Ultrafast Laser Pulse Makes Desktop Black Hole Glow
A desktop black hole created in a lab in Italy has been shown to emit light, a discovery that could seal one of the biggest holes in theoretical physics and pave the way for physicist Stephen Hawking to win a Nobel prize.
The eerie glow is called Hawking radiation, and physicists have been hunting it for decades. In 1974, Stephen Hawking calculated that, rather than gobbling up everything in their path and giving nothing back, black holes can radiate like the heating element in a toaster.
But astrophysical black holes, the ultradense gobs of mass that lurk at the centers of galaxies and are left behind when stars collapse, radiate too dimly to be seen. So instead of looking at real black holes, a group of physicists led by Francesco Belgiorno of the University of Milan, Italy, created a miniature analog by shooting short pulses of intense laser light into a chip of glass. The results will appear in Physical Review Letters.
“This is an extremely important paper,” said physicist Ulf Leonhardt of the University of St Andrews in Scotland, who built an artificial black hole in a phone line in 2008. “The experiment confirms that Hawking radiation can exist in principle.”
The basic idea behind Hawking radiation is that the quantum vacuum is not actually empty. Instead, it is a roiling mess of virtual particles and anti-particles that constantly pop into existence and eliminate each other when they meet. If one member of the particle/anti-particle pair is created on the wrong side of an event horizon — the edge of a black hole beyond which not even light can escape — the particles can never meet to destroy each other. An observer outside the black hole would see a perpetual stream of real particles.
But until now, no one had seen any evidence of these particles. Radiation from a black hole the mass of our sun would be 10 million times colder than the cosmic microwave background radiation, the ambient temperature of the universe left over from the Big Bang, which itself is only a few degrees warmer than absolute zero. Larger black holes would be colder still.
Luckily, conceptual counterparts to black holes and their event horizons are not hard to come by. In the 1980s, two physicists independently suggested this thought experiment: Picture a black hole as a river that flows faster and faster as it approaches a waterfall. Fleet-finned fish headed upstream can escape the falls, but at a certain point the water flows faster than the fish can swim. Any hapless fish caught behind that point are doomed to flop backwards over the falls. Replacing fish with light and the river with gravity yields a good simulation of a black hole.
Replace the fish with any other wave and the river with any fluid moving faster than that wave, and the likeness goes deeper. Physicists have found that the math describing light moving in the warped space-time geometry around a black hole is exactly the same as the math describing waves flowing through moving fluids. The analogy works for white holes, theoretical objects where nothing can get in rather than out, as well. And mathematically, Hawking radiation doesn’t need gravity or curved space-time at all. It just needs an event horizon.
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