Relativity is the property that determines the mass of one particle. It doesn’t change. It is equal to its rest mass. Unfortunately, relativity propagators like to talk of relativistic mass growing. Relativistic mass can be defined as the length of the energy-momentum 4vector in Minkowski space. Special relativity considers the length of any 4-vector an invariant. The problem gets more complicated if there are many electrons in a system, such as those found within an atom. System mass can be given to a system of particles even if they have no rest mass. The system mass is the length the energy-momentum 4vector of the system. Due to the equivalence between mass and energy, the total mass for a relativistic system is not simply the sum of all the constituents. It is always different.

When expressed in the most practical units, the relativistic invariant weight of an isolated electron is approximately 511 keV.