  There is a mathematical analogy between the propagation of fields in a
general relativistic space-time and long (shallow water) surface waves on
moving water. Hawking argued that black holes emit thermal radiation via a
quantum spontaneous emission. Similar arguments predict the same effect near
wave horizons in fluid flow. By placing a streamlined obstacle into an open
channel flow we create a region of high velocity over the obstacle that can
include wave horizons. Long waves propagating upstream towards this region are
blocked and converted into short (deep water) waves. This is the analogue of
the stimulated emission by a white hole (the time inverse of a black hole), and
our measurements of the amplitudes of the converted waves demonstrate the
thermal nature of the conversion process for this system. Given the close
relationship between stimulated and spontaneous emission, our findings attest
to the generality of the Hawking process.
