Frequently Asked Questions
about Particle Physics


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What is the meaning of the Lie Group SU/And E?     ( Lazaro Blanco)

Does light ( photons ) interact with protons ? If so, at which frequency does light interact the most with protons. At which frequency does light gives the most energy to protons ?    Follow-up question: A proton has a mass of 1.7x10-27 [kg] which is equivalent to a rest energy of 1 GeV, that's a lot of energy. Since light can interact with protons, is there a possibility that a photon beam could cause the proton to release it's mass energy ? Could a photon beam cause protons to loose gluon cohesion ? Thefore could protons be a source of energy in the future ? 
(Zack Hardwick)

Solar Power   (Robert Kneberg, UTAH)
Recently I became interested in solar power production. In looking into this type of energy production I became curious. If we can produce energy from a very small section of the electromagnetic spectrum aka visible light then why is it not possible to harness all or at least some of the parts of the electromagnetic spectrum? For example, UV panels, gamma panels, infared panels (which is probably the most abundant source) these panels could convert what is all around us to electricity. Have materials been identified that could absorb different parts of the spectrum and convert such to electricity? Perhaps focusing part of the spectrum is the way? If either of these were developed there could be free clean power day or night. Follow-up question: I understand that silicon is used for the visible light spectrum and that other parts of the spectra will not provide the desired results in silicon. I was just wondering if there were other materials that would produce an electric current from other parts of the electromagnetic spectrum or if a search for the desired materials was even in process? 

Answers to Questions

What is the meaning of the Lie Group SU/And E?     (Lazaro Blanco)

The subject of Lie groups is quite broad, and there are a number of books devoted to it. There is really no way to do the subject justice via a short explanation. However, hopefully this short explanation might help. SU(N) is a Lie Group of N by N unitary unimodular matrices. The "U" stands for Unitary, and a Unitary Matrix U satisfies U times its hermitian conjugate is Unity (i.e., UU+=1). Hence, its determinant is equal +1 or -1. "S" stands for special in that the determinant is restricted to be +1. Such a Lie group is characterized by N continuous parameters. Its generators Ta , a = 1,..,N satisfy the commutation relations [Ta, Tb ] = fabc Tc.  The f's are known as structure constants. They are antisymmetric in their indices. 

If you'd like to learn more and get more details, a couple of good references are:
   "Lie Groups fro Pedestrians" by Lipkin
   "Lie Algebras in Particle Physics : from Isospin to Unified Theories" by  Howard Georgi


Does light ( photons ) interact with protons ? If so, at which frequency does light interact the most with protons. At which frequency does light gives the most energy to protons ? (Zack Hardwick)

This is an interesting topic. Since a proton is charged it is effected by the electromagnetic force. The carrier of that force is a photon, so indeed light interacts with protons. Using low energy photons where the wavelength is much larger than the size of the proton (10-13 m) the interaction is the same as with an electron and a photon. When the photon energy gets large (i.e., it's wavelength is small compared to the proton's size), other interesting things happen. For example, the photon can directly interact with the constituents of the proton, the quarks. Furthermore, the photon may also act like a strongly interacting meson with the same quantum numbers as the photon, namely spin 1 and negative parity. Those mesons include the rho (r), omega(w), and phi (f). When these energies is reached the cross-sections grow and many particles are produced in the collision. This figure (see bottom plot) shows the measured total cross-section of photons on protons and deuterons. High energy photon beams have been made and used to study the structure of the proton and to produce other particles for study. Some of the best measurements of the properties of charm particles have recently been made with a photon beam at Fermilab by the FOCUS experiment ( see http://www-focus.fnal.gov/  ).

Does this mean that high energy photons break the protons into it's constituents ? 
Yes, and more quarks can be "popped out" of the vacuum as the energy increases.

Does the cross section always grow with the increase of energy ?
Yes and then it levels off at about 120 microbarns (see the PDG) http://pdg.lbl.gov/2002/contents_plots.html)

Should an ultra-high energy photon beam have an ultra large cross section or is there a limit ?
No, physical cross-sections should remain finite.

A proton has a mass of 1.7x10-27 kg which is equivalent to a rest energy of 1 GeV, that's a lot of energy.
Since light can interact with protons, is there a possibility that a photon beam could cause the proton to release it's mass energy ? Could a photon beam cause protons to loose gluon cohesion ? Therefore could protons be a source of energy in the future ? 
No, total energy and total charge is always conserved, as is net baryon number (at least in today's world). This means that if a photon
strikes a proton, whatever emerges must contain at least 1 baryon (such as a proton). It's possible at high energy for it to be other baryons, but the proton is the lightest, so you can't convert any of its mass into energy. 


Solar Power   
This is very insightful. In fact there are many scientists & companies devonting a great deal of time to improving our ability to convert solar power to electrical power. As you probably know, solar panels can be purchased to put on your roof. It turns out that visible light has the proper amount of energy to liberate electrons from the material in the solar cell (mostly silicon), which can be used to generate electrical current.
UV, Infra-red and gamma do not have the proper energies to make this process work; the photons are either too energetic or not energetic enough. Moreover, the peak of the sun's spectrum is in the visible, so this is the best place to capture and convert the sunlight. Another problem is that solar energy conversion is not very efficient. Even today's state of the art gives a conversion efficiency of about 10%. That is, only 10% of the incident energy is converted to electrical energy. If one does a fairly straightforward calculation, you will find out that solar power is not very economical at this point, as compared to coal or nuclear power.
     I do feel though that there is hope in the future for this source of energy.

Here are some nice "friendly" references which may be of interest to you.

 http://www.re-energy.ca/
 http://www.astropower.com/faq_solar_basics.htm

The gist of it is that there is not very much UV energy from the sun to mine as compared to the visible part. So, I don't think it is being heavily pursued. Here is a JPEG file showing the radiant intensity from the sun (and earth), and you can see it peaks in the visible, and not much in the UV.