For nearly 100 years, physicists have been trying to reconcile the physics of the smallest building blocks of the universe (quantum physics) with the physics of the galactically large (Einstein’s Theory of General Relativity). But they kept running into one requirement that was hard to swallow: their theories only worked if we live in a multiverse—if our universe is one of many.
For example, Erwin Schrödinger’s equation explaining the collapse of a quantum wave requires a multiverse. And String Theory, which postulates that the universe is made of infinitesimally small vibrating strings of energy, also must include a multiverse. Trouble is, no one can prove the multiverse exists, much less what it’s like, or whether we can interact with it. Still, the math looks promising.
That begs the question: if there are other universes, can we ever visit them?
Numerous popular movies are playing with that idea, including Everything Everywhere All at Once, Dr. Strange and the Multiverse of Madness, Loki, Spiderman: No Way Home, and others going back decades. But they always have some handy device for multiverse travel that physicists lack—a magician’s incantation, say, or a quantum Bluetooth device. Real physicists haven’t altogether worked out how we’d encounter other universes, but the answer partly depends on what kind of multiverse you’re talking about.
Many Worlds, Many Portals
If we live in the Many Worlds universe, we create new worlds or universes all the time. Every time we make a decision or choose a particular path, some version of us simultaneously chooses an alternate path—or possibly several others, depending on how many possible paths there are. So, if you decided to date someone, some version of you decided not to, and that version split off into a different timeline. Later, if you decide to marry your significant other, another version of you will decide not to, and another you is created.
This idea originated with Erwin Schrödinger’s equation on quantum wave function, which explains that a particle exists in a superposition, or wave of possibilities, until it is measured, at which point the wave collapses, leaving a single particle in a specific location. Unfortunately, no one has ever been able to explain why it collapses. American physicist Hugh Everett, who posited the Many Worlds Interpretation of the wave function in 1955, said that, in fact, the wave doesn’t collapse. Instead, all the probabilities happen. But you will only be aware of the one you experience. The others will branch off into different timelines. And since the whole universe exists as part of a single superposition probability wave, the universe splits itself into new timelines all the time.
Loki and Everything Everywhere All at Once seem to have borrowed from this notion. In Loki, decisions that created new timelines were deemed variants of “the sacred” timeline and pruned, so that only one reality would prevail. In Everything Everywhere, the characters jump into other timelines by doing wild stuff, like stapling their foreheads. Such choices, deliberately abnormal for their own lives, would help them get to other universes. (They also had a nifty Bluetooth device.)
Today, if we’re creating new timelines, physicists like Sean Carroll say, we’ll never know or experience them. But avid Many Worlds theorist David Deutsch hopes that a mature quantum computer would be able to detect and record the divergence. He also believes we’ll one day be able to upload ourselves into quantum computers, so maybe we’ll be able to see how our other selves fared.
Bubble, Brane, and Anti-Universes
Other multiverse theories postulate universes that exist in our timeline—they’re just too far away for us to see. These are suggested by the math in String Theory (remember the vibrating strings?) or the Eternal Inflation Theory, which states that the multiverse is expanding forever and creating new “bubble universes,” but they’re so far away we are unlikely to encounter them. They might be membranes or “branes” floating in space, each defined by a set of extra dimensions. Or they might be infinite probability bubbles formed by the inflationary stretch of space and time—called “Quantum Foam” or “space foam”—creating a quantum spacetime; each bubble could expand into a universe.
Some theories propose that the different laws of physics in these universes would cause them to collapse without creating stars, or that humans could never survive there. Others say that, if there are an infinite number of these universes, some of them would have identical physical laws to ours, in the same way that a couple who produced infinite children would wind up with at least two with the same gene combination.
Interacting with these universes presents a conundrum, though. It would be cool if they gently bumped into one another; imagine that, walking down the street, you were suddenly confronted with an alternate universe where the supermarket used to be. But that seems unlikely, as physicists predict that two bubble universes colliding would be a violent event, and may even be the source of the Big Bang.
Tunnels To Other Universes
Another way to get from one universe to another would be a wormhole, a tunnel comprised of two connected black holes where an incredibly dense mass has pulled spacetime into the shape of a funnel. Connect two and you can travel great distances quickly, kind of like going through a tunnel in a mountain, rather than going over or around it.
Lots of movies play with wormholes, including Interstellar and Contact. Einstein’s Theory of General Relativity predicts the possibility that a black hole—instead of ending in the crushing destruction of the gravity of a singularity—would actually funnel quantum information through to a “white hole” at the other end. He called these Einstein-Rosen bridges. A white hole has been described as a black hole running backward in time; it spews matter out as wormholes suck them in. So far, no white holes have been observed, but scientists are still looking. One theory suggests we have a mirror or “Bizarro Universe” running backward in time, connected to our universe. There’s also a theory that quantum entanglement could connect two ends of a black hole into a wormhole.
But even without wormholes, there are other possible modes of travel to get us to other universes. One day, we may discover a dimension that cuts across spacetime. Or a future generation or more advanced being could invent faster-than-light travel which, barring a wormhole, would be needed to span the extraordinary distances between these universes.
In the meantime, it’s fun to imagine what Bizarro-you might be up to out there.
