How Would Humans Know If They Lived in a Multiverse?
By Tanya Lewis, Staff Writer | June 02, 2014 01:44pm ET
Some theories in physics give rise to the idea of multiple universes, where nearly identical versions of the known universe exist. But if such a multiverse does exist, how would people know, and what would it mean for humanity?
There may be ways to find out if the known universe is one of many, said Brian Greene, a theoretical physicist and author at Columbia University in New York.
"There are certain versions of the multiverse that, should they be correct, might be most susceptible to confirmation," Greene told Live Science. [5 Reasons We May Live in a Multiverse]
Spotting a multiverse
For example, in the multiverse suggested by string theory, a model that says the universe is composed of one-dimensional strings, the known universe might exist on a giant 3D membrane, Greene told Live Science.
In such a world, "if the universe is a loaf of bread, everything we know about takes place on one slice," he said. Conceivably, debris from collisions that migrated off our slice into the wider cosmos might leave missing energy signatures, which a particle accelerator like the Large Hadron Collider at CERN might be able to detect, Greene said.
Some theories of inflation, the notion that the universe expanded rapidly in the first fractions of a second after the Big Bang, suggestanother kind of multiverse. The Big Bang could be one of many big bangs, each giving rise to its own universe — a cosmic bubble in a sea of other bubbles.
In such a scenario, the known universe might collide with another one, which might leave an imprint on the cosmic microwave background, the radiation signature left over from the Big Bang, Greene said.
Greene stressed that all of these notions are highly speculative — "There's reason to take the ideas seriously, but they are far from science fact," he said.
Is free will dead?
But if a multiverse does exist, it could have some wacky consequences. A world with an infinite number of universes would virtually ensure that conditions in one universe would repeat in another, Greene said. In other words, there would almost certainly be another version of you reading this article, written by another version of me.
In such a multiverse, you might decide to read the article in one universe and not read it in another. What would that mean for the notion of free will?
Perhaps it's a moot point. "I think free will bit the dust long before multiverse theory," Greene said.
Scientific equations describe the particles that make up all matter, including humans, Greene said. While more-complex structures arise that have no relevance to a single particle — temperature, for instance — everything still has a "fundamental microphysical underpinning," he said.
That means free will is merely a human sensation, not actual control.
"When I move my teapot, that sensation is absolutely real," Green said. "But that's all it is. It's a sensation."
Maybe in another universe, there's a Brian Greene that believes in free will.
5 Reasons We May Live in a Multiverse
Here are 5 different scientific theories on the possible existence of multiverses :-
( 1 )
Mathematical Universes
Credit: WGBH Educational Foundation
Scientists have debated whether mathematics is simply a useful tool for describing the universe, or whether math itself is the fundamental reality, and our observations of the universe are just imperfect perceptions of its true mathematical nature. If the latter is the case, then perhaps the particular mathematical structure that makes up our universe isn't the only option, and in fact all possible mathematical structures exist as their own separate universes.
"A mathematical structure is something that you can describe in a way that's completely independent of human baggage," said Max Tegmark of MIT, who proposed this brain-twistin gidea. "I really believe that there is this universe out there that can exist independently of me that would continue to exist even if there were no humans."
This story was provided by SPACE.com, a sister site to LiveScience. Follow Clara Moskowitz on Twitter @ClaraMoskowitz or SPACE.com@Spacedotcom. We're also on Facebook & Google+.
( 2 )
Daughter Universes
Credit: NASA/JPL
The theory of quantum mechanics, which reigns over the tiny world of subatomic particles, suggests another way multiple universes might arise. Quantum mechanics describes the world in terms of probabilities, rather than definite outcomes. And the mathematics of this theory might suggest that all possible outcomes of a situation do occur — in their own separate universes. For example, if you reach a crossroads where you can go right or left, the present universe gives rise to two daughter universes: one in which you go right, and one in which you go left.
"And in each universe, there's a copy of you witnessing one or the other outcome, thinking — incorrectly — that your reality is the only reality," Greene wrote in "The Hidden Reality."
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Parallel Universes
Credit: Shutterstock/Sandy MacKenzie
Bubble Universes
Another idea that arises from string theory is the notion of "braneworlds" — parallel universes that hover just out of reach of our own, proposed by Princeton University's Paul Steinhardt and Neil Turok of the Perimeter Institute for Theoretical Physics in Ontario, Canada. The idea comes from the possibility of many more dimensions to our world than the three of space and one of time that we know. In addition to our own three-dimensional "brane" of space, other three-dimensional branes may float in a higher-dimensional space.
Columbia University physicist Brian Greene describes the idea as the notion that "our universe is one of potentially numerous 'slabs' floating in a higher-dimensional space, much like a slice of bread within a grander cosmic loaf," in his book "The Hidden Reality" (Vintage Books, 2011).
A further wrinkle on this theory suggests these brane universes aren't always parallel and out of reach. Sometimes, they might slam into each other, causing repeated Big Bangs that reset the universes over and over again. [The Universe: Big Bang to Now in 10 Easy Steps ]
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Bubble Universes
Credit: Shutterstock/Victor Habbick
In addition to the multiple universes created by infinitely extending space-time, other universes could arise from a theory called "eternal inflation." Inflation is the notion that the universe expanded rapidly after the Big Bang, in effect inflating like a balloon. Eternal inflation, first proposed by Tufts University cosmologist Alexander Vilenkin, suggests that some pockets of space stop inflating, while other regions continue to inflate, thus giving rise to many isolated "bubble universes."
Thus, our own universe, where inflation has ended, allowing stars and galaxies to form, is but a small bubble in a vast sea of space, some of which is still inflating, that contains many other bubbles like ours. And in some of these bubble universes, the laws of physics and fundamental constants might be different than in ours, making some universes strange places indeed.
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Infinite Universes
Credit: Shutterstock/R.T.Wohlstadter
Scientists can't be sure what the shape of space-time is, but most likely, it's flat (as opposed to spherical or even donut-shape) and stretches outinfinitely. But if space-time goes on forever, then it must start repeating at some point, because there are a finite number of ways particles can be arranged in space and time.
So if you look far enough, you would encounter another version of you — in fact, infinite versions of you. Some of these twins will be doing exactly what you're doing right now, while others will have worn a different sweater this morning, and still others will have made vastly different career and life choices.
Because the observable universe extends only as far as light has had a chance to get in the 13.7 billion years since the Big Bang (that would be 13.7 billion light-years), the space-time beyond that distance can be considered to be its own separate universe. In this way, a multitude of universes exists next to each other in a giant patchwork quilt of universes. [Visualizations of Infinity: A Gallery]
