I’m at this point pretty confident that under the Copenhagen interpretation, whenever an intergalactic photon hits earth, the wave-function collapse takes place on a semi-spherical wave-front many millions of lightyears in diameter. I’m still trying to wrap my head around what the interpretation of this event is in many-worlds.
Under MWI, before the photon (a peak in EM field) could hit Earth, there were a lot of worlds differing by EM field values (“electromagnetic tensor”) - and, thus, with different photon directions, position, etc. Each of those worlds led to a variety of worlds; some, where light hit Earth, became somewhat different from those where light avoided it; so, integrated probability “photon is still on the way” decreases, while P(photon has been observed) increases. Whenever some probability mass of EM disturbances arrives, it is smoothly transformed, with no instant effects far away.
Under MWI, before the photon (a peak in EM field) could hit Earth, there were a lot of worlds differing by EM field values (“electromagnetic tensor”) - and, thus, with different photon directions, position, etc. Each of those worlds led to a variety of worlds; some, where light hit Earth, became somewhat different from those where light avoided it; so, integrated probability “photon is still on the way” decreases, while P(photon has been observed) increases. Whenever some probability mass of EM disturbances arrives, it is smoothly transformed, with no instant effects far away.