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- Path: sparky!uunet!ogicse!das-news.harvard.edu!husc-news.harvard.edu!husc8!mcirvin
- From: mcirvin@husc8.harvard.edu (Mcirvin)
- Newsgroups: sci.physics
- Subject: Re: What would a Photon be IF...
- Message-ID: <mcirvin.711729945@husc8>
- Date: 21 Jul 92 14:45:45 GMT
- Article-I.D.: husc8.mcirvin.711729945
- References: <l6c4lpINN559@pollux.usc.edu> <1992Jul17.130944.2127@pellns.alleg.edu> <mcirvin.711473575@husc8> <1992Jul20.213917.12361@mtu.edu>
- Lines: 45
- Nntp-Posting-Host: husc8.harvard.edu
-
- cescript@mtu.edu (Charles Scripter) writes:
-
-
- >Here's a QED puzzler for you. Suppose we examine the a free photon
- >using Feynman diagrams:
-
- [photon propagator with one-fermion-loop correction]
-
- >Can these contributions be viewed as actual processes, or do we
- >construe them be merely mathematical models?
-
- A fine thing to do a lot of thinking about. I think of them as
- terms in a perturbation expansion of what is really a continuous
- process: the EM field is excited and it causes some collateral
- excitation of the charged matter fields as well. The Feynman
- diagrams are a calculational device to get the outcome of the
- continuous process to nth order.
-
- >This second order
- >correction could be viewed as "photon decay", followed by an immediate
- >recombination.
-
- I was, of course, using the usual definition of "particle
- decay" as a situation in which the decay products last "forever," but
- as John Baez pointed out ominously a while ago, if we detect them, they
- haven't lasted forever.
-
- Which implies another puzzler: Are visible photons virtual? What does
- this imply for causality-- since, after all, virtual photons don't have
- to be *exactly* on their light cones? I know that this doesn't
- actually destroy causality, but I've never seen a very lucid explanation
- of why not. Any takers?
-
- >Although energy-momentum conservation is violated, we
- >are allowed to violate this on quantum time scales (how short is "short"?).
- >Energy and momentum are clearly conserved on macroscopic time scales.
-
- No! No! Energy-momentum is *always* conserved! It's the laws of
- motion that are violated! :) (The two views are really equivalent,
- and correspond, respectively, to old-fashioned and covariant perturbation
- theory.)
-
- --
- Matt McIrvin, grad student, Dept. of Physics, Harvard University
- mcirvin@husc.harvard.edu mumble mumble mumble mumble mumble
-