Holes of Silence

By Sean Carroll | June 12, 2009 11:39 am

Black holes are black because you can’t go faster than the speed of light. So what about the speed of sound?

Of course there is no problem in having something go faster than sound, but sound waves themselves are stuck with that speed limit. That fairly elementary fact inspired Bill Unruh years back to propose a clever idea: a black hole that you could make in the laboratory, but using sound rather than light. He called them dumb holes, although I’m not sure people get the right idea when they hear that name.

I used to think that this was an amusing thought experiment, but was believed to be unrealistic to actually attempt. But now Lahav et al. have apparently done it! (Via Swans on Tea and arXiv blog.)

A sonic black hole in a density-inverted Bose-Einstein condensate
Authors: O. Lahav, A. Itah, A. Blumkin, C. Gordon, J. Steinhauer

Abstract: We have created the analogue of a black hole in a Bose-Einstein condensate. In this sonic black hole, sound waves, rather than light waves, cannot escape the event horizon. The black hole is realized via a counterintuitive density inversion, in which an attractive potential repels the atoms. This allows for measured flow speeds which cross and exceed the speed of sound by an order of magnitude. The Landau critical velocity is therefore surpassed. The point where the flow speed equals the speed of sound is the event horizon. The effective gravity is determined from the profiles of the velocity and speed of sound.

The idea is simply that you get a fluid flowing faster than its speed of sound in some region, so that the sound waves cannot escape the “horizon” bounding that region. (The flow speed has to change within the material; taking a balloon full of air and putting it on a supersonic jet doesn’t count.)

But the reason this could some day be very exciting is when quantum mechanics gets into the game. Just like black holes, dumb holes should have “Hawking radiation” — but instead of particles, the holes should emit quantized sound waves (conventionally known as “phonons”). That would be very interesting to observe, although the experimental state of the art isn’t there yet.

To be clear, we wouldn’t be learning much about quantum gravity if we observed Hawking phonons from dumb holes. The underlying physics is still that of atoms (and, in this case, a Bose-Einstein condensate), not that of general relativity. Indeed, one of Unruh’s original motivations was to show that the physics on small scales didn’t affect the prediction of Hawking radiation. So the prediction of Hawking phonons should be rock-solid, no matter how little we know about quantum gravity. Still, it would be very cool.

  • Doug

    There’s also the old idea that if you get a massive enough star undergoing core collapse, the collapse speed of the material around the core could be faster than the speed of the rebound pressure wave, preventing a supernova.

  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    Note that this is Ori Lahav, not the famous astronomer Ofer Lahav.

    I’m sure Simon D. M. White would be amused. Yes, there’s another astronomer
    Simon. D. White without the M. Thus, Simon White’s use of two middle initials
    is justified to avoid confusion. (I wonder if the M. really stands for a third “first name”
    or was just invented to make literature references unambiguous, possibly with a nod
    to “dark matter” since Simon White was quite important in pointing out the need for
    dark matter in cosmology from a theoretical perspective.)

  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    By coincidence, this object (dumb hole) was mentioned in s.p.r recently:


  • rob

    I’ve always thought “mute hole” had a better ring to it, and it’s certainly more PC.

  • Ellipsis

    It also might be interesting to collide/merge two of them. (I wonder how one of these holes would interact with a second one.) Would detectable sound waves be emitted?

  • Kernal

    Experimental Black Hole models have also been investigated by using surface plasmons on a sheet with some irregularity (drop of some liquid or a peak, etc). The best part is that punching a hole in the sheet then mimics a wormhole, connecting the otherwise separated sides of the sheet.

  • Low Math, Meekly Interacting

    The Hawking radiation angle makes me wonder (not that I expect the answers have been worked out):

    Unlike Hawking radiation, which propagates in a vacuum, phonons would need some medium (presumably the condensate itself). I guess that means, unlike with relativity, phonons exist in an “ether” or a fixed frame of reference. Does that detract from the relevance of the physical analogy?

    Black holes have to get very small before they’re hot enough to detect. Presumably the “temperature” (or should I say loudness?) of a dumb hole increases as the size decreases as well. How small would the hole have to get before it started emitting detectable amounts of phonons? How big would those phonons be in relation to the hole? Could you get a phonon emitted with a wavelength bigger than the hole? That would be pretty wild. Since the hole is made of (highly smeared) ions or atoms, does granularity become a problem as the hole shrinks to the point where it emits prodigious Hawking radiation?

    Very cool stuff…

  • Clerk

    Wow… what a cool concept! I once heard Hawking make a joke on tv along the lines of. The discovery of Hawking radiation would surely win him the Noble Prize, but we will never get that close to a black hole to observe it, guaranteeing him to never win the prize. If this does progress and they do in fact observe Hawking radiation, will he get it?! …Wow… what a cool concept!

  • http://lablemminglounge.blogspot.com/ Lab Lemming

    If you submit these results to Nature, would they make a sound?

  • Fermi-Walker Public Transport

    Lab Lemming @9 Only if they are also submitted to the ArXiv preprint server.

  • http://crowlspace.com/ Adam

    Does the interior geometry of a Mute Hole form a Cone of Silence as it collapses towards an acoustic Singularity?

    Had to chuck in that reference. But as for ‘ether’ there was a “Scientific American” article about the prospects for acoustic black-holes and how they might imply an ‘ether’ for space-time structure after all. But it won’t be a 19th Century style ether – it’ll respect Lorentz invariance for starters.

  • brian

    how do we get one of these dumb holes into Fox New’s studio?

  • Gavin Polhemus

    Anechoic hole” is what I’d call it.

    Brian, It is fairly easy to produce one of these in a Fox News studio. We need to place a nozzle in the studio and then suck air out of the studio a super sonic speed. As air approaches the nozzle it will accelerate past the speed of sound. The place where the flow becomes super sonic is the horizon of the anechoic hole (dumb hole, whatever). As long as the commentator is within the horizon, the desired affect will be achieved.

    Of course, that is just the theory. It still needs experimental verification. I hope you can find funding to do this valuable research.

  • http://www.americafree.tv Marshall Eubanks

    I think that the proper analogue for black hole would be “silent hole.” It’s not that it’s not speaking, it’s that it can’t be heard.

  • Fourteener

    Is this really, truly, an example of sound science?

    In order to actually read the post, I had to wait for a Templeton Foundation ad to get out of the way. Folk wisdom has often noted that he who pays the piper calls the tune.

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  • tachyon

    “Of course there is no problem in having something go faster than sound, but sound waves themselves are stuck with that speed limit.”
    “Of course there is no problem in having something go faster than light but light waves themselves are stuck with that speed limit.”
    Might we hope?

  • pi

    interesting, fact is they did one in haifa and didn’t have hawking radiation, neither evaporated, and it was done only with 10.000 atoms in
    bosonic state. now they are going to mass strong force quarks at cern
    hundreds of thousands. If they become a boson, their strong force, 100
    times stronger than the electroweak force at haifa, will create a sucking hole 100^3 times faster in its absorbition. Speed of sound x 100^3=speed
    of light. Thus a quark hole is actually a black hole and will be created
    at cern and will not evaporate.
    interesting haifa has proved that a boson quark hole will be a black hole
    and it will not evaporate and it will be made at cern
    interesting nobody has thought of this at cern?

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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .


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