Some interpretations of quantum mechanics suggest that our entire universe is defined by a single universal wavefunction that constantly divides and multiplies, producing a new reality for every possible quantum interaction. That’s a pretty bold statement. So how do we get there?

One of the earliest facts in the history of quantum mechanics is that matter has a wave-like property. The first to propose this was the French physicist Louis de Broglie. light can act as both a particle and a wave .

Other physicists soon confirmed this radical idea, especially in experiments. electrons emitted through a thin foil before landing on a target. The way the electrons scatter was characteristic of a wave rather than a particle. But then a question arose: What exactly is a matter wave? How does it look?

Related: Are We Living in a Quantum World?

Early quantum theorists such as Erwin Schrödinger believed that the particles themselves dispersed into space in the form of a wave. He developed his famous equation to describe the behavior of these waves, which is still used today. However, Schrodinger’s idea was blown away by more experimental testing. For example, although an electron might behave like mid-wave flight, when it reached a target it landed as a single, compact particle, so it could not be physically expanded 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 have given to the wave-like property of matter, does not actually exist. Instead, it’s a mathematical convenience 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
However, the Copenhagen interpretation has several problems. As Schrödinger himself points out, it is unclear how the wave function goes to non-existence as soon as we observe from a cloud of probabilities prior to measurement.

Perhaps there is something more meaningful to the wave function. Perhaps it is as real as all the particles themselves. De Broglie was the first to propose the idea, but eventually joined the Copenhagen camp. Later physicists, like Hugh Everett, looked at the problem again 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 this super-special process that destroys the wavefunction. Instead, what we call measurement is actually a long string of quantum particles and wavefunctions interacting with other quantum particles and wavefunctions.

If you make a detector and throw electrons at it, for example 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 (consisting of more electrons) through a wire, which interacts with a screen, emitting photons that hit the molecules in its eyes, etc.

In this picture, each particle gets its own wave function and that’s it. All particles and all wave functions interact as they normally do, and we can use the tools of quantum mechanics (like the Schrödinger equation) to make predictions about how they will behave.

Photograph of a cat and Schrödinger’s wave equation. (Image credit: VICTOR de SCHWANBERG/SCIENCE PHOTO LIBRARY/Getty Images)
universal wave function
But quantum particles have a really interesting property because of their wave functions. When two particles interact, they don’t just bump into each other; For a short time, the wavefunctions overlap. When that happens, you can no longer have two separate wavefunctions. Instead, you should have a single wavefunction that describes both particles simultaneously.

When particles go their separate ways, they maintain this combined wave function. Physicists this quantum entanglement – what Albert Einstein referred to as “remote spooky action”.

When we follow all the steps of a measurement, a series of confusions arise from overlapping wavefunctions. The electron mixes with the atoms on the screen entwined with the electrons in the wire, and so on. Even the particles in our brains get mixed up. Soil circulating with all the light that comes and goes from our planet, down to every particle in the universe, with every other particle in the universe.

With each new entanglement you have a single wave function that describes all of the combined particles. So the obvious conclusion from 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 happened during the observation process. In quantum mechanics, we can never be sure what a particle will do – sometimes it can go up, sometimes it can go down, etc. In this interpretation, each time a quantum particle interacts with another quantum particle, the universal wavefunction splits into multiple parts, with different universes containing each of the different possible outcomes.

And this how do you get multiverse . By the sheer action of entangled quantum particles, you always end up with multiple copies of the universe being recreated. Each is the same except for minor differences in a random quantum process. This means you have multiple copies right now as you read this article, exactly the same except for some minor quantum details.

This interpretation also has its challenges – for example, how does this split actually come about? But it’s a radical way of seeing the universe, and a testament to how powerful quantum mechanics is as a theory – what started as a way to understand the behavior of subatomic particles can govern the properties of the entire cosmos.

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