Science fiction loves multiverses. Everyone from Rick and Morty to Spider-Man has walked through a portal and met their alter ego. My favorite use of this trope comes from the sitcom Communitywhere a simple roll of the die divides our heroes into seven alternate timelines. After the inhabitants of the ‘Darkest Timeline’ suffered spectacular misfortune worthy of a memorythey become evil and wear fake goatees to match their new villainous personas.
Writers often deploy this trope to incite team-ups or interdimensional showdowns. But some physicists like the idea of a multiverse for another reason: the existence of many unobserved domains could answer profound questions about our universe. There is, perhaps rightly, a multiplicity of ideas about how our universe might fit into a larger constellation of realities. Two of the most popular come from cosmology and quantum mechanics.
Shortly after the Big Bang, our universe experienced a brief period of ultrarapid expansion called inflation. During this growth spurt, tiny quantum fluctuations in the structure of the universe expanded to enormous proportions. In parts of space close enough for light to reach Earth during the life of the universe, these fluctuations created density variations that gave rise to the formation of galaxies.
On even larger scales – well beyond our cosmic horizon – quantum fluctuations could have created regions of space with radically different properties, says Andrei Linde, a retired Stanford University physicist and author of the theory of inflation. Invisible parts of space could have different particle masses and force strengths than our neck of the cosmos, Linde says. Electrons, for example, could be much heavier. Or gravity could behave differently. In such places, life might not exist.
Furthermore, even if inflation has stopped in our observable universe, it could continue elsewhere, eternally bursting more space bubbles with unique properties. These bubbles would be so distant and distinct that they would in fact constitute different universes.
For Linde and some other scientists, this scenario explains a big puzzle in cosmology: why particle masses and force forces in our universe seem perfectly adapted to life. If a multiverse exists, it’s not such a suspicious coincidence. Among the many bubble universes, living conditions inevitably had to appear somewhere.
Testing this idea may be possible. If our universe is just a bubble in an unfathomable foam, perhaps another universe has collided with ours, leaving a scar on our planet. the afterglow of the Big Bang. But “no one has yet seen the rings that would represent the scars of bubble collisions,” says physicist Paul Halpern of Saint Joseph’s University in Philadelphia.
Another idea of the multiverse comes from quantum physics, which says that particles can exist in a superposition of possible states until measured. “According to the traditional idea of quantum mechanics,” Halpern says, “once someone takes a measurement, this fuzzy scenario collapses into a single possibility.” For example, an electron that could exist in a set of possible locations is detected in only one location. “It’s a little strange, because it requires a human measurer,” says Halpern. If this is true, how did the universe function before humans existed?
In 1957, physicist Hugh Everett III proposed an explanation. Instead of observation causing a set of quantum possibilities to collapse into a single outcome, perhaps all possibilities unfold into alternate realities. For example, an observer splits into several copies of itself that each saw an electron in different locations.
“The versions separate seamlessly,” says Halpern. “They will never know each other and live in parallel universes.” (This is closer to the image of a multiverse seen in Community – although of course, for the sake of plot, these characters eventually meet and must defeat their evil counterparts.)
But this theory would be difficult to test. “We can’t have someone split an experience between two possibilities and ask each one how it was,” says Halpern. “If the theory is correct, you won’t notice it.”
The prospects of visiting other universes, if they exist, are equally dim. Hypothetical tunnels in the structure of space-time, known as wormholescould fill the realities. But “we don’t know if it’s possible to create them, and if it were…they would require so much energy and mass that they would far exceed current technology,” says Halpern. “So it’s not like you can have a wormhole in a secret closet in your bedroom and every night you open the closet door and jump in and travel to all these other places.”
This may be bad news for anyone who dreams of teaming up with their alter egos to save the day, Spider-Man style. But on the bright side, you’ll probably never have to fight an evil, goatish version of yourself.