Elsewhen
Elsewhen (1941) is a novella by Robert A. Heinlein, concerning time travel and parallel universes. It was first published as "Elsewhere" in the September 1941 issue of Astounding Science Fiction, under the pen name Caleb Saunders,[1] and was reprinted in the 1953 book Assignment in Eternity.
The story concerns five diverse students who attend a philosophy seminar. The professor explains that he learned how to use his mind to go back in time and correct a bad mistake in his life. Using hypnosis he lets them travel to alternate worlds of their choice. The religious person goes to her version of heaven. One couple eventually moves to a world at war with space invaders. They are helped in their conflict by the other couple and the professor who have settled on a Flash Gordon-like planet and share its technology with them.