Heroic & Dark Fantasy and Science Fiction Character created by Kevin L. O'Brien
ong a staple of certain forms of science fiction and fantasy, parallel universes, or multiverses as they now tend to be called, are gaining scientific respectability as evidence for their existence mounts. This essay will describe the basic ideas behind the concept in a way that hopefully will make them easy to understand.
There was a time when multiverses were thought to be separated by higher dimensions, such that there might be many other universes in existence, but we could never find them because we could not see through the dimensional barriers. New evidence has, however, largely changed all that. Gone are the higher dimensional planes for the most part; parallel universes are now separated by extreme distance and the expansion of the universe.
The First Level: Right Next Door
There are four types or "levels" of multiverses, the most basic being a Level 1 Multiverse (ML1). An ML1 is based on the idea that space is infinite or very nearly so, is essential flat, and is uniformly filled with matter and energy. Each individual universe is thus simply a region of this space that we haven't seen yet. Our current observational limit is about 42 billion light years, which is the distance light has been able to travel since the Big Bang. The area of space contained within a sphere described by this limit is called a Hubble volume. Note that the size of this volume is much greater than the current age of our universe, 14 billion years. This is because cosmic expansion has increased the physical distance light must travel.
There are at least two observations that indicate space is infinite, flat, and uniformly filled with matter and energy. If true, this would mean the ML1 we live in would be filled with an infinite number of Hubble volumes, each one a parallel universe. Each would be basically the same as ours except in the way the matter it contains is arranged. Our Hubble volume is estimated to contain 10 raised to the power 118 particles. This is ten multiplied by itself one hundred and eighteen times. The short way to notate this is as "10^118".
In any event, this many particles can form 2^10^118 arrangements. As such, there can be as many as 2^10^118 parallel universes that would contain all possible arrangements of the same number of the same kinds of particles. However, an infinite number of parallel universes also means that this finite number of possible arrangements will be duplicated many times. In other words, all these possible arrangements of particles, as many as there are, must repeat themselves. As such, not only are there parallel universes that would look different from ours, there are also parallel universes that would duplicate ours exactly, even down to duplicates of each of you reading this exact same essay. The average distance between exact duplicates has been calculated to be 10^10^118 meters, but the laws of chance dictate that an exact duplicate of our Hubble volume is likely to be closer than that.
Though referred to as separate universes, Hubble Volumes are all part of the same area of space. Hence, it is possible to visit them, if we can figure out how to cross the enormous distances involved. We cannot see them, though, because there hasn't been enough time for their light to reach us.
The Second Level: Bubbles in Spacetime
A Level 2 Multiverse (ML2) is the next step up from an ML1. It also consists of an infinitely large, flat region of space, but one that is devoid of matter and energy; this multiverse would then be filled with an infinite number of ML1s, each containing an infinite number of Hubble volumes.
Our ML2 is believed to have been formed with the Big Bang. Quantum fluctuations at that time created a quantum field called the inflaton, which in turn caused our ML2 to begin to expand rapidly. This inflaton will keep the multiverse expanding forever and, if certain observations are true, may even be accelerating the expansion. The lack of both matter and energy particles stems from the fact that the energy that would form these particles is tied up in the inflaton. For the most part, fluctuations on the quantum level prevent the field as a whole from decaying away, but in some local spots the field unaccountably looses strength and the expansion slows down, forming a kind of bubble in the fabric of spacetime. These bubbles then fill with particles as the energy released from the inflaton "condenses out". Each of these bubbles will form an ML1.
However, because of the quantum fluctuations, each ML1 can have radically different physical constants (such as the strengths of the four fundamental forces), different types of particles, and even different dimensionalities. For example, our ML1 has one temporal and three spatial dimensions, but there is no reason why that cannot be reversed, or that there cannot be different combinations of different numbers of either dimensional type. The result is that there can be ML1s where all stars explode despite their mass, all atoms are radioactive, complex structures cannot exist, deuterium or carbon cannot form, or even energy fields become unstable.
Despite this, the ML1s contained in an ML2 are still just separated from each other be extreme distance. Cosmic expansion will make it virtually impossible to ever see one of these parallel ML1s, but conceivably we could visit them if we could find a way to cross the enormous distances faster than they are expanding, and protect ourselves from the hostile conditions within them.
The Third Level: Infinite Possibilities
The next step up is the Level 3 Multiverse (ML3). An ML3 is based upon the idea that, according to quantum mechanics, every action or event has a number of possible outcomes, each described by a probability. These outcomes are known as quantum states. Each state has its own parallel universe, such that there is at least one parallel universe for every possible way the universe could be according to which quantum states unfold. Another way to put it is that for any state that has a non-zero probability, there must be at least one parallel universe in which that state occurs.
This is what people generally think of when they imagine parallel universes. Unlike ML1s and ML2s, the parallel universes of an ML3 are not separated by great distances, they are instead separated by virtue of the fact that they exist in an abstract realm called Hilbert space that permits all possible states to exist. We can at least speculate about how we could travel between ML1s in an ML2, even if we cannot conceive of how it would be done, but there is no basis for speculating about traveling between parallel universes in an ML3.
The ML3 does, however, present two intriguing possibilities. One is the principle of ergodicity, which states that ML3 parallel universes are equivalent to the more prosaic parallel universes in ML1s and ML2s. According to this idea, the quantum fluctuations that created the Big Bang did not generate particles and cosmological properties at random, but created a structured superposition in which all possible combinations of particles and all possible properties coexist simultaneously. These initial conditions then developed normally, but along separate quantum branches. They were unable to influence each other and they created all possible parallel universes that could exist in an ML3.
The point of ergodicity, however, is that this result is no different than if we assume, for example, an infinite number of Hubble volumes in an ML1. The arrangement of matter into all possible combinations, combined with the knowledge that these combinations will be duplicated endlessly, allows that all possible quantum states can exist in an ML1, but separated by distance rather than by quantum abstractness. Ergodicity would also apply to all the ML1s in an ML2; hence, postulating an ML3 does not add anything new to our knowledge. All possible parallel universes that can exist in an ML3 can also exist in an ML2, with all its ML1s and all their Hubble volumes. Thus we do not need to figure out how to go from one quantum branch to another, just from one Hubble volume to another.
The other intriguing aspect of quantum parallel universes is how they affect the nature of time. If one thinks of time as a way to describe change — i.e., matter has a certain arrangement one moment, then a different arrangement a moment later — then the idea of multiverses suggests a different approach. If for example the Hubble volumes of an ML1 contain all possible arrangement of matter, then time is simply a way to put those parallel universes into a sequence. The universes themselves are static; thus, change becomes an illusion, though a very interesting one. This idea also suggests that time as a dimension is very different from what we perceive it to be.
The Fourth Level: Mathematical Constructs
A Level 4 Multiverse (ML4) is the ultimate in parallel universes, because each parallel universe varies not just in its arrangement of particles and its cosmological properties, but also in its physical laws. Every ML1 in our ML2 would obey the same laws, but every ML2 in our ML4 would have different sets of laws. This would be the closest thing to the old idea of parallel universes, because the other ML2s would exist outside of our spacetime continuum within their own separate continua. We cannot at this time describe what such ML2s would be like except in abstract mathematical terms, but as with the pervious levels, where all possible arrangements of matter are represented by at least one Hubble volume and all possible sets of cosmological properties are represented by at least one ML1, all possible mathematical structures of the physical laws would be represented by at least one ML2 as well.
At this time, nothing can exist beyond an ML4; that is, we cannot conceive of a Level 5 multiverse made up of multiple ML4s. Even so, if we think of an ML4 as being an ensemble of possible mathematical structures, this leaves open the possibility of there being other ensembles that could account for different ML4s.
The consequences of multiverses goes beyond the simple existence of alternate Earth-like worlds, though we can now easily explain how they could exist: non-magical alternative worlds exist in separate Hubble volumes of our ML1, whereas magical alternative worlds exist in a separate ML1 of our ML2. Similarly, we can also account for such surrealistic planes of existence as the Dreamlands of H. P. Lovecraft or the Irish Otherworld. And they can serve as the basis for constructing a mythological cosmogony, as well help drive home the idea of the cosmic horror of outer space (with so much more room and so many more possibilities, virtually anything could be out there!). Nonetheless, the major point is that multiverses make it possible to account for the different nature of these other worlds and planes in ways that are in keeping with modern cosmological thought, and which do not invoke ideas that are untenable.
Sources / Further Reading
"Parallel Universes" by Max Tegmark, Scientific American, Vol. 288, No. 5, May 2003, pp. 40-51
"Parallel Universes" by Max Tegmark. Science and Ultimate Reality: From Quantum to Cosmos, honoring John Wheeler's 90th birthday. J. D. Barrow, P.C.W. Davies, & C.L. Harper eds. Cambridge University Press (2003)
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