Is there a name for two null lines that lie on the opposite sides of the null cone? Each line can be obtained from the other by reflection in the axis of the null cone (the time-axis). In terms of world-lines, this corresponds to two photons moving in the opposite directions. If there is not a standard name, what would you choose to call them as a pair?
Answer
There isn't an official standard name for opposite null lines. Note that opposite null lines are not a coordinate-independent geometric (invariant) notion, and hence it is not a very useful concept. If two null lines happen to lie on opposite sides of the light-cone in one reference frame, then they may not lie on opposite sides of the light-cone wrt. a boosted reference frame. Conversely, two different non-opposite null lines wrt. one reference frame may be boosted in such a way that they become opposite null lines wrt. the new reference frame.
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