This is taken from B. Schutz's First Course in General Relativity. Am I alone in considering this an abuse of language? After all, energy and spatial momentum are NOT the same concept, although they may be both expressed in kg in the units used (in which c=1). I would rather say that a photon's energy numerically equals their spatial momentum in units such that c=1, but not in any other units. I am puzzled because I consider Schutz's text quite rigorous.