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

[curtis osterhoudt <flutzpah@yahoo.com>]

  I agree with Jack that a good starting place is Feynman. However, the conclusion you can draw from Feynman is that photons exert all sorts of longitudinal and transverse forces (whatever those mean in the context of relativistic movements), especially at the scale of the atom. To wit, a parenthetical remark from QED, Chapter 3: 

"Perhaps you have heard somewhere that light comes in
only two states of polarization - for example, a photon going in the
Z direction can be polarized at right angles, either in the X or Y
direction. Well, you guessed it: in situations where the photon goes
a long distance and appears to go at the speed of light, the amplitudes for the Z and T terms exactly cancel out." 


I read this as being yet another manifestation of the difficulty in trying to temporally or spatially nail down photons; restrict their paths to go between a proton and an electron, and something has got to pop out the sides and give strong evidence of degrees of freedom. In this case, it's the photons' longitudinal polarizations which don't cancel out over short paths.
   One book which might be helpful for a not-very-detailed description of how these things fit together is Brian Duff's Fundamental Particles, An Introduction to Quarks and Leptons. It's not a comprehensive overview, and I vehemently disagree with some of his Feynman diagrams in the beginning (they purport to show scattering of the particles exchanging photons, but show no spacelike change whatsoever), but the big picture is nice, and not cluttered too much with lots of details. 

                    Regards, 
                            Curtis







________________________________


From: Jack Uretsky <jlu@hep.anl.gov>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Saturday, December 13, 2008 8:44:51 PM
Subject: Re: [Phys-l] Question about Quarks and the Standard Model

    There is at least one misconception here.  See below.
On Wed, 15 Oct 2008, 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?"
    No, the coulomb force does not come from the "exchange of gauge 
bosons (the photon)".  Photons exert only transverse forces; the coulomb 
force is longitudinal.

    Why are photons important?  It depends on what questions you want 
to answer.  If you want to explain the photelectric effect, photons are 
crucial.  If you want to explain the cosmic microwave  background,  photons are 
immensely helpful.
    An excellent starting point for all such questions is  the Feynman 
Lectures.  Start with I-2-7, and go on from there.  I do NOT recommend the 
Wikipedia article, or any other discussion that speaks of ``wave-particle 
duality'' which, in my view, is a nonsense concept (for the benefit of the 
more advanced practitioner, Bardeen, among others taught us that the 
relevant dichotomy is coherence and incoherence).
    If I were teaching about photons, today, I would focus (sorry 
about that) on the role of photons in vision.  The rods and cones in the 
eye are remarkably sensitive photon detectors, playing, respectively, 
quite different roles.  I hope you get a chance to enjoy learning about 
this stuff.
                Regards,
                    Jack


  >
> 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?  ;-)
>
> 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?
>
> Question 4
> In Your Humble Opinion, what Is the most important "thing" high school
> students should remember about quarks and/or the "standard model?"
>
> Thanks in advance,
> -Tony

-- 
"Trust me.  I have a lot of experience at this."
        General Custer's unremembered message to his men,
        just before leading them into the Little Big Horn Valley



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