AtD, p. 848 Ultraviolet Catatastrophe or When I Get Old, I Shall Wear Purple

[Monte Davis]

Mark Kohut sez:
 
> The ultraviolet catastrophe, also called the Rayleigh-Jeans catastrophe,
was a prediction  
> of early 20th century classical physics that an ideal black body at
thermal equilibrium  
> will emit radiation with infinite power. 
 
Infinite _frequency_, actually... James Clerk Maxwell and the gang had
worked out how the energy of an "excited" (oscillating, spinning,
stretching/relaxing) system would spread itself into the different modes of
motion. There was every reason, and consierable evidence, to suggest that
the same applied to the electromagnetic radiation -- light or whatever --
given off by atoms stimulated by, say, heat: the energy should spread evenly
over all available frequencies.
 
Trouble is, there's no upper limit to the frequency -- so the math implied
that the light emitted would just keep getting bluer, then ultraviolet and
beyond (although they hadn't detected & named the beyond yet).. Which it
doesn't.
 
This was one of the motivations for Planck's positing the quantum: that EM
energy could only be emittesd/absorbed in discrete packets, the energy per
quantum proportional to the frequency. If that were so, it would take more
energy for a quantum of visible light than of infrared, more for blue than
for red, still more for ultraviolet. That nudged the shape of the emitted
light curve -- frequency vs energy -- back to the shape observed in
experiments.
 
As has happened before and since in science, this was initially seen (by
Planck as well as others) as a fudge, a dodge, a bookkeeping convenience. It
took a few years, and Einstein showing how neatly quanta explained the
photoelectric effect (light generating a current in certain materials), to
convince everyone this wasn't an ad hoc "as if," but really happened. And
another 20 years to realize that *all* transfers of energy -- in chemical
reactions, good old mechanical push-pull, and everything else -- obey the
same rules.
 
By which time the meaning of "really" had turned very strange indeed.   
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