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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: 2) by hubie on Wednesday October 29 2014, @09:46PM
Excellent response, thank you.
Is that true in general, or do you need to have an inner boundary (the "white hole") to establish the proper dispersion relation to get the lasing effect as described in Steinhauer's paper? Is the lasing effect required to produce Hawking radiation, or is it just an effect to make it easier to detect?
(Score: 1) by boristhespider on Wednesday October 29 2014, @10:12PM
Certainly if you want to do it with a fluid system, you inevitably get the white hole if you want the black hole -- even on a classical level that's clear, since there's no physical way we can get something that moves faster and faster than the speed of sound and never slows back down again. (Failing anything else, we get a hard boundary at the edge of the fluid where the speed of sound is identically zero, so it *has* to pass back through the speed of sound, even if in a thin layer.) The dispersion relationship is more a factor of the actual type of fluid you're considering -- for superfluid Helium, for instance, it breaks down at the roton dip no matter what games you might play with the background setup -- but it's certainly true you need to ensure that if your dispersion relationship is only valid in a certain range of wavelengths that you can actually *contain* those wavelengths...
The details of Steinhauer's setup I'm afraid I can't comment on. I did my Masters on acoustic holes, but I've been twelve years out of this field so while I'm not strictly a layman in the area, especially since I've been in gravitational physics much of the rest of the time, I'm also not quite an expert.
(Score: 1) by boristhespider on Wednesday October 29 2014, @10:15PM
Also, sorry, I didn't quite answer your question. To get Hawking radiation all you need (in theory) is the presence of a horizon, and a source of (quasi-)particles. In reality, you're not going to have much luck in a lab setting up a black hole horizon without the white hole. If nothing else, the sheer focus of sound waves at the centre of your acoustic hole would obliterate your system pretty quickly.
(Score: 2) by hubie on Friday October 31 2014, @02:00AM
Thanks again. I did my research in cosmic rays, so I never had to worry about the particles until they got to our solar system. :)