i can see the mean of "[]" in the first row,it is the anticommunicater between \gamma{\mu} and \gamma{\nu}, but what it mean in the second row and the third?
Since E = mc^2, how can photons be massless? If a photon has no mass, then, according to Einstein's formula, its energy is given by E = 0 x c^2, which is 0. Yet, photons do have energy. This seems to be a complete contradiction. Please explain! Thank you.