why the photons are massless?

why the photons are massless?

The photon is the gauge boson of an unbroken gauge symmetry (the U(1) symmetry of the electromagnetic interactions). As such, it is exactly massless: there is no way to add a mass term for the photon to the Lagrangian describing electromagnetism without breaking the gauge symmetry. This is the same reason why the gauge bosons of the strong nuclear force, the gluons, are massless. They are gauge bosons of an intact SU(3) symmetry.

As others have noted, all experimental evidence is consistent with the photon's being exactly massless. As a quick mnemonic, photons from the farthest reaches of the universe come to our telescopes. This points to the electromagnetic force as having an infinite range (corresponding to zero mass for the photon).

Inside a superconductor, due to local interactions within the material, the U(1) symmetry is broken, leading the photon to behave as if it has a mass (the Meissner effect).

 

αναδημοσιευση απο το QUORA 26/6/2017