tend to focus on major decisions as having momentous effects, but
what if something as simple as a missed train could change the course
of your life? And what if you follow two different paths to see which
turned out better? That’s the premise of the 1998 film Sliding Doors, in
which Gwyneth Paltrow stars as a young woman named Helen who has just
been fired from her job with a prestigious public relations firm in
London. Dashing down the stairs of the Tube station, she gets to the
platform just as the car doors slide shut, missing her train.
Or does she? The next scene features a replay, in which a split-second change in timing results in Helen just barely catching her train. One Helen stands despondently on the platform as the train pulls away, while the other Helen takes a seat next to a friendly young man named James (John Hannah.). And events unfold quite differently for our two Helens over the remainder of the film, with profound implications for her future.
The notion of parallel worlds, or alternate timelines, in which we get to explore all the roads not taken, is a time-honored trope in science fiction—at least since 1923, when H.G. Wells published Men Like Gods, in which travelers cross into a parallel world that split from our own some 3000 years ago. Quantum mechanics was still in its infancy then, but 30 years later, a young physicist named Hugh Everett III proposed a controversial interpretation of quantum mechanics in which every possible forking of every possible path is realized in its own separate universe. Today it’s known as the many worlds interpretation.
It started out as a brainstorming session over sherry with colleagues, but by 1957, Everett had developed his idea into a full-length dissertation [pdf]. A philosophical sticking point in quantum mechanics is the idea of superposition of states. In any quantum system (e.g., a subatomic particle like a photon or an electron), every possible outcome for an experiment is present simultaneously in a sort of super-imposed limbo state. The sum, or superposition, of all those uncertain outcomes is described by an equation known as the wave function. It’s only when we check to see what happened by making a measurement that the wave function collapses and all those possibilities reduce to a single “real” event.
But what happens to those other possibilities once the wave function has collapsed? The strictest interpretation of quantum theory dodges the question and simply assumes they vanish by necessity. Everett suggested that perhaps the wave function continues to evolve instead. Every potential outcome contained in the wave function—a photon appearing as a particle or wave, or Helen catching or not catching her train, with all the subsequent repercussions thereafter—is realized in its own separate universe.
Worlds split when irreversible events occur, such as a choice, or in Helen’s case, a missed or not-missed train. (According to Everett, any measurement constitutes a kind of “choice.”) If we go back in time and change one of those choices, the result would be a separate universe where everything is identical up until that crucial decision is made. It’s the act of choosing that makes one of those options “our” reality, but Everett insisted that all other options still exist somewhere, in a parallel universe beyond our ken.
Every potential outcome contained in the wave function is realized in its own separate universe.
That’s the key: All these universes never, ever interact in any way; we can only observe “our” reality. It’s a buzzkill for science fiction writers, who cheerfully ignore the dictum all the time with characters confronting their own doppelgangers in parallel worlds. Sliding Doors, in contrast, largely honors this principle, with two minor exceptions: Helen’s philandering boyfriend Gerry catches a glimpse of the Helen from the other timeline (in which they have broken up) in a local pub, while “his” Helen sees visions of her alternate self in a dream sequence. But technically, no interaction occurs.
It all sounds just a wee bit crazy, right? Certainly most of Everett’s contemporary colleagues thought so. His dissertation quickly faded into obscurity and he left theoretical physics entirely. Indeed, in 1973, when Everett met his future business partner Donald Reisler, the latter exclaimed, “Oh my god, you’re that Everett, the crazy one who wrote that insane paper.” He was understandably bitter at the poor reception his ideas received. When he died in 1982, at the age of 51, he requested that his ashes be thrown out with the trash.
worlds has its current champions, and Everett’s son filmed a
documentary, Parallel Worlds, Parallel Lives, about his father’s
revolutionary work. The broader notion of a multiverse, while still
controversial, is very much an active field of inquiry in 21st
century theoretical physics. In fact, in 2009, Andre Linde and Vitaly
Vanchurin calculated just how many parallel universe there might be [pdf], concluding it should be around 101016.
So maybe Everett had the last laugh after all. I like to think that, in a parallel universe, Hugh Everett III lived to a ripe old age as a venerated visionary physicist.
Jennifer Ouellette is a science writer and the author of The Calculus Diaries and the forthcoming Me, Myself and Why: Searching for the Science of Self. Follow her on Twitter @JenLucPiquant.