In one minute

Rousseaux, Mathis, Maïssa, Philbin and Leonhardt target the mechanism at the heart of Hawking radiation: conversion of positive-frequency waves into negative-frequency waves at a horizon. Astrophysical Hawking radiation is too feeble to detect, so laboratory horizons are the way to demonstrate its physics. Instead of a quantum fluid they use water, where a horizon forms wherever the flow outruns the waves. Hawking's theory links three separate disciplines: quantum mechanics, general relativity and thermodynamics. They report the first direct observation of negative-frequency waves converted from positive-frequency waves in a moving medium. The measured conversion was significantly stronger than simple-model simulations predicted.

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