Photons Moving

Emc2 - Meanings of The Strict Mass–energy Equivalence Formula, E = Mc² - Massless Particles Contribute Rest Mass and Invariant Mass To Systems
... Two photons moving in different directions cannot both be made to have arbitrarily small total energy by changing frames, or by moving toward or away from them ... The reason is that in a two-photon system, the energy of one photon is decreased by chasing after it, but the energy of the other will increase with the same shift in observer motion ... Two photons not moving in the same direction will exhibit an inertial frame where the combined energy is smallest, but not zero ...