# Why are photons massless?

• khil_phys
It would be more accurate to say photons have no rest mass in special relativity, but they do have a rest mass in general relativity. Note that this is different from saying a photon has no mass in general relativity.f

#### khil_phys

According to Einstein's equation E=moc2, any object which has mass has energy, and, conversely, anything which has energy must also have mass. Then, why are photons, bosons and gluons said to be massless?

According to Einstein's equation E=moc2, any object which has mass has energy, and, conversely, anything which has energy must also have mass. Then, why are photons, bosons and gluons said to be massless?

Please start by reading the FAQ sub-forum in the Relativity forum.

Also note the flaw in the logic you are applying. An entity with a rest mass has an equivalent energy content. However, the reverse is not necessarily true. There is nothing that says that ALL energy must have a rest mass. This is different than saying that a clump of energy can be converted into an equivalent rest mass.

Zz.

Why is it so?

Why is it so?

Why is what so?

In any case photons with high enough energy (> 1.022 Mev) can be converted into electron-positron pairs (rest mass of each is .511 Mev) under the right conditions.

Another example is that if you had a perfectly mirrored box with a bunch of photons inside, they *would* contribute the rest mass of the ensemble. This may be initially quite mysterious, but consider analogy with an isolated atom versus a confined gas of atoms. The isolated atom has some rest mass. The confined gas of atoms contributes more rest mass to the ensemble than the sum of atom's rest masses. The difference is the the kinetic energy of the atoms - this also contributes to mass of the ensemble. Photons have kinetic energy but no rest mass.

As to why, there are really no good answers to such a question. If you assume special relativity is true, then anything whose speed is observer independent must have no rest mass.

Isn't saying a photon has 0 rest mass another way of saying it is never at rest?

in some sense yes