Remember what relativity does to simultaniety: As I begin to walk slowly across the room the answer to the question: "Has episode IV transpired yet in that galaxy far far away?" changes from "yes" to "no'; in those two frames before and after I began to walk.

The photon traverses a world line:
x = t
This equation remains unchanged in our other coordinate systems:
x' = t'.

The question "Where is the photon now?" becomes "For which value of x' does <x', ?> denote a point on the world line of the photon?".