MASSLESS ENERGY. “For example, how can we find the linear momentum or kinetic energy of a body whose mass is zero? This apparent paradox vanishes if we describe a photon as a relativistic particle.”
https://opentextbc.ca/universityphysicsv3openstax/chapter/the-compton-effect/#:~:text=The%20Compton%20effect%20is%20the,wavelength%20of%20the%20incident%20radiation