Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Unexpectedly large surface gravities for acoustic horizons?   Acoustic black holes are fluid dynamic analogs of general relativistic black
holes, wherein the behaviour of sound waves in a moving fluid acts as an analog
for scalar fields propagating in a gravitational background. Acoustic horizons
possess many of the properties more normally associated with the event horizons
of general relativity, up to and including Hawking radiation. They have
received much attention because it would seem to be much easier to
experimentally create an acoustic horizon than to create an event horizon. We
wish to point out some potential difficulties (and opportunities) in actually
setting up an experiment that possesses an acoustic horizon. We show that in
zero-viscosity, stationary fluid flow with generic boundary conditions, the
creation of an acoustic horizon is accompanied by a formally infinite ``surface
gravity'', and a formally infinite Hawking flux. Only by applying a suitable
non-constant external body force, and for very specific boundary conditions on
the flow, can these quantities be kept finite. This problem is ameliorated in
more realistic models of the fluid. For instance, adding viscosity always makes
the Hawking flux finite, but greatly complicates the behaviour of the acoustic
radiation --- viscosity is tantamount to explicitly breaking ``acoustic Lorentz
invariance''. Thus, this issue represents both a difficulty and an opportunity
--- acoustic horizons may be somewhat more difficult to form than naively
envisaged, but if formed, they may be much easier to detect than one would at
first suppose.

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The categories of this paper are: eess
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The categories of this paper are: adap-org
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The categories of this paper are: quant-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: atom-ph
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The categories of this paper are: econ
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The categories of this paper are: patt-sol
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The categories of this paper are: math-ph
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The categories of this paper are: math
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The categories of this paper are: cmp-lg
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The categories of this paper are: solv-int
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The categories of this paper are: stat
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The categories of this paper are: nlin
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The categories of this paper are: astro-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: acc-phys