= Photon separation under a collinear Lorentz boost
{title2=$d'=d\sqrt{(c+v)/(c-v)}$}
For two <photons> emitted simultaneously a distance $d$ apart in one frame and moving in the same positive direction, the world-line intercept $x-ct$ transforms to $\gamma(1+v/c)(x-ct)$. Their equal-time separation in the boosted frame is therefore the displayed constant $d'$. It is different from length contraction of the emitters, because the emission events are not simultaneous after the boost.
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