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Re: Question About Charged Particles.



At 07:05 PM 4/6/01 -0400, Robert B Zannelli asked about:
the possibility of a charged particle having a zero rest mass.

I think previous answers to this question have been overly theoretical. By
way of analogy, let's consider the following question:
Q1) Are there animals living wild in New York City today
having a mass greater than 10 tons each?

a) There is no particularly strong theoretical reason why such animals
could not exist.

b) OTOH if they did exist, even the most casual observations would have
detected them by now.

=====================

So... modifying Bob's question slightly, let's consider the question:
Q2) Are there particles similar to electrons in all respects
except having a smaller mass, by a factor of at least a million?

a) There is no particularly strong theoretical reason why such particles
could not exist. As I explained before, nobody AFAIK can calculate the
mass of the electron from first principles, so we are several leaps removed
from being able to disprove other possibilities in principle.

b) OTOH, if such particles did exist, even rather casual experiments would
have detected them by now.

For starters: There is such a thing as "light by light scattering". The
dominant physical mechanism is as follows: You start with a virtual
electron-positron pair. Such things spontaneously produce themselves
everywhere, all the time, but they don't last very long. You then whack
that virtual pair with an intense electric field (e.g. a tightly focused
multi-Joule pulse from a Ti-sapphire laser) and voilà -- you can
materialize the pair. A plain old real electron flies off in one
direction, and a real positron flies off in another direction.

In general, the physics of this is dominated by processes involving the
lightest charged particle. That's because the field required to do this
decreases radically as the mass of the particles decreases. You can
understand this using sophomore-level quantum mechanics ideas: evanescent
waves and all that.

Bottom line: I would say that there is overwhelming _experimental_ evidence
that the answer to Q2 is no.

==============

Postscript:

One could generalize Q2 to cover particles that couple more weakly to
familiar fields. The same sort of experimental arguments might or might
not be effective, depending on details.

Bob only asked about charged particles, so I will confine my remarks to
things that have a sizeable charge.