Some interpretations of quantum mechanics propose that our entire universe is described by a single universal wave function that is constantly dividing and multiplying, producing a new reality for each possible quantum interaction. That’s a pretty bold statement. So how do we get there?
One of the first realizations in the history of quantum mechanics is that matter has a wave property. The first to propose this was the French physicist Louis de Broglie, who argued that every subatomic particle has an associated wave, just like light can behave as a particle and a wave.
Other physicists soon confirmed this radical idea, especially in experiments where electrons dispersed into a thin sheet before landing on a target. The way the electrons scattered was more characteristic of a wave than a particle. But then, a question arose: What exactly is a matter wave? How does it look?
related: Do we live in a quantum world?
Early quantum theorists, such as Erwin Schrödinger, believed that particles themselves spread out through space in the form of waves. He developed his famous equation to describe the behavior of those waves, which is still used today. But Schrödinger’s idea passed unnoticed by further experimental evidence. For example, although an electron acted like a wave in mid-flight, when it reached a target, it landed as a single compact particle, so it couldn’t physically spread out in space.
Instead, an alternative interpretation began to gain ground. Today, we call it the Copenhagen interpretation of quantum mechanics, and it is by far the most popular interpretation among physicists. In this model, the wave function, the name physicists give to the wave property of matter, doesn’t really exist. Instead, it is a mathematical convenience that we use to describe a quantum mechanical probability cloud of where we might find a subatomic particle the next time we look for it.
tangle chains
However, the Copenhagen interpretation has several problems. As Schrödinger himself pointed out, it is not clear how the wave function goes from a cloud of probabilities before the measurement to simply not existing at the time we make an observation.
So maybe there is something more significant to the wave function. Perhaps it is as real as all the particles themselves. De Broglie was the first to propose this idea, but he eventually joined the Copenhagen camp. Later physicists, such as Hugh Everett, re-examined the problem and came to the same conclusions.
Making the wavefunction a real thing solves this measurement problem in the Copenhagen interpretation, because it prevents the measurement from being a super special process that destroys the wavefunction. Instead, what we call a measurement is really just a long series of quantum particles and wavefunctions that interact with other quantum particles and wavefunctions.
If you build a detector and shoot electrons at it, say, at the subatomic level, the electron doesn’t know that it’s being measured. It just hits atoms on the screen, which sends an electrical signal (made of more electrons) down a wire, which interacts with a screen, which emits photons that hit molecules in your eyes, and so on.
In this image, each particle has its own wave function, and that’s it. All particles and all wave functions just interact as they normally do, and we can use the tools of quantum mechanics (such as the Schrödinger equation) to make predictions about how they will behave.
The universal wave function
But quantum particles have a really interesting property because of their wave function. When two particles interact, they don’t just collide with each other; for a short time, their wavefunctions overlap. When that happens, you can no longer have two separate wavefunctions. Instead, you must have a single wave function that describes both particles simultaneously.
When the particles go their separate ways, they still hold this wave function together. Physicists call this process quantum entanglement what Albert Einstein called “spooky action at a distance”.
When we retrace all the steps of a measurement, what emerges is a series of tangles of superimposed wavefunctions. The electron gets entangled with the atoms on the screen, which get entangled with the electrons on the wire, and so on. Even the particles of our brain are entangled with Landwith all the light coming and going from our planet, down to every particle in the universe, getting entangled with every other particle in the universe.
With each new entanglement, you have a single wave function that describes all the particles combined. So the obvious conclusion from making the wavefunction real is that there is a single wavefunction that describes the entire universe.
This is called the “many worlds” interpretation of quantum mechanics. It receives this name when we ask ourselves what happens during the observation process. In quantum mechanics, we are never sure what a particle will do: sometimes it can go up, sometimes it can go down, and so on. In this interpretation, every time a quantum particle interacts with another quantum particle, the universal wave function splits into multiple sections, with different universes each containing different possible outcomes.
And this is How do you get a multiverse?. Through the mere fact that quantum particles become entangled with each other, multiple copies of the universe are created over and over again all the time. Each one is identical, except for the small difference in some random quantum process. That means there are multiple copies of you reading this article right now, all exactly the same except for a few tiny quantum details.
This interpretation also has difficulties; for example, how does this division actually play out? But it is a radical way of looking at the universe, and a demonstration of just how powerful quantum mechanics is as a theory: What began as a way to understand the behavior of subatomic particles can govern the properties of the entire cosmos.
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