🚀 go-pugleaf

In article <13379@ecs.soton.ac.uk>, dbc@ecs.soton.ac.uk (Bryan Carpenter) writes...
>In <1992Nov3.175155.27867@impmh.uucp> dsg@impmh.uucp (Dave Gordon) writes:
>>...
> 
>>This relates to something I have been wondering about for a while now:
>>is there an upper bound on the freqency of electromagnetic radiation?
> 
>You can increase the energy of a photon indefinitely just
>by boosting it into a fast enough reference frame.  So the answer
>is: no, there is no upper limit on the energy.

As I tried to make clear yesterday, this presupposes that the physics of 
high energy QED is the same as at low energy, a fact against which there
is at least some indirect suggestion (I hesitate to use the word "evidence",
though some would call it that).

Consider radiative corrections in QED.  If you allow that there is no new 
physics at high energy, then photons of arbitrarily high momentum contribute
an infinite amount to a variety of integrals which pop up when trying to 
evaluate Feynman diagrams.  It turns out to be unimportant when you want
to measure something like the Lamb shift, because these problems plague
a free electron as well as a bound one.  To measure an energy level, you
are interested in the energy *difference* between the free and bound states,
and all but a finite part of the effect ultimately cancels, allowing you
to accurately calculate energy levels.

But the fact remains that the theory includes a seemingly unphysical 
infinite contribution from high-energy photons.  It is therefore reasonable
to speculate that something new must happen at high energy which suppresses
the range of such photons, and which therefore allows us to escape the
"ultraviolet catastrophe." 

Clearly, you get into a nasty tangle trying to imagine such a modification,
and the implications that it would have for special relativity.  But this
does not imply, as Bryan infers, that it could not happen.  It just means
that we don't understand the behavior of photons at energies higher than 
we have ever studied them.  

This is a good example of how radiative corrections can give us information
about physics at energies higher than we can actually reach with our 
accelerators.  This is a common theme in particle physics.   Sometimes (not
in this case) it allows us direct experimental access to new physics. 
Some process,  which we cannot reproduce in the laboratory, nevertheless 
contributes,through some virtual intermediate state, to the radiative 
corrections of processes which we *can* observe directly.  A well-known
example would be recent attempts to infer the mass of the top quark from
how it modifies the radiative corrections to a variety of processes from
muon decay to B-zero mixing.

-Scott
--------------------
Scott I. Chase			"It is not a simple life to be a single cell,
SICHASE@CSA2.LBL.GOV		 although I have no right to say so, having
				 been a single cell so long ago myself that I 
				 have no memory at all of that stage of my 
				 life." - Lewis Thomas