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If there is a “holy grail” to be found in modern astrophysics, it probably has something to do with finding out what’s going on inside of black holes. Since no light escapes from their event horizons, studying their insides directly is impossible. As if that wasn’t bad enough, our best theories tend to break down inside the event horizon, limiting our ability to study them even theoretically with present models.
Despite all that, there are ways to get at the behavior of black holes. A recent line of work is approaching the problem in a different way—by analogy. Rather than trying to observe real black holes or trying to simulate them mathematically, researchers are constructing analogs of black holes. These constructions can be observed in a lab, right here on Earth.
Of course, scientists have no way of creating an actual gravitational singularity on a table-top, so they had to rely on the next best thing. The essence of a black hole is that it has an event horizon—a point of no return from which no light can escape. By analogy, in a fluid, there can be a point of no return for sound waves. If, for example, the fluid is moving faster than the speed of sound, no sound can outrun the fluid to escape in the opposite direction. That’s the basic idea behind a new experiment published in the journal Nature Physics (abstract) —an experiment that apparently makes a Hawking radiation laser out of a sonic black hole.
[Additional Coverage]: http://www.universetoday.com/115307/hawking-radiation-replicated-in-a-laboratory/
(Score: 1) by boristhespider on Wednesday October 29 2014, @08:25PM
No. This is a direct analogue. So long as you stay within the regime of the approximation, this is an exact analogy of a gravitational black hole. To get the setup working you need a fluid that is irrotational, inviscid and barotropic -- which the water swirling around the plug in your sink is absolutely not. You also need to ensure there are no other sources of noise, literally, since the analogue hole is mapped out by phonons, which are quantised sound waves, and there must be *no* other sources of noise, something which is only possible in a quantum fluid such as the Bose-Einstein condensate used here. BECs are practically the only fluid we've got where you can play this trick, since you also need the right dispersion relationship and other relatively easily-produced quantum fluids, such as liquid Helium, have totally knackered dispersion relations.
If anyone's interested, I made a few posts on the other site when this was discussed there (as AC since I was at work):
http://science.slashdot.org/comments.pl?sid=5820855&cid=48130377 [slashdot.org]
http://science.slashdot.org/comments.pl?sid=5820855&cid=48130453 [slashdot.org]
http://science.slashdot.org/comments.pl?sid=5820855&cid=48130667 [slashdot.org]
http://science.slashdot.org/comments.pl?sid=5820855&cid=48133843 [slashdot.org]
This one wasn't me: http://science.slashdot.org/comments.pl?sid=5820855&cid=48220377 [slashdot.org] but it's an interesting post.
(Score: 1) by boristhespider on Wednesday October 29 2014, @10:23PM
(I'd like to quantify the word "exact" slightly. It's an exact analogue of a gravitational hole, so long as we're working on a background level, which is applicable for Hawking radiation until the point at which the backreaction of the loss of energy to Hawking radiation on the hole becomes significant. That's certainly important for the analogue holes, because it could fairly strongly limit the time for which the analogue hole is a good approximation before the radiation has depleted the condensate to the level that the approximation breaks down. The same does happen for a gravitational hole but because the scales are so different it takes a hell of a lot longer unless we're looking at femtometre scale holes.
As soon as we look at perturbations on the hole, the analogy breaks down absolutely. So we can't, for instance, model the merging of two Schwarzschild holes by flinging two acoustic holes at one another. This is because while we've mimicked the kinematics of the background spacetime -- the geometry, basically -- the *dynamics* of the two systems are wildly different. An acoustic hole is obeying non-relativistic quantum physics or, if you're working with a speculative fluid, it's obeying absolutely Newtonian physics. A gravitational hole is obeying the Einstein equations, and these are obviously rather different. So while the analogy is perfect for a particular regime, we have to be careful that we don't leave that regime.)