= Charge invariance
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<Charge invariance> means that the total <electric charge> of a specified collection of particles is unchanged by a <Lorentz transformation>. It differs from <conservation of electric charge>, which compares the same isolated system at different times. For a convective charge distribution with rest density $\rho_0$, the <four-current> is $J^\mu=\rho_0U^\mu$, where $U^\mu$ is its <four-velocity>; its measured <charge density> increases under a boost while the simultaneous spatial volume contracts, leaving total charge unchanged.
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