Multiverse
lastmod 2015-11-05created 2015-11-051 edits
Multiverse theory is the theory that the universe routinely branches according to the various events and conditions that occur, and that universes in which different things happen are unfolding simultaneously in places unknown to people.
The multiverse is considered a useful theory for explaining inflation theory, M-theory, quantum mechanics, and so on, and it is related not only to the scientific community but also to art and philosophy. There is a view that, using multiverse theory, paradoxes caused by time travel would not arise, so it might be possible to build a device such as a time machine. This is because even if one went back to the past and had some effect, the universe affected by it and a universe unrelated to it would proceed in parallel.
There are also negative views of the multiverse. The argument is that, as long as it is impossible to observe other universes proceeding in parallel, most of which do not affect our universe, it is not reasonable to say that 'something unobservable exists'.
The multiverse and parallel universes are sometimes used interchangeably, but strictly speaking they are different concepts. Parallel universes are a sub-concept of the multiverse: the theory that the numerous branes described by the multiverse present other possible outcomes that our universe could take. Lisa Randall, who advocates the multiverse but rejects parallel universes, argues that each individual brane (universe) cannot be regarded as identical to the universe we currently live in. According to Randall's explanation, a 4-dimensional spacetime may exist like a brane within a space of 5 or more dimensions, but that other universe would be a world completely different from ours, and there could be no other version of us. Therefore, there is clearly a difference between the multiverse and parallel universes.