These days it often seems that if a theory has loose ends, its dangling threads are surreptitiously tied together out of view within the hidden fabric of a parallel universe. While some researchers recoil from introducing unseen aspects to a theory, others find that the invisible knots create an irresistibly pretty package.
Depending on one’s taste, there are so many types of parallel universes to choose from—alternative cosmos galore. If extra dimensions are not your thing, maybe bifurcating timelines would work. If an endless array of gigantic bubble universes seems intimidating, then perhaps a nursery of baby universes is more endearing. While there is not yet a GPS device or app to navigate through the cartography of scientifically sanctioned parallel possibilities, perhaps this guide to all things alternative will help.
Let’s start with the oldest, most basic idea and work our way toward newer, more complex models:
What if? Here is the simplest way to transport yourself to a parallel universe: Just imagine all the ways in which our universe might have turned out differently. Each of these might-have-been realities represents a parallel universe. The mathematician Gottfried Leibniz posited that we live in the “best of all possible worlds” (famously satirized by Voltaire in "Candide") and that all these other, unrealized, possibilities for creation would have been less desirable. His perspective has persisted for three centuries as a way of explaining why the cosmos is the way it is. Contemporary physicists who make use of the so-called Anthropic Principle argue that if the universe’s conditions were slightly different, it couldn’t have supported intelligent life, and we wouldn’t be here today to speculate about it. For example, if the inflationary era, a fleeting period of ultra-rapid growth in the very early universe, had continued for a long enough time, the stable structures we see in the cosmos today, such as stars and galaxies, couldn’t have formed. The super-quick expansion would have ripped them apart.
Alternative realities made possible by time travel: Science fiction writers relish the intricate plots woven by introducing time travellers into a story. Einstein’s general theory of relativity does not distinguish between space and time and hence hypothetically permits travels to the past, though the mechanics of such a journey are still largely beyond us. In recent decades, backward time travel ideas have been explored in serious articles published in reputable physics journals. If journeying back in time is possible, what would happen if someone changed history? Would they launch a new timeline, and hence a new universe, in which the chain of events was different? The answer won’t be known until backward time travel is either developed or ruled out.
Sum over histories: Physicist Richard Feynman had a practical, no-nonsense approach to physics, supporting notions that are potentially testable. Yet his approach to quantum field theory introduced the startling concept of reality as a weighted sum of alternative histories. For example, according to Feynman’s formulation, if two electrons approach each other, deflect and scatter, their overall behavior from start to finish must take into account every possible intermediate path—weighted according to each path’s likelihood. It is like assessing how tired someone will be after taking a walk in the woods by assuming that they somehow split up and took every possible route from entrance to exit—assigning more weight to the shortest (and therefore likeliest) paths, but still taking all of them into account.
Many-worlds interpretation of quantum mechanics: While Feynman did not assert that the ghostly alternative histories he described represented actual parallel universes, a young graduate student, Hugh Everett III (who shared the same research advisor as Feynman, John Wheeler), made the case that they are. Everett proposed a fundamental reinterpretation of quantum mechanics in which each time that particles interact, reality bifurcates into a set of parallel streams, each representing a different possible outcome. Researchers observing the outcome of such quantum experiments would similarly split up into multiple selves—each thinking that he or she is the only one. For example, suppose a physicist named Eve wants to measure the position of an electron and there are three possible outcomes. Upon taking the measurement, she would instantly divide into three distinct selves, each recording a different result. Each version of Eve would be convinced that she was the real one—wholly unaware of her near-doppelgangers.
Copycat regions of the universe: We now turn from the exceedingly small to the incomprehensibly large. If the universe is infinite, as many cosmologists surmise, then if you travel far enough you will eventually reach regions nearly identical to ours. That’s because if you take a finite number of elements and mix them into an infinite number of combinations, eventually chance will reproduce one of the previous arrangements. It is like playing tic-tac-toe—play enough times and you are bound to repeat yourself. Hence somewhere, by pure chance, there could be a near-parallel Earth where a nearly-identical version of you is reading this article on a parchment scroll illuminated by a glowworm.
Bubble Universes and Baby Universes: In general relativity, an energy field of the right variety can trigger space to grow explosively. Researchers use this phenomenon to explain how the universe expanded so rapidly during the inflationary era. However, they’ve come to realize that if explosive expansion took place in one part of space, it probably happened elsewhere, too. Hence, myriad bubble universes could have emerged from the primordial cosmic sea of energy. We would never have access to other bubble universes, though, because they would have since moved away from us well beyond the limits of observation. Baby universes represent a related idea, in which universes would be seeded in the extreme conditions of black holes. The embryonic regions of space would then grow into successor universes in their own right.
Higher Dimensions: For this type of parallel universe, we move beyond the three dimensions of space itself and consider the possibility of a higher, unseen dimension. While such a scenario sounds a bit like "The Twilight Zone," higher dimensions are a vital part of string theory and other attempts at unifying the natural forces. If a higher dimension exists beyond space and time, why can’t we travel through it? Theorists hypothesize that the particles of matter and light cling to our three-dimensional space, preventing us from entering or even observing the extra dimension.
While our bodies have remained in our own universe, our minds have completed an excursion through a weird assortment of parallel universe possibilities. Do any of these types of parallel universes exist? If so, how are they connected? Suggestions for testing these various hypotheses are too numerous to recount in this post. I refer the reader to several interesting proposals:
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.
Scientific American: Parallel Universes
In this article, physicist Max Tegmark explores four "levels" of multiverses.