Neumann János, Nikola Tesla, Albert Einstein
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neyoo #1071 " The gauge field now has one additional term in the free energy ~ A2. Remember that in a gauge theory, typically we only have terms like
∑ A
2. In k-space, this means that the dispersion relation of a photon is w2 = k2. However, once we have Ò A2, the dispersion becomes
w2 = k2 + Ò, and the coefficient of the A2 term is the square of the mass of photons, i.e. in a superconductor, photons become massive particles."
http://www-personal.umich.edu/~sunkai/teaching/Fall_2014/Chapter9.pdf