Hollywood is filled with movies about the multiverse. But what does science really say?
Over the last few years, the box office has been dominated by films like "Spider-Man: Across the Spider-Verse" that explore ideas of parallel universes, quantum worlds and alternate lives. There's some real theoretical physics behind those ideas, but they may not look exactly how we imagine.
You may have noticed a certain plot device appearing seemingly everywhere in sci-fi lately: multiverses.
Movies like "Spider-Man: Across the Spider-Verse," "Doctor Strange in the Multiverse of Madness," and "Everything Everywhere All at Once" have been exploding at box offices, all with some version of parallel universes.
There's an economic reason, of course, for Hollywood studios embracing the multiverse. But there's an emotional one too — one that resonates even far outside the Marvel cinematic experience.
“We're not gonna let Dr. Strange and Black Panther remain dead,” says Sean Carroll, a theoretical physicist and philosopher at Johns Hopkins University. Carroll is also a consultant credited for helping many big Hollywood movies, like the last two Avengers movies (Endgame and Infinity War), be as scientifically accurate as possible.
Parallel universes aren’t just writers room solutions to get characters from one studio's franchise to another. There’s actually some scientific theory to it.
On the latest episode of Seeking a Scientist, Dr. Kate Biberdorf (aka Kate the Chemist) speaks with a preeminent researcher about the real theories behind parallel universes, and separates some of the madness from the multiverse.
What does science actually say about the possibility of multiverses?
Multiverse movies explore the idea that there may be another version of you living in a world that resembles your own, but it’s just a little bit different.
However, Carroll — who wrote the book Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime — says there are “way, way more than millions of you running around.”
Carroll cites the “Many-Worlds Interpretation of Quantum Mechanics," a theory that dates back to the late 1950s.
This wacky but credible theory was proposed by American physicist Hugh Everett III as a twist on the predominant interpretation of quantum mechanics: the Copenhagen Interpretation.
Quantum mechanics, by itself, is the understanding of how atomic particles exist and interact with each other.
The Copenhagen Interpretation is the idea that electrons exist in a number of different positions until they are observed.
But the Many-Worlds Interpretation (MWI) suggests that the electrons can exist in a number of different positions and there is a separate, unique world where each of these outcomes exists.
For example, in the MWI, at every fork-in-the-road, your world splits into two. While you continue about your day in your world, You-2.0 moves about their entirely separate world created by this different branching outcome.
And then when either of you reaches another junction, you split again, and again, and again… until there are billions of parallel you’s running around in billions of different worlds.
In short, MWI gives us multiverses!
What does Schrödinger’s infamous cat have to do with this?
Back in 1935, Austrian physicist Erwin Schrödinger was talking to Albert Einstein, when he came up with a perfect analogy to help explain the flaws he saw with the Copenhagen interpretation of quantum physics.
Remember: The theory states that electrons exist in a number of different positions until they are observed. Schrödinger thought this theory was incomplete. So he proposed a thought experiment that is often referred to colloquially as “Schrödinger’s cat.”
In this (theoretical) experiment, a cat is placed in a box with a radioactive particle, a Geiger counter, a hammer, and a vial of poison. Once the particle decays enough to provide the Geiger counter with a specific reading, the hammer smashes the vial, releasing the poison, and ultimately kills the cat.
This idea explores the idea of quantum superposition. Basically, this tricky concept states that a particle can exist in multiple states at a time – until the point that it is measured. Therefore, the cat is simultaneously alive and dead, until we measure it by opening up the box and looking inside.
“The Shrödinger’s cat thought experiment was invented by Shrödinger, number one because he didn't like cats,” Carroll says. “But number two, because he wanted to emphasize how weird quantum mechanics is.”
According to Carroll, Everett had a problem with this thought experiment because of the way the cat was separated from the person performing it. If the cat is made up of quantum particles and the observer (you or me) is also made up of quantum particles, Everett argued that the same principles would apply to both the cat and the observer.
Everett “realized that in the existing Copenhagen Interpretation of the quantum mechanics framework, the first step was to say there's the quantum system you care about, and then separately there's an observer. So if your system that you care about is the whole universe, you can't separately have an observer outside of it. So you had to rethink how to do quantum mechanics.”
Let’s break that down: This means that we cannot separate the system from the surrounding — we have to look at the entire universe.
The “system” is Schrodinger’s cat, and the “surrounding” is everything outside of the cat: It’s the box, the table, you, me, the poison.
For the Copenhagen Interpretation of quantum mechanics, we just focus on the system (the cat) by itself.
What Everett proposed is that it’s better to look at the combination of the system and the surrounding, so the cat and everything else in the room.
Technically, both interpretations of quantum mechanics give us the same experimental results. The difference is how you think about it.
So how do we end up with the multiverse?
Everett’s theory, the Many-Worlds Interpretation of Quantum Mechanics, says that if we assume that the observer is entangled within the system, we end up with many separate worlds.
“Everett's big step is to say that you are a quantum system, also you're made of atoms. They obey the rules of quantum mechanics, therefore you obey the rules of quantum mechanics…And the answer is that when you open the box and you look at it, long story short, you and the cat evolve into an entangled superposition,” says Carroll.
This quantum entanglement is what gives us multiple universes! With entangled superposition, we cannot separate the cat from the box from the observer (us). We are all linked together.
“There is part of the wave function that says the cat's alive and you saw it alive. There's another part that says the cat's dead and you saw it dead,” says Carroll. “It is as if they are separate worlds.”
The benefit of the MWI is that it gives us enough scientific basis for movies like Sliding Doors and Parallel. (For what it’s worth, Carroll says that Sliding Doors is the “best” if you are looking for the most scientifically accurate multiverse movie).
However, in the truly scientific interpretation of the many-worlds theory, these universes can never interact. Each world is distinct, where you can never chat with You-2.0 to see if the right decision was made.
Despite how wild it may sound, Everett didn't back down from his theory. In 1957, Everett wrote his doctoral dissertation proposing the many-worlds interpretation, and to this day, scientists are still conducting experiments to test the theory.
Why are humans so obsessed with multiverses?
Beyond the intellectual property ramifications, multiverses maintain an emotional appeal for storytelling.
A big part of that is the feeling of regret — not knowing if you made the right decision at any point in your life.
What would have happened if you had gone to the doctor sooner? Was it the right time to start a family? What if you'd worked harder or given yourself permission to be happier? Or what if you had had the courage to express your feelings when it mattered the most?
With movie magic, it becomes possible to cross universes and see how you might have lived on a different timeline. But in the real world, even though those worlds do theoretically exist, they can never collide.
According to the science, we can't ask our alternative-selves if they are happy or living with regrets, too. All we can do is focus on being the best versions of ourselves, in this world.
Where can I hear more about this topic?
Listen and subscribe to Seeking A Scientist with Kate The Chemist, from KCUR Studios, available wherever you listen to podcasts.
Seeking A Scientist is a production of KCUR Studios, made possible with support from the Stowers Institute for Medical Research and design help from PRX.
This episode was produced by Dr. Kate Biberdorf, Suzanne Hogan and Byron Love, edited by Mackenzie Martin and Gabe Rosenberg, with help from Genevieve Des Marteau.
Our original theme music is by The Coma Calling. Additional music from Blue Dot Sessions.