From: aephraim@physics.Berkeley.EDU (Aephraim M. Steinberg)
Newsgroups: sci.physics
Subject: Re: Defining Photons
Date: 31 Jul 1992 03:13:27 GMT
Organization: University of California, Berkeley
Distribution: na
NNTP-Posting-Host: physics.berkeley.edu
Keywords: Relating photons E=MC^2 criticism

In article <1992Jul27.225249.12026@galois.mit.edu> jbaez@riesz.mit.edu (John C. Baez) writes:
>In article <24910@dog.ee.lbl.gov> sichase@csa2.lbl.gov writes:
>>Photons behave classically when there are alot of them around.  Electrons
>>behave classically when one of them is at high energy.  This is a fundamental
>>difference, which emerges directly from the different methods of quantization
>>used for fermion and boson fields, and directly leads to the dramatic
>>classical difference between EM fields and lumps of matter.  But in the
>>quantum regime this distinction blurs considerably, leading physicists to 
>>class all these objects - photons and electrons and everything else, together
>>as particles.
>>
>>Notice that masslessness have nothing to do with these questions.  

Well, actually photons behave classically when there are a lot of them 
around, AND they are in a coherent state (a particular kind of coherent
superposition of different number-states of photons which most closely
resembles a classical field, and yields the familiar Poisson counting
statistics one expects from independent random processes, just to define
the jargon in advance).  Recent work on squeezing is based entirely on
the concept that you can have many photons around but in a different
kind of state (you guessed it, a squeezed one, the simplest example of
which is a number state, e.g., exactly 500 quadrillion photons in a
cavity), and such a state will NOT act classically.  Most of the talk
centers around the fact that they "get around" the uncertainty principle
(of course, all they do is trade off one uncertainty for another), which
might make one think that they are "more classical" than a coherent state
(field emanating from an ideal laser, for example) or a thermal state
(field emanating from a candle, for example, or a star).  However, many
results have been proved for classical field theories, generally involving
the correlations of fields ("photon statistics"), which are violated by
squeezed states.  The Hanbury-Brown Twiss experiment is an example; as
someone recently pointed out, their result (of photon "bunching") can be
explained classically-- this is what thermal noise fields yield.  Photons
independent of one another (no HBT effect) can also be explained classically;
they arise from coherent states, as I mentioned earlier, and it can be
shown that any collection of classical currents radiates a coherent state.
On the other hand, photon "antibunching" (which is what a number state would
show, since if you see one out of the 5E17, there are only 5E17-1 left)
is impossible classically, and yet has been observed by several groups.

All these states certainly are well-defined for massive particles such as
the Z as well as for the photon, so long as they are bosons.  I am curious,
however, whether the superselection rule makes such states impossible for
charged bosons such as the W's?


-- 
Aephraim M. Steinberg                    | "WHY must I treat the measuring
UCB Physics                              | device classically??  What will
aephraim@physics.berkeley.edu            | happen to me if I don't??"
                                         |       -- Eugene Wigner