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By Root 1931 0

interested in, and I brazenly told him that I wanted to work on quantum gravity and unified theories. This was generally a conversation stopper, but for Nozick it presented a chance to educate a young mind by revealing a new perspective. “What drives your interest?” he asked. I told him that I wanted to find eternal truths, to help understand why things are the way they are. Naïve and blustery, for sure. But Nozick listened graciously and then took the idea further. “Let’s say you find the unified theory,” he said. “Would that really provide the answers you’re looking for? Wouldn’t you still be left asking why that particular theory, and not another, was the correct theory of the universe?” He was right, of course, but I replied that in the search for explanations there might come a point when we would just have to accept certain things as given. That was just where Nozick wanted me to go; in writing Philosophical Explanations he had developed an alternative to this view. It’s based on what he called the principle of fecundity and is an attempt to frame explanations without “accepting certain things as given”; without, as Nozick explains it, accepting anything as brute-force truth.

The philosophical maneuver behind this trick is straightforward: defang the question. If you want to avoid explaining why one particular theory should be singled out over another, then don’t single it out. Nozick suggests that we imagine we’re part of a multiverse that comprises every possible universe.7 The multiverse would include not only the alternative evolutions emerging from the Quantum Multiverse, or the many bubble universes of the Inflationary Multiverse, or the possible stringy worlds of the Brane or Landscape Multiverse. These multiverses wouldn’t, on their own, fulfill Nozick’s proposal, because you’d still be left wondering: Why quantum mechanics? Or why inflation? Or why string theory? Instead, come up with any possible universe whatsoever—it could be made of the usual atomic species, but a universe made solely of melted mozzarella would serve just as well—and it has a place in Nozick’s scheme.

This is the last multiverse we will consider, since it’s the most expansive of all—the most expansive possible. Any multiverse that’s ever been or ever will be proposed is itself composed of possible universes, and will therefore be part of this mega-conglomerate, which I’ll call the Ultimate Multiverse. Within this framework, if you ask why our universe is governed by the laws our research reveals, the answer harks back to anthropics: there are other universes out there, all possible universes in fact, and we inhabit the one we do because it’s among those that support our form of life. In the other universes where we could live—of which there are many since, among other things, we can certainly survive sufficiently tiny changes to the various fundamental parameters of physics—there are people, much like us, asking the same question. And the same answer applies equally well to them. The point is that the attribute of existence affords a universe no special status, because in the Ultimate Multiverse all possible universes do exist. The question of why one set of laws describes a real universe—ours—while all others are sterile abstractions evaporates. There are no sterile laws. All sets of laws describe real universes.

Curiously, Nozick noted that within his multiverse there’d be a universe that consists of nothing. Absolutely nothing. Not empty space, but the nothing that Gottfried Leibniz referred to in his famous query “Why is there something rather than nothing?” Not that Nozick could have known, but for me this was an observation of particular resonance. When I was ten or eleven, I came upon Leibniz’s question and found it deeply troubling. I’d pace my room, trying to grasp what nothing would be, often with one hand hovering behind the back of my head, thinking that the struggle to do the impossible—see my hand—would help me grasp the meaning of total absence. Even now, to focus on absolute true nothingness makes my heart sink. Total