If you stay along the line between the photons, in your frame their relative speed is just 2c, c in each direction. If you're moving relative to that line, in your frame they each travel at c but not in opposite directions, so their relative speed is less than 2c. If you are heading away from that line with speed v then you see their relative speed as 2c(1-v
2/c
2)
1/2.
Mike W.
(published on 02/19/2013)
