I am confused by most discussions of analog
Hawking radiation in fluids (see, for example,
the recent experimental result of Weinfurtner et
al. Phys. Rev. Lett. 106, 021302 (2011), arXiv:1008.1911). The starting
point of these discussions is the observation that
the equation of motion for fluctuations around
stationary solutions of the Euler equation have
the same mathematical structure as the wave
equation in curved space (there is a fluid metric
$g_{ij}$ determined by the background flow). This
background metric can have sonic horizons. The
sonic horizons can be characterized by an associated
surface gravity $\kappa$, and analog Hawking temperature
$T_H \sim \kappa\hbar/c_s$.

My main questions is this: Why would $T_H$ be relevant?
Corrections to the Euler flow are not determined by
quantizing small oscillations around the classical flow.
Instead, hydrodynamics is an effective theory, and
corrections arise from higher order terms in the derivative
expansion (the Navier-Stokes, Burnett, super-Burnett terms),
and from thermal fluctuations. Thermal fluctuations are
governed by a linearized hydro theory with Langevin forces,
but the strength of the noise terms is governed by the
physical temperature, not by Planck's constant.

A practical question is: In practice $T_H$ is very small
(because it is proportional to $\hbar$). How can you
claim to measure thermal radiation at a temperature
$T_H << T$?

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