The idea of parallel worlds and the tunneling in between is probably as old as mankind, if one looks e.g. at old mythologies (Hades, Olymp, earth and these tunneling guys) and religions. However the multiverse discussion got a major boost in the last century due to the dual nature of quantum mechanics and its various interpretations, including the many-worlds interpretation.
Last not least quantum computers make use of parallelism (which on the other hand doesnt mean that they can solve certain problems much faster then old fashioned digital computers (as was referred to in this randform post)). The trend of parallelism and the question of choice in everyday life was also discussed in focus and context part II .
Likewise the idea that there is a god or many gods or other creatures or noncreatures who created this our – what we perceive – human world as a kind of “simulated environment” is probably also as old as mankind. Whereas there are currently big discussions (IF we assume that there exists an or many outer existences) about the HOW of this “simulated environment” should look like, i.e. e.g. wether we were set into this world as more or less fully developped kind of avatars or not.
At this point it should be noted that the question of when a simulated environment is fake becomes in this context a real high-wire act of the mind.
By the way, the question wether there exists an “outer world”, which can not be accessed by humans — even not with all powers of mathematical abstraction — or the question wether “science progresses towards a sort of utopian ultimate understanding of the world” (citation from here) is again currently strongly debated in the math/physics community (see e.g. here)
Due to the growing capabilities of (sofar mostly digital!) computers a discussion which merges the two concepts of multiverse/parallelism and simulation or real got a boost often related to the term metaverse and likewise there are again modern discussions of the old questions.
Andreas Lange of Computerspielemuseum is currently collecting existing digital computer metaverses mainly in order to investigate their juridicial and social implications. This is his sofar collected list (besides second life and WoW):
(suggestions for more metaverses are welcome)
And last not least: Spreeblick has a very Berlin based ranking on metaverses, which is understandable under the premise that Berlin has a very active gay life.
And a last silly (?) remark: parallels meet at infinity…at least in a projective geometry math simulation…:)