A short derivation of the relativistic forms of energy and momentum

[PAllen]

Yes, in this way you define the total mass of a closed system, but that's not the mass of the particle. It's mass defined by $P_{\mu} P^{\mu}=M^2 c^2$, where $P^{\mu}$ is the (conserved!) total four-vector of the system.

Never, ever, did I say anything about particle mass. I explicitly said that’s not what I was referring to, from the very first post on this.

 

[vanhees71]

Sorry for the misunderstanding.