Movie · Avengers: Endgame (2019)
Name only. The word is real. The physics in the scene is not.
Pass through the Quantum Realm and you can step back in time, and the film's own rule is that changing the past spawns a new branch instead of rewriting your own future.
General relativity does allow time travel on paper: Gödel found solutions with closed timelike curves in 1949, and rotating or wormhole geometries have been proposed since. Quantum theory has models for them too. Deutsch's 1991 model keeps physics consistent without paradoxes, and Aaronson and Watrous showed in 2009 that with such a loop, quantum and classical computers become equally powerful: both can solve any PSPACE problem. In 2011 a group used teleportation and postselection to test, in the lab, a related model of such a loop (not Deutsch's).
The film's branching rule resembles how Deutsch's model is sometimes read through many-worlds, which is an interpretation and untested. If loops of this kind ever existed, computer science would change more than history would: a classical computer with such a loop would become as powerful as a quantum one, both reaching PSPACE. The site's own page on complexity classes notes that PSPACE is not proven to be larger than BQP, so even that gap is open.
See it in the swipe deck, with the other cards.