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Black holes and sound waves

Tormac

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So at lunch today, I read an article in CNN about a black hole "humming". The article went on to describe that

The sound waves are emanating from the Perseus Cluster, a giant clump of galaxies some 250 million light-years from Earth. A light-year is about 6 trillion miles (10 trillion km), the distance light travels in a year.
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I was under the impression that sound could not travel through the near vacuum of space. Have I missed some discovery about sound? Or is the article just poorly written, and they are talking about a different type of wave, but just call it a "sound wave"?

Who can fill me in on this?

edited to add the link to the article

http://www.cnn.com/2003/TECH/space/09/10/blackhole.music.reut/index.html
 
The sound waves that they're referring to are only present in the medium surrounding the black hole. The sound itself isn't propagating very far, in a cosmic sense, and definitely not anywhere near earth.

Think of it this way. Say you took some very very high speed pictures of someone hitting a drum. You could look at these pictures and actually see the ripples on the surface of the drum as it vibrates. By measuring the length of the ripples you could deduce the wavelength of the sound being emitted, without ever actually hearing it.
 
Tormac,

The article stated they were measuring concentric ripples in cosmic gas, which serves as the medium for these sound waves. Outside of the medium, there would be nothing on which the sound waves could propagate. Also, as the article says, the waves were generated in the medium by the gravitational effects of the black hole. Interesting stuff.

Eric
 
Tormac said:

I was under the impression that sound could not travel through the near vacuum of space. Have I missed some discovery about sound? Or is the article just poorly written, and they are talking about a different type of wave, but just call it a "sound wave"?
Click to expand...

Depends partly on what you mean by "sound". If you mean any traveling pressure wave in a gas, then all you need is SOME gas. There is some gas even in space, so you can get sound. If you only mean something in the audible frequency range, then no, sound can't travel in deep space.

One thing to keep in mind is that when you work up equations for pressure waves in a gas, you work with the assumption that it's a continuous medium, that is, you ignore working with individual atoms or molecules, and just talk about average properties like density, pressure, temperature, etc. That works quite well as long as the length scales you're talking about are much larger than the distance between atoms or (perhaps even more importantly) the mean free path (how far an atom or molecule can travel before it's likely to collide with another atom). In deep space, of course, these length scales are MUCH larger than the wavelength of audible sound, so audible sound can't travel in space. But if the wavelength is gigantic (as in this case), then you can still treat interstellar gas as a continuous medium, and you still get "sound".
 
NPR had a scientist explain it in simple terms...turns out it is a "b-flat" 57 octaves below middle C, and is now the longest known symphony.
 
This is quite like what I say in one of my talks, where I point out that "very low frequencies" are generally refered to as "barometer readings". Barometer readings are in the 1Hz to 30 picoHz range give or take...

The statement about wavelength being much longer than mean free path is well taken, btw.

What it means to have a wavelength that is shorter than the mean free path is not completely meaningless, but a variety of interesting mechanisms introduce themselves.


1.8 FemtoHertz Yes, that counts as a bass line. :D
 
I think those reports are overly cutesy. They have detected pressure waves or mechanical oscillation near a black hole. It's a stretch to call it "sound", and this is compounded by reporting the frequency in "octaves" instead as Hertz. I would prefer to see less cute, and better science.
 
arcticpenguin said:
I think those reports are overly cutesy. They have detected pressure waves or mechanical oscillation near a black hole. It's a stretch to call it "sound", and this is compounded by reporting the frequency in "octaves" instead as Hertz. I would prefer to see less cute, and better science.
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You have to admit that 1.8 FemtoHertz is a low frequency.

I think it was an attempt to explain "how low" to the layman, who can't understand 2^57 anyhow. My druthers would have been to say it both ways, but then again I rarely address lay audiences.

JJ
 
jj said:


You have to admit that 1.8 FemtoHertz is a low frequency.

I think it was an attempt to explain "how low" to the layman, who can't understand 2^57 anyhow. My druthers would have been to say it both ways, but then again I rarely address lay audiences.

JJ
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At least it explains that low, low B flat I hear all the time.
 
arcticpenguin said:
I think those reports are overly cutesy. They have detected pressure waves or mechanical oscillation near a black hole. It's a stretch to call it "sound", and this is compounded by reporting the frequency in "octaves" instead as Hertz. I would prefer to see less cute, and better science.
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There's a ton of pure math out there if you want it. This is something that gets people's attention, might get them to try to figure out what a black hole it.
 
For really low bass, how about the lowest note physically possible - the Big Bang cycle?
 
Ahh, so what the article is calling a "sound wave" is creted by pulsations of the jet of material being fired out of the hole.

I was trying to picture what medium the sound was propagating through, the best I could come up with was the disk of material getting drawn into the hole.

I had not considered that there was enough matter ejected by the black hole to be detectable at this distance. Comologists must have some realy neat toys to pick up on this.
 
Sundog said:


At least it explains that low, low B flat I hear all the time.
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Err, they can tell the frequency from this black hole because they can see it transversly...

But for you to HEAR it in the time domain, well, you'd need to be alive for an entire cycle to start to resolve the frequency.

You an immortal, by any chance? :D
 
jj said:


Err, they can tell the frequency from this black hole because they can see it transversly...

But for you to HEAR it in the time domain, well, you'd need to be alive for an entire cycle to start to resolve the frequency.

You an immortal, by any chance? :D
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Joke. I should have said so. I'm surprised Bill didn't nominate that yet; perhaps he's hidden in some sort of rolled-up extra dimensions.

Just assume that when I spout nonsense, I'm kidding. It will be a great smokescreen for the nonsense I actually spout. ;)
 
I dont know for certain, but I would think if the blackhole is producing any "sound", its just a laymens term to describe something along the lines of:
The black hole is emitting EM radiation in the form of X-rays (spinning gas creates friction and heats up allowing it to emit X-ray radiation... but I'm not sure how far it would travel and it doesnt penetrate the atmosphere very easily). The radiation would be detectable by "detection thingies", then translated into audible sound for humans to enjoy (Its the same process a radio uses).
 
Sundog said:


Joke. I should have said so. I'm surprised Bill didn't nominate that yet; perhaps he's hidden in some sort of rolled-up extra dimensions.

Just assume that when I spout nonsense, I'm kidding. It will be a great smokescreen for the nonsense I actually spout. ;)
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Oh, I know it was a joke.... I was wondering what you'd do with the Immortal thing, Duncan McCloud and all that :)
 
jj said:


Oh, I know it was a joke.... I was wondering what you'd do with the Immortal thing, Duncan McCloud and all that :)
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I figured you did. How small an mp3 could you compress that wav into?

It gives me an idea for a song but I'm going to have to seriously detune my low B string.
 
Yahweh said:
I dont know for certain, but I would think if the blackhole is producing any "sound", its just a laymens term to describe something along the lines of:
The black hole is emitting EM radiation in the form of X-rays (spinning gas creates friction and heats up allowing it to emit X-ray radiation... but I'm not sure how far it would travel and it doesnt penetrate the atmosphere very easily). The radiation would be detectable by "detection thingies", then translated into audible sound for humans to enjoy (Its the same process a radio uses).
Click to expand...

No, they are talking about a gas-propagated shock-wave of monstrous proportions...
 
The black hole is emitting EM radiation in the form of X-rays (spinning gas creates friction and heats up allowing it to emit X-ray radiation... but I'm not sure how far it would travel and it doesnt penetrate the atmosphere very easily).
Click to expand...

You could say that, or you could say 'sound'.
 
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