Physicspunks

[Jim Choate]

On Thu, 16 Aug 2001, Steve Thompson wrote:

> Quoting Anonymous (nobody at paranoici.org):
> > If a block of a metal is hollowed out and a small port is drilled to
> > see in, the radiance of the cavity is substantially higher than that
> > of the surface of the metal.  As if that weren't shocking enough, it
> > turns out that the radiance of cavities is the same no matter what
> > kind of metal is used.  (This is so counterintuitive that I almost
> > don't believe it!)
> 
> This is actually a phenomenal result.  If one makes a large enough apparatus
> so that the cavity is very, very large with a correspondingly small port is
> drilled, you've then got the makings of a great beam weapon.

You're missing the parameter where the walls have to be kept at a
temperature T. This implies that your environment around the ball is
controlled. In a lab environment you'd wrap the cavity in some sort of
heater blanket.

To help you get a better grasp of the scale of the effect, consider the
odds of a single photon entering the cavity port from the outside. What is
the odds that that photon will be re-emitted? Almost nill. It effectively
absorbs radiation at all frequencies (hence the 'black' term). Now,
consider the inverse, what is the odd that a photon of a given wavelength
will be emitted? The odds that a perfect black body absorber would radiate
a photon from inside the cavity to the outside environment is also low.

So, to make a black body radiator into a beam weapon implies so much
energy keeping the cavity at the temperature required that it would be
simpler to use that energy directly instead of trying to create a high
energy photon stream from a very(!) inefficient emission process.

The Dynamics of Heat
Fuchs
ISBN 0-387-94603-9


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