Tabletop measurements of Hawking radiation

Tabletop measurements of Hawking radiation. Stephen Hawking proposed in 1974 that black holes evaporate. In essence, vacuum fluctuations near a black hole’s horizon produce particle–antiparticle pairs. One of each pair falls into the hole while the other escapes. Since the escaping particle has energy, the black hole must lose energy. A blackbody temperature inversely proportional to the black hole’s mass can be assigned to the process, yet the temperature is so low—on the order of 100 nK for a solar mass—that the radiation is difficult to observe directly. But William Unruh (University of British Columbia) demonstrated an analogy between the behaviors of waves near the black hole and sound waves in moving fluids. Now, physicists Silke Weinfurtner, Matthew Penrice, and Unruh and engineers Edmund Tedford and Gregory Lawrence at UBC have used another analogous system, surface waves, to study the Hawking process. They put a streamlined object shaped like an airplane wing into a channel of flowing water to create a region of high-velocity flow. Long-wavelength surface waves created downstream could propagate upstream toward that region but were blocked by the obstacle and converted into short-wavelength waves. The figure shows the converted waves (bottom) and the interference between them and the incoming wave (top). The obstacle behaves like a so-called white hole, which, as a time-reversed black hole, lets no radiation in but does let radiation escape. The conversion is the analogue of stimulated emission, and the team’s measurements of the amplitudes of the converted waves matched the expected thermal distribution. Moreover, despite the system’s nonlinearities, turbulence, and viscosity, and along with prior numerical work by various groups, the new results demonstrate the generic nature of Hawking radiation. ( S. Weinfurtner et al., Phys. Rev. Lett. PRLTAO 1079-7114 106, 2011.)