The idea that reality or ‘Creation’ may contain multiple worlds is an old one. Hinduism, Buddhism, Judaism, Christianity, and numerous other religions have been contemplating the concept of multiple planes of existence in their own ways for millennia, often with regard to life before and after death.

As Danish scholar Helge Kragh points out, even pre-Socratic philosophers like Anaximander and Anaximenes were hashing these kinds of ideas out. Later generations of heavyweight thinkers (like Thomas Wright and Immanuel Kant, to name a few) have continued exploring the concept.

The actual term “multiverse” was coined in 1895 by American philosopher William James, but with a somewhat different meaning from the one it holds now. He described a world he found to be “all plasticity and indifference” and seemingly controlled by multiple forces.

Today, physicists, cosmologists, and philosophers use the word (in very broad terms) to identify the possibly infinite number of universes that exist as full-fledged manifestations of every possible variation, circumstance, and so on in the whole of time and space, right down to the quantum (think subatomic) level. Meaning everything that could occur or exist does exist in at least one universe—maybe even fairies.

WHAT DOES THE MULTIVERSE ACTUALLY LOOK LIKE?

Scholars who accept the theory of the multiverse are now working to pin down what form that multiverse might actually take and how it would function. For example, they're asking if the multiverse emerged from one Big Bang or from many Big Bangs (or even from an ongoing series of them).

In recent years, two popular frameworks for how the multiverse is arranged have been put forth: one comes from Massachusetts Institute of Technology cosmologist Max Tegmark, and the other from theoretical physicist and string theorist Brian Greene of Columbia University. 

Tegmark explains in his Scientific American essay “Are Parallel Universes Unscientific Nonsense? Insider Tips for Criticizing the Multiverse” that the four models in his taxonomy of the multiverse—which, he argues, are all implied by extant theories in physics—are based on the ideas of there being an inflating (or expanding) universe, a landscape of inflating universes, one that’s subject to collapse-free quantum mechanics, and an external reality. 

His Level I multiverse, for example, is “a single space so large that it contains many universe-sized regions,” while Level II, in contrast, features “a single space containing universe-sized regions with each kind of space.” And with his Level IV multiverse model, Tegmark makes the original argument “that if there’s an external reality completely independent of us humans, then there’s a fourth type of multiverse realizing all mathematically possible universes.”

Greene, on the other hand, proposes a full nine possible forms for the multiverse in his book The Hidden Universe, some of which have some overlap with Tegmark’s. They include: a quilted multiverse, an infinite scenario in which every possible event will occur an infinite number of times in a series of layered universes (but in which we can't see the other layers because of the speed of light); a quantum multiverse, in which new universes are created each time “a diversion in events" happens (i.e. there's now a universe in which you picked a different article to read, maybe even with Schrodinger’s cat curled in your lap); and a cyclic multiverse, which has multiple universes (as three-dimensional branes) colliding, re-emerging and re-forming through Big Bangs, and repeating.

Some of these multiverse models have been sci-fi mainstays for decades, while others are just starting to gain traction in our imaginations. The show Futurama, for example, plays with a version of Greene’s latter idea in an episode that has three characters stuck in a time machine which can only travel forward. After happily discovering that their universe is cyclical in nature, the gang is able to return to their right time (or an almost identical, regenerated version of it) simply by bringing the machine around through a couple full loops of that universe. 

A personal favorite, taken from Greene’s options, is defined by the holographic principle. As Motherboard explains, “some researchers believe [it could reconcile] Einstein’s theory of relativity and quantum mechanics” with its postulation that “every three-dimensional object we know and cherish is a projection of tiny, subatomic bytes of information stored in a two-dimensional Flatland” (an idea which, if true, is “a really important insight," theoretical physicist Daniel Grumiller told the site).

A lot of this brainstorming, Greene says, comes from string theory, a broad research area and potential ‘theory of everything.’ Among other things it suggests that one-dimensional strings of matter might reach throughout the universe, tying it to the same physical laws throughout (and possibly into other laws, too).

These unifying strings may not just be string-shaped, according to Greene. The theory “also allows objects that look like large flying carpets, or membranes, which are two dimensional surfaces,” he told NPR. “And what that means, within string theory, is that we may be living on one of those gigantic surfaces, and there can be other surfaces floating out there in space."

SO, WHERE DOES OUR UNIVERSE FIT IN?

One way of explaining the idea is that, if all possible combinations of universes are being played out—ones in which the Earth is completely covered in hydrochloric acid, or has sunlight and water but no breathable atmosphere, etc.—we’re in one that has all the necessary factors for sustaining life, one that exists out of physical and mathematical necessity.

Hard to imagine infinite liveable Earths? To visualize this, Greene suggests we imagine a deck of playing cards:

"Now, if you shuffle that deck, there's just so many orderings that can happen. If you shuffle that deck enough times, the orders will have to repeat. Similarly, with an infinite universe and only a finite number of complexions of matter, the way in which matter arranges itself has to repeat."

Think about the film Groundhog Day, for example, and how Bill Murray’s character Phil endures numerous variations of the same day—a day he gorges on diner food, a day he does a Thelma and Louise-style cliff-dive, and on and on. For the sake of argument, we can imagine that Phil is personally working through a variety of possible February 2s by hopping between universes in which he himself is the only variable feature. Because, in that neck of the multiverse, each single universe has had the exact same order of cards in its deck—that is, until the fateful winter day when Phil’s crew arrived in Punxsutawney.

NOT EVERYONE’S ON BOARD WITH THE MULTIVERSE, THOUGH.

Some critics of the multiverse simply think the whole thing’s hogwash, and many contend that, since the idea isn’t necessarily falsifiable (i.e. it can’t be disproven according to the scientific method, though Greene has proposed a way of confirming its existence using the Large Hadron Collider), further investigation isn’t worth our time.

In response to such criticism, Tegmark points to cosmologist Sean Carroll, who “believes we should rethink the whole [theory-testing] process,” Tegmark says. The "interplay between theory and experiment” isn’t being viewed from the right angle at the moment (in this universe, at least), Carroll explains in an essay:

"We can't (as far as we know) observe other parts of the multiverse directly. But their existence has a dramatic effect on how we account for the data in the part of the multiverse we do observe. [The idea’s] virtue is not that it's a neat idea or fulfills some nebulous principle of reasoning, it's that it helps us account for the data. Even if we will never visit those other universes."

Greene also noted to Fresh Air's Terry Gross that, even if theories about the shape and nature of the multiverse are difficult to prove, the potential consequences of doing so make his and his colleagues’ work worthwhile: 

"You almost can't avoid having some version of the multiverse in your studies if you push deeply enough in the mathematical descriptions of the physical universe [...] There are many of us thinking of one version of parallel universe theory or another. If it's all a lot of nonsense, then it's a lot of wasted effort going into this far-out idea. But if this idea is correct, it is a fantastic upheaval in our understanding."

WHAT CAN THE MULTIVERSE DO FOR ME RIGHT NOW?

The next time you’re on a date with someone who doesn’t seem to be digging it, try telling them that, in the next universe over, they’re deeply charmed by your wit and elegance, and that both universes (and countless others) are pressed together like a stack of flying carpets. They might buy it, or they might not. 

Either way, you can now take comfort in the possibility that in at least one other universe, they did.