Hi PhysicsForum, I'm currently reading Spacetime Physics by Taylor and Wheeler and I can't wrap my head around the concept of mass in SR. In the textbook, mass is described as the magnitude of the momenergy 4-vector and is invariant no matter which inertial reference frame you choose. So does (relativistic) mass of an object increase with velocity or does mass remain invariant (since it would be possible to pick a reference frame at the same speed as the object and thus it's mass would only be its rest-mass)? The latter seems like the correct concept, and if so, how is the impossibility of an object to ever attain the speed of light explained? Would it be correct to say, an object moving at the speed of light would theoretically have an infinite amount of energy (E = mc2/√(1-v2/c2), denominator goes to 0 as v → c)? Thanks for the time to read such a seemingly elementary question!