Re: Question About Charged Particles.

["John S. Denker" <jsd@MONMOUTH.COM>]

At 07:05 PM 4/6/01 -0400, Robert B Zannelli wrote:
>         A question came up on another list concerning the possibility of a
> charged particle having a zero rest mass. It would seem that this would be
> impossible for several reasons, the most obvious being, that there is an
> electromagnetic mass associated with a charged particle.  Also Gluons are
> believed to have zero rest mass even though they carry a color charge, plus
> neutrinos carry a weak charge and were once thought to be massless. Therefore
> is the answer to this question conditioned on the range of the force
> involved?

IMHO this question deserves a whole lot more respect than it has
gotten.  This is one of the best questions that has appeared on this list
in a long time.  We should thank Bob for asking such a fine question.

The question is perfectly clear.  The fact that I don't know the answer
does not detract from the question!

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

Here are some partial answers:

1) As a rule, short-range interactions are mediated by massive
particles.  This is a quantitative relationship.  For instance, given the
range of the strong force you can estimate the mass of the pion (or vice
versa).  The physical basis for this can be understood, roughly, in terms
of the propagation of evanescent waves.  Suppose you need to provide 140
MeV to knock a free pion out of a proton.  If you do _not_ provide the 140
MeV, you can imagine an unfree pion trapped in a well 140 MeV deep.  Its
evanescent wavefunction will extend outside the well for a short
distance.  It's essentially like the light which is totally internally
reflected at the side of an aquarium;  its evanescent wavefunction extends
outside the glass for a short distance, as you can verify by pressing your
thumb against the glass.

The photon has zero rest mass and mediates an infinite-range interaction,
namely electromagnetism.

2) A straightforward calculation of the mass of the electron includes a
contribution representing the mass of its electromagnetic field.  If the
electron is a point particle, this contribution is infinite -- which poses
obvious theoretical difficulties.  If the electron is not a point particle,
it must have some internal structure.  This poses extreme difficulties
also, because there is a conspicuous lack of evidence for such internal
structure.

Here is a scenario for how the future of physics might unfold:

  A) Somebody is able to calculate the mass of the electron from
     first principles.  (Note that this would have to be considered
     a _postdiction_ not a prediction.)
  B) Somebody is able to postdict the masses of all other known
     particles, charged and otherwise.
  C) Somebody is able to predict the mass of one hitherto-unknown
     charged particle.
  D) Somebody is able to make a list of all possible particles (ruling
     out the existence of any unlisted particles), and to calculate from
     first principles the mass of each listed particle.

Now, as far as I can tell, the physics community has not even gotten to
step (A) yet.  The people I know who are working on this are quite careful
to be modest about their accomplishments to date.  When we get to step (D),
we will be able to give a definite answer to Bob's question.

In the meantime:  If it were announced tomorrow that somebody had
discovered a particle with the same charge as the electron but 1/100th of
the mass, I would be somewhat surprised but I wouldn't die of shock.  As
for zero rest mass, or something so small as to be indistinguishable from
zero, I would be somewhat more surprised.  Therefore my best-guess answer
to Bob's question is that it is a remote possibility.  But we are getting
waaay beyond what I actually know.