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By Root 2100 0

or a mere handful. Big things don’t stand outside the basic mathematical law of quantum mechanics, as Bohr thought, but they do allow probability waves to acquire enough variations that their capacity to interfere with one another becomes negligible. And once two or more waves can’t affect one another, they become mutually invisible; each “thinks” the others have disappeared. So, whereas Bohr argued away by fiat all but one outcome in a measurement, the Many Worlds approach, combined with decoherence, ensures that within each universe it appears as though the other outcomes have vanished. Within each universe, that is, it’s as if the probability wave has collapsed. But, compared with the Copenhagen approach, the “as if” provides for a very different picture of the expanse of reality. In the Many Worlds view, all outcomes, not just one, are realized.

Uncertainty at the Cutting Edge

This might seem like a good place to end the chapter. We’ve seen how the bare-bones mathematical structure of quantum mechanics leads us by the nose to a new conception of parallel universes. Yet you’ll note that the chapter still has a fair way to go. In those pages I’ll explain why the Many Worlds approach to quantum physics remains controversial; we will see that the resistance goes well beyond the queasiness some feel about the conceptual leap into such an unfamiliar perspective on reality. But in case you’ve reached saturation and feel compelled to skip ahead to the next chapter, here is a short summary.

In day-to-day life, probability enters our thinking when we face a range of possible outcomes, but for one reason or another we’re unable to figure out which will actually happen. Sometimes we have enough information to determine which outcomes are more or less likely to occur, and probability is the tool that makes such insights quantitative. Our confidence in a probabilistic approach grows when we find that the outcomes deemed likely happen often and those deemed unlikely happen rarely. The challenge facing the Many Worlds approach is that it needs to make sense of probability—quantum mechanics’ probabilistic predictions—in a wholly different context, one that envisions all possible outcomes happening. The dilemma is simple to state: How can we speak of some outcomes being likely and others being unlikely when all take place?

In the remaining sections, I’ll explain the issue more fully and discuss attempts to address it. Be warned: we are now deep into cutting-edge research, so opinions vary widely on where we currently stand.

A Probable Problem

A frequent criticism of the Many Worlds approach is that it’s just too baroque to be true. The history of physics teaches us that successful theories are simple and elegant; they explain data with a minimum of assumptions and provide an understanding that’s precise and economical. A theory that introduces an ever-growing cornucopia of universes falls way short of this ideal.

Proponents of the Many Worlds approach argue, credibly, that in assessing the complexity of a scientific proposal, you shouldn’t focus on its implications. What matters is the fundamental features of the proposal itself. The Many Worlds approach assumes that a single equation—Schrödinger’s—governs all probability waves all the time, so for simplicity of formulation and economy of assumptions, it’s hard to beat. The Copenhagen approach is surely no simpler. It, too, invokes Schrödinger’s equation, but it also includes a vague, ill-defined prescription for when Schrödinger’s equation should be turned off, and then an even less detailed prescription regarding the process of wave collapse that is meant to take its place. That the Many Worlds approach leads to an exceptionally rich picture of reality is no more a black mark against it than the rich diversity of life on earth is a black mark against Darwinian natural selection. Mechanisms that are fundamentally simple can give rise to complicated consequences.

Nevertheless, while this establishes that Occam’s razor isn’t sharp enough to pare away the Many Worlds approach,