It's the stuff of science fiction — parallel worlds that fan out in time and space.
But do such parallel worlds exist?
It turns out that at least some physics theories do allow for the existence of parallel universes — at least on the quantum level.
In several interpretations of quantum mechanics, like the Many-Worlds interpretation and the Pilot Wave theory, the universe can be described by a single giant equation, known as a quantum wavefunction. Any time a quantum (or subatomic) process occurs anywhere in the universe, this wavefunction splits in two, meaning parallel universes are constantly created.
But these interpretations have never been shown to be correct, and they have some major weaknesses that prevent them from being widely accepted.
Related: If we live in a multiverse, where are these many worlds hiding?
The problem of measurement
Quantum mechanics is the physics framework that describes the behavior of tiny particles. One quirk of this theory is that no one is sure what results they get until they look. For example, the canonical interpretation of the physics theory says that electrons exist in multiple states at once. Then once someone makes a measurement, the electron "picks" one of those states.
This idea can be pretty frustrating, because the whole point of physics is to make predictions for how the objects in our universe will behave. If I throw a ball to you, you can use your knowledge of physics (for example, Newton's laws) to predict where the ball will go. But if I throw an electron at you, you have no way of knowing exactly where it will land.
However, quantum mechanics does give us one tool to make predictions: the Schrödinger equation. The Schrödinger equation assigns something called the wavefunction to every particle, and describes how that wavefunction evolves with time. In the standard picture of quantum mechanics, that wavefunction is a cloud of probability that describes where there's a chance to see the particle once people look for it. Where the wavefunction has high values, there's a strong possibility, and where it has low values, there's a small possibility.
However, this standard picture runs into a problem when scientists actually make a measurement. When they're not looking, the wavefunction evolves on its own according to the Schrödinger equation. No big deal. But when scientists make a measurement, this wavefunction "collapses", essentially disappearing, with the particle appearing at one of the possible locations.
Introducing many worlds
How can the quantum world have two completely different sets of rules for how the wavefunction behaves? In the standard picture, the wavefunction obeys Schrödinger's equation when people are not looking, and then immediately collapses when people are. That seems…weird.
In response to this, some other interpretations of quantum mechanics, most notably the Many-Worlds Interpretation and Pilot Wave theory, promote the wavefunction from a mere mathematical tool into a real, existing object. In these interpretations, there's no such thing as measurement. There's no special process or magic trick that makes the wavefunction disappear. Instead every particle in the universe gets assigned its own private wavefunction, and those wavefunctions just keep on evolving according to the Schrödinger equation without end.
When particles interact, their wavefunctions briefly overlap. In quantum mechanics, once this happens those particles are forever linked: a single wavefunction describes both particles simultaneously, a process known as "quantum entanglement." When scientists make a measurement, they are just triggering a series of entanglements beginning with the particle hitting a detector, and ending with molecules shifting around in their brains to make them consciously aware of what just happened.
But the entanglements don't stop there: every particle in the universe becomes entangled with every other particle, leading to a single universal wavefunction that describes the entirety of the cosmos in one fell swoop.
But even with a universal wavefunction, randomness is still a fact of life in quantum mechanics. To account for this, these interpretations say that the wavefunction splits every time a quantum interaction takes place, with each duplicate universe containing one of the possible results. So if we send an electron through a screen and it has a 50/50 chance of going up or down, for example, there's one universe where the electron goes up and one where it goes down.
This process creates a quantum multiverse. Because essentially every interaction is at some level a quantum interaction, there are universes containing every possible alternative choice you could have made in your entire life. In fact, you are being constantly split at this very moment, fragmenting and splitting into multiple copies of you with every choice, every movement, and every action.
This is where the multiverse starts to get a little heavy, because it's not just conscious decisions that lead to splits, but every quantum interaction. Just by reading this article on a device, you are triggering the splitting of countless universes that are exactly identical except for the tiny, insignificant quantum details happening inside the electronics.
That's…a lot. But there's a bigger issue. Humans experience consciousness as seamless, and it takes time for the brain to integrate all sensory inputs into a conscious experience of the world. But if we're constantly splitting and fragmenting, how can we maintain a consistent history of our own identity?
Beyond that, none of these physics theories explain how this splitting of the universes actually takes place. How quickly does it happen, and why can't people detect it? And how do people recover the probabilities of quantum mechanics with all these splitting universes — in other words, how do the universes "know" how much splitting to produce with every quantum interaction?
These questions are areas of active research, so it's not clear if the quantum multiverse truly exists or not.
This is part of an ongoing series describing potential interpretations of quantum mechanics.
OK, but Copenhagen is the stuff of fantasy.