Hello. I am independently studying modern physics. I have a basic special relativity question. Consider a completely inelastic collision Of course, objects are moving at a constant velocity before and after the collision with respect to a stationary observer. Let's call him Rick. Suppose that Rick calculates the difference between the sum of the masses of the objects when they are apart and the mass of the composite object they make after they stick together. In his frame, he finds M. Here is the question. I know other inertial observers in different frames will measure different values of momentum and kinetic energy for the objects. I think, though, they will measure the same mass defect (M)? If not, why? P.S. No force fields around.