It's my understanding that there is a clearly defined electron rest mass. In atoms, electrons in higher energy levels have a larger amount of energy than electrons in lower energy levels. I think e=mc^2 shows mass-energy equivalence. So seeing all of this, I thought that electrons' masses in different levels must be different. Is this true? I've also read that the electron's mass cannot change, or else the universe would be completely different, if not completely disappear. So my question is, can an electron's mass change?