Recently (for the past couple months, in fact) I’ve been down quite the rabbit hole. More or less, it can be described as an as-in-depth-as-I-can-manage exploration of “multiverse theories.” I’d like to give a relatively brief synopsis of my findings thus far, with the caveat that none of these descriptions of various theories will be exhaustive (though I’ll provide links to further reading, if you’re interested), hoping to arrive at a point that rang a little truer for me recently: speculation is healthy, sometimes even necessary, in order to more fully grasp the concrete and actual. That’s not at all to say speculation can’t be outlandish, especially when it becomes masturbatory speculation for speculation’s sake; but when it remains rooted in the actual and concrete, when it moves from the mortifying realm of “conspiracy theories” to something more like mapping possible upcoming roads, I believe it’s quite useful. I believe the immediate use for writing will be apparent, but this perspective seems to me to be broadly applicable to other fields as well.
Modal Realism and Platonic Selectors
My deliberate leaning over the rabbit hole (followed by an arguably accidental slip of footing and a headlong dive) started with interest in the speculative metaphysics of Eric Steinhart (whom I’ve mentioned before), which in turn led to a study of David K. Lewis’ “modal realism” (which you can find outlined in his book On the Plurality of Worlds). Lewis, ever the philosopher, specializes in what’s called modal logic, which deals primarily in logical statements which utilize what does not exist. An example would be, “I could have been born with red hair”; I was not born with red hair (dirty blond, with more cowlicks than I’d like), but something about that previous statement still rings true. There’s nothing to state that I could not have been born with red hair (so long as my gene pool allows for it), yet it never happened—so in what sense is it true?
Long story short (and this is cutting out a lot here, so I highly recommend you look into modal realism for yourself), and fully acknowledging how radical a view this is, Lewis suggests that this may indicate that all possible worlds (all worlds one can reasonably imagine in modal logic) are not merely abstract possibilities but concrete actualities. They exist. There is a world where I have red hair. These worlds are organized on a principle of maximal fecundity; modally logical statements rely, in Lewis’ view, on the existence of these other worlds for their authoritative weight, and so these other worlds are quantified by a principle of maximal fecundity—for any modally logical statement, if it is true, then it represents another actual world.
It’s important to state that these “other worlds,” in modal realism, do not interact with each other. But for every possibility there is an actual world, in this view. For a better introduction to this concept, I recommend a short video by Philosophy Tube on the subject.
In addition to Lewis, I also looked into the work of another philosopher, John Leslie, who thinks in explicitly Neoplatonic terms. Leslie engages the question raised by Gottfried Leibniz: why this particular arrangement, rather than nothing at all? In other words, rather than nothing, why does any world, let alone this particular one, exist? Leslie’s line of thought is quite complex and draws on a number of other thinkers, but I’ll try to be brief; for more details, however, you might try his Immortality Defended.
In a Platonic worldview, reality is divided in two: concrete realities and abstract realities. Concrete realities are things like planets, people, and puppies. Abstract realities are things like mathematical objects; no matter what you use to express the number 2 (II, two, dos, etc.), no one symbol is the number itself—the number 2 exists independently of them all. But the number 2 isn’t sitting somewhere in the universe, waiting to be summoned; it exists as an abstract object beyond space and time. Other abstract realities might include more complex things, like mathematical structures, or even descriptions of other possible worlds (like ones found in modal logic). If no concrete things existed, abstract things would still exist; concrete things depend upon abstract things for their existence, not the other way around.
This then adds nicely to Leibniz’ initial question: if there are, say, a whole slew of abstract descriptions of other possible worlds, then why is this one the actualized one and not some other? Leibniz suggested that possibilities as such (including possible worlds) push of their own accord for their own actualization, and that the strength of that drive toward actualization is directly related to their degree of perfection. In other words, the more perfect a possible world, the stronger its drive to actualize itself. Leibniz concluded that we live in the best of all possible worlds (explaining suffering in our world to be necessary to some possible higher purpose, while maintaining this world’s perfection; in other words, perfection doesn’t necessarily entail zero suffering whatsoever).
John Leslie refines this point just a bit. Returning to the question of why the super-cosmic library of all possible worlds would choose our particular one for reality and not others, he suggests a mediating something which filters numberless possibilities into finite actualities. He borrows Baruch Spinoza’s idea of the universe being the collective thoughts of a divine mind, but it’s not an inherently religious suggestion. Rather, Leslie says that this filter, this Selector, selects from all possible universes only certain ones to actualize—he then asks what criteria that Selector would then use in its choosing process. This Selector, for one, is not a person, but a principle, which means it’s not picky but precise.
Leslie borrows Plato’s view, comparing his Selector to Plato’s idea of “the Good.” The Good doesn’t exist as an object in the universe or a concrete thing (like a pencil or a puppy dog), or even as an abstract thing (like the number 2 or a possible world), but as the Selector of our world: this world, and not some other, exists because the Good deemed it so. Leslie refines this into the concept of the Selector.
But who’s to say that this Selector chooses the best of all possible worlds? Why not the worst, or some other? First, Leslie says that if we can admit that there is an objective difference in quality between a world of maximal suffering and a world of maximal pleasure, then we admit that all possible worlds can be arranged into a hierarchy of differing value. If one world is worse than another in itself, then these possible worlds possess not only descriptions of their contents, but valuations of those contents. Leslie calls this axiology (sometimes ethics), and his principle that the Selector selects only the most axiologically perfect possible world he calls axiarchism.
But even if we have this hierarchy of valuated worlds, who’s to say the Selector would select the best of them? Again, why not the worst, or some other? There are no differing probabilities, only possibilities, all equally probable. One solution is that the Selector works on these possible worlds like gravity works on sandbags and pulleys. Were we to arrange some pulleys and two sandbags, then ask, were we to let the system go on its own, which sandbag would hit the ground and which would rise into the air—we would consider this a system with not only possible outcomes but probable ones. If weighted properly, we wouldn’t say that the sandbags might not hit the ground at all because there are several possible places they could be along the pulley system; there are several possible places they could be, but the ones with the highest probability are those nearest the ground and nearest the top of the system (where the non-grounded sandbag would end up). Many possibilities, but different probabilities, and thus most likely only two options—in this case, which sandbag hits the ground; in the axiarchic case, which criterion characterizes our Selector.
So let’s take that analogy to the Selector and suggest that the corresponding equivalent of either sandbag is the choice of “worst possible universe”|”best possible universe.” It may be fair to say that as unlikeable as our universe may sometimes be, it can’t necessarily be the worst, which seemingly disqualifies that criterion as that on which our Selector operates; and it’s unlikely to be something between best and worst because their probabilities are so low relative to best and worst. So our Selector’s criterion is “best possible universe.”
There are of course issues to raise: if the worst universe is devoid of anything good (and thus can’t be this universe), then this universe can’t necessarily be the best because such a restriction could then imply that the best possible universe is devoid of anything bad. Enter Eric Steinhart, who points out the possibly illogical nature of “best” anyway, and instead adapts Leslie’s axiarchic principle: rather than selecting the “best possible universe,” the Selector would select the ever-better possible universes, eternally surpassing all its previous selections. The process theologian Charles Hartshorne referred to God (as Nature, in essence, in process theology) as the ultimate self-surpassing Surpasser; Steinhart applies this to the multiverse as a consequence of its Selector. He sees a multiverse not like David Lewis’, which actualizes all possibilities, but one like Leslie’s, which exhibits a trajectory: something like, “become more complex,” or “become more axiologically perfect,” adapting in ways analogous to Darwinian natural selection or machine-learning.
Max Tegmark and the Mathematical Multiverse
Now, as much as I love philosophy (and I’ve recently kindled a bit of an affair with natural philosophy specifically), theories like the above mentioned occasionally leave a bad taste in my mouth. They’re intriguing, no doubt, and they sincerely address issues within their fields (modal logic is not quite as straight forward as an outside observer may presume, and Plato may have more to say today than we may otherwise assume), they seem terribly thin. Lewis and Leslie are no strangers to the possible ramifications of their theories (Lewis even seems not to be a diehard advocate for modal realism), but at the end of the day it feels a little more like a matter of semantics than anything else.
As a result, and being no stranger to the sci-fi iterations of multiverse theories, I decided to take my interests to the hard sciences. There are a few people I could mention here (Lawrence Krauss and his A Universe from Nothing among them), but the book that really led my way at this juncture was Max Tegmark’s Our Mathematical Universe: My Quest for the Ultimate Nature of Reality. Partly autobiographical, Tegmark takes the first half of his book to walk the reader through the history of science, especially detailed accounts of the biggest breakthroughs of the last century (which alone made this book worthwhile); the second half, however, contained what I was looking for—Tegmark’s tiered model of various cosmological theories of the multiverse.
The Level-I multiverse relies on the theory of inflation, currently the most popular theoretical model for explaining why our universe is expanding at its present rate. If inflation is eternal, says Tegmark and others, then it would result in an eternal spread of space and matter. Furthermore, Tegmark states that one can in fact generally count how many quarks are in our observable universe. The number is outlandishly high, as are the number of possible ways one can arrange them, but the number is still finite; combine that with infinite space and matter, in eternal inflation, and you have an infinite ocean of universes, all playing out an infinite number of variations on, and copies of, our own universe.
I should note here that what Tegmark means by “universe” is the approximately-bubble shaped area we can observe of our own universe. Because of the speed of light and vast distances, the farther a telescope looks out into the universe, the farther back in time it sees; rather than seeing other galaxies in this present moment, telescopes see galaxies, depending on how many light years away they are, that existed that many years ago in our universe’s life. The farther you look, the younger the universe looks, until you hit the “cosmic microwave background,” a proverbial wall that represents how the universe appeared about 400,000 years after its rapid expansion. What is beyond that is anyone’s guess; it could be nothing, or more likely it could be more universe like that which we inhabit.
In any case, Tegmark says at this level of multiverse, various copies of ourselves (plus worlds where you or I do not exist) are actual. And, metaphorically speaking, students in these other universes would learn the same things you and I did in physics class, but they would learn different things in history class.
Tegmark extends inflation into the Level-II multiverse, and I think it may involve hints of string theory (as evidenced by this TED talk from physicist Brian Greene) as well. This one comes naturally from Level I and can be explained with a similar metaphor: in this level of multiverse, students learn different things from us in both history and physics classes. Our fundamental laws of physics are the same, but our apparent laws are not: the amount of dark matter or dark energy, for instance, could very, or perhaps the difference in charge between a proton and an electron. This is when we start into seeing some really barren universes, because the range of possibilities are so broad that they include possible universes that could never support life in the first place, or perhaps even for galaxies, let alone stars or planets. Tegmark says these universes are separated by rapidly expanding space, which means that even if we could theoretically reach one of them, we could never traverse more than a few or these other universes.
The Level-III multiverse pans in a bit on our universe, borrowing from the work of the late physicist Hugh Everett III, who pioneered what is now typically called the “Many-Worlds Interpretation” (MWI) of quantum mechanics. If you understood David Lewis’ modal realism, then you understand Everett’s many worlds, at least in concept. One key difference is that Lewis’ many worlds never interact, but Everett’s theoretically can. Everett’s worlds do not exist in space (like Level-I and –II universes) but in a special dimension called Hillbert space. They exist in parallel to one another, branch off form one another whenever multiple possibilities occur, and they actualize each possibility in proportion to all possibilities according to what that possibility’s probability of occurrence was.
Another key difference between Everett’s theory and Lewis’ is that the former is a possible consequences of the empirical findings of quantum physics. In brief, quantum physics finds that subatomic particles (like electrons) do not behave like big objects (puppies, pencils—you get it) do on our scale; rather than existing in one particular place with a single, definite history, they in fact exist in a superposition of all possible positions they could exist in, arranged by probability, not only in space but in time as well. It’s uncertain as to how an electron in superposition becomes a person in a single position. One famous body of interpretations—known as the Copenhagen interpretation, pioneered by scientists like Niels Bohr—suggested that the superposed electron underwent some kind of “collapse” upon being “observed.” Thus we get our “observed” world of definite positions and histories while maintaining the quantum superposed world of the electron. But this school of thought was never clear what “observation” meant (despite some baseless New Age insistence to the contrary), and folks like Einstein weren’t pleased because this implied that when a superposition “collapsed,” where it would in the end find itself was completely random (“God does not play dice with the universe,” or something to that effect, according to Einstein).
So here comes Hugh Everett, who in his dissertation for his doctorate suggested that maybe the superposition never collapses at all. Maybe it plays itself out, all possibilities arranged by probability, all the way up to our own scale—and that we are therefore just one arrangement of possibilities among others playing themselves out in parallel to each other. Tegmark also mentions the birth of the concept of “decoherence,” which goes a little like this: an electron in a superposition of positions is considered “coherent,” but it is knocked into “decoherence” (it decoheres) when one portion of its superposition (one of its many positions) makes contact with one of the positions of another superposition. Say, a photon hits the electron. Except in this case, rather than being a singular particle hitting another singular particle, it’s more like a forest of closely-kept trees, their branches brushing against each other, entangling; this is decoherence, and each point of contact between the various branches of two (or more) trees is a possible world playing itself out. Decoherence is also a bit like a beam of light hitting a prism, splitting into a rainbow; when superposed particles collide, their constituent positions split into this rainbow of all possible worlds. This was Everett’s view, and decoherence has added a scientific explanation for why we don’t consciously experience other worlds as actually as we experience our own.
The Level-IV multiverse is a bit more complex, but consider it another level up in the hierarchy of these multiverses and you might get the picture. In any case, I highly recommend you read Tegmark’s book yourself—it’s well worth the time.
Lee Smolin and Terra Firma
As a brief aside, I’ve recently started into two books by physicist Lee Smolin, The Life of the Cosmos and The Singular Universe and the Reality of Time: A Proposal in Natural Philosophy. In the former, he suggests something like a possible unified theory—similar to Everett’s attempt though on the scale of something more like the Level-II or –IV multiverse than Level-III—of the multiverse, but instead of saying that the multiverse simply plays out all possible arrangements (Lewis, Everett, Tegmark), it rather follows super-cosmic laws similar to those which govern natural selection on our scale (similar to Leslie, and Steinhart especially), resulting in a “survival of the fittest” universes. So far as I can tell, Smolin suggests that even laws of nature evolve, and that universes persist through natural selection depending on their black hole production, which seems to influence whether they (likely asexually, though not necessarily) reproduce new universes. Our universe, he suggests, may be the result of this super-cosmic evolution, thus explaining our fine-tuning.
It’s his second book, however, with philosopher Roberto Mangabeira Unger that seems to bring us back to solid ground from the heady space of other possible worlds. In The Singular Universe and the Reality of Time, Smolin and Unger (from the perspectives of a physicist and a philosopher, respectively) insist that modern cosmology has strained itself with two axioms that may prove to be more cumbersome than helpful: 1) that the universe can be easily dissected into discrete parts, rather than being a cohesive whole; and 2) that this apparently changing universe is the result of a changeless, eternal something underlying it, and that time itself, therefore, is only illusory. Smolin and Unger say this won’t do and give some interesting arguments as to why these questions are not only not yet settled, but why there may be equally (if not more) productive answers.
If Smolin’s suggestion (and it’s only a suggestion, so calm down, you) that even natural laws evolve is true, then this has profound consequences. For one, mathematics goes from being an exploration of our universe’s eternal foundations to an attempt to catalogue what it happens to be doing this eon in particular. Everett’s Level-III multiverse wouldn’t be shaken too much by the possibility that all reality is a cohesive, interrelated, interdependent whole, but how would Levels-I, II, and IV appear? Or string theory? If nothing’s discretely separate, does it makes sense to speak of “multiple worlds or universe” at all?
In that same vein, in the above mentioned Our Mathematical Universe, Max Tegmark is careful to note on a few occasions that the models of possible multiverses he describes are not themselves theories, but consequences of theories. Rather than these models themselves being authoritative, they rest on more fundamental theories in cosmology and physics (such as eternal inflation and interpretations of quantum physics), which, if true, would suggest these multiverse models.
I choose to end at least our mutual trip down the rabbit hole here (you can go on further on your own if you like; I know I will), because I think it reminds us of the bounds of our exploration, where empiricism ends and speculation begins, and even where healthy speculation may begin and end. While reading Tegmark’s book especially, my mind was swamped in the best of ways with strange new ideas and possibilities: are there other versions of me out there? Who lives the better life? How different could they be before they aren’t “me”? And, of course, in the spirit of Level-IV: is all this merely describable by mathematics, or is it all a highly-complex mathematical structure among other possible ones? The entire two or three weeks through which I read this book, these questions buzzed in my head from the time I woke up to the time I went to bed. But once the book was done, while the questions persisted, the buzzing stopped, so to speak. Suddenly, no longer chin-deep in multiverses and metaphysics, I found myself precisely where I was—at a vantage point that proved little, suggested much, and only ever teased anything further. And that’s okay.
And if may only be temporary. Tegmark mentions how one of our all too human tendencies is to assume not only that we’re the center of reality, but that we have a good grasp at how big reality is. We assume the Earth is the center of Creation, then we get a Copernicus who says it’s the sun; then we get a Bruno who says our sun is even one of many; and the pattern goes on. Many stars because many galaxies, which end up speeding away from each other (sometimes faster than light, since space itself can move faster than light even if matter can’t move through space at such a velocity), and eventually perhaps many galaxies in a single universe will become just one universe in a vast expanse of several others like and unlike our own to varying degrees. And who can say if the pattern goes on further, like Tegmark hypothesizes?
For now, however, there’s something to be said about scientific and philosophical terra firma. The philosopher Ludwig Wittgenstein wrote that “Whereof one cannot speak, thereof one must be silent”—there’s wisdom there, even if it can easily become a cudgel. I love speculation, and I think we should keep it up (I want more Tegmarks, more Leslies and Lewises, more Steinharts and Smolins, more Everetts and maybe even more universes!), but there’s something to be said about remaining mindful of the boundary, however (im)precise we may be, between empiricism and speculation.
On Not Being a Total Bummer
That said, I really like speculation. And I think it’s useful, too.
Think of it like this: you’re a cartographer on some far-future mission to map an alien planet’s surface. Satellite imagery has given a pretty crisp view of the terrain, but because we’re cheap (and this metaphor’s not perfect) you’re sent to survey things directly. So you’ll want to plan. But it won’t do to suggest to yourself just any old plan; after all, what’s more probably, vicious dinosaurs or sharp rocks? You don’t need to plan for every possibility, just what’s most probable, and those satellite images can give you that. So you plan from those, making speculations as to what kind of terrain may be awaiting you, and you prepare accordingly.
That’s good speculation. And I think it applies marvelously here. The multiverse is not a new concept, believe it or not; it’s arguably popped up here and there throughout the millennia of religious and philosophical thought. Lewis may belong to an ancient tradition when he suggests modal realism; such a school of thought could see the aforementioned alien planet through an advanced telescope, know it was there, tell how far it was from its local star, and perhaps even what other planets might be near it. But Everett belongs to a new school of thought that can now go there and take some (albeit grainy) pictures of the surface through a remote probe. Eventually there will be an even newer school of thought that may get to put boots on that proverbial planet’s surface. And thus, in the meantime, we have all the reasons in the world to plan and speculate about what may be waiting for us.
It may only serve to emphasize my point if I mention that Tegmark insists on a few occasions in his book that our minds were shaped by Darwinian processes to focus on our immediate surroundings and needs: we were built to detect predators, find food sources, reproduce, etc. The universe itself may have different processes in mind, ones far different than the ones our distant ancestors grew up with in the Fertile Crescent. So it shouldn’t shock us that science may produce models of the universe that are counterintuitive, that defy our instinctual, gut-feeling expectations. All the more reason to pay close attention and plan.
A multiverse is a big place to live, especially if it’s like Lewis’ and contains all possible worlds, not just a Darwinian selection of them. Philosophy should catch up, and science should continue its own precision. People like Smolin and Unger should continue to critique, no matter how much of a minority camp they may represent, so that science can maintain that precision. And people like Leslie and Steinhart should continue to adapt and modernize even ancient ideas, and continue to anticipate new modes of speaking of and living meaningfully in a way that may seem much bigger and busier than our own little pocket of reality appears to be.
There’s also something to be said about simply discussing a topic. I don’t see eye-to-eye with Leslie in all things, but it was because of him I gained a clearer view of abstract|concrete divisions of reality, or of the idea of a Selector. These were valuable concepts. Smolin and Steinhart provide the possibility of speaking of a multiverse with a trajectory, not just a static fecund of all possible worlds. The intellectual game of speculation, with its top-down (or may bottom-up) implications for philosophy, science, or any other field really, is about tracing all possibilities, learning their probabilities with increasing precision with accrued experience and data, and allowing lesser possibilities to fall away so greater ones can take their place.
It’s fairly Darwinian language. It also feels like another multiverse—one familiar, I think, to each of us.