Tuesday’s big post on harmonic oscillation spun off an Agents of S.H.I.E.L.D. episode where they used a bit of magical technology to visit an alternate world. This sort of thing is a common trope in science fiction tv, also showing up in this week’s episode of The Flash. And, of course, this has a long and distinguished past, from the show Fringe (which built a whole series mythology around a parallel world), 90′s-era shows like Sliders and Quantum Leap, and all the way back to the classic Star Trek episode featuring an evil parallel-universe Spock with a goatee.
The emotional appeal of these kinds of stories is easy to see– basically anybody who has ever made an important decision has at some point wondered what would’ve happened if things had gone differently. Alternate-world fiction is a way of playing with that idea, giving those different results tangible reality, at least within the context of a particular fictional story. It’s no wonder that authors and screenwriters return to this over and over again.
Writers employing this trope can also find encouragement in modern physics, which offers a number of ideas that seem tantalizingly like realizations of alternate-world fiction. These are some of the most popular of the exotic theories dreamed up by physicists, but also some of the most badly misunderstood, in large part because there are a whole bunch of different concepts– extra dimensions, parallel universes, the Many-Worlds interpretation of quantum physics– that sort of get mashed together. This post is an attempt to sort these out a bit, though I will warn you up front that I’m going to be a bit of a buzz-kill, here, and also explain why the reality of physics means that you’ll never get to pay a visit to the version of you who made a slightly different choice at a key moment some years back.
Extra Dimensions The idea of “extra dimensions” gets folded in here largely through linguistic confusion. Writers have a tendency to use “dimension” in the sense of “plane of existence” (thus Terry Pratchett’s Discworld is frequently menaced by gibbering horrors from the Dungeon Dimensions). When physicists talk about “extra dimensions,” though, they have a precise technical meaning in mind that is much less colorful.
“Dimension” in a physics context just means “possible direction of motion.” In the everyday world, we experience three dimensions of space, corresponding to motion in the north-south, east-west, and up-down directions. the exact choice of directions is arbitrary, provided the three are mutually perpendicular– you could perfectly well define the three dimensions of space as northeast-southwest, northwest-southeast, and up-down, if you prefer, but you’ll never find more than three ways to move that are at right angles to one another.
The idea of “extra dimensions” in physics arises because certain equations in physics take on a particularly simple form if you express them using more than three dimensions of space– nine or ten work especially well. If these dimensions had some physical reality– if particles could move in a bunch of different directions that are at right angles to the usual set of north-south, east-west, and up-down– it would make life a lot easier for high-energy theorists. So a great deal of effort has been devoted to investigating ways that the universe might really be ten-dimensional, and just look three-dimensional to us. Most of these involve restricting the size of these extra dimensions so that it’s impossible to move any detectable distance in those extra directions (via complicated mathematical structures). Another scheme involves three-dimensional slices of space existing within ten-dimensional space, in a manner analogous to a film of soap, which is effectively a two-dimensional space in our three-dimensional world. These are generally called “branes” thanks to the analogy to thin membranes in three dimensions.
Thinking about theories with multiple extra dimensions has been a rich and fruitful source of mathematical physics, though it’s not clear yet whether these theories correspond to our physical reality. The big stumbling block is that there’s no obvious way to test these theories, at least not yet.
Why can’t you visit there? Well, in cases where the “extra” dimensions are just tiny, they’re not alternate worlds at all, just extra directions of motion. You’re already in them, you just can’t tell because there’s nowhere for you to go. Brane-world scenarios look more like the classic alternate-universe scenario, but those models have material objects trapped on their home branes, unable to move off through the “bulk” to reach a different brane.
Alternate Earths: Moving along in our normal three dimensions of space, there are several versions of scenarios involving real, physical alternate worlds, which break into two broad classes. The simplest of these is just a matter of probability, relying on the fact that the universe in which we live is effectively infinite.
That might seem like a strange thing to say, given that we know how much time has passed since the Big Bang (13.7 billion years, give or take a hundred million years or so), but the best current theories of cosmology mostly include a period of “inflation,” a tiny instant after the Big Bang, when the space of the infant universe was driven outward at an increasing rate for a short period. This means that while we can only see light from objects within several billion light-years of us, the universe as a whole extends far beyond what we can ever hope to see. The scale of the expansion is so mind-bogglingly huge that it might as well be infinite.
This allows for a sort of monkeys-typing-Shakespeare version of an alternate universe. The idea is that there are only so many ways to stick material particles together, in the same way that there are only so many ways to stick letters from a keyboard together. In an infinite universe, all possible combinations must eventually appear. Which means that somewhere out there, in the vast universe that we can’t see, there must be a volume of space the same size as the Earth containing a planet that is identical to our home world down to the placement of the last quark in the last atom. And also versions that differ by trivial and insignificant re-arrangements of particles, and versions that differ in more significant ways, like the life choices made by the collection of particles that is otherwise identical to you.
Why can’t you visit there? Basically, because of probability. The number of ways to stick particles together is incomprehensibly gigantic, which means that the chances of a mirror Earth showing up anywhere within reach of us are absurdly tiny. It’s conceivable that there could be such a planet in a part of the Milky Way that we haven’t seen yet– in a truly infinite universe, it’s inevitable that there will be a “Milky Way” that contains two “Earths”– but that’s not the way to bet.
The Multiverse: A more extreme variant of the real, physical alternate world case involves the idea that what we think of as the Big Bang was not a singular event. In these theories, there are whole other universes out there, either arise from completely separate “Big Bang” events, or different bits in the immediate aftermath of our Big Bang where inflation proceeded in a different manner.
This picture will necessarily involve alternate versions of Earth for the same monkeys-typing-Shakespeare reasons as in the infinite single universe picture. It also involves vast numbers of other universes where the laws of physics work according to different rules. In fact, this has been proposed as a kind of solution to the question of why the physical parameters of our universe (the masses of fundamental particles, the strengths of fundamental interactions) have the values they do. In a “multiverse” containing an effectively infinite number of universes with different physical laws, it’s inevitable that one of them will have the exact rules and parameters we observe.