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 every possible quantum interaction. That’s a pretty bold statement. So how do you get there?

One of the earliest 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 asserted that each subatomic particle is associated with a wave, just like light can behave both as a particle and as a wave .

Other physicists soon confirmed this radical idea, especially in experiments where electrons scattered on 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? What does it look like?

Related: Do we live in a quantum world?

Early quantum theorists like Erwin Schrödinger believed that the particles themselves were spread out in space in the form of a wave. He developed his famous equation to describe the behavior of these waves, which is still used today. But Schrödinger’s idea ran into more experimental tests. For example, even though an electron acted like a wave in midair, when it hit a target, it landed as a single compact particle, so it could not be physically extended in space.

Instead, an alternative interpretation began to gain traction. 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’s a mathematical convenience that we use to describe a cloud of quantum mechanical probabilities of where we might find a subatomic particle the next time we go looking for it.

Entanglement chains
The Copenhagen interpretation, however, presents several problems. As Schrödinger himself pointed out, it is unclear how the wave function goes from being a cloud of probabilities before the measurement to simply not existing at the time we make an observation.

So maybe there is something more meaningful in the wave function. It is perhaps 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 like Hugh Everett looked into the problem again and came to the same conclusions.

Making the wave function a real thing solves this measurement problem in the Copenhagen interpretation, because it prevents the measurement from being this super special process that destroys the wave function. Instead, what we call a measurement is really just a long series of quantum particles and wave functions interacting with other quantum particles and wave functions.

If you build a detector and send electrons to it, say, at the subatomic level, the electron doesn’t know it’s being measured. It just hits the atoms on the screen, which sends an electrical signal (made up of more electrons) down a wire, which interacts with a screen, which emits photons that hit the molecules in your eyes, etc.

In this image, each particle has its own wave function, and that’s it. All particles and wave functions interact as they normally do, and we can use the tools of quantum mechanics (like the Schrödinger equation) to predict their behavior.

A photograph of a cat and Schrödinger’s wave equation. (Image credit: VICTOR de SCHWANBERG/SCIENTIFIC PHOTO LIBRARY/Getty Images)
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; for a brief moment, their wave functions overlap. When this happens, you can no longer have two separate wavefunctions. Instead, you should have a single wave function that describes both particles simultaneously.

When the particles separate, they still retain this united wave function. Physicists call this process quantum entanglement – What Albert Einstein called “remote spooky action”.

When tracing all the steps of a measurement, what emerges is a series of overlapping tangles of wave functions. The electron tangles with the atoms in the screen, which tangles with the electrons in the wire, and so on. Even the particles of our brain intertwine with Earth with all the light that comes and goes from our planet, down to every particle in the universe that gets entangled with every other particle in the universe.

With each new tangle you have a single wave function that describes all the particles combined. So the obvious conclusion of making the wave function real is that there is only one wave function that describes the entire universe.

This is called the “many-worlds” interpretation of quantum mechanics. It gets this name when we ask what happens during the observing process. In quantum mechanics, we are never sure what a particle is going to do – sometimes it can go up, sometimes it can go down, and so on. In this interpretation, each 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 it’s how to get a multiverse . By the simple fact that quantum particles become entangled with each other, you get multiple copies of the universe being created over and over again all the time. Each is identical except for the small difference in a random quantum process. This means that there are multiple copies of you reading this article right now, all of them exactly the same except for a few tiny quantum details.

This interpretation also presents difficulties – for example, how does this split actually unfold? But it’s a radical way to view the universe and a demonstration of the power of quantum mechanics as a theory – what started out as a way to understand how subatomic particles behave can govern the properties of the whole world. cosmos.

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