Four-current

In <a href="/facts/Special_relativity/vsFFc63E">special</a> and <a href="/facts/General_relativity/jVsoIg8X">general relativity</a>, the four-current (technically the four-current density) is the <a href="/facts/Four-dimensional_space/vFQd0M7T">four-dimensional</a> analogue of the <a href="/facts/Current_density/IlUKSe2B">current density</a>, with units of charge per unit time per unit area. Also known as vector current, it is used in the geometric context of four-dimensional <a href="/facts/Spacetime/uRIN1x4b">spacetime</a>, rather than separating time from <a href="/facts/Three-dimensional_space/KAqKWUbS">three-dimensional</a> space. Mathematically it is a <a href="/facts/Four-vector/89GpELMA">four-vector</a> and is <a href="/facts/Lorentz_covariant/K5AEuGKg">Lorentz covariant</a>.
This article uses the <a href="/facts/Einstein_notation/UXwS43wU">summation convention</a> for indices. See <a href="/facts/Covariance_and_contravariance_of_vectors/s3HGMkhz">covariance and contravariance of vectors</a> for background on raised and lowered indices, and <a href="/facts/Raising_and_lowering_indices/cPsSVpgI">raising and lowering indices</a> on how to switch between them.

Four-current open-in-new

Four-current