Over the event horizon

Where analogues really come into their own is with objects in the universe that we know exist, but that are impossible to investigate directly. Black holes are a good example. These cosmic monsters are predicted by Einstein’s theory of gravity, the general theory of relativity. They form when large stars collapse and die, and supermassive versions are thought to skulk at the heart of most large galaxies. They are also portals to the ultimate prize of physics – a theory that explains what happens when the quantum particles of matter meet the extremes of gravity, the only force not covered by the quantum rules of the standard model. But given that it emits no light, it is not  easy to discern exactly what a black hole is doing. Silke Weinfurtner at the University  of Nottingham, UK, aims to lift the veil in the lab, using just water and laser light to simulate a black hole’s emission of Hawking radiation. This process, proposed by the physicist Stephen Hawking in the 1970s, is thought to occur when a fluctuation in the quantum vacuum near a black hole’s event horizon – its point of no return – causes a quantum entangled pair of matter and antimatter particles to form. If one of the pair falls into the black hole while the other is just far enough away to escape it, the two particles  can separate, with one trapped inside the  black hole forever and one radiated away. Weinfurtner’s analogue actually simulates a “white hole” that, instead of sucking everything in to it, deflects everything away. Reverse the direction of time in the underlying equations, however, and conclusions drawn for white holes are just as valid for black ones. The analogue consists simply of water flowing along a channel containing a smooth obstacle. The team induced ripples on the surface of the water travelling in the opposite direction, and used a 2D sheet of laser light to analyse the properties of the surface waves as they hit the obstacle region and are reflected off. They found that the amplitude and spread of wave frequencies corresponded to those expected of Hawking radiation at a black hole’s event horizon  (arxiv.org/abs/1302.0375). “It was a very clear, conclusive detection of the effect,” says Weinfurtner. “It was a big surprise to us how robust these experiments are.” The work has already triggered theoretical studies into how an entirely classical-physics experiment can even crudely reproduce aspects of Hawking radiation, which is a fundamentally quantum effect. A full-blown lab demonstration – one that also shows that the particles remain entangled as predicted by the theory – would require a more sophisticated, quantum analogue. Together with her Nottingham colleague Peter Kruger, Weinfurtner is working out how to detect the effect using supercooled atoms.