First Acoustic Black Hole Created 165
KentuckyFC writes "One of the many curious properties of Bose Einstein Condensates (BECs) is that the flow of sound through them is governed by the same equations that describe how light is bent by a gravitational field. Now, a group of Israeli physicists have exploited this idea to create an acoustic black hole in a BEC. The team created a supersonic flow of atoms within the BEC, a flow that prevents any phonon caught in it from making headway. The region where the flow changes from subsonic to supersonic is an event horizon, because any phonon unlucky enough to stray into the supersonic region can never escape. The real prize is not the acoustic black hole itself but what it makes possible: the first observation of Hawking radiation. Quantum mechanics predicts that pairs of phonons with opposite momentum ought to be constantly springing in and out of existence in a BEC. Were one of the pair to stray across the event horizon into the supersonic region, it could never escape. However, the other would be free to go on its way. This stream of phononic radiation away from an acoustic black hole would be the first observation of Hawking radiation. The team hasn't gotten that far yet, but it can't be long now before either they or their numerous competitors make this leap."
Doppler Shift? (Score:1, Interesting)
I think it would be interesting to see a 3D graph of the magnitude of a 'chirp' sent across the "black hole" and compare it with doppler shift equations, and then overlay that with a graph of purposed light behaviors about a black hole. Experimental proof vs mathematical?
maybe we can use this to... (Score:3, Interesting)
Not the first Acoustic Black Hole (Score:5, Interesting)
www.iop.org/EJ/article/1742-6596/8/1/013/jpconf5_8_013.pdf
The argument basically goes that when you unplug your bathtub, there's a certain point at which waves generated past the "event horizon" near the hole never escape the hole. It's an interesting read, but I was under the impression that this is basically the same thing, albeit not an effect that arises from quantum field theory.
Weaponize it and use it against car stereos. (Score:3, Interesting)
Re:Sort of Hawking Radiation (Score:5, Interesting)
That's somewhere in between a metaphor for Hawking Radiation and the real thing.
Not a physicist, but here's how I think the metaphor between the experiment and the real thing is supposed to work:
Speed of light: maximum speed information can travel through a vacuum ("the void")
Speed of sound: maximum speed information can travel through a medium composed of atoms ("substance")
(When aircraft go supersonic, the air they run into is incapable of "preparing" to be hit, in a manner of speaking...)
We can't create stuff that goes faster than the speed of light, but we can create stuff that goes faster than the speed of sound. And just as you can't go fast enough to come back through an event horizon, information can't propagate fast enough in the experiment to go back across the subsonic/supersonic boundary. This shows us what it looks like to be in a situation like that of a black hole.
By the way, there's a similar, cheaper experiment you can do: pop a hole in a pressurized container. The gas cannot escape it (at the outlet) faster than the local speed of sound, which is obtained whenever the ratio of pressure inside to pressure outside exceeds a critical value. One gas dynamics professor said I can think of it like this: "even though a higher pressure ratio creates a greater pressure potential difference, the gas inside the tank cannot 'learn' of the greater difference because that would require information to go *into* the tank, *against* the gas that is escaping at the speed of sound"
Kind of like in the setup described in the article...
Re:Not Hawking Radiation (Score:4, Interesting)
Re:Sort of Hawking Radiation (Score:3, Interesting)
That's somewhere in between a metaphor for Hawking Radiation and the real thing.
This isn't really a metaphor exactly. If the equations governing two systems are the same, then we expect the behavior to be the same, and we can describe them in the same terms. Phonons themselves are a good example of this: a phonon is hardly the sort of thing that you would intuitively think of as a particle, but because the equations governing phonons are the same as those governing quantum mechanical particles, physicists describe phonons as particles. Subatomic particles themselves bear very little resemblance to the 'billiard ball' particles that most people imagine. I think that it would be better to say that Hawking radiation is just an effect predicted for systems obeying certain equations, and in that sense, both the acoustic and traditional black holes exhibit completely real Hawking Radiation.
It is true that getting 'acoustic Hawking radiation' wouldn't constitute absolute proof that Black Holes do the same thing - our model may be wrong. What it will do do is provide proof that, assuming our model is correct, Hawking radiation is real, and there isn't some unanticipated effect which invalidates the theory.
I take your point, and you may easily have more expertise than I do (non-specialist grad quantum mechanics classes and a couple undergrad astro classes along with some casual enthusiasm for the subject). My understanding of Hawking radiation is that the split virtual pair explanation isn't physically accurate, but that tunneling of particles through the event horizon is the more physically valid explanation.
1) I'm not aware of an analogous effect that will work for phonons. Tunneling itself is on the wrong lengthscale.
2) Since the virtual particle pair splitting explanation also satisfies the radiation equations, maybe this difference doesn't have much significance. Were someone to convince me of this, I'd fully agree with you.
Re:Sort of Hawking Radiation (Score:4, Interesting)
Arguably, light travels faster in a Casimir cavity than in a vacuum. Really, there's no reason to suppose that "emtpy space" represents the medium through which light flows the fastest, merely that it's somewhere close.
what would happen... (Score:3, Interesting)
we all know photon pairs are connected. an observation on changes the state of the other. (quantum entanglement) but what happens if one of a pair of photons enters a black hole and the other remains outside?
does quantum entanglement still exist fo these photons? does the photon still exist inside the black hole or does it disintegrate or change state? if so: what would happen to the other photon outside?
i call whatever will happen the Fuzzums effect ;)