Somewhere in the vastness of the cosmos, there are planets resembling Earth. And on these planets live other versions of you, who have made radically different life choices.
This tantalizing idea has hijacked pop culture this year. Take “Everything Everywhere All At Once,” a critically acclaimed thriller about a woman with the power to access parallel realities. Or Marvel’s “Doctor Strange in the Multiverse of Madness,” in which a sorcerer takes a wondrous journey across universes, battling his “multiversal selves” along the way.
But for all the recent multiverse rage, the concept actually dates back thousands of years, and its history is nearly as convoluted as anything Doctor Strange encountered on his adventures.
As documented by Tom Siegfried in his 2019 book “The Number of the Heavens,” the idea of the multiverse began more than two millennia ago in ancient Greece, with the philosopher Democritus, who speculated that there were an infinite number of universes. Centuries later, a Roman scholar, Lucretius, took the idea and ran with it, writing that “there are other orbs of earth in other regions of space, and various races of men and generations of beasts.”
But the multiverse bombed in the ancient marketplace of ideas. Plato and his disciple Aristotle offered a simpler, more logical proposition: There’s a single universe, the one we live in. This became the conventional wisdom.
The Bible pushed that same belief, so when Christianity took over the West, church leaders doubled down on the theory of the singular universe. It reigned supreme until the Middle Ages.
Ironically, it was a bishop who in 1277 plucked the multiverse out of the dustbin of history. That fateful year, the Bishop of Paris, Étienne Tempier, issued 219 “condemnations” to theologians, basically telling them which opinions they should never espouse, under punishment of excommunication. Number 34 forbade them from claiming that God “could not make several worlds.” Tempier’s rationale made sense: If the Almighty is truly omnipotent, as the Bible teaches, then he can create an infinite number of universes.
But the divine was in the details. Christian scholars argued that God could create multiple universes, not that he had actually done so.
Fast-forward to the Renaissance, however, and that’s pretty much what Italian polymath Giordano Bruno contended. In “De l’infinito universo et mondi,” published in 1584, he wrote: “Thus is the excellence of God…he is glorified not in one, but in countless suns; not in a single earth, a single world, but in a thousand thousand, I say in an infinity of worlds.”
Bruno ended up burned at the stake, sentenced to death by the Inquisition, which tells you how well his musings on the multiverse were received.
Throughout the next centuries, the concept of the multiverse floated around. Immanuel Kant even toyed with it. But few scholars were willing to dedicate their careers to it — until the 1950s and an iconoclastic American physicist named Hugh Everett III.
As recounted in a biography by Peter Byrne, in 1953 Everett started his PhD at Princeton, with a focus on quantum mechanics, or how particles behave at the microscopic level. The equations of quantum mechanics hold that a particle can be in two or more locations at the same time. But when physicists use machines to detect a particle, they see it in only one location — a conundrum known as the quantum measurement problem.
“[It] was a kind of taboo subject in physics for about 50 years,” said David Wallace, a physicist and philosopher at the University of Pittsburgh and a leading expert on the multiverse. In Everett’s day, as Wallace put it, physicists devised “a fudge” to explain the quantum measurement problem: “The equations of quantum mechanics only apply when no one’s looking.”
The biggest proponent of that explanation was Nobel laureate Niels Bohr. As one physicist quipped, in the field of quantum mechanics, “Bohr was God.”
Everett disagreed with God. His PhD thesis offered a radically new explanation for why a physicist never sees a particle in multiple locations at once. It isn’t because the laws of quantum mechanics suddenly change. Instead, as Everett argued, it’s because “the observer himself has split into a number of observers, each of which sees a definite result of the measurement.”
In other words, as the physicist sees the particle, he gets instantly cloned. Each of his clones then observes the particle in a different location. If the scientist doesn’t appear to have divided into multiple clones, that’s because the clones don’t inhabit the same physical reality. Each one is spawned into a distinct and closed-off universe. Crucially, the clone is never aware that he’s just been cloned.
Everett’s theory is dubbed the many-worlds interpretation of quantum mechanics. Its implications are staggering: We live in a multiverse where new universes — parallel realities — are constantly springing up. This means there are universes where you have another profession, married another person, or weren’t even born.
And this has been going on for billions of years. Therefore, there are also universes where evolution unfolded differently and the human species doesn’t exist. As Wallace said: “Probably most of the multiverse has non-human, non-hominid, God-knows-what creatures!”
Almost no one took Everett seriously back in the ’50s. Predictably, when Bohr got wind of Everett’s theory, he hated it.
Everett’s PhD supervisor, John Wheeler, admired Bohr. While recognizing his student’s brilliance, Wheeler had Everett delete three-quarters of his thesis.
Dejected, Everett quit academia and got a job in the arms industry. He later struggled with alcoholism and died at 51 in 1982. His theory was rediscovered in the ’70s by physicist Bryce DeWitt and later found devotees at Oxford. Wallace, who studied and taught there, is among them.
Today, Wallace said, the multiverse is hotly debated by physicists. But it’s more widely accepted than ever. Some cosmologists and string theorists have also posited its existence, though each has a different conception of the multiverse.
One question remains: Could we, like Doctor Strange, travel to other universes and meet our other selves?
“The equations of quantum mechanics say that it can’t be,” explained Wallace. “There’s a property called linearity in the equations. … What linearity means is that once you’ve got a separation of branches [parallel realities], you can never transfer information from one branch to a different branch.”
Wallace mused that “maybe the next generation of physics will add a non-linearity to the equations, which would allow communication across branches.” But don’t get your hopes up that you might someday cross into another, better universe. Wallace cautioned, “There’s absolutely no reason to expect that.”