Pulses in an optical fibre can mimic the space-bending physics of collapsed stars.
A black hole has been made in the laboratory. It’s not, needless to say, a collapsed star with super-strong gravity, like the black holes thought to exist in deep space. Rather it’s a kind of toy black hole made from light — and “completely harmless”, its creators say.
It might seem odd to make an analogue of a black hole — so-called because even light cannot escape its gravitational tug — from light itself. But Ulf Leonhardt of the University of St Andrews, UK, and his colleagues demonstrate that pulses of light travelling down an optical fibre can be made to affect other light waves in much the same way as a black hole does1.
Laboratory analogues of black holes have been proposed before, including an exotic ultracold gas of metal atoms that might act as a ‘sonic black hole’2, but none has previously been demonstrated. Lab versions of black holes might permit a key aspect of their theory to be put to the test: whether they are not in fact perfectly black, but instead emit some particles and radiation.
Although the model is made in an optical-fibre cable with special properties, the team says that almost any pulse — including those used in fibre-optic telecommunications — should be a black-hole mimic to some degree. “We’re making them all the time,” says Leonhardt.
A river runs through it
The group’s light pulses provide not only optical black holes but ‘white holes’ too. Whereas no light can escape from within a region called the event horizon surrounding the centre of a black hole — so light hitting it always falls in and becomes trapped — incoming light is repelled by the event horizon of a white hole.
The St Andrews team point out that black holes are like rivers flowing over a waterfall, carrying fish with it. The waterfall is the hole’s ‘singularity’, where fish ‘disappear’; a little way back upstream lies the event horizon, beyond which no fish can swim fast enough to escape from the brink.
A white hole, meanwhile, is like a fast-flowing river entering the sea: when the speed of the river current exceeds that of fish in the sea, no fish can enter the river mouth. “These are more than analogies,” the researchers say. “The propagation of waves in moving fluids is mathematically equivalent to wave propagation in spacetime.”
Altered by the light
The laboratory analogy involves a special, 'non-linear' optical fibre, in which light actually changes the fibre’s light-bearing properties. This mimics the way that a black hole’s gravity warps the space around it, altering the way light travels through it.
Because of this property of the fibre, a stream of light behind but faster than the ‘hole’ pulse gets slowed down as it catches up, and then is reflected backwards (like the fish in the sea that cannot enter a stream). So light can never cross into the trailing edge of the pulse, making it like a white hole. Just as with a cosmic white hole, the light also has its wavelength shortened by this process.
The 'black horizon' at the pulse front is more complicated. In a cosmic black hole, incoming light is also frequency-shifted as it approaches, until its wavelength is so small that it requires new and unknown physics to describe how the light behaves. In the optical fibre, the alteration of wavelength is not so extreme. The result is that the light actually bounces away. That sounds as if it's the opposite behaviour to a real black hole, but in fact the maths governing the behaviour is the same.
The ultimate test
The researchers speculate about making such pulses act like mirrors — which might then be used to trap light in a way that could lead to ‘black-hole lasers’. In the meantime there is a more likely application for the toy: testing an unproven theory about black holes.
In 1974, Stephen Hawking predicted that black holes should actually emit some particles and radiation. This should happen, he said, because empty space is constantly fizzing with ‘virtual particles’ — pairs of particles and their corresponding antimatter particles, which annihilate one another almost as soon as they are formed. Near the event horizon of a black hole, one member of a pair would occasionally fall across the horizon before annihilation happens, so the other would escape destruction. The surviving particle could then escape from the edge of the black hole.
This ‘Hawking radiation’ means that black holes should have an intrinsic temperature (albeit very, very low) caused by the emission of these particles. But no one has ever been able to test this, and the prospect for doing so looks bleak: their temperature would be much lower than the pervasive afterglow of the Big Bang.
Leonhardt and his colleagues predict that their optical black holes should emit photons of ultraviolet light that are equivalent to Hawking radiation, which should be detectable. “They are of very low intensity,” says Leonhardt, “so seeing them will need great luck. We’d be overjoyed if we found this was happening.” It will also be tricky to distinguish these photons from the hole pulse itself, although the photons ought to have a characteristic fingerprint.
But the challenges clearly don’t deter the St Andrews team. “Working on artificial black holes seems seriously addictive,” they write in a press statement. “Maybe we are beyond the point of no return.”
Philbin, T. G. et al. Science 319, 1367-1370 (2008).
Garay, L. J., Anglin, J. R., Cirac, J. I. & Zoller, P. Phys. Rev. Lett. 85, 4643-4647 (2000).
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Ball, P. 'Black hole' made from light. Nature (2008). https://doi.org/10.1038/news.2008.651