Under general relativity, a black hole is inescapable. Once something travels beyond the event horizon into the heart of the black hole, there’s no return. So intense is the gravitational force of a black hole that not even light - the fastest thing in the Universe - can achieve escape velocity.
Under general relativity, therefore, a black hole emits no electromagnetic radiation. But, as a young Stephen Hawking theorised in 1974, it does emit something when you add quantum mechanics to the mix.
This theoretical electromagnetic radiation is called Hawking radiation; it resembles black body radiation, produced by the temperature of the black hole, which is inversely proportional to its mass (watch the video below to get a grasp of this neat concept).
This radiation would mean that black holes are extremely slowly and steadily evaporating, but according to the maths, this radiation is too faint to be detectable by our current instruments.
So, cue trying to recreate it in a lab using black hole analogues. These can be built from things that produce waves, such as fluid and sound waves in a special tank, from Bose-Einstein condensates, or from light contained in optical fibre.
“Hawking radiation is a much more general phenomenon than originally thought,” explained physicist Ulf Leonhardt to Physics World. “It can happen whenever event horizons are made, be it in astrophysics or for light in optical materials, water waves or ultracold atoms.”
These won’t, obviously, reproduce the gravitational effects of a black hole (a good thing for, well, us existing), but the mathematics involved is analogous to the mathematics that describe black holes under general relativity.
This time, the team’s method of choice was an optical fibre system developed by Leonhardt some years ago.
The optical fibre has micro-patterns on the inside, and acts as a conduit. When entering the fibre, light slows down just a tiny bit. To create an event horizon analogue, two differently coloured ultrafast pulses of laser light are sent down the fibre. The first interferes with the second, resulting in an event horizon effect, observable as changes in the refractive index of the fibre.
The team then used an additional light on this system, which resulted in an increase in radiation with a negative frequency. In other words, ‘negative’ light was drawing energy from the ‘event horizon’ - an indication of stimulated Hawking radiation.
While the findings were undoubtedly cool, the end goal for such research is to observe spontaneous Hawking radiation.
Stimulated emission is exactly what it sounds like - emission that requires an external electromagnetic stimulus. Meanwhile the Hawking radiation emanating from a black hole would be of the spontaneous variety, not stimulated.
There are other problems with stimulated Hawking radiation experiments; namely, they are rarely unambiguous, since it’s impossible to precisely recreate in the lab the conditions around an event horizon.
With this experiment, for example, it’s difficult to be 100 percent certain that the emission wasn’t created by an amplification of normal radiation, although Leonhardt and his team are confident that their experiment did actually produce Hawking radiation.
Either way, it’s a fascinating achievement and has landed another mystery in the team’s hands, too - they found the result was not quite as they expected.
“Our numerical calculations predict a much stronger Hawking light than we have seen,” Leonhardt told Physics World.
“We plan to investigate this next. But we are open to surprises and will remain our own worst critics.”
The research has been published in the journal Physical Review Letters.
