Every now and then a discussion of quantum theory and its interpretation involves photons, perhaps propagating in some interferometer; e.g. when I wrote about the interpretation problem I used the picture of a Mach-Zehnder interferometer.
And then a discussion of the many worlds or the 'collapse' of the wave function often follows.
But notice that the wave function of a photon is actually a problematic concept, e.g. "...it is possible to define position operators and localized states for massive particles and for massless particles of spin 0, but not for massless particles with spin."
Certainly, the electromagnetic field is not the wave function of a photon, as sometimes implied by discussions of interference in (quantum) optics experiments.
Why do I mention this? Well, it seems to me that some people consider wavefunctions as something very real, while others (including me) think of them as descriptions of reality; It might be helpful to consider single photons before discussing the many worlds.