Whether you're a fan of the popular TV show Rick and Morty or just a connoisseur of modern physics theories, you may know a thing or two about the multiverse theory. In brief, this theory states that the universe we live in may not be the only one out there.
It also proposes the idea that there are, in fact, an infinite number of universes, known as the multiverse.
This concept may seem a little insane, but it gets even weirder when you actually start thinking about its implications. If the multiverse theory is correct, then there's a universe out there where everything is exactly like this universe.
However, in some alternate universe, you may well be reading this article in a clown costume while drinking a cup of maple syrup. Or, any other combination of differences you can imagine.
Jokes aside, the multiverse theory isn't just physics quackery, it's a trail of thought that has infatuated scientists and philosophers alike. While some physicists argue the multiverse is not a legitimate topic of scientific inquiry, let's take a look at a few things some physicists point to as proof that we might be living in a multiverse.
Somewhere out there is another exact copy of you, maybe
Consider space-time. Scientists really can't be sure what the actual shape of space-time is. This is because the shape of the universe depends on its density.
If its density is greater than the critical density (the average density of matter required for the Universe to just halt its expansion), the universe will be closed and form a curved shape; if less, it will form a shape a bit like a saddle. But if the actual density of the universe is equal to the critical density, as scientists think it is, then it will extend forever like a flat piece of paper.
If that were the case – which many scientists think is likely – then if you travel far enough you will eventually reach regions nearly identical to ours. After all, an infinite series of numbers has an infinite number of possibilities, but there are only a finite number of ways that particles in space can be arranged.
That means that at some point, there's going to be repetition.
Hence somewhere, by pure chance, if you look far enough down the space-time timeline, there could be a near-parallel Earth where a nearly-identical version of you is reading this article in a clown suit while drinking maple syrup.
Our observable universe goes only as far as light has traveled in our universe's existence. That equates to a diameter of roughly 93 billion light-years. If we assume our current laws of physics are correct, we can set limits on the minimum size the Universe must be before it curves back on itself.
Observations tell us that if the Universe does curve back on itself and close, that the unobservable part of the Universe must be at least 250 times the radius of the observable part — at least 23 trillion light years in diameter, and with a volume more than 15 million times as large as the volume we can observe.
However, it's also possible that the entire Universe is just barely larger than the part we can observe, and that there is an "edge". If so, the space-time beyond that boundary would essentially be considered its own universe. Some argue that there has to be something beyond our universe. It might be another universe.
Eternal inflation might spawn new universes
The principle of inflation suggests that the universe expanded rapidly after the Big Bang, a bit like a giant balloon. When this happened on a massive scale, different pockets of space might have stopped inflating at different times, according to top cosmologists.
This would essentially be like bubbles forming in the universe. These other bubble universes would have then moved away from us, well beyond the limits of observation.
Some cosmologists suggest that our own universe exists as one of those bubbles. This could also mean that if there were multiple universes, each would have its own unique laws of physics.
The case of the multiple researchers
In the theory of quantum mechanics, there might be other ways that multiple universes can arise. For example, physicist Richard Feynman's approach to quantum field theory introduced the possibility of reality as a "weighted sum of alternative histories".
This theory proposed that the overall behavior of two electrons as they approach each other, then deflect and scatter, must take into account every possible intermediate path—and that these should be weighted according to the likelihood of each path occuring.
Another way to think of this is by trying to find out how tired a person will be after a walk in the woods by assuming that they split themselves up and take every possible route. More weight would be assigned to the shortest (and therefore most likely) path, but all of the paths need to be taken into account.
A graduate student, Hugh Everett III, took this further by proposing a reinterpretation of quantum mechanics in which each time that particles interact, reality separates into a set of parallel streams, each representing a different possible outcome. So, researchers observing a quantum experiment would similarly split up into multiple selves. Each version of the researcher would be convinced that they were the real one and would be unaware of the existence of the others.
2 + 2 = Another Universe
Scientists also debate the role of math in alternate universes. Is math a construct that we use to describe our universe, or is it really the essence of our universe?
If math is just a system that we use to understand our universe, then maybe there are other mathematical structures that exist in different universes. Under this theory, maybe there's another universe where 2+2=5. However, if math is a fundamental reality, then every single universe would be defined by it equally.
A mathematical structure can be defined completely independently of human experience or even understanding. Whether you understand a mathematical principle or not, it still is. This theory of the universality of math, which many scientists think is accurate, would suggest that there are likely to be universes out there that would exist regardless of human experience.
Yes, our brain hurts too.
Whether or not the concept of the multiverse is true or not, it will probably remain one of those unanswerable questions. At least in this Universe.
However, it is fun (for some) to theorize about, even if we can never really prove it. Or can we?