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Re: Forces:



The "Strong force" is really an artifact of the color interaction and is
mediated by the exchange of mesons. The long range interaction is
dominated by low mass spin zero mesons and is attractive for particles of
like "strong charge". The short range interaction is dominated by the
exchange of more massive vector mesons, and is repulsive between particles
of like "strong charge".

The effect of the Pauli Exclusion principle is entirely separate from the
effects discussed in the previous paragraph. The Pauli principle raises
the pressure of a Fermi gas by imposing a phase space limitation on the
occupation of momentum states. When fermi particles are added to an
ensemble they must have quantum numbers different from those already taken
by the members of the ensemble. Usually this means the added particles
have more momentum and hence contribute more to the pressure. The effect
of the Pauli Principle is, therefore, repulsive. BUT...there is no
interaction term in the Lagrangian, and no exchanged particle, to support
the idea that the Pauli Exclusion Principle is an interaction, or a
"force".

A good way to approach these interactions would be to begin with
electrostatics. This interaction has an infinite range because the photon
is massless and does not decay. The inverse square part is just a
cross section!
F=Qq exp(-t/tau)/r^2
(tau=infinity=lifetime of photon)

The interactions of the Z,W+,W-, and mesons are not so lucky. The
exchanged particles are massive and decay. Tau is inversely proportional
to their mass (Heisenberg uncertainty). The weak interaction is feeble
not because the charges are small, but because the lifetime of the
exchanged particles is so short! If you can get this concept (due to
Yukawa) across, it will inform all of your explanations of elementary
particle physics.

The "four forces" are really an historical artifact from about 30 years
ago. The modern equivalent might be three forces. It refers to grouping
the interactions by the particles which mediate them. Thus the
electro-weak forces are mediated by the photon-W-Z group of particles, and
the strong forces are mediated by the gluons (and by the short lived
mesons bound by those gluons), and the third force is gravity. Thirty
years ago Ward, Weinberg, Salaam, and others started to figure out the
details of grouping the photon with the (then hypothetical) W+.W-, and Z.

Now if we really knew the details of how gluons work!

PS. I just subjected a student to a discussion of leaky capacitors.
Thanks!

On Sat, 15 Nov 1997, LUDWIK KOWALSKI wrote:

Date: Sat, 15 Nov 1997 14:47:44 -0800 (PST)
From: John Mallinckrodt <ajmallinckro@CSUPomona.Edu>
Subject: Re: forces

With regard to "the four forces" ...

Isn't it really time to start saying "the three forces"?

John
-----------------------------------------------------------------
I know you are being sarcastic. Who am I to make a decision of that
kind? Didn't people who know so much more than I discover such things
as -W, +W and Zo bosons which, as they say, "mediate week forces".
I just want to know which particles interact by exchanging these very
heavy bosons (91 GeV for Zo, for example) and what are dirctions of
forces.

I know that my background is not sufficient to understand their physics.
But is this not true for at least 99% of those who teach introductory
physics courses (high schools and universities)? I may be wrong, please
do not be insulted. Even if the number is only 75%, or 50%, my question
is valid:

WHY DO ELEMENTARY TEXTBOOKS INSIST THAT THERE ARE ONLY FOUR FUNDAMENTAL
FORCES? How can I explain this to students without being Aristotelian?

Ludwik Kowalski
P.S.
I think that experts would say that a "strong" force between nucleons
is attractive at longer distances and repulsive at short distances. The
repusion, they would add, is due to the Pauli principle. The strong force
does not trouble me (as long as I treat nucleons as elementary particles
and as long as I am not asked which potential is more realistic and why).
But the weak force does. Can somebody share a good way of teaching about
week forces?