The concept of a “multiverse,” a collection of different universes existing together and offering skewed perspectives of familiar concepts, has become incredibly prevalent in pop culture, playing a central role in movies like Spider-Man: No Way Home, Doctor Strange in the Multiverse of Madness, and the Oscar award-winning Everything Everywhere All At Once. And now, DC Comics’ resident Scarlet Speedster is taking his own jaunt through a multitude of different worlds via timeline meddling in the new Warner Bros film The Flash.
But the multiverse portrayed on the silver screen usually depicts a situation that is very different from the term’s scientific reality. The traditional multiverse of fiction actually resembles another theory in physics—not called a multiverse at all—but referred to as “the many worlds interpretation of quantum physics,” initially dreamed up to explain one of the most fundamental mysteries of nature.
To explore these scientific multiverses, consider a hypothetical situation in which you are accused of a spate of petty crimes you know you did not commit. Yet as you stand trial, eye-witness testimony, video evidence, and even forensics all point your way. In fact, the only difference between you and your criminal doppelgänger seems to be their rather stylish goatee.
This grabs your attention, and you immediately begin searching for physics books in the prison library that concern the concept of a multiverse, finding it to be pleasingly well-stocked.
To mount your defense, you will call on two well-qualified experts as your star witnesses: Russian-American theoretical physicist Andrei Linde, Ph.D., a professor at Stanford University, and Russian-Israeli physicist Lev Vaidman, Ph.D., a professor at Tel Aviv University in Israel.
Searching for Clues in a Cosmological Multiverse
As the term “multiverse” became widely adopted in cosmology during the 20th century, the term “universe” no longer represented all there is. Instead, scientists started to think of our universe as possibly one of many.
The main reason for this expansion of our cosmic horizons came from the discovery that our universe underwent a period of rapid inflation that we call the Big Bang. This led scientists like Linde to wonder if there could be separate disconnected regions of space, out of sight to us, because of the speed at which our cosmic bubble grows. These regions may also undergo their own spates of inflation, and this could be, in theory, never-ending, in what is known as “eternal inflation.”
This gives rise to the concept of an infinite number of universes spread across a “cosmic canvas,” engaging in bouts of rapid and chaotic inflation and perhaps collapse, all separated by boundaries of some sort, sometimes called a “brane,” that act as domain walls.
“So once the universe started inflating, in many different versions of this model, this inflation happens in some parts of the universe, not everywhere, thus producing new kinds of universes in a multiverse,” Linde tells Popular Mechanics.
Linde says his reason for initially theorizing a multiverse was the fact of how “right” our universe is to form large-scale structures like galaxies, stars, planets, and life. A multiverse suggests it is just right because life like us could only exist in such a universe; there are other regions of space far less hospitable and incapable of forming structures.
For Linde, this explains why the universe we see is so uniform on a large scale, and why it has the right characteristics to form life, even if only on one tiny planet tucked away in orbit around a very average star. There are other universes within the multiverse that simply would not allow this.
These “new kinds of universes” may differ radically from our own, and that goes way beyond a rotten version of you with a hairy chin—they may have laws of physics that are radically different from our own. Of course, our universe alone is an estimated 94 billion light years across, so getting to the boundary between our universe and another would be no walk in the park.
“You cannot go and jump from one universe to another unless, by some stupid luck, you live near the boundary,” Linde says. “To jump from one universe to another, you must find this boundary which will probably be exponentially far away from you.”
This difficulty is compounded by the fact that the boundary is estimated to expand at a rate of 41.9 miles per second per 3.3 million light years — for widely separated regions of space — which means universes are moving away from one another faster than the speed of light.
Even if a prospective “multinaut” is lucky enough to be near such a boundary, Linde thinks this could be where their luck runs out. An attempt to cross the division between two universes could be messy, to say the least.
“It might be possible to jump through the boundary, then most probably you will immediately die,” Linde explains. “Different laws of physics operate in each of them, so particles from which your body is built may not exist in this new universe.”
And these universes could have even more radical differences. Linde points out that string theory suggests that our universe began with as many as 11 dimensions, which collapsed and left us with the three dimensions of space and the one dimension of time, in other words, the 4-dimensional world we now perceive.
Different laws of physics, and possible additional dimensions, would likely spell instant and messy death for a universal interloper. They are hardly likely to make it to Earth to commit a series of pretty crimes, and if they did, coming from a universe with 11 dimensions could cause them to stand out in our 4D world.
The cosmological multiverse that Linde and other theoretical physicists describe doesn’t sound much like the multiverse presented in movies like Across the Spider-verse and Everything Everywhere All at Once, or the alternative timelines that the Flash explores in his movie. Clearly, the cosmological multiverse isn’t a good place to hunt for our law-breaking doppelgänger.
Many Worlds, Many Suspects
Disheartened by the revelation that an actual multiverse wouldn’t be likely to harbor another “you,” the cosmology textbooks are put aside, and you turn instead to a stack of tomes concerning quantum physics. You’ve heard about the strange counterintuitive nature of this realm and wonder if there is room within its theories for other “yous.”
You quickly discover the work of Hugh Everett III, who, as a Ph.D. student at Princeton in 1955, came up with an interpretation of quantum mechanics that suggests when the universe is faced with “quantum choices,” it “splits” into different worlds. This is known as the “many worlds interpretation of quantum mechanics.”
The concept isn’t widely accepted, but the way it deals with a fundamental problem in quantum physics makes it appealing to Lev Vaidman and many other physicists and philosophers.
“There is a kind of strange situation, a real scandal, with quantum mechanics. It has existed for over a century, and yet even today, people are fighting about what it means,” Vaidman tells Popular Mechanics. “From my point of view, if you accept many worlds, then the scandal disappears.”
Vaidman is referring to the “measurement problem” of quantum physics, which states that things like photons (which are tiny particles of light), electrons, and larger conglomerations of atoms can exhibit wave or particle-like behavior.
This particle-wave duality in nature means the characteristics of a quantum system or its state can be described by wave mathematics, and waves can overlap and exist in a superposition. This means existing in a superposition of states which could be contradictory in nature.
What physicists found was that attempting to measure this superposition of states or interactions with other systems caused the superposition to be destroyed, thus causing the system to adopt one set of values or one state. This is called “wave function collapse.”
Everett’s solution was novel because it did away with the measurement problem because the wave function collapse never happens in the many worlds interpretation. Instead, Everett suggested the wave function grows to include the experimenter, the lab, and the entire universe. There is a separate world created for each possible quantum outcome.
Here’s where you (and the other you) come in. When the universe “splits” into separate worlds, according to Everett, it “remembers” everything that has happened to it up until that point. That means each new world contains a new you, and from the moment of the split, there is the possibility of divergent characteristics, including criminal ones, developing.
Vaidman describes a situation in which he uses a quantum experiment he calls a “world splitter” in his lab to decide if he should attend a conference in Italy.
As long as he strictly follows the outcome of the machine, his experiment creates two worlds with very different Vaidmans. One has all the memories and knowledge from that conference, and one does not. If Vaidman meets you at that conference and you get chatting, there are now two different “yous” that are no longer identical. One has the memory of a fascinating chat and Vaidman, and the other does not.
Could such a world-splitter experiment have caused a chain of events that created a world in which your quantum doppelgänger was influenced to adopt a life of crime?
Before heading off to Vaidman’s lab to smash his world-splitter and protect your other doppelgängers from its nefarious influence, you realize there remains a problem. Linde was clear, even if a cosmological multiverse could contain another you (and it wouldn’t), they would be unable to cross into our world.
Vaidman points out that despite a many worlds situation containing another you, one of the key principles of many worlds theories is that different worlds in this interpretation shouldn’t be able to interact with each other.
“One world cannot interact with another, but in principle, another world could interfere with us,” he explains. However, this interference, like other quantum effects, manifests on a scale no larger than hundreds or possibly thousands of atoms. You and your many world doppelgängers are composed of around seven billion billion billion atoms (7 x 10²⁷), so they (or we) couldn’t even exert the tiniest influence over another world.
Thus, Vaidman describes the possibility of meeting an “alternative you” from another world as “the next level of impossibility.” Yet even though a many worlds interpretation is out as an alibi in our scenario, it is the most viable interpretation of quantum mechanics there is. Thus he is somewhat wary of how pop culture addresses this quantum multiverse concept.
“I don’t think it’s science fiction, I believe it is there. My reason is that it is the only way to have physics, which has no randomness and no action at a distance,” Vaidman says. “I’m good company too. Einstein didn’t like these violations either. It’s unfortunate that he didn’t live until Everett invented the many worlds interpretation. I speculate that he would have adopted it strongly.”
There may be a world in which Einstein and Everett discussed the concept of many worlds, and as a result of Einstein championing it, the theory currently enjoys the status of the preferred interpretation of quantum mechanics. That, sadly, isn’t ours. Everett’s theory barely ruffled feathers in 1957, and he would die at 51 in July 1982, not living to see it adopted by physicists like Vaidman or lauded by writers across the globe and loved by consumers of science fiction, books, cinema, comic books, and TV.
Cosmologist Andrei Linde expresses the utility of such outlets of creativity when it comes to breaking out of the rigidity of the constraints of science to arrive at out-of-the-box ideas.
“Science most often works by experiment, but if you say ‘this is how science must work,’ then you put science into a box and do not allow it to develop,” Linde says. “Science fiction helps to overcome this rigidity of thought, it may be useless at the moment when you watch it, but it pushes you somewhere.
“Sometimes you need to change the rules of the game. So science fiction is sometimes helpful in that regard, just like philosophy, and just like all other aspects of culture.”
As you are brought to the dock to mount your defense, you consider this statement. With your multiversal alibi out of the window, you clear your throat and croak: “Your honor, are you familiar with cloning?”
Robert Lea is a freelance science journalist focusing on space, astronomy, and physics. Rob’s articles have been published in Newsweek, Space, Live Science, Astronomy magazine and New Scientist. He lives in the North West of England with too many cats and comic books.
