SEMINAR: Waves and Currents: Hawking Radiation in the Hydraulics Laboratory

We are all aware that nothing, not even light, can escape from a black hole. Well, maybe not! In 1974 Stephen Hawking proposed that black holes emit a form of radiation, which has become known as Hawking radiation.

This radiation originates at the black hole horizon – the spherical surface inside of which light is trapped. The black hole horizon is effectively a control surface for light: the interior of the sphere is said to be superluminal and the exterior, subluminal. An interpretation of Hawking’s analysis is that when particle-antiparticle pairs are formed at a black hole horizon, one falls into the hole while the other escapes and reduces the energy in the black hole.

Unruh (1981) showed that there is a mathematical analogy between the process described above and the behavior of water waves propagating upstream against a current. I will report on the results of the experiments of Weinfurtner et al. (2011) that tested this analogy in a 6.2 m long and 0.15 m wide flume.

The fate of free surface water waves propagating upstream toward the crest of a streamlined obstacle has been examined. As the waves propagate toward the crest of the obstacle they slow down, both because the flow velocity in the channel increases, and because their phase speed decreases as they shoal. As their wavelength decreases so too does their group velocity and eventually the waves are arrested and are converted into pairs of short waves.

Both waves have a downstream group velocity, but one has an upstream phase velocity and the other a downstream phase velocity. These wave pairs are analogous to the particle-antiparticle pairs of Hawking radiation and represent the closest analogy to Hawking radiation observed to date.

Hawking, S.W. (1974) Nature 248, 30. Unruh, W.G. (1981) Phys. Rev. Lett. 46, 1351. Weinfurtner, S., E.W. Tedford, M.C.J. Pennrice, W.G. Unruh & G.A. Lawrence (2011) Phys. Rev. Lett. 106, 021302.

Biography: B.E., (W. Aust.), M.S., Ph.D. (Berkeley), P.Eng. Research Interests: Environmental fluid mechanics, hydraulics, hydrodynamic stability and mixing, physical limnology, water quality management.



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