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Hawking Radiation Mimicked In the Lab
Annanag writes *Nothing* escapes a black hole, right? Except 40 years ago Stephen Hawking threw a spanner in the works by suggesting that, courtesy of quantum mechanics, some light particles can actually break free of a black hole's massive pull. Then you have the tantalizing question of whether information can also escape, encoded in that so-called 'Hawking radiation'. The only problem being that no one has ever been able to detect Hawking radiation being emitted from a black hole. BUT a physicist has now come closer than ever before to creating an imitation of a black hole event horizon in the lab, opening up a potential avenue for investigating Hawking radiation and exploring how quantum mechanics and general relativity might be brought together.
"I'm having a hard time grasping how sound waves can behave like subatomic particles in this way."
It's done with phonons -- quantised fluctuations that in the classical limit are sound waves.
"Sound is a wave through some medium, so how can they pop into existence in a vacuum? Are particles of some kind (and what are they? Hydrogen atoms? Helium?) popping into existence long enough for them to physically interact with one another so a physical wave can propagate from one particle to another before they pop back out of existence, and thus "sound waves" are appearing?"
No, it's literally that pairs of phonons can be produced from the "sound vacuum" in the same way that pairs of photons can be produced in a normal vacuum. If you like, you can think of it as quantised shifts in the structure of the quantum fluid (superfluid helium or a Bose-Einstein condensate, or what have you) generating these phonons. It's not, strictly speaking, true but at least it's a physical picture.
"Seems the signal to noise ratio would be pretty bad."
I'm a long time out of this field -- I did my Masters in acoustic holes but that was a long time back -- but I'd also expect signal to noise to be pretty lousy. However, any signal at all would be awesome.
It tells us how horizons behave. The production of Hawking radiation in a gravitational black hole relies (and relies only) on the presence of a horizon. In an acoustic hole, we've got a horizon for phonons, rather than for photons, but it's still a horizon. The actual structure of the geometry in the simplest cases is Schwarzschild, but one can play some interesting games to get a more complicated setup which is more usable - and in any event, it also exhibits a horizon. Therefore, while the effective field theory describing the phonons holds, and while the system exhibits a horizon, any observation of Hawking radiation will directly test the processes by which we believe Hawking radiation is produced in actual gravitational holes. It's based on basically the same physics. What it *won't* tell us is anything about the backreaction of Hawking radiation on a gravitational hole, because while the kinematics of an analogue hole are the same as a gravitational hole (at an unperturbed level), the dynamics are completely different. Further, the system will eventually produce enough Hawking radiation that the condensate will be depleted to an extent that the analogy is no longer valid even at a background level. However, while the analogy is valid -- and that can be quantified and therefore controlled -- we can still exploit it. And when the analogy isn't valid we're still learning useful things about the behaviour of supercold fluids.
When it comes to the derivation of Hawking radiation, surprisingly, yes. It relies on there being a quantum vacuum for some type of particle (in gravitational radiation, photons; in the analogue case, phonons), and it relies on there being a horizon (in the gravitational case this is an event horizon; in the analogue case it's an acoustic horizon). It also relies on the analogue medium producing phonons of the right form -- a form where (in the appropriate regime) the phonons have a quantum theory that acts like photons do. In particular, you have to have a medium where the phonons have a "squeezer" Hamiltonian in the vicinity of the acoustic hole, meaning that pair production will happen. The derivation of Hawking radiation from this point takes the same form in both a gravitational hole and an acoustic hole. Of course, when the conditions change, as they inevitably will, the analogy breaks down but in its regimes of validity there's no issue, and we can quantify the extent to which the analogy is holding.
One thing is off in your explanation: it is not anti-matter that falls into the black hole. Anti-matter still has positive net energy (âoeweightâ), so throwing antimatter into a black hole will make it more massive, not less massive. The actual idea is that a of a negative-energy particle (the total energy of the virtual pair is zero, so if one particle becomes real, with positive energy, the other had to have negative net energy) falling in, *not* a matter anti-particle (like a positron). Quoth wikipedia:
Antiparticles should not be confused with virtual particles or virtual antiparticles.
The equilibrium is not maintained because if a virtual particle outside the event horizon becomes real, it will always end up having positive net energy: real negative-energy particles do not exist.
I think "vacuum" in the article means the Bose-Einstein condensate with no phonons, which is the analog to the true vacuum and not actually a vacuum.