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?"

Slightly OT: Unruh effect

[TeknoHog]

My first thought from the headline was Unruh effect [wikipedia.org]. It's a kind of Hawking radiation you can get in a particle accelerator. It just happens that with black holes, the acceleration is due to gravity, but other sources of acceleration also work. There are huge decelerations from c to nearly 0 at heavy ion collisions, for example.

I first heard of the effect when some fellow physicists were considering the idea of tiny black holes created in particle physics experiments. It turned out that the presence of Hawking-like radiation doesn't necessarily mean a black hole.

Well, it also turns out that this has nothing directly to do with the article, but might be +i, interesting nevertheless.

Re:Thinking in circles anyone?

[Chris Burke]

You have a theory how the world behaves. You do a numerical simulation based on that theory, and amazingly, it proves true.

Well that's not exactly the case. We have a theory of how the world behaves, and Hawking Radiation is a predicted emergent property of that theory. It's not an axiom, it's a predicted consequence, so it isn't a given based on the theory. Here, we have a detailed simulation that shows that yes, if the underlying theory is correct, then we should expect to see Hawking Radiation.

It is true that this is in no way a real-world observation that shows that the theory accurately models reality. However it does have a non-trivial and non-circular implication for our theory.

Re:Black holes should radiate anyway

[harryjohnston]

Not quite my understanding. Positron-electron pairs don't get created spontaneously as this would violate the conservation of energy. However, a pair of what are called "virtual" particles can appear spontaneously if one of them has a negative mass.

Now, a negative mass particle can't normally exist for very long, so it has to recombine in short order with the original particle and they cancel each other out.

However if the negative mass particle is trapped by the event horizon, "not very long" gets stretched out indefinitely by the time dilation, and the positive mass particle can escape. The total mass of the black hole goes down because the particle entering it has a negative mass.

Of course, this is all just a way of visualizing what goes on so that it seems to make sense. It doesn't necessarily correspond in any meaningful way to reality.