Re: blackbody radiation

[John Denker <jsd@AV8N.COM>]

Justin Parke wrote:
> So the difference between [atomic transitions] and [antennas] is the
> number of different available states, the states of the hydrogen atom
> being given by the solution of the schroedinger equation (and being
> discreet) and the states for the blackbody being essentially
> continuous (because of the number of different ways radiation can be
> absorbed/emitted) and the distribution of those states obeying planck
> statistics.  Right?

That hits the main points.

At the next level of detail you might want to consider:

1) The radiative process requires not just "available states"
but also a suitable coupling, specifically a transition
matrix element.
 -- When the coupling is lacking in an atom we say we have
a forbidden transition, aka a forbidden line in the spectrum.
 -- Analogous things can happen in antennas.  Suppose there
is a wave-generator connected by a waveguide to a horn-type
antenna.  At frequencies below the waveguide cutoff, there
are plenty of modes in the generator and plenty of modes in
the far field, but no coupling between them.

2a) In an atom, the states are not totally discrete.  Think
of them as oscillators with a Q ... where the Q can be
impressively high (a million or more) but not infinite.  So
if you've got 'enough' atoms, you can do what you need to
do by relying on off-resonant transitions.

2b) Also there are lots of things that can broaden the
observed linewidth beyond the 'intrinsic' atomic value,
such as Doppler shifts (if/when the atoms are moving
around in a gas) and pressure shifts (if/when they're
bumping into things).

Nothing is completely transparent ...
 -- on the other hand some things have remarkably low
    attenuation, e.g. glass fibers for optical communication
    have a tiny fraction of a dB per kilometer ...
    -- on the third hand, they have to work real hard
       to make that stuff.