Why are photons massless, but have energy and momentum?
The photon is massless, has no electric charge, and does not decay spontaneously in empty space. A photon has two possible polarization states and is described by exactly three continuous parameters: 1) the components of it's wave vector, 2) it's wavelength, and 3) the direction of it's propagation. If the photon were not massless, it would not move at the speed of light in a sealed vacuum. It's speed would be slower and would depend on it's frequency. It has energy because it is a tiny packet of energy that is produced by electrons becoming more energetic and "jumping" from energy level to energy level at the atomic level. The summation of all these tiny packets of energy create light, which to this present point in time, is defined by the wave-particle dual theory of light behavior.