As I just addressed dimensions in physics, it seems natural to address universes next. Like misinterpretations of dimensions, people frequently conceptualize a multiverse with alternate universes that are essentially “higher dimensions” in the mystical/spiritual sense, science fiction writers imagine travelling to other universes in e multiverse, and finally it is only natural to suppose that by “multiverse” physicists mean that our universe isn’t the only one (despite the prefixation).
But the truth is that there is so much more (and less) to multiverse theory and it doesn’t usually involve any actual other universes (at least not in the sense often thought). Let’s begin at the beginning (it seems an à propos place).
How split-ends create universes: The Many-Worlds Interpretation of Quantum Mechanics
A long time ago (the 50s) in a galaxy far, far away (the milky way, which is still far, far away, just not from our perspective), there lived an individual by the name of Hugh Everett III. As you can imagine given this pretentious name, Dr. Everett retired from physics early in order to make millions. However, this wasn’t his plan. In his doctoral thesis (written under the supervision of the great J. A. Wheeler), Everett proposed the basics of what is now called the many-worlds interpretation (MWI) of quantum mechanics. He proposed a way of dealing with the so-called “collapse of the wave-function.” Put as briefly as possible, quantum mechanics is statistical/probabilistic in nature, but the equations that define how quantum systems “evolve” in time (such as the famous Schrödinger equation) are deterministic. So our math tells us that the system behaves one way, but when we try to measure the system it acts quite differently: The state of the system “jumps” or “collapses” upon measurement in a very unsatisfactory way (it’s wayyy more involved than this but I’m massively simplifying for brevity). One big problem with this mysterious “collapse” notion is that it doesn’t do a good job of explaining how the quantum realm, which is supposed to be the foundation for reality, yields the classical world we experience. Everett proposed we resolve this by understanding that the possible outcomes are really “actual”, and that our classical world is constantly emerging from infinitely many realized outcomes of infinitely many quantum interactions, and each outcome is a “branch” realized by this infinite splitting of universes into others.
So, when you get split ends, the universe splits. But this multiverse theory isn’t really different universes (or rather, it’s more like different histories of the same universe). It’s not like there’s “our” universe which is just one among many. After all, “our” universe is constantly splitting too, so there isn’t really even a single “us” (granting the MWI is true, of course). Another way to think about it that won’t help you is that the “wave function” never collapses because there’s only one wave function that encompasses all reality. The possible outcomes of quantum mechanics aren’t possibilities, they’re just “branching worlds” of this wave function. If it were possible to travel to one of these alternate “universes”, there would be no point to MWI. Everett (who didn’t use the term “many-worlds”, which was coined by DeWitt), didn’t really even go as far as suggesting there exist alternate histories in which e.g., World War I remained “The War to End All Wars” (i.e., no WWII).
Not a multiverse, just a universe with some really deep pockets
The next “multiverse” theory is even less like a multiverse. The good news is it’s much simpler. It’s well-known that the universe is expanding. It’s even more well known that a very widely (although not universally) accepted theory (the Big Bang theory) posits that this expansion began a while ago (probably even before the 50s!) from a “point” out of which the entire universe emerged. It’s not very widely known how problematic the nature of this “bang” is. Obviously, the laws of physics breakdown at the “bang” itself, but they continue to fail after the initial moments of the universe (expansion faster than light, no atoms, immensely high temperatures and pressures, etc.). With some fairly minimal assumptions, the Big Bang theory also gets us a multiverse (it’s a “buy one get one free” sort of thing). Simply put, as the universe expanded you can think of it as sort of “ripping” into various pieces. Our piece is bounded by our particular cosmic horizon-a sort of limit that prevents us from observing anything beyond it. These pieces are often called universes, and this is (an incredibly simplified) version of the multiverse theory: a set of universes originating from the same cause, with the same laws of physics, impossible to “reach” or “travel to”, and fairly boring. In fact, some physicists don’t like to call these pieces “universes” at all:
“Some refer to the separate expanding universe regions in chaotic inflation as ‘universes’, even though they have a common causal origin and are all part of the same single space–time. In our view (as ‘uni’ means ‘one’) the Universe is by definition the one unique connected1 existing space–time of which our observed expanding cosmological domain is a part. We will refer to situations such as in chaotic inflation as a multidomain universe, as opposed to a completely causally disconnected multiverse.”
Ellis, G. F., Kirchner, U., & Stoeger, W. R. (2004). Multiverses and physical cosmology. Monthly Notices of the Royal Astronomical Society, 347(3), 921-936.
From Branches to Bubbles, Pieces to Pockets: Inflationary Cosmology Take 2
But the story of “universes” resulting from inflation doesn’t end here. In a similar multiverse theory, not only do the “pieces” or “pocket universes” differ more radically, but the “gaps” in the multiverse allow for “bubble universes” that not only have different laws of physics but perhaps the possibility of interaction (they can careen into one another, which isn’t exactly the kind of interactions between universes from sciences fiction). However, I’m not going to explain this one. I’m going to use it as an example to show that my explanations aren’t as bad as they seem by quoting another, fairly non-technical introductory piece on multiverse cosmologies:
“In the fashionable variant known as eternal inflation, due to A. Vilenkin and A. Linde, our “universe” is just one particular vacuum bubble within a vast–probably infinite–assemblage of bubbles, or pocket universes. If one could take a god’s-eye-view of this multiverse of universes, inflation would be continuing frenetically in the overall superstructure, driven by exceedingly large vacuum energies, while here and there “bubbles” of low-, or at least lower-, energy vacuum would nucleate quantum mechanically from the eternally inflating region, and evolve into pocket universes. When eternal inflation is put together with the complex landscape of string theory, there is clearly a mechanism for generating universes with different local by-laws, i.e. different low-energy physics. Each bubble nucleation proceeding from a very large vacuum energy represents a symbolic “ball” rolling down the landscape from some dizzy height at random, and ending up in one of the valleys, or vacuum states. So the ensemble of physical by-laws available from string theory becomes actualized as an ensemble of pocket universes, each with its own distinctive low-energy physics. The total number of such universes may be infinite, and the total variety of possible low-energy physics infinite, but stupendously big.”
Davies, P. C. W. (2004). Multiverse cosmological models. Modern Physics Letters A, 19(10), 727-743.
Naturally, you not only use the word “frenetically” in everyday discourse, but of course are more than well aware of the ways in which extra dimensions required by string theory are explained in terms of compactification to space-like regions in which they determine the physical laws for each particular region.
“As Above, So Below”: Combining the Multiverse with Many-Worlds
Certain physicists have decided that, as long as we’re admitting the possibility of infinitely many bubble universes eternally popping into existence and having differing laws of physics, and because this sounds a lot like the many-worlds interpretation of quantum mechanics, it would be a good idea to say that these quite independently developed theories formulated to address fundamentally distinct issues are nonetheless the same. I have no illusions about my inability to condense into a paragraph anything remotely resembling a clear account of how some physicists derive an equivalence between the MWI and multiverse cosmology. So I’ll leave this one with “nothing is possible, because every possibility is actualized”.
“I found God! He was hiding in a holographic anthropic multiverse”
Scientists may not be as objective as we’d like, but at least they rely on fairly minimal assumptions as opposed to e.g., historians of prehistory or theologians. Except when they don’t:
“Despite the growing popularity of the multiverse proposal, it must be admitted that many physicists remain deeply uncomfortable with it. The reason is clear: the idea is highly speculative and, from both a cosmological and a particle physics perspective, the reality of a multiverse is currently untestable…For these reasons, some physicists do not regard these ideas as coming under the purvey of science at all. Since our confidence in them is based on faith and aesthetic considerations (for example mathematical beauty) rather than experimental data, they regard them as having more in common with religion than science…To the hard-line physicist, the multiverse may not be entirely respectable, but it is at least preferable to invoking a Creator. Indeed anthropically inclined physicists like Susskind and Weinberg are attracted to the multiverse precisely because it seems to dispense with God as the explanation of cosmic design” (emphases added)
Carr, B. (2007). Introduction and Overview. In B. Carr (Ed.). Universe or Multiverse? Cambridge University Press.
I like the 2nd bolded portion, mostly thanks to
Amoroso, R., & Rauscher, E. (2009). The Holographic Anthropic Multiverse: Formalizing the Complex Geometry of Reality (Series on Knots and Everything Vol. 43). World Scientific.
What could this obscure sounding combination of cosmology, theoretical physics, and mathematics have to do with why some physicists like the multiverse because it “seems to dispense with God as the explanation” of the cosmos? Because Amoroso and Rauscher’s cosmology is (in their words) “a theistic cosmology”, despite being a multiverse cosmology and despite espousing the holographic principle (yes, it’s related to holograms; no, the world isn’t a hologram like the holograms we’re familiar with are).
Wrapping this up (finally)
I could keep going for some time and not scratch the surface. But the point is that invariably even the most exotic multiverse theories aren’t the kind described in everything from popular science to science fiction films. Even those which hold that some 101029 miles away there is an identical version of you are still really (if real, that is) part of this universe, and the very thing that seems to warrant calling them universes is that they are untestable, undetectable, and impossible to ever travel to or visit (even if you had a Delorean equipped with a flux capacitor AND a warp drive).