The multiverse is a theory in which our universe is not the only one, but states that many universes exist parallel to each other. These distinct universes within the multiverse theory are called parallel universes. A variety of different theories lend themselves to a multiverse viewpoint.
Multiverses in religion and philosophy
The idea of a physical multiverse came later to physics than in some other areas. The Hindu religion has ancient concepts that are similar. The term itself was, apparently, first applied by a psychologist, rather than a physicist.
Concepts of a multiverse are evident in the cyclical infinite worlds of ancient Hindu cosmology. In this viewpoint, our world is one of an infinite number of distinct worlds, each governed by its own gods on their own cycles of creation and destruction.
The word multiverse was originated by American psychologist William James in 1895
(the word “moral” is excluded from some citations of this passage):
“Visible nature is all plasticity and indifference, a [moral] multiverse, as one might call it, and not a [moral] universe.”
The phrase rose in prominence throughout the 20th century, when it was used regularly in science fiction and fantasy, notably in the work of author Michael Moorcock (though some sources attribute the word to the earlier work of author and philosopher John Cowper Powys in the 1950s). It is now a common phrase within these genres.
In some theories, there are copies of you sitting right here right now reading this topic in other universes and other copies of you that are doing other things in other universes. Other theories contain parallel universes that are so radically different from our own that they follow entirely different fundamental laws of physics (or at least the same laws manifest in fundamentally different ways), likely collapsing or expanding so quickly that life never develops.
Not all physicists really believe that these universes exist. Even fewer believe that it would ever be possible to contact these parallel universes, likely not even in the entire span of our universe’s history. Others believe the quantum physics adage that if it’s possible, it’s bound to happen somewhere and sometime, meaning it may be inevitable that quantum effects allow contact between parallel universes.
According to MIT cosmologist Max Tegmark, there are four levels of parallel universes:
Level 1: An infinite universe that, by the laws of probability, must contain another copy of Earth somewhere
Level 2: Other distant regions of space with different physical parameters, but the same basic laws
Level 3: Other universes where each possibility that can exist does exist, as described by the many worlds interpretation (MWI) of quantum physics
‘ Level 4: Entirely distinct universes that may not even be connected to ours in any meaningful way and very likely have entirely different fundamental physical laws
The following sections look at each of these levels in more detail.
Tegmark’s approach is one of the only attempts to comprehensively categorize the concepts of parallel universes in a scientific (or, as some see it, pseu-doscientific) context. The full text of Tegmark’s 2003 paper on this topic is available at his MIT Web site, space.mit.edu/home/tegmark/ multiverse.pdf, for those who don’t believe that these concepts are scientific. (They may not be scientific, but if that’s the case, then at least they’re unscientific musings by a scientist.)
Plurality of worlds: A hot topic
Early astronomy provided some support for the existence of a plurality of worlds, a view that was so controversial that it contributed to at least one man’s death. These plurality of worlds, and the eventual parallel worlds, were rooted in the ideas of an infinite universe, as are the ideas of parallel universes presented in this topic.
The Italian philosopher Giordano Bruno (15481600) was executed for a variety of heresies against the Catholic Church. Though Bruno was a supporter of the Copernican system, his abnormal beliefs went far beyond that: He believed in an eternal and infinite universe that contained a plurality of worlds. Bruno reasoned that because God was infinite, his creation would similarly be infinite. Each star was another sun, like our own, about which other worlds revolved. He didn’t feel that such viewpoints were in opposition to the scriptures.
In fairness to the Catholic Church, Bruno wasn’t executed merely for believing in other worlds. His list of heresies was long and varied, including denial of Mary’s virginity, the divinity of Christ, the Trinity, the Incarnation, and the Catholic doctrine of transubstantiation. He also believed in reincarnation and was accused of practicing magic. This is not to say that any (or all) of these viewpoints warranted death, but given the time period, it would be hard to get out of such accusations alive.
In 1686, the French writer Bernard le Bovier de Fontenelle wrote Conversations on the Plurality of Worlds, which was one of the first topics to address the popular audience on scientific topics, being written in French rather than scholarly Latin. In Conversations, he explained the Copernican heliocentric model of the universe and contemplated extraterrestrial life. Though other enlightenment thinkers — possibly even John Adams and Benjamin Franklin, by some accounts — were agreeable to such viewpoints, it would be many years before the plurality of worlds extended to the plurality of universes.
In 1871, the French political malcontent Louis Auguste Blanqui wrote — while in prison — a brochure titled Eternity by the Stars: Astronomical Hypotheses, in which he said that an infinite universe would have to replicate the original set of combinations an infinite number of times to fill up the infinite space. This is, to my knowledge, the first inkling of the transition from “plurality of worlds” to “parallel worlds” — copies of you sitting reading this same topic on another planet.
Level 1: If you go far enough, you’ll get back home
The idea of Level 1 parallel universes basically says that space is so big that the rules of probability imply that surely, somewhere else out there, are other planets exactly like Earth. In fact, an infinite universe would have infinitely many planets, and on some of them, the events that play out would be virtually identical to those on our own Earth.
We don’t see these other universes because our cosmic vision is limited by the speed of light — the ultimate speed limit. Light started traveling at the moment of the big bang, about 14 billion years ago, and so we can’t see any further than about 14 billion light-years (a bit farther, since space is expanding). This volume of space is called the Hubble volume and represents our observable universe.
The existence of Level 1 parallel universes depends on two assumptions:
The universe is infinite (or virtually so).
Within an infinite universe, every single possible configuration of particles in a Hubble volume takes place multiple times.
In regard to the first assumption, inflation theory predicts that the universe is actually far larger than our Hubble volume. Recall that eternal inflation implies that universes are constantly being created and destroyed by quantum fluctuations, which means that space is actually infinite under the most extreme application of this theory.
The regions created in an eternal inflation model trigger every single set of initial conditions, leading to the second assumption. This means that there’s another region of space that consists of a Hubble volume that has the exact same initial conditions as our universe. If it has exactly the same initial conditions, then such a region would evolve into a Hubble volume that resembles ours exactly.
If Level 1 parallel universes do exist, reaching one is virtually (but not entirely) impossible. For one thing, we wouldn’t know where to look for one because, by definition, a Level 1 parallel universe is so far away that no message can ever get from us to them, or them to us. (Remember, we can only get messages from within our own Hubble volume.)
In theory, however, you could get in a spaceship that can travel at nearly the speed of light, point it in a direction, and head off. Time for you would slow, but the universe would continue to age as you moved throughout the entire expanse of the universe looking for your twin. If you’re lucky, and dark energy is weak enough that eventually gravity causes cosmic expansion to end, you might eventually be able to get to your twin’s planet.
Chaotic and eternal: Two facets of inflation
The theories of eternal inflation and chaotic inflation can be quite confusing, as I discovered in writing this topic. Most people, even physicists, use them fairly interchangeably. This is an excellent example of how concepts on the cutting edge of science can get blurred, even between different experts in the field.
In eternal inflation, the quantum fluctuations in the vacuum energy result in “bubble universes” (or “pocket universes” or “island universes” . . . will the naming confusion never cease?!). The possible energies that such a universe could have (called the false vacuum) are represented by a graph that looks kind of like a mountain range, which is often referred to as an energy hill. The true vacuum of our universe is represented as one of the valleys in such a graph.
In 1983, Paul Steinhardt and Alex Vilenkin both presented the key ideas of eternal inflation, which is that quantum fluctuations can cause the triggering of new inflationary cycles. The assumption at the time was that each new cycle of inflation would start at the top of the energy hill and, during the inflationary cycle, would progress down toward the true vacuum. The energy state of the universe is decaying into a ground state.
In 1986, Andrei Linde wrote a paper called “Chaotic Inflation,” in which he pointed out that
these universes can be created anywhere on the energy hill, not necessarily at the peak. In fact, the hill itself may not even have a peak; it might continue on forever! He furthermore showed that chaotic inflation is also eternal, because it spawns continued creation of new bubble universes.
Several sources make chaotic inflation sound like a specific type of eternal inflation theory. Max Tegmark’s 2003 article uses “chaotic inflation” in a way that sounds, to me, more like eternal inflation. Wikipedia has an article on chaotic inflation, identifying it as a “sub-class of eternal inflation,” but has no article on eternal inflation itself!
But Vilenkin, in his 2006 topic, Many Worlds in One: The Search for Other Universes, is adamant that chaotic inflation is an entirely different theory, seeming a bit frustrated that they’re so often interchanged, a frustration that certainly seems justified, unless Vilenkin is the one who’s applying the term imprecisely.
Time will tell what consensus cosmologists reach over this distinction between chaotic inflation and eternal inflation. For now, though, it’s useful to know that most chaotic models will yield eternal inflation (but not all of them), and many eternal inflation models are not chaotic.
Level 2: If you go far enough, you’ll fall into Wonderland
In a Level 2 parallel universe, regions of space are continuing to undergo an inflation phase. Because of the continuing inflationary phase in these universes, space between us and the other universes is literally expanding faster than the speed of light — and they are, therefore, completely unreachable.
Two possible theories present reasons to believe that Level 2 parallel universes may exist: eternal inflation and ekpyrotic theory. Both theories were introduced in topic 14, but now you can see one of the consequences in action.
In eternal inflation, recall that the quantum fluctuations in the early universe’s vacuum energy caused bubble universes to be created all over the place, expanding through their inflation stages at different rates. The initial condition of these universes is assumed to be at a maximum energy level, although at least one variant, chaotic inflation, predicts that the initial condition can be chaotically chosen as any energy level, which may have no maximum, and the results will be the same. (See the nearby sidebar “Chaotic and eternal: Two facets of inflation” for more information.)
The findings of eternal inflation mean that when inflation starts, it produces not just one universe, but an infinite number of universes.
Right now, the only noninflationary model that carries any kind of weight is the ekpyrotic model, which is so new that it’s still highly speculative. (Ironically, both the eternal inflation model and the ekpyrotic model were partially created by cosmologist Paul Steinhardt.)
In the ekpyrotic theory picture, if the universe is the region that results when two branes collide, then the branes could actually collide in multiple locations. Consider flapping a sheet up and down rapidly onto the surface of a bed. The sheet doesn’t touch the bed only in one location, but rather touches it in multiple locations. If the sheet were a brane, then each point of collision would create its own universe with its own initial conditions.
There’s no reason to expect that branes collide in only one place, so the ekpyrotic theory makes it very probable that there are other universes in other locations, expanding even as you consider this possibility.
In other words, modern cosmology — regardless of whether inflation or ekpy-rosis are true — virtually demands that Level 2 parallel universes exist. (Some alternate cosmological theories presented in topic 19, such as variable speed of light cosmology and modified gravity, don’t have this demand.)
As in the Level 1 universes, these universes would be created with essentially random initial conditions, which, averaged out over infinity, implies that there are other universes that are virtually (or completely) identical to our own. These new universes are continually formed, so many (infinitely many, in fact) are still undergoing the inflationary phase of their evolution.
Unlike in a Level 1 universe, it’s possible that a Level 2 universe could have different fundamental properties, such as a higher (or lower) number of dimensions, a different array of elementary particles, fundamental force
strengths, and so on. But these universes are created by the same laws of physics that created ours, just with different parameters. These universes could behave quite differently from our own, but the laws that govern them would — on a very fundamental level — be exactly the same.
Unfortunately, Level 2 universes are pretty much impossible to reach. Not only are there an infinite number of universes, but there are an infinite number of inflating universes, which means the space between our universe and a parallel universe is expanding. So even if we could move at the speed of light (and we can’t), we’d never be able to get to another universe. Space itself is inflating faster than we can move between our universe and another Level 2 universe.
Level 3: If you stay where you are, you’ll run into yourself
A Level 3 parallel universe is a consequence of the many worlds interpretation (MWI) from quantum physics. In this interpretation, every single quantum possibility inherent in the quantum wavefunction becomes a real possibility in some reality. When the average person (especially a science fiction fan) thinks of a “parallel universe,” he’s probably thinking of Level 3 parallel universes.
The many worlds interpretation was presented by Hugh Everett III to explain the quantum wavefunction, the Schrodinger equation. The Schrodinger equation describes how a quantum system evolves over time through a series of rotations in a Hilbert space (an abstract space with infinite dimensions). The evolution of the wavefunction is called unitary. (Unitarity basically means that if you add up the probabilities of all possible outcomes, you get 1 as the sum of those probabilities.)
The traditional Copenhagen interpretation of quantum physics assumed that the wavefunction collapsed into a specific state, but the theory presented no mechanism for when or how this collapse takes place. The collapse turned the unitary wavefunction, which contains all possibilities, into a non-unitary system, which ignores the possibilities that never took place.
Everett took a tactic similar to that taken later by string theorists, assuming that each “dimension” predicted mathematically by the wavefunction (an infinite number of them) must be realized in some way in reality. In this theory, all quantum events result in a branching of a universe into multiple universes, so the unitary theory can be treated in a unitary way (no possibilities ever go away).
Alternate history across many worlds
Of all the types of parallel universes, Level 3 universes have most captured the imagination of popular culture, spawning their own genre of science fiction and fantasy: alternate history. These are stories written with settings that are based on our own universe, but with the assumption that some historical event went differently, resulting in consequences different from those in our own universe. (For the non-science fiction fan, think of It’s a Wonderful Life.) In these fictional universes, it’s possible (and common) that visitors from one universe can interact with a Level 3 parallel universe.
Obviously, in these fictional universes, the author (and reader) care about the macroscopic differences, but the many worlds interpretation applies to all levels. If a particle decays, or not, different worlds represent those events. No one
observing would be able to tell the difference between them. However, if they were observing with a Geiger counter, which detects radioactive decay, the quantum split would result in further splits. The Geiger counter is triggered in one universe and not the other. The scientist who detects the decay would react differently, perhaps, than the one who does not detect the decay. So, in principle, this is how these tiny quantum universes become full-fledged parallel universes.
In fiction, the effects are generally more dramatic, such as the southern states winning the American Civil War or the Byzantine Empire never collapsing (both of which have been explored by alternate history author Harry Turtledove, called “the Master of Alternate History” by his fans).
Level 3 parallel universes are different from the others posed because they take place in the same space and time as our own universe, but you still have no way to access them. You have never had and will never have contact with any Level 1 or Level 2 universe (I assume), but you’re continually in contact with Level 3 universes — every moment of your life, every decision you make, is causing a split of your “now” self into an infinite number of future selves, all of which are unaware of each other.
Though we talk of the universe “splitting,” this isn’t precisely true (under the MWI of quantum physics). From a mathematical standpoint, there’s only one wavefunction, and it evolves over time. The superpositions of different universes all coexist simultaneously in the same infinite-dimensional Hilbert space. These separate, coexisting universes interfere with each other, yielding the bizarre quantum behaviors, such as those of the double slit experiment in topic 7.
Of the four types of universes, Level 3 parallel universes have the least to do with string theory directly.
Level 4: Somewhere over the rainbow, there’s a magical land
A Level 4 parallel universe is the strangest place (and most controversial prediction) of all, because it would follow fundamentally different mathematical laws of nature than our universe. In short, any universe that physicists can get to work out on paper would exist, based on the mathematical democracy principle: Any universe that is mathematically possible has equal possibility of actually existing.
Scientists use mathematics as their primary tool to express the theories of how nature behaves. In a sense, the mathematics that represents the theory is the meat of the theory, the thing that really gives it substance.
In 1960, physicist Eugene Wigner published an article with the provocative title “The Unreasonable Effectiveness of Mathematics in the Natural Sciences,” in which he pointed out that it’s kind of unreasonable that mathematics — a construct purely of the mind — would be so good at describing physical laws. He went further than this, supposing that this effectiveness represented a deep level of connection between mathematics and physics, and that by exploring mathematics you can figure out ways to approach sciences in new and innovative ways.
But the equations that work so well to describe our universe are only one set of equations. Certainly a universe could be created, as physicists have done on paper, with only two dimensions and containing no matter, which is nothing but expanding space. There could be a vast anti-de Sitter space, contracting, right next to it.
Why, then, do we observe the specific set of equations, specific set of laws, that we do? In other words, to use the phrase of British cosmologist Stephen Hawking (from his 1988 A Brief History of Time), what is the force that “breathes fire into the equations” that govern our universe?
Throughout this topic, you explore concepts that are on the cutting edge of theoretical physics — the bosonic string theory, the various superstring theories, AdS/CFT correspondence, Randall-Sundrum models — but that clearly don’t match our own universe. Most physicists leave it at that, with the understanding that some “pure math” just doesn’t apply directly to the physical universe we live in. However, according to the principle of mathematical democracy, these universes do exist somewhere.
In this controversial view, our equations aren’t preferred, but in the multi-verse, every equation that can have life breathed into it will. This makes up the Level 4 multiverse, a place so vast and strange that even the most brilliant among us can only conceptualize it with the tools of mathematics.
