I've looked in several sources but haven't seen this addressed specifically.(adsbygoogle = window.adsbygoogle || []).push({});

If you have a ball of static electric charge, then the four-current is timelike. If the ball is moving, then you can transform to its rest frame, in which the four-current's spacelike components are all zero.

If you have a wire carrying current and it has no net charge density, then the four-current is spacelike. If the whole wire is moving, one can transform intoitsrest frame, in which the four-current's timelike component is zero.

It seems that one can always decompose a spacelike vector into two timelike vectors: for example, the four-current of the moving electrons and the four-current of the stationary protons.

As for null four-current, I wonder if there is any special significance, other than the fact that it is null in all reference frames? (Even by itself, this seems pretty intriguing.)

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# Timelike vs. null vs. spacelike four-current

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