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Hawking radiation in the lab

A highlight of the recent NZ Institute of Physics conference was the Dan Walls medal talk given by Matt Visser. Matt has been working on general relativity. That's not desparately unusual for a physicist, but Matt has been successful in working on some of the crazier aspects of relativity and getting it published - wormholes, dumb holes and the like. He gave an entertaining talk - perfect for closing the conference.

I was particularly taken by the description of the analogies between light and sound. It's unsurprising that there should be analogies between the physics of light and the physics of sound in that both are waves, but the extent to which the analogy can go surprised me. For example, it is possible to get Hawking radiation with sound. 

Hawking radiation is predicted to be radiated from black holes. I say 'predicted' because experimental evidence is still scant. It allows black holes to 'evaporate' by emitting radiation from their event horizons (Within the event horizon nothing escapes the black hole - not even light. Once you've passed that boundary, you have a one-way ticket to a singularity). There's an analogy between the event horizon of the black hole and an acoustic shock-front (sonic boom) created by an object moving faster than sound. In the case of the former, once you are past the event horizon you can't get back out, and in the case of the latter, it's not possible for a perturbation that occurs behind the shock front to have an effect in front of it - in order to do so it would need to go faster than sound. 

It turns out that many of the equations governing the situations are similar, including those necessary to produce Hawking radiation. The implication is that one should be able to create Hawking radiation from shock fronts created with supersonic fluid flow. And indeed it has been done - what one might consider an effect of general relativity demonstrated in a fairly simple lab experiment. Quite beautiful. Black holes (well, OK, certain aspects of them) on your lab bench.


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