[NB: This post is actually a repost of an essay I wrote back in 2008-- here.]
Philip Pullman's His Dark Materials trilogy (The Golden Compass, The Subtle Knife, and The Amber Spyglass) is a wild metaphysical ride-- imagine Tom Robbins for kids-- that takes the reader through multiple alternate universes, many of which appear to be variations on our own, but at least one of which features an earth on which no human life evolved (the mulefa of that world are sentient, but not human: imagine elephants on motorcycles). We encounter only an infinitesimal fraction of the universes out there; more are born every moment.
The manner in which Pullman's universes are born is boilerplate sci-fi (for a classic example, see Larry Niven's short story, "All the Myriad Ways," in his collection of the same title): as sentient beings are faced with choices, each choice results in a mitotic split by which new universes are born, each universe containing an alternate version of the sentient being who has passed beyond the moment of choice. If a certain Being X has twenty possible choices at a given moment, then twenty different universes will be born, each one instantiating one of those twenty choices. Of course, assuming the existence of libertarian free will, each sentient individual actually faces an infinity of possible actions each moment, so each individual is "producing" infinities upon infinities of universes every moment. If you think that's complicated, apply that scenario to every sentient being.
The idea that we live in an ever-burgeoning froth of universes is evocative, but is also, in my opinion, unworkable. I want to talk first about the narrative problems it poses for Pullman's plot (this will require explaining the story a bit), and later on about the philosophical problems inherent in a frothy metaphysic.
1. Narrative Problems
Pullman obviously can't lead us through every single universe; for his story to have any coherence, he must confine his narrative to just a few universes. The ones we encounter are:
1. Lyra Belacqua's world
2. Will Parry's world (which is also our world)
3. The world of Cittàgazze (characterized by the predominance of Italian culture, the presence of Coca Cola, and the general lack of adults in the big cities)
4. The world of Lord Asriel's fortress
5. The world of the mulefa, where Mary Malone constructs her amber spyglass
Beings from other universes appear in the story, but we never visit those places.
All the parallel/alternate universes are connected, however, by the existence of Dust, which is particles of consciousness. When matter evolves to the point where sentience appears, there Dust is found. The universes are also connected to the Abyss: interdimensional explosions that rip the fabric of space-time can create holes in many alternate worlds at once, and the Dust from those worlds will begin to drain into that singular Abyss.
It is possible that the universes are also connected "at the top": the idea that all the universes are the products of a single creator God is alluded to in the books, although God is never actually seen, and God's existence is never confirmed. Much of the story focuses instead on The Authority, the first and greatest angel to be formed from the coalescence of Dust. The Authority crowned himself God and told all who followed that he was the creator.
Angels can pass easily between alternate universes without disturbing the overall frothy structure of the Great Metaphysic (my term for the sum total of all universes, not Pullman's). It seems that angels, despite being the most highly sentient of sentient beings, do not produce new universes with their choices. Pullman never directly addresses this issue. Humans, too, can pass from one universe to another; in fact, many doors between worlds remain open because the humans who created them have forgotten to reclose them.
If I've read Pullman correctly, the human ability to travel between worlds began about three hundred years ago in the world of Cittàgazze, where someone or a group of someones created a tool called the "subtle knife." The blade is of modest size and two-edged; one edge cuts through any material (reminiscent of a lightsaber); the other edge, when the proper owner of the knife achieves the correct state of mind, cuts through the fabric of one's universe and, depending on the direction of one's concentration, can slice a window or hole into an alternate world. Shifts in cuts and concentration are what allow the knife wielder to open doors to different worlds. A conscientious user of the knife can step through the threshold and reseal the tear, if he so wishes.
But over the course of three centuries, the various users of the subtle knife have secretly entered different universes, pilfering items and technologies found in them, often leaving the doors between worlds open. Each tear allows a little of the Abyss to peek through, and soul-eating Specters, the children of the Abyss, are created every time a cut with the subtle knife is made. As a result, a good part of the trilogy is devoted to the question of how to repair the open doors, stanch the flow of Dust into the Abyss, and stop the spread of the Specters.
It's all quite complicated, and I'm afraid it's also unworkable from a narrative point of view. The problem is this: if there's one Cittàgazze world, there are many-- an infinity of them, in fact. The moment the subtle knife was created, there wouldn't have been only one such knife, as Pullman's story implies: there would have been an infinity of them, too, with an infinity of people doing an infinite amount of damage to the Great Metaphysic. The plot of the trilogy wraps itself up far too neatly (and happily), and this is problematic because Pullman obviously wants to write a smart story for smart kids, a story that works on many levels. Astute young readers will catch on to the same problems I'm talking about here, and will have the same doubts about the conclusion of Pullman's trilogy.
With a blossoming infinity of subtle knives out there, a simple resolution is quite impossible. How would you track down and stop the wielders when each wielder is producing an infinity of new wielders at every moment? I conclude that Pullman bit off more than he could chew when he decided on such a freewheeling many-worlds scenario for his story. He could have avoided the chaos by hewing to a more modest alternate-universe paradigm, such as can be found in CS Lewis's Narnia series, where God's anteroom is a forest filled with still pools of water, each pool a gateway to a self-contained universe, and little to no interpenetration between universes except whatever God allows. Pullman could also have gone for an even more restricted scenario, such as the one in Stephen R. Donaldson's Thomas Covenant series, which deals with only one alternate world created by a being who, in our world, appears to be a Hindu monk. In terms of narrative, neatness counts, and the more I think about Pullman's story, the less I like this aspect of it. What a contrast with that other well-known series, the Harry Potter heptalogy! JK Rowling offers us only one world, one with quite enough action to keep us occupied, thank you very much. When put next to Pullman's trilogy, Rowling's series looks relentlessly linear.
2. Philosophical Problems
Now let's turn to the matter of the frothy metaphysic itself.
I'm a big fan of Occam's Razor, which states that we should "not multiply entities beyond necessity." This is normally interpreted to mean "the simplest, most elegant explanation for a given state of affairs is probably the correct one," but in the case of Pullman's Great Metaphysic, there's no need to reinterpret Occam: Pullman's story quite literally multiplies entities beyond necessity!
But let's think for a moment in terms of simple, elegant explanations. Which explanation for the current state of affairs strikes you as simpler and more elegant?
1. There is only one universe.
2. There is an infinity of universes, with new ones being created all the time as sentient beings make choices.
The idea that this one reality (and there can only ever be one reality, as I explained back in this post) contains one universe strikes me intuitively as correct. Parallel universes seem to me to feed an anthropocentric need to spread our egos as far and wide as possible: what a nice fantasy to think that somewhere out there is an alternate Kevin who is at this very moment sipping Mai Tais and surrounded by gorgeous women!
So the froth model seems to fail the test of Occam's Razor, a truly subtle knife if ever there was one. I also think the notion of a frothing reality presents us with a problem only vaguely alluded to earlier: the problem of freedom.
Freedom, conventionally defined, is the ability to do otherwise than what one has done. This suggests that, at a given choice-moment, there is the actual choice made and, potentially, an infinity of counterfactuals, the ghosts of alternatives unexplored. In the froth model, however, there are no counterfactuals: all possibilities are actualized! Stepping back to the God's-eye view, we can see that this means there is no freedom, no shadowy "otherwise." Those "otherwises" actually exist in-- as-- other worlds.
Let's simplify the situation and pretend that at moment M, when Kevin makes a choice, reality suddenly switches to the froth model, and that only Kevin is the generator of universes. What this means, from the God's-eye perspective, is that Kevin is a being whose true shape spreads across a multiverse and resembles a great, branching structure. That structure contains no potentiality, because every single one of Kevin's choices is actualized in some universe somewhere. The shape of this structure is therefore fixed: the branch-Kevin, taken as a whole, is not free. If we follow Kevin along only one world-line, we can see how he might think of himself as free-- how, from his limited perspective, he might come to regret the would-haves and could-haves in his life. But Kevin in his entirety, the infinitely ramified Kevin, isn't free at all: his plural existences cover all possibilities, leaving no counterfactuals.
I somehow doubt that reality is this complicated. I may be wrong, but Occam's Razor is quite persuasive: it's more fruitful to think we all inhabit a single, non-frothing reality, and that counterfactuals, whatever they are and whatever their ontological status, drop away as we pass through each moment of choice.* It also makes little sense, thermodynamically speaking, to say that we, or that our decisions, somehow create whole universes. Easier to adopt the creaturely view that we arise out of a universal matrix, retain some coherence for a time, and then slough back into the cosmic churning-- scattered and dissipated, and never to return exactly as we were.
In conclusion, then: while I found Pullman's trilogy to be a great read, it may have failed in the exploration of one of its most central ideas-- the notion of a ramifying multiverse. The neat conclusion of the trilogy did not take the metaphysic seriously enough, and as a result, the conclusion rang false.
*The same could be said for quantum-level fluctuations in the structure of abiotic matter. Why should sentience be the sole producer of universes?