The protagonist of the new film Yesterday is a failing singer-songwriter who gets hit by a bus and wakes up in the hospital to find himself in a sort of alternative reality in which the Beatles were never a band. The singer-songwriter remembers all of their music that never was. His career, needless to say, improves.

This sort of thing has long been the stuff of science fiction. It can be a big “What if?” For example: What if Hitler had been the first to get his hands on nukes? (The Man in the High Castle) Or it can be a medium-sized change in the reality we know: What if an anomaly in time finds a modern tank crew throwing in with Lt. Col. George Armstrong Custer at the Battle of the Little Bighorn? (The Twilight Zone episode “The 7th Is Made Up of Phantoms”) Or even the smallest of alterations: One might ask how the world would be different if 66 million years ago a butterfly had its already brief butterfly life shortened, squished under a time traveler’s boot.