theory, Schrödinger’s thought experiment splits the universe into two parallel copies, one with a live cat, the other with a dead one (and each of them with a version of you). Physicists who look favorably on this interpretation—and many distinguished ones have, among them Richard Feynman, Murray Gell-Mann, and Stephen Hawking—claim that each universe splits into copies every second numbering something like 10 followed by one hundred zeros, all of them equally real. Yet, since quantum theory forbids these parallel worlds from interacting in any but the ghostliest of ways, their reality cannot be experimentally observed.
Another version of this backward argument for multiple universes was championed by the late Princeton philosopher David K. Lewis. Lewis shocked his fellow philosophers by claiming that all logically possible worlds are real—just as real as the one we call the “actual” world. Why did he think such a thing? Because, he said, their reality would crisply solve a wide range of philosophical problems. Take the problem of counterfactuals. What does it mean to say, “If JFK hadn’t gone to Dallas, the Vietnam War would have ended earlier”? According to Lewis, the counterfactual statement is true only if there is a possible world very similar to the actual world in which JFK didn’t go to Dallas and the Vietnam War did end earlier. Lewis’s possible worlds are also useful for making sense of propositions beginning, “If pigs could fly …”
Such dubious arguments in support of the multiverse idea have evoked equally dubious arguments against it—such as these three:
(1) It’s not science. Both Paul Davies and Martin Gardner submit that the proposition “the multiverse exists” has no empirical content, and hence amounts to empty metaphysics. But some of the theories that imply the existence of a multiverse—like the theory of chaotic inflation—do lead to testable predictions; moreover, these predictions have been borne out by the evidence collected so far. And improved measurements over the next decade of the microwave background radiation and of the large-scale distribution of matter may further confirm these theories—or overturn them. That looks like real science.
(2) Alternative universes should be shaved away by Occam’s razor. Both Davies and Gardner complain that the multiverse notion is too extravagant. “Surely the conjecture that there is just one universe and its Creator is infinitely simpler and easier to believe than that there are countless billions upon billions of worlds,” Gardner writes. Is it? Our universe came into being with the Big Bang, and (as the Canadian philosopher John Leslie has observed) it would be exceedingly odd if the mechanism behind this world-engendering event bore the label “THIS MECHANISM OPERATED ONLY ONCE.” A computer program that prints out the entire sequence of numbers is simpler than one that prints out only a single, very long number.
(3) The multiverse, if real, would reduce our own world to a Matrix-like simulation. This objection, voiced by Davies, is surely the most bizarre of the lot. If there really were myriad universes, Davies argues, then some of them would contain advanced technological civilizations that could use computers to simulate endless virtual worlds. These virtual worlds would vastly outnumber the actual universes that made up the multiverse. So, he continues, taking the multiverse theory at face value, it is much more likely that we ourselves are creatures in a virtual world than in an actual physical universe. If the multiverse theory is true, Davies says, “there is no reason to expect our world—the one in which you are reading this right now—to be real as opposed to a simulation.” He takes this to be a reductio ad absurdum of the multiverse idea. But Davies’s argument is a poor one, for at least two reasons. If it were valid, it would rule out the existence of technologically advanced civilizations in this universe, since they too would presumably create simulated worlds in great abundance. And the hypothesis that we are living in a simulation itself has no empirical content. How could it be verified or falsified? We cannot even talk about it coherently, as Hilary Putnam has pointed out, since our words could refer only to things “inside” the alleged simulation.
Among those who take the idea of the multiverse seriously, perhaps the greatest disagreement is over how many distinct versions of it there are. Is, for instance, the “quantum multiverse” the same as the “inflationary multiverse”? The quantum multiverse, as I mentioned earlier, is the version that is invoked to make sense of quantum weirdness. First put forth in the 1950s by the