Thanks to all who wrote to address my question about the status of
the Pauli exclusion principle as a force.
The question brought out a rarely seen vicious side of Bill B. who
simply wanted to pinch my finger with a pair of pliers made of
solid neutronium. Or maybe he was trying to make a point. ;-)
John D. pointed out that the degeneracy pressure can only be
observed via interactions with a container that are mediated by a
more conventional force, (like the electrostatic part of the
electroweak force presumably.) He also provides an analogy with
the forces we feel when we try to reorient the axis of a
gyroscope. I appreciate both of these responses as enlightening,
but I still don't find that they address the central question.
Jack U. gave what may well be the appropriate technical
answer--that there is no force term corresponding to degeneracy
pressure in the Hamiltonian, that it arises purely from the
properties of the wave function. It seems to me, however, that if
this *is* the only way to understand why it is not considered a
force, then we will have to content ourselves with not really
being able to explain that fact in any appreciable way to
nonphysicists. I'm only a little dismayed by this possibility.
Frankly, it has always seemed to me that it is even harder to
explain why the weak part of the electroweak force *is* considered
a force.
Chris H. has obviously thought longer and harder about this
question than I have and expresses my sentiments (and, I suspect
those of my sophisticated layperson friend) eloquently.
At any rate, I think my answer *might* now be *something* along
these lines:
Within the restrictions of the Newtonian model, the concept of
"force" is associated exclusively with two phenomena--gravity and
electromagnetism--in the sense that all Newtonian "forces" can be
understood in terms of them.
Quantum physics deals poorly with the concept of "force" and
replaces it with the concept of "interaction." We now understand
electromagnetism as the long range part of the electroweak
interaction and we add the nuclear strong interaction for a total
of three (not four) "fundamental interactions." The Newtonian
"forces"--and, therefore, Newtonian physics--are based on the
properties of the long range interactions.
Within quantum physics, we deal with mathematical entities known
as "operators" which represent interactions and "wavefunctions"
which represent entities. A "fundamental interaction" is
identified by the fact that it shows up in the operators (and we
*believe* that gravity will someday show up in those operators!)
Degeneracy pressure, however, arises from properties of the
wavefunctions themselves.
I look forward to constructive criticism of the above!