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 The explanation for why a massless spin-1 particle has only two polarization states, both perpendicular to the particle's momentum, is found as a result of considerations that arise in quantum field theory, so we will just accept it here. |
You can think of the source of the E transitions as being the charge probability distribution of the system, and the source of the M transitions as being the charge probability current distribution of the system... so the moments are polar vectors in one case and axial vectors in the other. Hence the difference in the parity selection rule.
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A convenient list of particle accelerators, past and present, can be found here.
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Magnetic monopoles were studied by Dirac in 1931; their hypothetical existence can be used to explain charge quantization. However, experimentally there is no trace of such particles in our universe. Monopoles do feature in some efforts to extend physics beyond the Standard Model. You might have fun thinking about how Maxwell's equations would be modified if monopoles existed in everyday life--- the modifications are simple and straightforward and make the equations much more symmetric. Where are they? [Recent biography of Dirac.] |
An article about the history of magnetic monopoles in modern physics. |
In the Standard Model there is no obvious reason why CP-violating processes should not be observed for strong interactions. It was suggested that if the free parameter (an angle θ) in the CP-violating term were actually a pseudoscalar field, with a 0− boson called the axion, the suppression of such processes could be understood. Axions have not been detected (they would have properties much like the π0 meson but with much, much smaller mass, probably much less than 1 eV), but their properties are so fascinating that there has been a lot of effort to detect them in various ways.
 Maxwell's equations with axions! |
 Shining light through solid walls! In a strong magnetic field, photons can convert to axions, or axions can convert to photons. |
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The axion could couple to photons, gluons, leptons and quarks. It is also a candidate for “dark matter,” as we will discuss later.
By the way, since Maxwell's equations are already relativistically correct, it is straightforward to put them in covariant form, without changing much of anything other than notation.
Have you ever heard of the incredible war between proponents of vectors and proponents of quaternions, which had a drastic effect on the history of physics in England in the late 19th Century?
A famous modern application of quaternions is to avoid the notorious problem of gimbal lock.