At one time or another, all of us have looked to the stars and wondered what might be out there: a higher power, life on other planets, all of the above? Heavy thoughts like these — and the question of whether it would be more frightening for us to be alone or not alone in the universe — only get more intense when you ponder that the universe itself might not be alone.

The theory

Many stars in the night sky with view of galaxy
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As you likely surmised from its name, the multiverse theory posits that the reality we inhabit is one among many that exist rather than the only one. Terms like alternate and parallel universe refer to the same concept, which in some cases is expanded to infinity and beyond — that is to say, there are literally infinite universes, and any reality you can imagine exists in one or several of them. That’s a lot to take in, which is why so many of us can only spend so much time reflecting on such a possibility before retreating to the comfort of our cosmically insignificant lives.

We’re hardly the first to have these thoughts, of course. The theory has been popping up in all manner of fields — religion, astronomy, physics, philosophy, etc. — for more than a hundred years, though it wasn’t given serious consideration until the latter half of the 20th century. That’s partially thanks to physicist and Nobel laureate Erwin Schrödinger, whose thought experiment involving a cat you may have heard of. In a famous 1952 lecture that he admitted would "seem lunatic" to some, Schrödinger revealed that equations he’d been working on suggested that parallel universes were "not alternatives but all really happen simultaneously."

Eleven years earlier, Argentinian writer Jorge Luis Borges described “an infinite series of times, a growing, dizzying web of divergent, convergent, and parallel times," in his short story "The Garden of Forking Paths." Borges's story continued: "That fabric of times that approach one another, fork, are snipped off, or are simply unknown for centuries, contains all possibilities. In most of those times, we do not exist; in some, you exist, but I do not; in others, I do and you do not; in others still, we both do.”

As for the term itself, we have an American psychologist and philosopher named William James to thank. He pondered in 1895 that “visible nature is all plasticity and indifference, a multiverse, as one might call it, and not a universe.” James didn't mean it the way we do today, however. Rather, he was suggesting that our existence is inherently chaotic and doesn’t become a cohesive universe until it’s grafted with meaning — which, for him, came from god.

Going back even further, Gottfried Leibniz argued in his 1710 book "Théodicée" that we live in “the best of all possible worlds” — a turn of phrase made even more famous by Voltaire’s “Candide” nearly 50 years later. Inherent in the concept is the implication of a multiverse: for our world to be the best, others must also exist.

Just think of it: anything that can happen not only will happen, but is happening right now. What's so endlessly fascinating (and, for many, frightening) about this is that it flies in the face of everything we know about the world. defines "universe" as "the totality of known or supposed objects and phenomena throughout space; the cosmos," which sounds fairly open-and-shut. Even the root word, “uni,” obviously means “one” — as in the only one.

In science

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True to its nature, there isn’t actually one unified theory of the multiverse. Max Tegmark, a cosmologist and physicist, has proposed a classification system consisting of four levels in his book “Our Mathematical Universe: My Quest for the Ultimate Nature of Reality.” His work is fittingly heady, but the levels can be described in simple terms as:

1. an extension of our own universe
2. far-away universes that are too far away for us to ever observe
3. a manifestation of the many-world interpretation (MWI) of quantum mechanics
4. the mathematical universe hypothesis (MUH), also known as the ultimate ensemble theory

Another well-known theory-within-a-theory comes courtesy of theoretical physicist Brian Green, whose nine types of multiverses are described as Quilted, Inflationary, Brane, Cyclic, Landscape, Quantum, Holographic, Simulated, and Ultimate. There's overlap between the two scientists, especially as pertains to the ultimate multiverse — which, not unlike Walt Whitman, contains multitudes. The basic idea is that every mathematically possible universe exists within it, with each following its own rules of physics.

In fiction

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Science fiction and fantasy have been exploring alternate realities for many moons, from classic novels like "The Time Machine" and "Through the Looking-Glass" to popular TV shows like "Lost," "Stranger Things," and "The Good Place." Fewer concepts lend themselves to speculative fiction as well as the multiverse, and it’s easy to see why: it allows for storytellers to bend the very rules of space and time.

You’re likely aware of the Marvel Cinematic Universe by now, and perhaps have seen at least a couple of the 23 movies that constitute said universe. In its more recent installments, MCU has begun seriously exploring the possibility (and, just as important, consequences) of parallel realities. In high-concept narratives like this, our heroes are often tasked with venturing into alternate dimensions in order to save the one we inhabit.

Sometimes these worlds are used as a kind of antagonist (i.e., what if another world encroaches on our own and changes it for the worse?), while others serve as wish-fulfillment: what if visiting one of these other worlds could allow us to correct a mistake in our own? Beauty is in the eye of the beholder, and so are the implications of the multiverse theory — by its very nature, it’s neither good nor bad.

Beyond personal interpretation, however, it might be more accurate to say that it’s all of these things at once. If you could open a door that allowed you to glimpse — and even experience — one of these alternate realities, with no way of knowing what version of yourself and the world itself were on the other side of it before turning the handle, would you do it?