Pondering the possibility of a Multiverse has astounding implications about the nature of reality.
https://en.wikipedia.org/wiki/Multiverse#History_of_the_concept
The Many-Worlds Interpretation of Quantum Mechanics https://plato.stanford.edu/entries/qm-manyworlds
https://en.wikipedia.org/wiki/Many-worlds_interpretation
https://www.nature.com/articles/d41586-019-02602-8
https://www.nature.com/articles/4651010a
Of course it eventually worked its way into fiction.
https://en.wikipedia.org/wiki/Multiverse_(DC_Comics)
https://en.wikipedia.org/wiki/Multiverse_(DC_Comics)#Fictional_history_and_structure
https://en.wikipedia.org/wiki/Multiverse_(Marvel_Comics)
https://en.wikipedia.org/wiki/Multiverse_(Marvel_Cinematic_Universe)
The parallel universe that's part of the Mythology of the Fringe TV series
https://en.wikipedia.org/wiki/Mythology_of_Fringe#The_parallel_universe
The Fringe_(TV_series) was fantastic science fiction and quite thought provoking entertainment.
https://en.wikipedia.org/wiki/Mythology_of_Fringe#Alternate_timelines
https://collider.com/fringe-tv-show
https://tvtropes.org/pmwiki/pmwiki.php/Series/Fringe
https://fringe.fandom.com/wiki/Observers
https://en.wikipedia.org/wiki/Mythology_of_Fringe#The_Pattern
https://en.wikipedia.org/wiki/David_Deutsch#The_Fabric_of_Reality
https://thedigitallabyrinth.blogspot.com/search?q=the+multiverse
https://thedigitallabyrinth.blogspot.com/search?q=The+Cosmic+Pattern
