is an excellent way to talk about how much relativity is having an impact on distance, time, and mass. Einstein realized that in order to rewrite Newton’s laws for relativity he needed to be careful about whose time he was talking about. But that wouldn’t be consistent with the interpretation of…Gravitational mass, which creates gravity and pulls other matter toward it, is the second property of matter. For example, if you had a crazy big laser beam (all energy, no matter), that beam would have some gravity. And the kinetic energy of an object with mass M is given by , which is very, very close to (the “rest mass energy” plus the “Newtonian energy“).