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From: sra@oddjob.UUCP (Scott R. Anderson)
Newsgroups: net.physics
Subject: Re: More on repulsion and attraction
Message-ID: <1002@oddjob.UUCP>
Date: Thu, 17-Oct-85 06:56:24 EDT
Article-I.D.: oddjob.1002
Posted: Thu Oct 17 06:56:24 1985
Date-Received: Sat, 19-Oct-85 04:50:21 EDT
References: <326@bcsaic.UUCP> <608@mtung.UUCP> <10634@ucbvax.ARPA> <613@mtung.UUCP>
Reply-To: sra@oddjob.UUCP (Scott R. Anderson)
Organization: University of Chicago, Department of Physics
Lines: 58
Summary: 

In article <613@mtung.UUCP> jhc@mtung.UUCP (Jonathan Clark) writes:
>rimey@dali.berkeley.EDU (Ken Rimey) writes:
>>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?

Massive, yes, unstable, no.  As mentioned often in this group, the
Heisenberg Uncertainty Principle places limits on how long a virtual
particle can exist.  If the particle is a massless spin-zero particle
like the photon, the probability of interacting with it at a distance r
from its source goes like 1/r, the familiar electromagnetic potential.
Yukawa was the first to show that a virtual spin-zero particle with
mass m (e.g. the pion) has an exponentially small probability of being
at distance r,
			1
			- exp(-r/R)
			r

where R = h-bar/mc.  This is the reason that a force mediated by such
a particle is so short-ranged (remind me to tell net.internat that any
extended ascii set should include h-bar).  Note that the more massive
the particle is, the smaller the range R is.

>Now turning to the complementary question about why unlike
>forces attract, well that's the way the universe works.
>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.

I'm afraid that you are mixing your classical with your quantum-field-
theoretical here.  If you accept the existence of the EM field, then,
as you say, an electron reduces its potential energy in this field by
transferring the energy to the field, i.e. creating photons.  But no
virtual particles are necessary!  The latter are a *replacement* for the
concept of the field.  It is difficult to think of them in classical
particle terms, though, because their primary characteristic is the
momentum they carry (not to be confused with "the direction they are
travelling"; this has no meaning).  This momentum can be both pointing
away from *and* pointing towards the source particle.  What is unusual
is that like-charged and oppositely-charged particles pick up one or
the other, exclusively.

>My walk has become rather more silly lately.

Mine's random!

				Scott Anderson
				ihnp4!oddjob!kaos!sra