parallel universe or alternative reality is a hypothetical self-contained separate reality coexisting with one's own. A specific group of parallel universes is called a multiverse, although this term can also be used to describe the possible parallel universes that constitute reality. While the terms parallel universe and alternative reality are generally synonymous and can be used interchangeably in most cases, there is sometimes an additional connotation implied with the term alternative reality that implies that the reality is a variant of our own.
Dear Manuel, for a formal definition of the concept of Local Universe see here:
https://www.researchgate.net/publication/263104630_A_simple_definition_of_Time?ev=prf_pub
M-theory is a theory that is based on strings as the 'foundamental brick' of the universe, but, my opinion, it has also trapped inside the Relativities failure vicious circle. Since, GR is not a valid theory, but it is only a nice hypothesis (see many discussions here in RG), every theory that will try to build our cosmos and is based on false foundations, is doomed to fail. That is the reason why, despite of the non existences of point singularities, the super-strings theories have never been verified in Lab, almost 30 years after their first presentation and with CERN in full capacity...
The existence of many Local Universes can be accepted, among other reasons, from the Axiom that Nature should be more complicated (in general) than any of its subsets (for example: from Mathematics).
Article A simple definition of Time
No. The equations that describe natural phenomena have many solutions-additional criteria are required to deduce whether all such solutions are physical, which are, or that none are. By construction, any description of phenomena, that takes place in more than four spacetime dimensions, describes multiple four-dimensional universes: the multiplicity is due to the extra dimension(s). The question is, therefore, whether such a description can lead to, in principle, measurable effects in any one universe.
So, while M-theory *can* describe many four-dimensional universes, since it's a theory in eleven spacetime dimensions, whether quantities measured in any one of them are sensitive to the others is an independent question.
I can't comment on M-theory demonstrating parallel universes, aside to say that until M-theory is proved, I doubt it. However, parallel universes from quantum mechanics have been proved with a physical, table top experiment that I conducted, multiple times, which anyone can read about here - https://www.prlog.org/12613870-new-similar-experiments-dramatically-achieve-rainer-plaga-suggestion-to-prove-parallel-universes.html .
The bottom line is that retrocausality, time travel paradoxes, and how Everett/Wheeler works is all reconciled under John A. Wheeler's Participatory Universe model and this dramatic experiment proves it.
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