Actually, **ZenBeam**, that is exactly what **Ring** is doing. “Spacetime Physics”, by Taylor and Wheeler (the worst book on relativity I’ve ever seen, btw), uses relativistic mass throughout.

On p.232, the problem is to calculate the relativistic invariant quantity E[sup]2[/sup]-(pc)[sup]2[/sup] for different systems of photons and particles. They call this the “system mass”, probably as a way of making the invariance seem more natural.

I should be clear that the idea of relativistic mass is not wrong. It’s just different language. If you redefine the mass, then momentum is still p=mv, which some people find more natural. But a lot of physicists find it unnecessary – rest mass, energy and momentum are more than sufficient, and more natural, IMO, since a particle’s mass is not well-defined outside of its rest frame.