Parallel worlds and parallel universes are staples of science fiction. In the original Star Trek series, I seem to remember an evil Kirk from an alternate dimension who had a pointy ear friend with a goatee.1 Comic books are rife with parallel worlds. In both the Marvel and DC Comics universes, the mechanism of choice is quantum branching. DC Comics had their Crisis on Infinite Earths series, and Marvel had the Secret Wars story arc.
In the CW network's television series The Flash, how many parallel Earths has the hero Barry Allen been to? The television series Sliders had an endless slew of universes. The heroes were even confronted by Kromaggs, a race descended (probably) from a different evolutionary branch than Homo sapiens. The entire raison d'etre of the television series Fringe is the mysteries presented by a single parallel universe.
Plenty of examples of alternate universes pop up in movies. Edge of Tomorrow is based on a very enjoyable book with the scary title All You Need Is Kill written by Hiroshi Sakurazaka. The hero is stuck in a time loop and relives the previous day each time he dies in a war with alien invaders. Each redo changes the day, creating a parallel timeline from the one in which he died.
In books, try the Merchant Princes series by Charles Stross. The series is about a family with an inheritable trait (this trends to fantasy) that grants the ability to travel between parallel Earths. The family works as drug runners who do their businesses across Earths.2
I think you get the idea. Parallel worlds are a science fiction staple. But being popular in fiction does not mean they aren't real. This chapter covers different theories that can explain an alternate Earth or an alternate version of you, the reader. They are all mathematically consistent and reasonably justified. Be warned, however, that none have been scientifically proven yet.
Brace yourself, because each one requires really, really large numbers.
PARALLEL WORLDS FROM MATH
The equations that describe the big bang have more than one solution. Each solution could be interpreted as another version of the universe. In fact, string theory has 10500 different solutions for the big bang.3
PARALLEL WORLDS FROM DISTANCE
We live in a large universe, a place larger than what we can observe. If we assume that the universe is infinitely spread out but not necessarily infinitely old (remember that the best estimate of its age is 13.8 billion years) then combinations of events will repeat.
Consider how unlikely your existence is. The odds against the combination of atoms coming together to make you—you, not a clone or a facsimile—are so fantastically small. The odds are probably less than 1 in 102685000 lifetimes (a 10 followed by 2,685,000 zeros). Nevertheless, here you are reading this book. Congratulations on winning the life lottery!
Let's say you get a comic book every time you win a game of Jenga. It turns out that you reliably win about one out of every five games. If you played fifteen times, how many comics would you expect to win? The answer is three.
I'm not trying to insult you with simple math. I'm preparing you for the next paragraph.
Imagine that you played an infinite number of Jenga games and your average number of wins is a meager 1 in 102685000. It sounds pathetic, but, given enough time, you will win. Given even more time, you will win again. (Don't go out and buy a lottery ticket because you suddenly feel lucky.) For this example, I assumed infinite time instead of infinite space, but I think you get the idea.
This theory uses probability to prove that, within an infinitely large (or near infinite) universe, other regions of space will be like ours and, no matter how unlikely, there is another planet like Earth and another version of you. And because we know some combination of atoms have created at least one Earth (the one you're living on right now), there has to be a nonzero chance for another one somewhere really, really far away.
Whatever else you can say about a 1 in 102685000 chance, it isn't zero. So given enough space, there will be another you.
Something to ponder: if we add infinite time to the infinite space scenario then there not only exists another you somewhere contemporaneously (meaning in the now) in a huge universe, but it also implies that other you(s) have existed in the past and will exist in the future.
The distance argument, in general, makes a good case for the existence of nonhuman extraterrestrial life. If the universe is big enough, even given a low probability, other intelligence must evolve somewhere.
PARALLEL WORLDS FROM BRANCHING
This is the many-worlds interpretation of quantum mechanics described in chapter 2. I hope you recall the wave equation, where the size of a particle's wavelength dictates all the possibilities of the particle's position. And I hope that you remember that particles are both particle(ly) and wave(y).
The many-worlds interpretation considers a standing wave that holds up many branches. No, this is not a mixed metaphor. Any real possibility within the probability wave becomes a separate branch of the universe. The wave never collapses to a single outcome.
The Man Who Folded Himself, written by David Gerrold (he also wrote the fan-favorite Star Trek episode, “The Trouble with Tribbles”), is about some funky paradoxes caused when his character time travels to be with himself. Each time he travels, a new branch is created, and that new branch contains yet another him. Many versions of him, along with gender changes, appear in the story.4
Robert J. Sawyer's Neanderthal Parallax Trilogy (Hominids, Humans, and Hybrids) is set on a parallel Earth where the Neanderthals evolved and Homo sapiens went extinct.
In the movie The Butterfly Effect, a college student discovers he can make an alternate version of his present time by having his younger self make small changes in the past. (The actual term butterfly effect comes from chaos theory and will be covered in chapter 11.)
This movie is a lot like Edge of Tomorrow where the hero is trying to find the best future for himself (himselves, actually). The big difference between the movies is character placement. One lives in the present and manipulates his past self while the other lives in the present trying to affect the future.
MEM(BRANE) THEORY
Membrane theory is a derivative of string theory. This cosmology theorizes higher dimensions. The theory proposes that we live on a three-dimensional membrane situated within a wider multidimensional space. We (might) share this space with many different universes, and each universe can have very different physical laws, constants, and initial conditions.
Is life still possible in a universe with different laws of nature? This is an idea David Brin explored in his novel The Practice Effect. The protagonist travels to anomalous worlds in alternate universes where the physical laws are different.5
Can these different universes ever meet? String theory math shows that they can. But for us, let's hope not, at least for a very long time. When they do, it's a collision—a big bang. At least according to this part of string theory.
THE WE LIVE IN THE BEST OF ALL WORLDS THEORY
Forget the earth or the sun as the center of the universe. The anthropic principle puts life at the center. This is more philosophy than science because it cannot produce a falsifiable prediction or any testable experiment, but a lot of scientists have given it consideration. This is an idea I will not spend much time on. It is a lot of tail chasing.
The universe appears fine-tuned for life. If gravity were just slightly stronger, stars would compress more tightly and burn out after only a few million years rather than billions. Ergo, life would never get the chance to evolve. If the strong nuclear force were just a bit stronger, all the protons in the early universe would have paired off and water would not exist.
For this theory to come close to science, the anthropic principle relies heavily on the existence of a vast number of universes. This ties into string theory because some versions of string theory predict a multiverse wherein each “verse” formed with different constants. If we focus on regions of the multiverse where life forms, then we will (probably) find the constants predicted by string theory.
More on this circular thinking in a few paragraphs, but first we need to define the weak and strong versions of the anthropic principle.
Weak anthropic principle: We live in a special place and time in the universe during which life exists. Surely the universe contains all the necessary parameters for life to exist because (guess what?) we are here.
Strong anthropic principle: The laws of physics are biased toward life. The universe must be obliged to contain all the necessary parameters so that life can exist because (guess what?) we are here.
The difference between these definitions is nuanced, but (possibly) has philosophical existential consequences. The weak anthropic principle puts limits on certain properties of the universe. The fact that at least part of the universe contains carbon-based life observers puts constraints on what the whole universe can be like. For example, the universe must be at least old enough for evolution to have occurred. The strong anthropic principle implies that the universe is compelled to have properties compatible with intelligent life.
The unavoidable and uncomfortable part of this is the observation selection effect, also called anthropic reasoning. This is when the thing being studied is correlated with the observer. If humans hadn't evolved, humans wouldn't be around to study the probability of their evolution. Everything we observe is being observed by, you guessed it, us. To be observed, there must be an environment conducive for the observer. Tail chasing.
A nonreligious mechanism behind the anthropic principle (philosophy) might be Chaotic Inflation. This twist on the big bang redefines the cosmic inflation described in chapter 4. Inflation still occurs in different regions of the universe, just not necessarily all parts at the same time.
The standard theory considers inflation a one-time event. If Chaotic Inflation is true then different areas of space are undergoing inflation and evolving into separate universes. In turn, this chaotic inflation repeats itself in each new universe. Among this infinite number of universes, all different physical laws would exist. Through the pure odds allowed by an infinite number of universes, one of them must function under the laws that allow for stars, atoms, and life. Most of the rest would have different laws of physics and be barren.
PARTING COMMENTS
Parallel universes can be justified theoretically, but there is no practical evidence that they exist. It is all speculation, and no test that can prove their existence is anywhere in sight. In none of the proposed theories of parallel worlds—whether from math, distance, quantum mechanical branching theory, the bulk of string theory, or (dare I suggest) philosophy—is there any way for us to interact with anyone or anything in a different universe.
The idea is especially fun in science fiction. And, for our sanity's sake, it makes interpreting some of the physics of the universe easier (disclaimer: easier doesn't make it right). For example, the branching theory is a convenient solution to the grandfather paradox. This doesn't mean branching is real.
In science fiction, who cares?