stuff, I'm no physicist so i can be wrong, but in black holes there is nothing there is nothing that can resonate thus there can be no sound.

Originally posted by Choreant
While sound is my stuff, I'm no physicist so i can be wrong, but in black holes there is nothing there is nothing that can resonate thus there can be no sound.

I was under the impression black holes were formed from matter, like gaziillions of tons of it compressing down so fast the gravity gets so strong that even light can't escape, so why do you think there is nothing nothing nothing in a black hole?
And think of something not quite a black hole, a neutron star, that is where matter has condensed so far that it is something like a quark soup, squashed down so dense it is all but a black hole but enough repelling force is still left to keep it from the fate of a black hole, whatever that is.
So consider the speed of sound in that stuff, where a teaspoon of it would mass many tons, if the density of matter determines the speed of sound then surely the speed of sound in a neutron star is many many times faster than in air or water, so the next step in this analogy, a black hole....

Originally posted by sonhouse
I was under the impression black holes were formed from matter, like gaziillions of tons of it compressing down so fast the gravity gets so strong that even light can't escape, so why do you think there is nothing nothing nothing in a black hole?
And think of something not quite a black hole, a neutron star, that is where matter has condensed so far that i ...[text shortened]... many many times faster than in air or water, so the next step in this analogy, a black hole....

As far as I know black holes are ultra-dense objects formed of matter. Due to the high density, black holes are incredibly heavy and even photons cannot escape holes' gravitational field, thus they are called black holes. They cannot be seen because light cannot escape them. However, I'm not sure how it is with the sound. Maybe the density restricts particle movement and there is no sound. But I'm not the one to tell.

If a tree falls in a black hole and there's no one there to hear it...

Originally posted by AThousandYoung
If a tree falls in a black hole and there's no one there to hear it...

Then as the tree gets stretched longer and longer by the gravitational tidal forces operating there, a lot of sound energy would be generating, going faster and faster as the poor tree decends into the abyss. If you are in a position to actually here it you are in deep doo doo.

I'm liking where this is going,.......a perplexing question it turns out to be!

Originally posted by joe shmo
I'm liking where this is going,.......a perplexing question it turns out to be!

Don't know if you ever heard of Brian Greene, he is a world class physicist and he has a series and a book I think, called The Elegant Universe.
I went to his site at Columbia and asked him that same question. I may get an answer, he answered another question of mine a couple years ago.
I also asked that question of my son in law, Gandhi, also a physicist. So maybe I will get some kind of real answer.
It also brings to mind another question: Since matter would be so devastatingly compressed inside a black hole, can light even propagate at all inside one? Light at ANY wavelength?
I can see sound propagating before light inside a black hole.

Originally posted by sonhouse
I was under the impression black holes were formed from matter, like gaziillions of tons of it compressing down so fast the gravity gets so strong that even light can't escape, so why do you think there is nothing nothing nothing in a black hole?
And think of something not quite a black hole, a neutron star, that is where matter has condensed so far that i ...[text shortened]... many many times faster than in air or water, so the next step in this analogy, a black hole....

ok,I used a shortcut, mayby unclear. I just suggested, and as I told I can be wrong, that the pressure in a black hole is so great that nothing, cannot exist inside the black hole, so there is nothing to vibrate, thus prolonging the sound wave.

About this neutron star stuff, I've got no impression.

Originally posted by Choreant
ok,I used a shortcut, mayby unclear. I just suggested, and as I told I can be wrong, that the pressure in a black hole is so great that nothing, cannot exist inside the black hole, so there is nothing to vibrate, thus prolonging the sound wave.

About this neutron star stuff, I've got no impression.

But think about it, what causes the pressure?

it seems to me that once the mass moves at the speed of light, then it becomes the actual light wave energy form and light and sound converge, as mass and light converge as mass approaches the speed of light as well.

I don't believe a word of it though, any more than I believe in those 50% off sales at department stores.

Originally posted by coquette
it seems to me that once the mass moves at the speed of light, then it becomes the actual light wave energy form and light and sound converge, as mass and light converge as mass approaches the speed of light as well.

I don't believe a word of it though, any more than I believe in those 50% off sales at department stores.

In a pressure wave, the individual particles do not move anything like the speed of the actual wave so it could be the same in a black hole.
Here is a link to an article about sound generated by a black hole, friggin low frequency though, 10 million years per cycle! They said it has been doing that steady for 2.5 billion years or 2500 cycles have been generated since then:
http://chandra.harvard.edu/press/kits/perseus/blackhole_sound.html

There can be no sound in a black hole. A black hole is a singularity, essentially a point mass. Sound requires a medium to propagate in: it is the result of the collisions of many particles; there can be no sound within an infinitesimal volume where there are no particles.

There are definitely sound waves in neutron stars. I looked around on the web and found this:

http://www.journals.uchicago.edu/ApJ/journal/issues/ApJL/v470n1/5329/5329.html

which contains this sentence:

"Using a variational method in which they constrained the equation of state to have subluminal sound velocity and be stable against microscopic collapse, they proved that, in the regions where it is uncertain, the equation of state that produces the maximum neutron star mass is the one for which the sound speed is equal to the speed of light."

--

I understand very little of that, but I think it means that the speed of sound on a neutron star increases as the mass of the neutron star (and hence its density) increases, until the speed of sound reaches the speed of light and the neutron star collapses into a black hole.

...very interesting question indeed: because why not go even further? here's another quote from the same article:
"The connection between the zero frequency sound velocity being greater than the speed of light and violation of causality, while physically plausible, is a tricky question, due to the frequency dependence of the sound velocity and sound wave damping. We are not aware of a general proof yet that the ground state of matter must obey dP/dρ ≤ c^2."
(where dP/dρ is the definition for the square of the speed of sound, and c is the speed of light)

However, it is an article from remote 1996.....

Originally posted by Hengst
...very interesting question indeed: because why not go even further? here's another quote from the same article:
"The connection between the zero frequency sound velocity being greater than the speed of light and violation of causality, while physically plausible, is a tricky question, due to the frequency dependence of the sound velocity and sound wave d ...[text shortened]... eed of sound, and c is the speed of light)

However, it is an article from remote 1996.....

That brings to mind a test, assuming you found yourself inside something with a huge gravitation, how do you know if you are in a black hole? If that idea is right, if the speed of sound = the speed of light, you are in a black hole, then whacking something and producing a sound then measuring its velocity would give you an independent test of what medium you were in, wouldn't it?