= Four-vectors and experimental charge invariance
A <Lorentz four-vector> is a set of four components transforming as $A'^\mu=\Lambda^\mu{}_{\nu}A^\nu$ 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 $A_\mu B^\mu=A^0B^0-\mathbf A\cdot\mathbf B$ 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. https://doi.org/10.1103/PhysRevLett.5.562[Search for a Small Charge Carried by Molecules (1960), hydrogen and helium efflux measurements]{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.
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