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Re: [Phys-l] Question about Quarks and the Standard Model

On 10/15/2008 06:09 PM, Tony wrote:

Question 1 ..with parts
Protons feel the electromagnetic force due to an exchange of gauge bosons,
(the photon). Why is this exchange particle important? Is the particle a
signal to one proton that a like charge is near? And do we know what the
proton, or any particle, does in response to receiving an "exchange
particle?"

Here's a diagram of the ideas:

1: Electrostatics; action at a distance
||
(time dependence + ||
relativistic causality) ||
\/
2: Electrodynamics; action mediated by a wave
/\
(wave/particle duality) ||
\/
3: Electrodynamics; action mediated by a particle


To say the same thing in words: 1: Electrostatics is fine in the
limit where the timescales are long (compared to distance/c) and/or
the distances are short (compared to time*c). You have the force
from A acting on B, and if asked how the force gets from A to B
you can say "hypotheses non fingo".

2: If you look at dynamics, with sufficiently short timescales and
sufficiently long length scales, relativistic causality requires
you to have something that carries the force from A to B, i.e.
something that can exist in the space between A and B. For centuries
there was a debate over whether the carrier was a wave or a particle.

3: Quantum mechanics tells us that asking about the difference
between wave and particle is asking the wrong question. QM
requires you to be able to treat the carrier as a wave, as a
particle, as both, or as neither. IHMO the "neither" option
is the best: there is no such thing as a particle (strictly
speaking) and no such thing as a wave (strictly speaking);
there is just /stuff/. Sometimes stuff acts approximately
like a wave and sometimes stuff acts approximately like a
particle.

=========

To answer another part of the question: The canonical model
for introducing the idea of a force mediated by the exchange
of particles is as follows: You have two persons wearing
ice skates on a very slippery ice rink. They play catch.
Whenever either person throws the ball, he recoils backwards.
Whenever either person catches the ball, he recoils backwards.

This is a nifty model for a repulsive force. Extending the
model to attractive forces is possible but messier.

Like all models, this model is imperfect. For starters, it
involves the exchange of a /real/ ball instead of a virtual
ball. The difference between real and virtual is closely
analogous to the difference between "above cutoff" and
"below cutoff" for waves in a waveguide. That in turn
corresponds to propagating versus evanescent waves. You
can easily observe evanescent waves by pressing your thumb
against the side of an aquarium. Your thumbprint is
visible because it absorbs the evanescent wave, ruining
the total internal reflection.


Why is this exchange particle important?

Because electromagnetism is important. We are using the exchange
of virtual photons to represent *all* of the electromagnetic
interaction.

Is the particle a
signal to one proton that a like charge is near?

That's an unnecessarily crude way to put it. The electromagnetic
force is much more than a "signal". The exchange of virtual
photons *is* the electromagnetic force, i.e. the electromagnetic
interaction.

And do we know what the
proton, or any particle, does in response to receiving an "exchange
particle?"

We know that quite precisely.

The interaction between a charged particle and a virtual photon
is governed by the same laws as any other interaction between
a charged particle and the electromagnetic field. (There is a
quirk associated with the polarization of the virtual photon,
but that's more than you need to know right now.)



Question 2
A couple of sites I ran across, e.g. http://arxiv.org/abs/astro-ph/0606093 ,
mentioned "Quark Stars." Are there any other applications/explanations using
quarks? I trying to come up with tangible reasons for studying quarks that
go beyond their cool strangeness? ;-)

Quark stars are IMHO pretty far down on the list of "applications" of
quarks.

Quarks were invented because there is a "periodic table" of elementary
particles. There are patterns and regularities in the properties of
the particles, and in the interactions among the particles. These
regularities are best explained in terms of quarks. Actually the
history was the other way around: the quark hypothesis facilitated
the construction of the table. The pièce de résistance was that the
constructed table had a gap in it, which was immediately interpreted
as predicting a hitherto-undiscovered particle, the Ω-. When that
particle was discovered a couple of years later, with the predicted
properties, it was Nobel prizes all around.

It doesn't get any better than that. What are quarks good for? They
are good for constructing periodic tables and explaining the properties
of known particles ... as well as not-yet-known particles.

Question 3
Finally, I've been reading various edu sites on the net about quarks and the
standard model. Does anyone have a recommendation of a intro level book or
article(s) that I could read for more information?

http://en.wikipedia.org/wiki/Eightfold_way_(physics)
http://en.wikipedia.org/wiki/Quark_model
http://35.9.69.219/home/modules/pdf_modules/m282.pdf


Question 4
In Your Humble Opinion, what Is the most important "thing" high school
students should remember about quarks and/or the "standard model?"

Here's a useful exercise: Start with the periodic table of the
plain old elements: hydrogen, helium, et cetera. Then have the
students work out what it would look like if all electrons, rather
than having two spin-states (up and down) had three states (+, 0, -).
-- What happens to the periodic table? How many columns per row?
-- What happens to the Pauli exclusion principle?
-- What does the new covalent bond look like?


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


What follows is probably not the answer you were expecting, but
it does fall within the scope of the question:

The photon (virtual or otherwise) is the _gauge boson_ of the
electromagnetic field.

That is reeeeally important ... but it doesn't mean much unless
you know what a gauge is, and what gauge invariance is.

I'm not going to explain how to get from gauge invariance to a
gauge boson ... but I will say that the idea of gauge invariance
is more age-appropriate and generally more useful than everything
we have said about quarks.

I am quite insistent about this. I love asking students (and
everybody else) why a voltmeter has two leads, not one. Hint:
the answer I am looking for is "gauge invariance".

Once upon a time, I was recruiting a guy who had done his PhD
thesis on the noise in amplifiers, including common-mode
rejection, line-noise rejection, et cetera. Among other things,
he found there were some fundamental limits that had nothing
to do with e or kT or ℏω but were instead explainable in terms
of gauge invariance. He started to explain what he meant by
that. I told him he didn't need to explain. He kept going.
I told him again, "you don't need to tell me what gauge
invariance is". He was shocked; he said I was only the second
person he had ever encountered who knew what he was talking
about. Then it was my turn to be shocked.

Understanding the symmetries of the natural world is IMHO the
heart and soul of what physics is. Gauge invariance is high
on the list of important and useful symmetries.

The concept extends far beyond voltmeters and circuits. I get
a couple dozen hits from
http://ipv6.google.com/search?q=gauge+invariance+site:nobelprize.org

If you want something that students can play with hands-on to
get a feel for electrostatics including the gauge invariance
thereof, check out
http://www.av8n.com/physics/laplace.htm

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

You should re-read
Feynman volume I chapter 52
"Symmetry in Physical Laws".