= Static magnetic grating in a moving frame
{title2=$K'^2=-k_u^2$}
Boosting a static spatially periodic <magnetic field> produces temporal oscillations together with an <electric field>. Its spatial-grating four-wavevector remains spacelike, rather than the null four-wavevector of a vacuum <plane wave>. For a boost along the grating axis, $|E'|/|B'|=v<c$. Ultrarelativistic motion can make the field approximately wave-like along a prescribed trajectory without making it an exact source-free radiation field.
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