Two photons, photon A and photon B, are traveling in the same direction at the speed of light, $c$. How fast does photon B appear to be traveling from the perspective of photon A?
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$A$ doesn't have a perspective, there are no coordinate frames moving at $c$.
Nevertheless, you can naively use the velocity addition fromual:
$$ v_{21} = \frac{v_1+v_2}{1+v_1v_2/c^2}=(c-c)/(1+(c)(-c)/c^2)=0/0$$
well I guess you can't. See 1st paragraph.
JEB
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