ความหนาแน่นกระแสสี่มิติ

In special and general relativity, the four-current (technically the four-current density) 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.