First Acoustic Black Hole Created

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."

Comments

Sort of Hawking Radiation

[Colonel Korn]

That's somewhere in between a metaphor for Hawking Radiation and the real thing. It's not true HR, but it would be a nice demonstration if they were to get it to work, especially if they could show some sort of analog to black hole "evaporation," which is the main implication of HR. I suppose that should naturally happen as the separation of the pairs sucks energy from the BEC and slows the fluid inside, shrinking the event-horizon-analogue.

Also, let's get properly flowing BEC layers in our noise canceling headphones!

Re:Sort of Hawking Radiation

[blincoln]

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.

We can't create stuff that goes faster than the speed of light in a vacuum. We create things that travel faster than the speed of light in other media all the time. The blue Cherenkov Radiation glow in fission reactors is caused by particles exceeding the speed of light in water, and creating a light shockwave analogous to the sound shockwave that e.g. supersonic aircraft produce.

/nitpick

Re:Sort of Hawking Radiation

[clone53421]

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"

I really don't like that explanation... it makes it seem like the pressure differential is "known" to the gas inside the cylinder via some sort of acoustically-transmitted information. My initial reaction was "HUH" and my secondary reaction was "ok, I don't buy that."

After a little work on Google, I discovered that the effect really exists, but I think this link describes it better (emphasis mine):

However, once the downstream pressure reaches or is less than the critical pressure, the compressible mass flow rate does not increase even though the downstream pressure is decreased further.

This specific phenomenon is called as "choked flow", and the energy difference between the choked exit and ambient conditions is dissipated by shock wave and/or turbulence.

Also, choked flow does not occur for thin-plate orifices, which wouldn't make sense relative to the explanation as given by that prof. If the flow was really being limited by the non-propagation of information against a gas flowing at the speed of sound, the thickness of the plate shouldn't matter.

Not Hawking Radiation

[Big_Breaker]

There is an analogy there in the macro physics but that doesn't mean the small scale stuff like QM will be mirrored.

You can model gravity in the orbital mechanics sense with a marble and vertical cone that tapers at 1/square(height). That doesn't mean it will do anything relativistic or quantum mechanical.

Re:From the tone of the description

[ShieldW0lf]

Here, I'll fix it.

The real prize is not the acoustic black hole itself but what it might makes possible: the first observation of something analogous to Hawking radiation. The Theory of Quantum mechanics predicts that pairs of phonons with opposite momentum ought to be constantly springing in and out of existence in a BEC. Theoretically, 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 anything even vaguely resembling Hawking radiation. The team hasn't actually achieved any of this though. It can't be long now before they or their numerous competitors make this leap. Unless their hypothesis is totally wrong, which is entirely possible. But that doesn't sound particularly impressive, so we'll just forget about that bit...

Re:Phonon ey?

[Chris Mattern]

Everything is quantized if you're looking at it at a small enough scale.