neal@denelcor.UUCP (Neal Weidenhofer) (11/07/83)
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I have a couple of items that have been bothering me for some time
about what's happening in physics lately. If anyone is both willing and
able to explain to me what I'm missing, I'd be very grateful. First some
background (as I understand it):
1. One of the earliest verifications of Special Relativity was in
observing that certain particles produced by cosmic rays entering
our atmosphere were reaching the ground in larger numbers than
predicted by classical physics. The explanation was that,
because these particles are traveling at a significant fraction of
c, their "internal clocks" are running slower than our clocks and
that therefore, any given particle has a larger probability of
reaching the ground before it decays because less time has passed
for the particle than for us.
2. Many of the recent experiments in particle physics have involved
colliding some particle and it's anti-particle (e.g., a proton and
an anti-proton) which produces a *very* high energy photon (or
more than one??) which, in turn, decays to produce (hopefully) the
particles that the experimenter wishes to observe.
My questions:
1. How does the photon in (2) "have time" to decay? Since according
to S.R., it's "internal clock" has stopped completely--at least
for our observations, it must be decaying in zero time (as
measured on its "internal clock") while it decays in a fixed
finite time (as measured by our clocks).
2. Is this related to the fact(?) that, in at least some of these
experiments, the photon is a "virtual" photon (i.e., so short
lived that it's not required to conserve momentum)?
3. Now that physicists are debating the possibility that neutrinos
have non-zero rest mass, has anyone considered the question that
photons might also? I realize that would negate many of the
arguments Einstein used to derive S.R. and G.R., but they're both
well enough verified by now to stand without their original
derivations (like taking down the scaffolding when the building
is finished.)
Thanks in advance for taking the time to point out my mistakes.
Regards,
Neal Weidenhofer
Denelcor, Inc.
<hao|csu-cs|brl-bmd>!denelcor!nealhalle1@houxz.UUCP (11/07/83)
I recall when studying for my comps a problem to calculate the upper bound for the mass of a photon. Using Heisenberg and other stuff, there is an upper limit - quite small, even on that scale, but non-zero. I don't remember the details now, but I thought the problem was interesting. Can anyone supply the details?
stekas@houxy.UUCP (11/07/83)
Real photons do not decay. The process of creating an
electron-positron pair from a photon is called "conversion", and
it occurs when a photon collides with a charged particle.
During the collision, the photon is "absorbed" by the charged particle
which later emits a massive "virtual" photon. The decay of the
massive "virtual" photon gives the electron-positron pair.
Photon decay would violate the conservation of energy-momentum.
The simplest way to see this is to realize that the electron-positron
system has a non-zero mass (by mass I mean the "rest mass") while the
photons mass is zero. Only when some energy and momentum can be shed
to some other particle can an electron-positron pair be created from
a photon without violating energy and momentum conservation.
JimMJackson.Wbst@PARC-MAXC.ARPA (11/08/83)
(1) and (2): A photon travelling in free space can't decay; the (relativistic) relationship between energy and momentum is determined by a particle's rest mass, and for a zero-rest-mass particle splitting apart doesn't work, four-vector-wise. For particle-antiparticle collisions, energy and momentum can be conserved by producing two photons (p = 0 in center-of-mass frame, so has to be at least two). Other things (for which quantum numbers such as charge and strangeness sum to zero) could be produced instead, and in analyzing such reactions one finds it convenient to represent the intermediate state (between input and output) as a single photon; this is a "virtual" photon for which the E-p relationship need not be the usual. (3) The zerocity of the rest mass of the photon is sensitively tested by the degree to which n = 2 in the electrostatic field expression E = q/(r**n) and that is very well established indeed. Mark
Shinbrot.WBST@PARC-MAXC.ARPA (11/08/83)
"Photon decay would violate the conservation of energy-momentum." Quite right - momentum transfer is needed to conserve both momentum and energy in pair-production. - Troy