Obviously there are various multiverse theories such as open multiverses, bubble theory, Everett’s interpretation, ensemble theory, string landscapes etc. but supposing we accept that there are multiple / infinite realities and that perhaps the whole universe is simply part of an atom that in turn is making up part of a grain of sand in someone’s pocket that is in turn part of a universe that is a divisible part of another atom etc. then – and I am struggling to even crystallise the question given the enourmity of the concept – are we ever going to come up with a truly unified ‘theory of eveything’ or should we just give up since it is all too mind blowing? What limitations / arguments are there against the concept detailed and does it conflict with accepted theory that there may be multiple laws of physics for different multiverses i.e. one multiverse full of antiquarks, another without any, one where light travels at a different speed, another that is closed, another open etc etc?
Also, both from a physics and even a philosophical perspective, how would one travel between multiverses and would it even be possible? How would they be bound, especially if physics / governing rules were indeed different in each entity? Even with 4 dimensions we can’t even map our own universe due to relativity so how could we expect to map multiple universes in relation to each other?

My head hurts.

The universe might be all twisted in on itself and just be a speck of dust in your own pocket. The main problem I have with the idea of multiverses is this: ITS NOT SIMPLE! Remember Occam’s razor…given two explainations for a phenomenon, the simplest explaination is most likely to be correct. The heliocentric model of the solar system was eventually accepted because it was conceptually simpler than believing in epicycles and celestial spheres. When it comes to interpretations of quantum mechanics and "theories of everything" the same rule should apply. Personally I find it easier to believe in one universe governed by probability (or by non probabalistic laws that we just don’t yet understand) than to believe in an infinite number of undetectable parallel universes. Maybe I’ll believe in multiverse theory when it makes testable predictions that differ from other interpretations of quantum mechanics. However, when it comes to understanding nature I try to let simplicity be my guide. When I see an airplane, I just accept that it is an airplane and not a UFO filled with gelatenous chess playing monsters shaped like an airplane.