Re: What are the shapes of subatomic particles?

Dear Rob,

Current particle physics experiments are able to probe the radii of
fundamental particles down to 10^-18 meters.  So far, electrons
and quarks appear to have no internal structure, that is, no volume
and nothing inside them.  If electrons or quarks are really spheres
or some other shape, this object is smaller than 10^-18 meters across
and will not be discerned until the next generation of particle
physics experiments which will incorporate higher energies and hence
be able to resolve smaller lengths.

In contrast, particles which are not fundamental but composite, such
as the proton, are experimentally distinguishable from points; they
have non-zero volume with smaller objects (quarks in this case)
inside them.  However, even they do not have a well-defined surface
because the sub-atomic world is governed by the theory of Quantum
Mechanics which blurs particles and spreads their probability of
existing over large volumes.  If you look in a particle physics
book, you will see the proton radius quoted as around 1 fermi =
10^-15 meters.  This means that if you experimentally measure the
proton radius, you will get a value around 1 fermi 95% of the time,
but it is also possible though less likely to get a value of 2 fermi,
or 5 fermi, or 100 fermi.  The proton should be modeled not as a
hard sphere but rather as something closer to a cloud with high density
near the center out to 1 fermi, then a rapidly decreasing density out
to larger distances.  The Quantum Mechanical world is fuzzy.

Now let's suppose that a new high-energy experiment determines that
quarks have structure.  Is it likely that the surface shapes of quarks
have anything to do with their properties and combination possibilities?
Not if recent history is any guide.  The most successful theories
of particle interactions (in terms of matching the experimental data)
are quantum field theories which incorporate Quantum Mechanics and
Einstein's Special Relativity.  The quarks in these theories interact
by exchanging gauge bosons: gluons mediate the strong nuclear force,
photons mediate the electromagnetic force, etc.  These theories are
so fantastically good at making predictions that there does not seem
to be room for a theory of surface shapes.

--Randall J. Scalise    http://www.phys.psu.edu/~scalise/