How can we said that photon does not have a mass?

[aloshi]

how can we sad that photon does not have a mass? in my book they are a piece that there write at the photon influence of gravitation?

 

[tiny-tim]

how can we sad that photon does not have a mass? in my book they are a piece that there write at the photon influence of gravitation?

Hi aloshi!

Mass is energy, energy is mass.

A photon does not have rest-mass, but it does have mass.

A box into which photons are inserted (from outside) weighs more, and exerts more gravity.