> Mass and energy are equivalent. Mass is energy at rest, hence equivalence with exception of massless particles like photons that have zero mass and non-zero

> Mass and energy are equivalent. Mass is energy at rest, hence equivalence with exception of massless particles like photons that have zero mass and non-zero energy. Also, photons travel with the speed of light in vacuum and cannot be found at rest in any frame of reference. Modern physics is fun, isn't it? P.S. Neutrinos were thought to have zero mass as well, but according to the Standard model they have mass.