Re: Virtual Particles

From: jdff (jdff1001_at_hotmail.com)
Date: 10/11/04 

Date: Mon, 11 Oct 2004 08:52:31 +0000 (UTC)

Igor Khavkine <igor.kh@gmail.com> wrote in message news:<cjs0q6$sv2$1@lfa222122.richmond.edu>...
SNIP
>
> With that in mind, let me tell you how virtual particles come up in
> calculations. I'm not going to tell you what "real" is, but I'll tell you
> how we decide that a particle is there or not. Take some process and put
> detectors around it. The detectors make localized measurements that tell
> you the energy and momentum of something. You say that this something is a
> particle. Let us not belabor the "reality" of this scenario because I've
> not even introduced virtual particles yet.
>
SNIP
>. In principle, three come to mind at the
> moment, but there could be more:
>
> 1) Do some perturbative calculations that involve scribbling diagrams on
> paper with solid and wavy lines that look awful lot like photons and
> electrons, and evaluating integrals associated with them.
>
> 2) Concoct some large matrix representation of your states and operators,
> then go to your futuristic supercomputer and make it solve some matrix
> differential equations.
>
> 3) Write down the path integral formulation of the same problem and go off
> to another futuristic supercomputer and make it crunch some numbers to
> evaluate this integral.
>
SNIP
> So if you start
> asking yourself about the reality of virtual particles, I think you should
> start asking yourself whether the paths taken by electrons in the path
> integral and the superpositions and finite dimensionality of the matrix
> approximation are real.
>
> Well, are they?
>
> Igor
I would answer as follows: if you put the detectors in an accelerated
frame of reference, you will indeed observe some "virtual" particles
as on mass-shell and real. Of course, the other two formulations will
also produce identical observational predictions of the events. And
each would have their own (but rather convoluted) explanation.

In this sense, on-mass-shell particles are no more or less real than
off-mass-shell particles.

Another example would be (in condensed matter physics), electrons with
an effective mass in a crystal lattice, or quasiparticles in the
Fractional Quantum Hall Effect. They are all simply the easiest way of
picturing the physics, and hence getting the insight required to push
forward. Which to me is shorthand for saying that all Physics is
perturbation theory, done in one's head, just find the "right" basis.

So to go back to the question, no, the second two methods aren't
"real", because for whatever experimental situation at hand that you
assumed, they were not the lowest-order perturbation, because you
needed a futuristic supercomputer to get the results. But for some
experimental set-ups, they may provide the lowest-order perturbation
results, and so they would be real for that system.

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