It was one of Stephen Hawking’s finest insights: the 1974 prediction that black holes are not totally black, but emit a steady stream of radiation. Experimental confirmation of Hawking radiation would probably bring the 68 year-old British cosmologist a Nobel Prize in Physics. Unfortunately, no-one has been able to detect a black-hole signal because it would be so faint compared with the universe’s background radiation.
However Hawking’s chances at a Nobel may be rising, thanks to a paper that will soon be published in the journal Physical Review Letters. In this work, Italian physicists describe what many believe to be the first measurement of Hawking radiation from a black hole “analogue” in the lab.
The research has ignited a debate over what truly constitutes Hawking radiation, and whether lab-based evidence could help make Hawking a serious contender for a Nobel prize.
“We don’t have any observational evidence from astrophysical black holes regarding the existence of the Hawking effect, and it is extremely unlikely that we will ever have such evidence, so any way of verifying Hawking’s prediction is of tremendous importance to the scientific community,” says Matt Visser, an expert in gravitational analogues at the Victoria University of Wellington, New Zealand, who was not involved with the research.
Origins in quantum mechanics
Hawking’s theory stemmed from the uncertainty principle in quantum mechanics, which tells us that pairs of particles are continually popping into existence, even in a vacuum. Most of the time these particles annihilate one another almost as soon as they are born, but this would not be true at the edge of a black hole, known as the event horizon, where gravity becomes so strong not even light can escape. If a particle pair is born straddling this point, one particle would have to be sucked in while the other would escape – and this latter one would become Hawking radiation.
Any way of verifying Hawking’s prediction is of tremendous importance to the scientific community. Matt Visser, Victoria University of Wellington
Because Hawking radiation is currently impossible to observe for real black holes, physicists have recently been looking to black hole analogues in the lab that can mimic the behaviour of their astrophysical counterparts. One type of analogue employs lasers to simulate an event horizon, because intense light can alter a medium’s refractive index, which governs light propagation speed. In simple terms, shining a powerful laser through glass creates a refractive index peak: any other photons in front this peak can travel forward, while those behind and trying to travel forward are slowed to a halt – they are trapped, as in a real black hole.
This is the type of system employed in the latest work by Daniele Faccio and colleagues of the University of Insubria and other Italian institutions. They placed a photon detector and spectrometer at right angles to the direction of the laser beam passing through the glass to catch any photons born spontaneously at the simulated event horizon. Amid noise coming from fluorescent defects in the glass, Faccio’s group was able to pick out a signal of photons with wavelengths between 850 and 900 nm. Because there is no known fluorescence emission in this window, the researchers claim, these photons must be Hawking radiation.
Some agree, but not all
Some independent researchers already agree, notably Ulf Leonhardt, a physicist at the University of St Andrews, UK, who pioneered the basis of the experiment two years ago. But others are not so sure.
One problem is that Faccio’s group cannot show that the emission is a continuous “black body” spectrum, as an astrophysical black hole’s would be – even if they did have an apparatus to make such a thorough measurement, their system is so dispersive that the black-body spectrum would likely be ruined. Another possible issue is that Hawking radiation should be emitted only in the direction of the laser and not perpendicular, although this could be because the strong refractive-index profile bends the light outwards. The question is, do shortcomings like these render the “Hawking radiation” claim untenable?
“This is partially a semantic question,” says Renaud Parentani, who specializes in Hawking radiation at Paris-Sud 11 University, France. Parentani believes that no-one has yet defined what should constitute Hawking radiation proper, and that researchers should concentrate on pinpointing which aspects of the phenomenon an experiment has succeeded in demonstrating. “We have to somehow make a list of specific properties that characterize standard Hawking radiation,” he adds.
Entanglement could be the clincher
One way to convince doubters might be to measure the photons generated on both sides of the refractive-index peak simultaneously. If they are entangled in the quantum-mechanical sense, this would be solid evidence that they were born together at the horizon. Leonhardt told physicsworld.com that he expects to have results for such an experiment in a year or less.
As for Hawking’s chances of a Nobel prize, physicists seem almost unanimous in thinking it’s too soon to tell. But with the near-impossibility of making an astrophysical measurement of Hawking radiation, and the official clause that bars posthumous nominations, there is the chance – albeit remote – that the Swedish committee rules laboratory proof of Hawkins’s theory sufficient for a decoration. Already, one group based in Canada has found evidence for what it claims is Hawking radiation in a classical, water-based system (see arXiv: 1008.1911), and many other experiments are likely to shore up lab evidence over coming months.
A preprint of the article is available at arXiv: 1009.4634.