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How to Communicate Across the Quantum Multiverse

What is a multiverse in cosmology?

A century of astronomical observations has told us a lot about the age, size, and evolution of galaxies, stars, matter and the four dimensions we sum up as spacetime.

One thing we know with great confidence is that everything we see now is expanding at an accelerating rate. This logically implies the Universe, at least the one we live in, used to be a lot smaller.

We can theoretically squeeze all of the matter of the Universe down to a point where the concentration of energy reduces atoms to a soup of simpler particles and forces combine until we can’t tell them apart. Any smaller than that? Big shrugs.

If we go with what’s known as a cyclic model of cosmology, the parent universe preceded ours in some way. It might even be a lot like this one, only running in reverse compared with ours, shrinking over time into a concentrated point only to bounce back out for some reason. Played out for eternity, we might imagine the respective universes bounce back and forth in an endless yo-yo effect of growing and collapsing.

Or, if we go with what’s known as a conformal cyclic model, universes expand over trillions upon trillions of years until their cold, point-like particles are so spread out, for all mathematical purposes everything looks and acts like a brand new universe.

If you don’t like those, there’s a chance our Universe is a white hole – the hypothetical back end of a black hole from another universe. Which, logically, just might mean the black holes in our Universe could all be parents, pinching off new universes like cosmic amoebae.

What is a multiverse in quantum physics?

Early last century, physicists found theories that described matter as tiny objects only told half of the story. The other half was that matter behaved as if it also had characteristics of a wave.

Exactly what this dual nature of reality means is still a matter of debate, but from a mathematical perspective, that wave describes the rise and fall of a game of chance. Probability, you see, is built into the very machinery that makes up the gears of a universe like ours.

Of course, this isn’t our daily experience as vast collections of atoms. When we send a bucket of molecules called a rocket to the Moon as it zooms past 300,000 kilometres away, we’re not rolling dice. Classical old physics is as reliable as tomorrow’s sunrise.

But the closer we zoom in on a region of space or time, the more we need to take into account the possible range of measurements we might find.

This randomness isn’t the result of things we don’t know – it’s because the Universe itself is yet to make up its mind. There’s nothing in quantum mechanics explaining this transition either, leaving us to imagine what it all means.

In his 1957 doctoral dissertation, American physicist Hugh Everett suggested the range of possibilities are all as real as one another, representing actual realities – separate universes, if you like – just like the one we’re all familiar with.

What makes any one universe in this many worlds interpretation distinct is how each wave correlates with a specific measurement taken of other waves, a phenomenon we call entanglement.

What ‘we’ means, and why ‘we’ experience one entangled set over waves over another, isn’t clear, and in some ways presents an even bigger problem to solve.

In the Many Worlds interpretation of quantum mechanics, the universal wavefunction is the reality, encompassing all possible histories and futures and all exist. But we are only sensitive to a slice of the wavefunction corresponding to our “world”, and due to the superposition principle our world can happily do its thing unperturbed by other parts of the wavefunction – other “ripples,” or worlds. And while it may seem like it would be physically impossible to have any connection between worlds, it may turn out to be entirely possible to communicate between them.

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