Movies can’t get enough of exploring these questions. From superhero blockbusters like Dr Strange in the Multiverse of Madness to my independent darling Everything everywhere all at once, science fiction stories are full of creative interactions between alternate realities. And depending on which cosmologist you ask, the concept of a multiverse is more than pure fantasy or a practical storytelling device.
Humanity’s ideas about alternate realities are old and varied – in 1848, Edgar Allan Poe even wrote a prose poem in which he imagined the existence of an “unlimited succession of universes”. But the concept of the multiverse really took off when modern scientific theories attempting to explain the properties of our universe predicted the existence of other universes where events take place outside of our reality.
“Our understanding of reality is not complete, by far,” says Stanford University physicist Andrei Linde. “Reality exists independently of us.”
If they exist, these universes are separate from ours, inaccessible and undetectable by any direct measurement (at least until now). And that has some experts wondering if the search for a multiverse can ever be truly scientific.
Will scientists ever know if our universe is the only one? We break down the various theories about a possible multiverse – including other universes with their own laws of physics – and whether many versions of you could exist there.
What is a multiverse?
The multiverse is a term scientists use to describe the idea that beyond the observable universe, other universes may also exist. Multiverses are predicted by several scientific theories that describe different possible scenarios – from regions of space in different planes of our universe, to separate bubble universes that constantly pop up.
The one thing all of these theories have in common is that they suggest that the space and time we can observe is not the only reality.
Ok…but why do scientists think there could be more than one universe?
“We can’t explain every feature of our universe if there’s just one,” says science journalist Tom Siegfried, whose book The number of heavens investigates how designs for the multiverse have evolved over millennia.
“Why are the fundamental constants of nature what they are? asks Siegfried. “Why is there enough time in our universe to make stars and planets? Why do stars shine the way they do, with just the right amount of energy? All of these things are questions to which we have no answers in our physical theories.
Siegfried says there are two possible explanations: First, we need newer and better theories to explain the properties of our universe. Or, he says, it’s possible that “we’re just one of many different universes and we live in the one that’s nice and comfortable.”
What are some of the most popular multiverse theories?
Perhaps the most scientifically accepted idea comes from what is called inflationary cosmology, which is the idea that in the tiny moments after the big bang, the universe expanded rapidly and exponentially. Cosmic inflation explains many of the observed properties of the universe, such as its structure and the distribution of galaxies.
“This theory looked like a piece of science fiction at first, although very imaginative,” says Linde, one of the architects of the theory of cosmic inflation. “But it explained so many interesting features of our world that people started taking it seriously.”
One of the theory’s predictions is that inflation could happen over and over again, possibly ad infinitum, creating a constellation of bubble universes. Not all of these bubbles will have the same properties as ours – they could be spaces where physics behaves differently. Some of them might be similar to our universe, but they all exist beyond the realm that we can directly observe.
What are the other ideas?
Another compelling type of multiverse is called the many-worlds interpretation of quantum mechanics, which is the theory that mathematically describes the behavior of matter. Proposed by physicist Hugh Everett in 1957, the Many Worlds Interpretation predicts the presence of branching timelines, or alternate realities in which our decisions play out differently, sometimes producing very different outcomes.
“Hugh Everett says, look, there’s actually an infinite number of parallel Earths, and when you do an experiment and you get the probabilities, all that basically proves is that you live on the Earth where it’s was the result of that experiment,” says physicist James Kakalios of the University of Minnesota, who has written about the physics (or not) of superheroes. “But on other Earths, the result is different.”
According to this interpretation, versions of you could live the many possible lives you could have had had you made different decisions. However, the only reality that is perceptible to you is the one you inhabit.
So where are all these other Earths?
They all overlap in dimensions that we cannot access. Max Tegmark of MIT refers to this type of multiverse as a Tier III multiverse, where multiple scenarios take place in branching realities.
“In the many-worlds rendition, you always have an atomic bomb, you just don’t know exactly when it’s going to explode,” Linde explains. And maybe in some of these realities, that won’t be the case.
In contrast, the multiple universes predicted by some theories of cosmic inflation are what Tegmark calls a Level II multiverse, where the fundamental physics may be different from universe to universe. In an inflationary multiverse, Linde says, “you don’t even know if in some parts of the universe atomic bombs are even in principle possible.”
So if I want to meet, how do I get there? Can we travel between multiverses?
Unfortunately no. Scientists don’t think it’s possible to travel between universes, at least not yet.
“Unless a lot of the physics that we know of that’s pretty solidly established is wrong, you can’t travel through these multiverses,” says Siegfried. “But who knows? A thousand years from now, I’m not saying someone can’t understand something you never imagined.
Is there direct evidence to suggest multiverses exist?
Even though certain features of the universe seem to necessitate the existence of a multiverse, nothing has been directly observed to suggest that it actually exists. So far, the evidence supporting the idea of a multiverse is purely theoretical and, in some cases, philosophical.
Some experts say it may be a grand cosmic coincidence that the big bang forged a perfectly balanced universe that is perfectly suited to our existence. Other scientists believe it is more likely that a number of physical universes exist and that we simply inhabit the one that has the right characteristics for our survival.
An infinite number of small alternate pocket universes, or bubble universes, some of which have different physics or different fundamental constants, is an interesting idea, Kakalios says. “That’s why some people take these ideas seriously, because it helps solve some philosophical problems,” he says.
Scientists debate whether the multiverse is even an empirically testable theory; some would say no, since by definition a multiverse is independent of our own universe and inaccessible. But maybe we just haven’t found the right test.
Will we know one day if our universe is only one among many others?
We might not. But the multiverses are among the predictions of various theories that can be tested in other ways, and if those theories pass all of their tests, then maybe the multiverse holds up as well. Or maybe a new discovery will help scientists determine if there really is something beyond our observable universe.
“The universe isn’t limited by what some blobs of protoplasm on a tiny little planet can understand or test,” says Siegfried. “We can say it’s not testable, so it can’t be real, but that just means we don’t know how to test it. And maybe one day we’ll figure out how to test it, and maybe we won’t. But the universe can do whatever it wants.