What If— is a science-fiction short story by Isaac Asimov.
A standalone story, it was first published in the Summer 1952 issue of Fantastic. It was later collected in 1969's Nightfall and Other Stories.
Summary[]
A married couple on a train trip from Boston to New York meets a mysterious man who calls himself "Mr. If." He possesses a small, portable viewing device that can display alternate versions of the past. He shows the couple scenes from their own lives, illustrating how their relationship might have unfolded if certain pivotal, minor events had happened differently.
After viewing several alternate scenarios, they come to a profound realization: even if those key moments had changed, the ultimate outcome—their life together—would have remained essentially the same. The story suggests that some relationships are destined, and the paths of our lives are resilient to minor deviations.
Trivia[]
The story's core concept of a device that explores alternate realities based on "what if" questions was notably used in two episodes of the animated series Futurama ("Anthology of Interest I & II"), where Professor Farnsworth demonstrates a very similar invention called the "What-If Machine."