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From: jhc@mtung.UUCP (Jonathan Clark)
Newsgroups: net.physics
Subject: More on repulsion and attraction
Message-ID: <613@mtung.UUCP>
Date: Wed, 16-Oct-85 14:24:08 EDT
Article-I.D.: mtung.613
Posted: Wed Oct 16 14:24:08 1985
Date-Received: Fri, 18-Oct-85 00:06:19 EDT
References: <326@bcsaic.UUCP> <608@mtung.UUCP> <10634@ucbvax.ARPA>
Reply-To: jhc@mtung.UUCP (Jonathan Clark)
Organization: AT&T Information Systems Laboratories, Holmdel, NJ
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<>
In response to an earlier posting of mine, responding to a
question about the mechanism whereby two electrons repel
each other and any photons that might be involved,
rimey@dali.berkeley.EDU (Ken Rimey) writes:
>No, the force between two charged particles does not involve real photons.
>Hold two identically charged pith balls near each other.  You will not
>detect any light or radio waves between them.

I suspect that the author has forgotten that photons are the
name that we give to the particles that mediate the
electromagnetic force. They are not limited to that part of
the spectrum which we interpret as radio and light. Call
'photons' 'gauge particles' if it makes more sense. So yes,
the Coulomb force *is* mediated by the gauge particles of
the electromagnetic force, viz. photons.

>You say that virtual particles exist only for short times, and therefore
>shouldn't be able to travel large distances.  Indeed, this is the reason
>that the strong nuclear force has a finite range. But moving clocks run
>slow, and clocks on photons don't run at all.  This is why the force
>between charged particles can be felt even at large distances.

Doesn't this have more to do with gluons (all right, strong
force gauge particles) being massive and therefore being
unstable?

Now turning to the complementary question about why unlike
forces attract, well that's the way the universe works. As
to the *mechanism*, when an (isolated) electron and a
positron are attracted to each other, they are losing
(electromagnetic) potential energy, and thus are emitting
photons (thus adhering to the law of conservation of
energy). If the electron moves left (towards the positron),
then the photon(s) go right (away from same), thereby
conserving momentum.

Questions like 'how does the photon "know" whether it just
got absorbed by an electron or a positron' are very close
to meaningless. An energy and momentum transfer took place,
that's all. If the question was 'how does the electron know
that there is a positron out there' then remember that the
electron is emitting and reabsorbing photons (virtual ones)
all the time, and because it exists in the EM field of the
other, it is (statistically) more likely to emit the said
photons in such a manner that it moves closer to the
positron. Again, this is the way the universe works. Of
course, it may work differently tomorrow when someone
discovers how to generate a probability field or a new GUTS
theory crops up.

Comment to diehard QMers out there: it was quite painful to
write the last couple of paragraphs. I beg your indulgence
as to the use of both language and concepts (like "exist"!).
-- 
Jonathan Clark
[NAC]!mtung!jhc

My walk has become rather more silly lately.