So I have only just been introduced to the concept of rest mass in Special Relativity.
Do we assume that the rest mass of a fundamental particle is constant in all inertial reference frames? i.e. is the rest mass of an electron if it is travelling at constant velocity c/2 (relative to the distant stars) the same as the rest mass of the electron if it is travelling at velocity 0 relative to the distant stars?
Is the rest mass simply the "mass" of a particle in inertial frame of reference which has the particle at it's origin (assuming the particle is actually in an inertial reference frame)?