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One theory is almost certainly the most incredible idea in all of science.

The evidence needed to verify it is scarce if not impossible to obtain, but it is the favoured explanation of many scientists for strange phenomena that they have seen in our universe.

If this theory were true, it would answer many long-standing and difficult questions in physics, but at the same time create a myriad more questions.

It’s called the ‘multiverse’.

You might already have an idea of what it is. It’s been used as a plot device a thousand times in movies, comics, books, and TV, including in Avengers: Endgame.

It is the idea that there are alternative universes to the one we’re living in now, many of which have alternative Earth’s, and alternative you’s.

We’re going to look at what scientists have seen that suggest the existence of the multiverse, how the multiverse answers their questions, and what it means if our universe is not alone.

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The biggest mystery of all

You don’t need big particle accelerators or space telescopes to do it either.

All you need is an assistant, a simple torch, a piece of photo paper, a metal plate with some holes cut into it, and a very long empty room.

Stand in front of the torch, and ask your assistant to turn it on. Hundreds of trillions of light particles (called photons) will erupt from the torch and hit your retinas in a torrent, like a continuous spray of bullets from a machine gun.

With so many photons coming out of the torch, what you’ll see in front of you is a single, unflickering, bright light.

Walk down the long empty room, and eventually the light will begin to dim, then flicker. The photon bullets will start to miss you, and the light appears to twinkle.

If you walk one hundred million kilometres down the empty room, the distant light seems to switch on and off. Only a single photon bullet will now hit your retinas once per day, making the light appear to flash on just for a brief moment.

You call your assistant and she can verify that yes, the torch has been turned on and steady the whole time.

(In reality scientists had to make the experiment more feasible by passing the light through dark filters instead, and use special light sensors to see the very faint light).

So far everything in the experiment is explained by science as we know it, so we haven’t discovered multiple universes yet. But as it turns out, the devil is in the details.

What we know for sure

This is where you set up a metal plate with two slits cut into it in front of the light. It can be set up at any point in the long room.

The slits are one-fifth of a milimeter apart, and are there to let some light through.

Past the plate you set up a piece of photo paper. Photons that hits the paper will leave a bright mark.

You’re looking out for the pattern that appears on the other side of the slits.

You might assume that you’ll get two bright bands on the other side as the light shines straight through. But this is not what happens.

After a while you check the paper and it looks like this:

It’s a familiar pattern to many scientists, known as an interference pattern. It most often occurs when waves collide with and scatter each other.Β In this case, photons going through one slit seem to collide with photons going through the other slit, and they end up scattered across the photo paper in an interference pattern.

But this is where things get weird.

The unexplained phenomena

If you set up the plate and photo paper one hundred million kilometres away from the torch, you might remember that only one photon travels through the slits per day. But the same pattern appears on the paper.

There are no others to collide with. So why do the photons end up scattered across the paper?

This is the great mystery that’s at the heart of science.

This experiment may seem pretty niche and trivial, but its simplicity is deceptive. Physicist Richard Feynman was fond of saying that all of quantum mechanics can be gleaned from carefully thinking through the results of this single experiment. Similar experiments have been done with other particles, and it seems that almost all quantum mechanical sized particles (that is to say the particles that make up everything around us) behave the same way.

In other words, this behaviour is the true nature of the building blocks that make up the universe.

Explaining the unexplainable

There are many competing explanations for why this happens.

One theory, called the ‘Copenhagen Interpretation’ explains that before the photon hits the photo paper or your retina, it exists in a very strange state called a ‘superposition,’ where it is in many places at once. The photon only locks in its place when it strikes another particle, like the surface of the photo paper.

The Copenhagen Interpretation could certainly end up being true, but it seems reminiscent of the clumsy attempts to justify Geocentrism, the idea that the Earth lies at the centre of the solar system.

The competing many universes interpretation is simpler but far more mind-bending.

The theory goes that there are a huge number of universes, layered on top of each other like pages in a book.

When a single particle like a photons travels, it is never really alone. It interacts with photons from other universes, and they ricocheting off each other and scatter each other wherever they go.

This is why the interference pattern occurs at one hundred million kilometres way from the torch. There are trillions of other photons to collide with. It’s just that they belong to other universes.

So this interference pattern could be our first piece of evidence in the existence of universes other than our own.

An exercise in humility

This is almost certainly a difficult concept to wrap your head around. But it’s worth remembering that other mind-minding concepts have been proposed before, and are now accepted as the new normal.

In 1609, Galileo Galilee peered through his telescope and discovered that the planet Jupiter was orbited by several moons. It prompted his theory that the Earth was not the centre of the solar system. Perhaps the Sun was, and that the uncountable billions of stars at night in the Milky Way were other Suns with their own planets orbiting them, which may include alternative Earths.

Jupiter and its moons on a clear night, visible next to our own Moon through a telescope

This was one of the most radical ideas of its time, but is now accepted as practically self-evident.

In 1920, Astronomer Heber Curtis reported that an object in the sky, called the Andromeda nebula, had certain ‘dark lanes’ in its shape, which was eerily similar to what the Milky Way galaxy looked like. From this and other observations he speculated that it was another galaxy, containing billions more stars and worlds.

Andromeda Galaxy by Rob Kirby. Find more of his shots on Facebook here.

At the time it was called an ‘island universe’, because we couldn’t wrap our head around the idea of a anything being outside the gargantuan Milky Way galaxy.

Over time more galaxies were discovered, and modern estimates put the number of galaxies in the universe at around 2 trillion, and the number of stars they contain at 10^24.

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At the beginning, each of these theories were just creative guesses based on shreds of evidence. But over time the evidence grew, and by necessity so did our humility.

Discovering a multiverse will surely be the biggest scientific discovery of all time, perhaps even exceeding discovering life on another planet.

What’s going on in those other universes?

It’s not easy to guess what the nature of these other universes may be. Once again, there are competing theories.

This leads to an infinite amount of universes, all existing in parallel, each one representing a slightly different version of reality where a single particle went a different way, since the big bang. The crazy thing is, the mathematics of this idea makes sense. It is truly possible.

When we look back at history, we see a single past. Humanity invented farming, Archduke Ferdinand was assassinated which led to the two world wars, and your parents met at just the right time.

Our history is a timeline that stretches back to include all of these events.

But the many worlds hypothesis says that this is a serious lack of imagination.

What if that first crop was never planted? Or that a stray bullet hit the Duke of Wellington and won the battle for Napoleon? Or that the Archduke was never assassinated and the world wars never happened?

Each of these events was surely a turning point in history. It feels like

car went left, resulted in ____. Car went right, and ____.

But what if, and this is pretty out there, at each major turning pointΒ a new universe was created?

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Ben McCarthy

Ben McCarthy

Ben is the Founder of Discover Earth and the author of the Big Ideas Network.