Could our universe just be one of countless universes constantly bubbling into existence? Science journalist Dan Falk looks at arguments on both sides of one of the most contentious debates in science - the multiverse.
If your childhood was like mine, you likely went through a phase when you “extended” your mailing address to indicate the big picture – the really big picture. First your name, then street address, then city, then country, followed by Planet Earth, Milky Way, Local Galactic Group, Local Supercluster, The Universe.
I believed I had provided the most complete address possible, and, were I actually to write it on a piece of mail, I figured my local post office would appreciate my specificity. I was confident there was nothing more to add – the universe is all there is.
But is it? What if our universe is just one among many?
What if my childhood mailing address, to be truly accurate, needed to specify which universe I inhabited, as if my mail could wind up in the wrong universe entirely?
The question of multiple universes is an ancient one, and it continues to provoke debate – often heated – to this day.
Within just the last five years, the idea has garnered attention in popular science books, in particular The Number of the Heavens by journalist Tom Siegfired (2019), and The Allure of the Multiverse by physicist Paul Halpern (2024). Siegfried points out that although the idea of a multiplicity of worlds goes back centuries, the first person to employ the phrase “parallel universe” in something like its modern usage was the science fiction writer H.G. Wells in his 1923 novel Men Like Gods (which means the multiverse might be said to have recently celebrated its 100th birthday).
Granted, some people feel the very idea of multiple universes carries a whiff (or more) of science fiction. For its part, Hollywood has embraced the concept, sparking all manner of entertainment, especially in the last few years. In writing about the multiverse recently in a feature story for Discover, I made a point of highlighting the many appearances the notion has made in science fiction, from the classic Star Trek episode “Mirror, Mirror” to the 2022 Oscar winner, Everything Everywhere All at Once.
But it’s not all sci-fi: There really are arguments rooted in science that suggest the multiverse is real, though physicists and cosmologists (and sometimes philosophers) argue over the merits of those arguments. The issue has proven deeply divisive: Some find the pro-multiverse arguments convincing; others find them profoundly flawed and unscientific.
The arguments for the possible existence of a multiverse come from three different directions, inspired by three distinct branches of physics – inflationary cosmology, string theory, and quantum mechanics. Let’s look at each of these lines of argument in turn.
Inflationary Cosmology
It was just a century ago, in the 1920s, that astronomers found that distant galaxies are moving away from each other – implying that the universe is expanding. This discovery eventually gave rise to the big bang model of cosmology. A refinement of the original big bang model, developed in the 1970s and 80s, provided a more sophisticated view of the universe’s earliest moments. It was called “inflation,” and it posited that the universe underwent a period of exponential growth when it was less than a millionth of a trillionth of a trillionth of a second old. That accelerated expansion took the miniscule irregularities inherent in quantum mechanics – “quantum fluctuations” – and blew them up to macroscopic size.
But if inflation could happen once, why not a million times – or an infinite number of times? This is how the idea of “eternal inflation” was born. In this view, tiny quantum fluctuations are continuously turning into full-sized universes. Stanford physicist Andrei Linde, one of eternal inflation’s greatest champions, has made an analogy between these baby universes and a pot of boiling water: the way he sees it, new universes are popping into existence all the time, just as new bubbles are always popping up in a boiling pot.
But many cosmologists remain unconvinced. Princeton physicist Paul Steinhardt, who helped develop inflation into its modern form, worries that a theory giving rise to an infinity of universes ends up saying nothing about what any one particular universe will be like. In other words, the theory lacks predictive power.
One of the most vocal critics has been physicist and YouTuber Sabine Hossenfelder, who laid out her anti-multiverse arguments in her books Lost in Math (2018) and Existential Physics (2022). Because the existence of these other universes can never be confirmed, she argues, it makes no sense to take them seriously. She describes multiverse theories as “ascientific”, meaning that they lie outside of science. Even so, proponents of the idea have not budged. Linde, for example, told colleagues he’d nearly bet his life that the inflationary multiverse is real.
String Theory
Meanwhile, around the turn of the 21st century, another idea from the frontiers of physics appeared to lend support to the many-universes idea. This new approach came from string theory – the idea that the universe is made up of tiny vibrating strings, far smaller than anything we could see through our best technology (or even detect with our most powerful particle accelerators).
But string theory, rather than describing one universe, seemed to allow for a staggeringly vast array of universes. Stanford physicist Leonard Susskind described the resulting picture as a “landscape” of universes – seemingly echoing the multiverse given by eternal inflation (and in fact, many physicists believe the two ideas are related).
Quantum Mechanics
This brings us to the third pro-multiverse argument: The so-called “many worlds” interpretation of quantum mechanics. In classical physics, one might talk about the speed or momentum of a particle – but in quantum physics, these quantities are replaced by something called a wave function, a mathematical structure associated with the probability of a physical system being in some particular state (a counterintuitive idea even Einstein struggled to accept).
The really weird thing about quantum mechanics is that we can’t know what state a quantum system is in until we measure it. Prior to measurement, it’s said to be in a superposition of states.
Take an electron: According to quantum mechanics, an electron can spin in two different ways (physicists call the two spins “up” and “down”). Before you look at the electron, the theory says it’s both spin-up and spin-down. But when you actually measure the electron’s spin, the wave function is said to “collapse”; the superposition goes away and you’re left with one spin or the other. This view is called the “Copenhagen interpretation” of quantum mechanics, after the city where its first proponents, Niels Bohr and Werner Heisenberg, worked.
What if not only electrons, but things in the macroscopic world, followed these bizarre rules? The question inspired Erwin Schrödinger’s famous thought experiment involving a simultaneously alive and dead cat.
In the decades following the development of quantum mechanics, the Copenhagen interpretation reigned, more or less by default. But in the 1950s, a graduate student named Hugh Everett developed a bold alternative. Like many physicists, Everett was troubled by the idea of wave function collapse. What causes the collapse, and exactly how does it happen?
Everett suggested that the wave function doesn’t collapse. Instead, he suggested that when we make a measurement, the universe divides or branches, creating a brand new universe – one for each possible outcome (some experts caution against this phrasing, but it will do for our purposes.) When we look at that electron, the universe splits in two, with one universe containing a spin-up electron and one containing a spin-down electron. The same logic applies to Schrödinger’s cat: In one universe, we observe a living cat; in another, we see only its lifeless body. These universes also contain multiple copies of you: in some universes, you see a living cat, in others, a different “copy” of you sees the less fortunate one.
This idea of new universes constantly being birthed seems so bizarre and counter-intuitive that many people dismiss it outright. Some criticisms fall under the broad umbrella of concerns over falsifiability: If we can’t see, touch, or measure these parallel universes in any way – if the theory cannot be falsified – why should we believe those universes actually exist?
In a critical essay published in Aeon, science writer Philip Ball dismisses the many worlds view as incoherent, suggesting it is “not even wrong,” the put-down that Wolfgang Pauli reserved for ideas he felt deserved only ridicule. Ball also cites the philosopher of science Robert Crease, who has called the many worlds view “one of the most implausible and unrealistic ideas in the history of science.”
Proponents of the many worlds theory see it differently. They argue that we don’t need to test every prediction a theory makes to take it seriously. Quantum mechanics, for example, has proven to be incredibly successful, passing test after test; if it makes other predictions we can’t directly confirm, so be it.
Some argue that the many worlds view is not only simple but elegant. Physicist and podcaster Sean Carroll, for example, has argued that the many worlds view is actually the leanest and most straightforward version of quantum mechanics. If you just take the idea of quantum wave functions seriously, Carroll suggests, this is where you inevitably end up. While critics like Hossenfelder believe physicists have fallen into the trap of taking their equations too seriously, Carroll believes it’s equally problematic to not take the equations seriously enough. In his 2019 book Something Deeply Hidden, Carroll defends the many worlds interpretation of quantum multiverse, describing the theory’s array of unseen universes as “indisputably real.”
And Carroll is not alone; other staunch Everettians include Max Tegmark at MIT, David Deutsch at Oxford, Brian Greene at Columbia, and David Wallace at the University of Pittsburgh. As Deutsch put it in his provocative 1997 book The Fabric of Reality: “The quantum theory of parallel universes is not the problem, it is the solution. It is not some troublesome, optional interpretation emerging from arcane theoretical considerations. It is the explanation – the only one that is tenable – of a remarkable and counter-intuitive reality.”
***
Inflationary cosmology, string theory, quantum mechanics – what do these varied arguments for the multiverse add up to? As Siegfried put it when I interviewed him for my Discover story: “Whether there’s a multiverse or not does not hinge on any one theory being right or wrong. There are different possible ways there could be a multiverse – and we don’t know if any of them are correct... But we have reasons to take some of these ideas seriously.”
While Halpern’s book takes a relatively neutral tone, Siegfried’s sympathies are clear: He admits we can’t know for sure (and may never know for sure) that the parallel universes exist, but feels the pro-multiverse arguments deserve to be heard. In fact, he turns some of the standard anti-multiverse arguments around, using them to defend the possibility of a multiverse.
In The Number of the Heavens, Siegfried writes that “the multiverse deniers... are presupposing a definition of science that rules out multiverses to begin with. And that’s not scientific.”
The physics community has not reached anything like a consensus on the issue of multiple universes. But then, at the frontiers of science, this is often the case. People argue about the multiverse just as they argue about the existence of free will or the nature of consciousness.
As the late Nobel Laureate physicist Steven Weinberg suggested, we may simply have to reconcile ourselves to the possibility of never truly settling these most challenging questions. As we investigate the nature of reality, Weinberg mused, there may always be “some irreducible mystery,” adding that this is “just one of those tragedies we have to live with.”
At the same time, the arguments do not seem fruitless; rather, they bring difficult questions into sharp (or at least sharper) focus, helping us think and see more clearly. In the meantime, children can safely end their mailing addresses at “The Universe,” even as they ponder what might lie beyond.
Dan Falk is a science journalist based in Toronto. His books include The Science of Shakespeare and In Search of Time. Follow him on X at @danfalk and on Instagram/Threads @danfalkscience.
