10 Reasons the Multiverse Is a Real Possibility

Before Everett and his concept of multiverses came along, physicists were stuck in a bedeviling bind. They had to use one set of rules for the subatomic world that quantum mechanics focuses upon, and another set of rules for the large-scale, everyday world that we can see and experience. The complexities of this shift in scale forced them to twist their brains into some strange shapes.

For example, in quantum mechanics, particles don't have set properties when no one's looking at them. Instead, their nature is described by something called a wave function, which includes all of the different possible properties that a particle could have. But in a single universe, all those possibilities can't exist at once, so when you look at a particle, it settles upon one single state. That idea is illustrated metaphorically by the famous Schrödinger's cat paradox — that is, if there's a cat in a box, it's both alive and dead at the same time, until you open the box to check. Your action forces the cat to become either a warm, breathing kitty or carcass.

But in a multiverse, you don't have to fret that you're going to kill the cat with your curiosity. Instead, whenever you open the box, reality splits into two versions. Sure, there's one realm in which you're thinking, "Ewww!" as you gaze at a dead cat. But there's another version of the event in which you might be scratching its ears as it purrs [source: Merali].

In a 2011 interview, Columbia University physicist Brian Greene — of the book "The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos," — explained that we're not really sure how big the universe is. It could be a really, really huge but finite. Or else, as you travel away from Earth in any direction, space could just stretch on forever, which is probably the way that most of us imagine it.

But if space is infinite, that means it must be a multiverse with infinite parallel realities, according to Greene. Here's the reasoning: Think of the universe, and all of the matter in it, as the equivalent of a deck of cards. Just as there are only 52 cards in a deck, there are only so many different forms of matter. If you shuffle that deck enough times, eventually, the order of the cards you deal must repeat itself. Similarly, in an infinite universe, matter eventually would have to repeat itself, and arrange itself in similar patterns. A multiverse, with an endless number of parallel realms containing similar but slightly different versions of everything, provides a neat, easy way to accommodate the need for repetition [source: Greene].

There's something about being human — perhaps our brains' tendency for pattern formation — that makes us want to know the beginning and the end of every story. That includes the story of the universe itself. But if the Big Bang was the start of the universe, how was it triggered, and what existed before it? Will the universe someday end, and what will happen after that? Inquiring minds want to know.

A multiverse could provide an explanation for all of those things. Some physicists have hypothesized that the infinite regions of the multiverse are something called braneworlds. These braneworlds exist in many different dimensions, but we can't detect them, because we can only perceive the three dimensions of space, plus the fourth dimension of time, in our own braneworld.

Some physicists believe that these braneworlds are slabs, bunched together like slices of bread inside a plastic bag. Most of the time, they stay separate and out of reach. But occasionally, the braneworlds bump into each other. Hypothetically, those collisions are cataclysmic enough to cause repeated Big Bangs — enabling the parallel universes to restart themselves, over and over [source: Moskowitz].

The European Space Agency's Planck orbital observatory is gathering data on the cosmic microwave background, or CMB — background radiation that still lingers from an early, hot stage of the universe's existence.

But that research also has yielded what could be evidence of a multiverse. In 2010, a team of scientists from Great Britain, Canada and the U.S. discovered four odd, seemingly unlikely circular patterns in the CMB. They hypothesized that the marks essentially were bruises that our universe got from bumping into other universes [source: Zyga].

In 2015, ESA researcher Rang-Ram Chary made a similar discovery. Chary took a model of the CMB out of the observatory's picture of the sky, and then removed everything else that we know about — stars, gas, interstellar dust, you name it. At that point, the sky should have been pretty much empty, except for some background noise.

But it wasn't. Instead, in a particular frequency range, Chary could detect scattered patches in the map of the cosmos, areas that were about 4,500 times brighter than they should have been. The researcher came up with another possible explanation: The patches are imprints from a collision between our universe and a parallel one.

As Chary told EarthSky.org, unless someone comes up with another way to explain the patches, "one has to conclude that Nature may be playing dice after all, and we are just a random universe among a multitude of others" [sources: Byrd, Sokol].

There's a possibility that a multiverse exists, even though we haven't seen any of those parallel realities, because we can't prove that it doesn't exist.

We know that sounds like a convenient rhetorical trick, but think of it this way: Even in our world, we've discovered many things that we previously didn't know existed, and we've had events occur — the 2008 global financial meltdown is a good example — that nobody ever had thought possible. There's even a name for the latter: black swan events, named after the philosopher David Hume's observation that people would assume all swans were white until they saw a black one [source: Taleb].

The universe's scale is the factor that makes the possibility of a multiverse all the more believable. We know that the universe is really, really big, and possibly even infinite in size. That means we may not be able to detect everything that exists in the universe. Since scientists have determined that the universe is approximately 13.8 billion years old, that means we can only detect light that's been able to reach us in that time. If a parallel reality simply was farther away than 13.8 billion light years, there would be no way for us to know it was out there, even if it existed in dimensions that we were capable of perceiving [source: Ball].

As Stanford University physicist Andrei Linde explained in a 2008 interview, if the physical world operated by even slightly different rules, life wouldn't be able to exist. If protons were just 0.2 percent more massive than they actually are, for example, they'd be so unstable that they would break up into simpler particles, and atoms wouldn't be possible. And if gravity was just a bit more powerful, the result would be woeful. Stars such as our sun would be compressed tightly enough that they would burn through their fuel in a few million years, long before life on a planet such as Earth had a chance to evolve. This is known as the "fine-tuning problem."

Some see that precise balance of conditions as evidence of the deliberate hand of a supreme being who created everything, which would put atheists in a serious bind. But the possibility of a multiverse, in which this might simply be the particular realm in which all of these life-giving factors are present, gives them an out.

As Linde put it in an interview with Discover magazine: "For me the reality of many universes is a logical possibility. You might say, 'Maybe this is some mysterious coincidence. Maybe God created the universe for our benefit.' Well, I don't know about God, but the universe itself might reproduce itself eternally in all its possible manifestations" [source: Folger].

The enduring popularity of "Back to the Future" (BTTF) movie trilogy has made a lot of us fascinated with the idea of time travel. Since the movies came out, alas, nobody actually has developed a trash-powered DeLorean that's able to zoom back and forth between decades or centuries. Even so, some scientists think that time travel might be at least theoretically possible [source: Russon].

Assuming that this could happen, we could run the same danger that BTTF protagonist Marty McFly encountered — the risk of inadvertently doing something to alter a past event, which in turn triggers a series of events to alter the course of history. In the initial BTTF, as you might remember, McFly, accidentally prevented an event that led to his parents dating and falling in love, which threatened to erase him from the family's snapshots. That necessitated a lot of plotting and McFly's part to ensure that they actually did get together and procreate.

But as Gizmodo writer George Dvorsky noted in a 2015 article, the existence of a multiverse would make all of those antics unnecessary. "The presence of alternate worlds means there isn't a single timeline to screw up," he wrote. Instead, if a person went back in time and changed things, he'd simply spawn a whole new set of parallel universes.

All this stuff about parallel universes that we've talked about so far is pretty mind-blowing, but here's an idea that is even stranger.

In 2003, philosopher Nick Bostrom, director of the Future of Humanity Institute at the University of Oxford, raised the question of whether what we think of as reality — that is, our particular parallel universe — is simply a digital simulation of another universe. In Bostrom's conception, it would take 10 to the 36th power calculations to create a lifelike, detailed replica of the entirety of human history.

A really advanced alien civilization — we're talking beings whose technological sophistication would make us look like Paleolithic flint carvers — conceivably might have enough computing power to pull it off. In fact, simulating every human who ever lived might not even gobble up that much of their electronic resources, so that there might be more computer-generated beings in existence than real ones [source: Choi and Kestin].

And that might mean that we're living in an actual version of the digital world in "The Matrix" movies.

But wait, it gets even weirder. What if the civilization that's simulating us is itself a simulation?

This isn't hard proof, of course. But it's intriguing to remember that old saying, variously attributed to Picasso or sometimes the writer Susan Sontag, that if you can imagine something, it must exist.

And there might be something to that. After all, long before Hugh Everett sipped his sherry, numerous people in history imagined different versions of a multiverse.

Ancient Indian religious texts, for example, are filled with descriptions of multiple parallel universes [source: Sanskriti]. And in her book "Worlds Without End: The Many Lives of the Universe," Wesleyan University religion professor Mary-Jane Rubenstein notes that the ancient Greeks had the Atomist school of philosophy, which held that there were an infinite number of worlds scattered through a similarly infinite void.

In Medieval times, the idea of multiple universes also resonated. In 1277, for example, the Bishop of Paris even argued that Greek philosopher Aristotle had been wrong to say there was only one possible world, because that questioned an omnipotent God's power to create parallel ones. The idea was resurrected once again in the 1600s by Gottfried Wilhelm Leibniz, one of the leaders of the Scientific Revolution, who argued that there were many possible worlds, each with different physics [source: Wilkinson].

Bizarre as the notion of a multiverse might seem, in a way it actually fits the progression through modern history of how humans view themselves and the universe.

As physicists Alexander Vilenkin and Max Tegmark noted in a 2011 Scientific American essay, people in western civilization have been successively more humbled as they've discovered the nature of reality. They started out thinking the earth was the center of everything. We learned that wasn't true, and that even our solar system was just a not particularly significant part of the Milky Way.

The multiverse would take that pattern to its logical extreme. If multiverses exist, that means that we're not special at all, because there are infinite versions of each of us.

But some think that we're just at the beginning of a mind-expanding trip. As Stanford University theoretical physicist Leonard Susskind has written, it could be that a couple of centuries from now, philosophers and scientists will look back on our time as "a golden age in which the narrow, provincial 20th century concept of the universe gave way to a bigger, better [multiverse] ... of mind-boggling proportions" [source: Vilenkin and Tegmark].