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A Lorentz four-vector is a set of four components transforming as $A^{\prime \mu }={\Lambda }^{\mu }{}_{\nu }{A}^{\nu }$ under a Lorentz transformation. Examples include the four-position and four-velocity. A Lorentz invariant is a quantity unchanged by that transformation. With metric signature $(+---)$, the scalar product $A_{\mu }{B}^{\mu }=A^0{B}^0-\mathbf{A}\cdot \mathbf{B}$ is invariant. The components of a four-vector are generally not individually invariant.

The proton's total electric charge is taken to be a scalar: it does not acquire a factor $\gamma$ when the proton moves. Experimental evidence comes from sensitive charge neutrality tests of matter. Hydrogen molecules and helium atoms remain neutral despite different binding and internal Electron and nuclear motions. If constituent charge scaled with kinetic energy as relativistic mass does, the same cancellation of fixed numbers of proton and Electron charges would generally fail in different bound systems.

In a gas-efflux test of charge neutrality, gas escapes from an electrically isolated vessel connected to an electrometer. Any net charge per escaping molecule would leave a measurable opposite vessel charge, after residual ions and charged contaminants are removed. The hydrogen/helium efflux experiment supports velocity-independent additive charge. ^[ref]

This is evidence for charge invariance, not merely conservation of electric charge: the latter concerns constancy during a process, while invariance compares descriptions or internal motions without rescaling charge by a boost factor. Net-matter neutrality constrains combinations of constituent charges; it should not be described as an exact proof covering every possible velocity-dependent microscopic model.