Four-current

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In special and general relativity, the four-current (technically the four-current density)^[1] is the four-dimensional analogue of the current density, with the dimension of electric charge per time per area. Also known as vector current, it is used in the context of four-dimensional spacetime, rather than separating time from three-dimensional space. It is a four-vector and is Lorentz covariant.

This article uses the summation convention for indices. See Covariance and contravariance of vectors for background on raised and lowered indices, and raising and lowering indices on how to translate between them.