Charge invariance refers to the fixed electric charge of a particle, regardless of speed. For example, an electron has a specific charge at rest. At a velocity v, the charge remains the same (as opposed to the energy increasing). Some particle characteristics are relativisticly invariant (electric charge and rest mass). Others, like energy, change with velocity. Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... e- redirects here. ... The invariant mass or intrinsic mass or proper mass or rest mass or just mass is a measurement or calculation of the mass of an object that is the same for all frames of reference. ...

Typically, we write:

where the subscript v represent the quantity at speed v. The key factor is:

which is a function of speed relative to rest frame. The first equation above describes relativistic energy, the second – invariant mass (or rest mass), the third – charge invariance. Notice that γ does not appear in the second and third lines. Notice also that as v (speed) increases, γ increases, and so E increases too. The invariant mass or intrinsic mass or proper mass or rest mass or just mass is a measurement or calculation of the mass of an object that is the same for all frames of reference. ...

The property of charge invariance follows from the vanishing divergence of the charge-current four-vector , with . In relativity, a four-vector is a vector in a four-dimensional real vector space, whose components transform like the space and time coordinates (ct, x, y, z) under spatial rotations and boosts (a change by a constant velocity to another inertial reference frame). ...