First "Observation" of Hawking Radiation

KentuckyFC writes "Italian physicists are claiming the first observation of Hawking radiation, but not from a black hole. Instead they've spotted it streaming from a sonic horizon in a Bose Einstein Condensate (abstract on the arXiv). That's consistent with previous predictions but they're claiming the 'first' even though the experiment was only a numerical simulation. Does that really count?"

Re:Black holes should radiate anyway

[ExecutorElassus]

yes, but Hawking radiation is somewhat different, and requires no material in the vicinity. It works like this: there are always opposite particles (say, positron-electron pairs) that spontaneously appear at the subatomic level. But in the normal universe, they immediately re-collide, giving back to the local space whatever energy was used to create them. That's a bad explanation (because I'm not a physicist), but it gives a rough picture. At the edge of an event horizon, however, there is a small - though nonzero - chance that one or the other particle will get sucked down the gravity well before it can remerge with its opposite. Thus the one that survived ambles off into space, no doubt pondering its cosmic parthenogenesis. The energy of the particle is - for reasons unknown to me - taken from the black hole. Also, this process steadily accelerates as the black hole continues to lose mass through the process: it eventually pops out of existence in a burst of gamma radiation.

So, it's a little more complicated/interesting than you described; I'm sure it would be even better if someone here could describe it from an actual background in physics, instead of the armchair variety I can muster.

Re:Doesn't Count

[Beetle B.]

but you may argue that they have produced evidence that supports the theory.

No, you can't.

Without knowing the details of both theories, it's hard for me to judge. Basically, if their formalism is more or less isomorphic to Hawking's (without their realizing it) - then all they've done is do Hawking's work over again.

If they used independent formalism to get Hawking radiation, then it's a good sign, and shows that their theory is consistent with Hawking's (and perhaps later someone will link the two).

In either case, they did not produce any evidence. At best, they're saying, "If you look at this our way, it is consistent with what Hawking predicted."

Story is distorted (what a surprise)

[Doug Merritt]

The story is grossly distorted -- what a surprise. I was going to say that at least it was distorted by author of the linked-to news item, rather than by the /. submitter, but now I see they seem to be the same person ("KFC" and "KentuckyFC").

The abstract that is linked to merely claims "numerical evidence", not "first observation", and to get from that unobjectionable claim to the more sensational false accusation, one must distort the paper itself ( http://arxiv.org/pdf/0803.0507v1 [arxiv.org] ), which says:

...a recent work [11] has anticipated that the presence of Hawking radiation in this setting can be unambiguosly revealed from a very peculiar feature of the correlation function of density fluctuations.

Here we report numerical experiments that nicely confirm this prediction. Differently from most previous works on analog models [6], our calculations are based on the application of microscopic many-body techniques to an experimentally realistic system and never involve concepts of gravitational physics.

In this way, our observations can be considered as a first independent proof of the existence of Hawking radiation and rule out the frequent concerns on the role of short wavelength, "trans-Planckian" physics on the Hawking emission.

So for one thing, they never claimed "first observation", they said "first independent proof", which is sharply different.

For another thing, they softened even that claim; they said "our observations [of the simulation] can be considered" proof, not that it is proof.

At any rate, it's interesting in general; they're talking about predictions that Hawking-Unruh radiation might be found in many settings unrelated to domains involving gravity or acceleration, and that their simulation might be an independent confirmation of those predictions.

Re:Black holes should radiate anyway

[Anguirel]

The energy comes from the black hole because the mass comes from the black hole.

The easiest way to conceive of it, in very basic terms, is that the Electron/Positron pair spontaneously converts to mass from the energy surrounding the black hole. The positron falls into the hole, and annihilates with an electron's worth of mass already in the singularity. The electron from the initial pair escapes. The black hole has been reduced in mass/energy by the amount of one electron.

If the electron, instead, falls into the hole, the positron escaping will annihilate with an electron being pulled toward the hole (probably) and release a burst of energy, leaving a net gain of no mass for the black hole as a particle that would have added to it no longer reaches that point.