Aparticle of rest mass m, travelling at speed v in the x-direction, decays into two photons, moving in the positive and negative x-direction relative to the original par ticle. what are their energies? what are the photon energies and directions if the photons are emitted in the positive and negative y-direction relative to the original particle (i. e., perpendicular to the direction of motion, in the particle's rest frame)