An observer may categorize a photon by two 4-vectors, k and epsilon. The 4-vector k is the energy-momentum 4-vector of the photon. The time-component of k is the frequency. If we multiply this by Planck's constant we obtain the energy for that observer. If we multiply the spatial components by h/c we get the momentum vector of the photon for that observer. The 4-vector epsilon is the polarization 4-vector of the photon.

Now two 4-vectors means 8 numbers, but an electromagnetic field has 6 components, so not all of these numbers can be independent. First of all, k^2 must equal zero for all observers because the photon is a massless particle. Second, by convention the polarization has a magnitude of -1 (time-like metric) or +1 (space-like metric), so there are six independent numbers. But wait a minute! By convention, k and epsilon are orthogonal 4-vectors, so there are only 5 independent numbers. Fortunately everything is OK, because there are really only 5 independent components for the electromagnetic field of a photon. The electric field and the magnetic field are both 3-vectors (6 numbers), but for a photon they are orthogonal, so there are only 5 independent quantities.

There is no intrinsic energy of any particle, massless (like a photon) or massive. Observers record different energies and momenta, but all agree that E^2 - (pc)^2 = (m*c^2)^2 where m equals zero for a photon or the rest mass in the case of a massive particle.

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