New Journal of Physics Observation of negative-frequency waves in a water tank: a classical analogue to the Hawking effect?

doi:10.1088/1367-2630/10/5/053015 Abstract. The conversion of positive-frequency waves into negative-frequency waves at the event horizon is the mechanism at the heart of the Hawking radiation of black holes. In black-hole analogues, horizons are formed for waves propagating in a medium against the current when and where the flow exceeds the wave velocity. We report on the first direct observation of negative-frequency waves converted from positive-frequency waves in a moving medium. The measured degree of mode conversion is significantly higher than that expected from the theory