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By Root 2103 0

the proposal’s surfeit of universes does yield a potential problem. Earlier I said that in applying a theory, physicists need to tell two kinds of stories—the story describing how the world evolves mathematically and the story that links the math to our experiences. But there’s actually a third story, related to these two, that the physicist must also tell. It’s the story of how we’ve come to have confidence in a given theory. For quantum mechanics, the third story generally goes like this: our confidence in quantum mechanics comes from its phenomenal success in explaining data. If a quantum expert uses the theory to calculate that in repeating a given experiment we expect one outcome to happen, say, 9.62 times more often than another, that’s what experimenters invariably see. Turning this around, had results not agreed with the quantum predictions, experimenters would have concluded that quantum mechanics wasn’t right. Actually, being careful scientists, they would have been more cautious. They would have called it doubtful that quantum mechanics was right but would have noted that their results didn’t rule out the theory definitively. Even a fair coin tossed 1,000 times can have surprising runs that defy the odds. But the larger the deviation, the more one suspects the coin is not fair; the larger the experimental deviations from those predicted by quantum mechanics, the more strongly the experimenters would have suspected that quantum theory was mistaken.

That confidence in quantum mechanics could have been undermined by data is essential; with any proposed scientific theory that has been suitably developed and understood, we should be able to say, at least in principle, that if upon doing such and such an experiment we don’t find such and such results, our belief in the theory should diminish. And the more that observations deviate from predictions, the greater the loss of credibility should be.

The potential problem with the Many Worlds approach, and the reason it remains controversial, is that it may undercut this means for assessing the credibility of quantum mechanics. Here’s why. When I flip a coin, I know there’s a 50 percent chance that it will land heads and a 50 percent chance that it will land tails. But that conclusion rests on the usual assumption that a coin toss yields a unique result. If a coin toss yields heads in one world and tails in another, and moreover, if there’s a copy of me in each world who witnesses the outcome, what sense can we make of the usual odds? There’ll be someone who looks just like me, has all my memories, and emphatically claims to be me who sees heads, and another being, equally convinced that he’s me, who sees tails. Since both outcomes happen—there’s a Brian Greene who sees heads and a Brian Greene who sees tails—the familiar probability of there being an equal chance that Brian Greene will see either heads or tails seems nowhere to be found.

The same concern applies to an electron whose probability wave is hovering near Strawberry Fields and Grant’s Tomb, as in Figure 8.16b. Traditional quantum reasoning says that you, the experimenter, have a 50 percent chance of finding the electron at either location. But in the Many Worlds approach, both outcomes happen. There’s a you who will find the electron at Strawberry Fields and another you who will find the electron at Grant’s Tomb. So, how can we make sense of the traditional probabilistic predictions, which in this case say that with equal odds you’ll see one result or the other?

The natural inclination of many people when they first encounter this issue is to think that among the various yous in the Many Worlds approach, there’s one who’s somehow more real than the others. Even though each you in each world looks identical and has the same memories, the common thought is that only one of these beings is really you. And, this line of thought continues, it’s that you, who sees one and only one outcome, to whom the probabilistic predictions apply. I appreciate this response. Years ago, when I first learned about these ideas,