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Fibre-optical analogue of the event horizon

In 1974 Stephen Hawking published in Nature [1] a theoretical prediction that has been one of the intellectually most influential and tantalizing results of theoretical astrophysics: he predicted that black holes are not black, but they radiate due to the physics of the quantum vacuum. The vacuum is teeming with virtual quanta that are continuously created and annihilated, but if they are created on opposite sides of the event horizon they can no longer annihilate each other and are forced to materialize. The quanta constitute a radiation with a characteristic temperature; the black hole is a black-body radiator. Hawking’s theory supports Bekenstein’s idea [2] that the horizon carries entropy that is proportional to its area. Hawking’s effect thus connects three vastly different areas of physics — thermodynamics, quantum mechanics and general relativity. In modern attempts for finding a quantum theory of gravity, such as loop quantum gravity [3] and superstring theory [4], the correct prediction of the Bekenstein-Hawking entropy has been used as a benchmark. However, the Hawking temperature of solar-mass black holes lies about eight orders of magnitude below the temperature of the cosmic microwave background; so, most probably, there is no chance of directly observing Hawking radiation in astrophysics. The benchmark of some of the most advanced theories of physics seems destined to remain theory.