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at the risk of boring everybody, I have a question that has probably been asked before.

We all know that Special Relativity is based upon the concept of invariant light speed. Let some light travel a distance s in time t, then

(ct)^2 - s^2 = 0 should hold for any observer (in an inertial system).

Let anything else be at rest in some inertial system, then (ct)^2 - s^2 for this body should be the same for all observers.

OK. This leads to time dilatation and space contraction. But how about the relativistic mass formula? It seems to me that we need some extra assumptions to get that. Or do you disagree?