As far-fetched as the idea of a multiverse might seem, “it’s not like we come up with these crazy ideas and try to shove them into our theories,” said theoretical physicist Brian Greene at Columbia University, author of “The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos,” which explores a range of possible multiverses. “Our mathematical theories naturally yield these unfamiliar notions of other universes, and we’ve got a great deal of respect for mathematics, because over the centuries it’s proven itself a very potent guide to reality. This doesn’t mean the conclusions of mathematics are necessarily right, but they’re sufficiently compelling to make us willing to investigate them.”

The inflationary multiverse

What if our Big Bang was just one of many? Cosmologists think that, in the moments after the Big Bang, the universe experienced a rapid growth spurt called inflation. “When one explores the mathematics of inflation, one finds that in most versions of inflation, the Big Bang should not be a unique event,” said Greene. “Instead, our Big Bang should be one of a series, perhaps an infinite series, of Big Bangs.”

Physicists call this scenario eternal inflation, and it suggests that our universe is just one bubble “within a grand collection of a larger multiverse” that is regularly popping out new bubble universes, Greene said. “It doesn’t take much energy to spawn a universe, because inflation has this wonderful property of mining energy from a reservoir of energy contained in the gravitational field that, in most models, one cannot ever use all up.”

It might even be possible to test this idea by looking for evidence that such bubble universes have collided with one another. “If our universe got hit by an expanding bubble, there should be some imprint of that collision in the cosmic microwave background (CMB) radiation, the heat left over from the Big Bang,” Greene said. “People have been scouring CMB data looking for the cosmic fingerprints of such collisions, looking for telltale signs of other realms by performing observations on our own realm.”

The quilted multiverse

Not convinced? There’s more than one way to build a multiverse, and the quilted multiverse starts with the simple assumption that the universe goes on forever. If the universe is really infinite, then it contains infinite variations—and infinite copies—of everything. You know the one about the monkey typing Shakespeare? Given enough time, he won’t peck out just one edition of the Complete Works; he’ll bang them out infinitely. “You can imagine there are realms out there that are close copies or identical copies of ours somewhere else in the universe, or versions of ours that are modified in mind-bending ways,” Greene said. “These chunks of space would be like patches in a quilted patchwork, which is where the name for this idea comes from.”

To test the quilted multiverse hypothesis, scientists have to find out if space really is infinite. “There have been suggestions that space might not go on forever, but might, for instance, have the shape of a giant donut, or torus as we call it,” Greene said. “In this configuration where the universe does not go on forever, if you look at distant stars or galaxies, the light from them can travel multiple cycles, yielding multiple images of the same object, and if one sees those, that would be pretty strong evidence the universe does not go on forever.”

The quantum multiverse

In the quantum world, there is no way to predict with certainty the outcome of a measurement before it’s made. A system exists in a “superposition” of states until the moment that it is observed. So how does a single state emerge from that jumble of possibilities? Perhaps, some theorists say, each outcome really does come to be–in its own distinct universe.

When quantum mechanics “came blistering onto the scene of physics in the 1920s and 1930s, it introduced a new idea into physics — that there are experiments where one can only ever predict the possibility of one outcome or another, as opposed to the older idea of Newtonian physics, which suggested one can predict what will happen with absolute certainty,” Greene said. “That raised an interesting puzzle: If a particle has a 30 percent likelihood of being here and 40 percent chance of being there and 20 percent likelihood of being way over there, when experiments find particles at one location or another, what happens to the other outcomes? In the 1950s, Hugh Everett, then a grad student at Princeton, suggested all possible outcomes do occur, each in its own universe.”

There are alternative interpretations of quantum mechanics, such as Ghirardi–Rimini–Weber (or GRW) theory, named after its developers, which suggests each event can only have one possible outcome, Greene noted. Since many of these other interpretations make predictions that scientists can potentially test, maybe one day researchers can find support for them, which would weaken the case for a quantum multiverse. Until then, it’s possible that every possible timeline actually exists, including ones where fictional characters such as Sherlock Holmes actually lived.

The brane multiverse

String theory proposes that, underneath it all, the universe is made up of little one-dimensional threads that vibrate in different ways to make the particles with which we’re familiar. In the mid-1990s, theorists suggested that these strings could form higher-dimensional structures they dubbed membranes, or branes for short. The universe as we know it occupies one such brane.

“If one can have a big 3-D structure, it could be equivalent to our universe, and theory allows multiple such 3-D objects, or multiple universes,” Greene said. “An analogy I like to use is that everything we know of our lives in essence takes place on a giant slice of bread, and there’s this larger cosmic loaf that has other slices, with each slice being a universe.”

The most powerful particle accelerator on Earth, the Large Hadron Collider, might be able to test whether the brane multiverse is real. “If you collide protons on our ‘slice of bread’ and the collision is strong enough, some of the debris can be ejected off our slice of bread, and you can notice if this happened by a loss of energy in the output,” Greene said. Such tests could begin when the Large Hadron Collider restarts in 2015.

The simulated multiverse

One of the wildest multiverse concepts begins with the idea that our universe is actually just a computer simulation. It sounds like the premise for “The Matrix,” but the question of whether our world is just a fantasy is ancient, dating back at least to Chinese philosopher Zhuangzi, who more than 2,000 years ago described waking from a dream unsure of whether he was a man who dreamed he was a butterfly or a butterfly dreaming that he was a man.

In 2003, philosopher Nick Bostrom, director of the Future of Humanity Institute at the University of Oxford, posed a modern take on this ancient contemplation of the true nature of reality. He estimated it would take 10 36 calculations in total—a 1 with 36 zeroes behind it—to create a simulation of the whole of human history that was indistinguishable from reality, and that a planet-sized computer based on current electronics might conceivably perform 10 42 operations per second.

Simulating every person who has ever lived might only take a tiny fraction of an advanced civilization’s resources, Bostrom reasoned. In that case, the number of computer-generated people and other entities within simulations could vastly outnumber all the real minds that have ever lived. By this logic, the odds are that we all are simulated, not real. In turn, it may be possible that our simulators are themselves simulated, and their simulators are simulated, and so on.

Seems rock-solid, right? The problem, says Greene, “is that it’s very hard to imagine falsifying the simulated multiverse notion — even if we can find out we are simulated, the simulator can erase that memory if they wanted, so any proof that the universe is simulated would go away. There are a number of physicists who nod to this idea as a mind-bending curiosity, but it’s not going to affect our day-to-day lives or work.”

Beyond reach?

We may never find proof that other universes exist. But even if we could, there is virtually no known way to reach them.

Scientists have speculated that wormholes, “shortcuts” in space and time, might allow one to reach other universes. However, wormholes may not actually exist, and even if they do, it might not be possible to traverse them. “It’d be lovely go get into a spaceship to go through a wormhole to travel back and forth to other universes, but it doesn’t look to me like the equations will allow us to do so,” said theoretical physicist Leonard Susskind at Stanford University.

Even if we can never travel to another universe, that does not mean we are alone. “Even in our single lonely universe, there is a gargantuan number of opportunities for life to exist,” Greene said. “The many discoveries of extrasolar planets are thrilling, and that may lead one to seriously consider that there may be other life forms out there in our universe.”

Go Deeper
Editor’s picks for further reading

FQXi: Philosophy of the Multiverse
In this essay, discover why many theorists are drawn to the idea that our universe is just one among many.

NOVA: Parallel Worlds, Parallel Lives
Discover web resources associated with NOVA’s “Parallel Worlds, Parallel Lives,” a film about the life and work of Hugh Everett III.

The Hidden Reality
Brian Greene’s 2011 book asks whether our universe is the only universe.

Video Credits

Produced, animated, and edited by Greg Kestin

Editorial help from Kate Becker, Anna Rothschild, and Lauren Aguirre

A special thanks to Andrew Friedman

Original Footage
© WGBH Educational Foundation 2014


Night Music, Air Prelude, Not as it Seems, and Comfortable Mystery 3 by Kevin MacLeod ( CC By 3.0

Bicep2: Courtesy of National Science Foundation
Big Bang, Galaxies, and Cosmic Web: Courtesy of NASA


Many physicists believe there is a strong connection between the inflationary multiverse and an important feature of string theory; for more see:

Although there is not yet evidence favoring the quantum multiverse over several other interpretations of quantum mechanics, it is still consistent with the results of every quantum experiment ever conducted.

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