Einstein's Unfinished Revolution by Lee Smolin

Author:Lee Smolin
Language: eng
Format: epub
Publisher: Penguin Publishing Group
Published: 2019-04-08T16:00:00+00:00

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THE EVERETT THEORY is a hypothesis about the nature of reality. It posits that all that exists is a wave function evolving deterministically. From the imaginary perspective of a godlike observer outside the universe, there are no probabilities, because the theory is deterministic. All branches of the wave function exist; all are equally real.

The Everett theory asserts that each of us leads many parallel lives, each defined by a branch that has decohered. The theory also tells us that each of these branches exists, with certainty. So if this theory is right, since there is no Rule 2, there are no objective probabilities at all. Let us call this Everett’s hypothesis.

But we are not godlike; we are observers living inside the universe, and, according to the hypothesis, we are part of the world that the wave function describes. So that external description has no relevance for us or for the observations we make.

We are then faced with a puzzle. Where in this world do we find the probabilities that ordinary quantum mechanics claims to predict, which are to be compared with frequencies counted by experimentalists? With no Rule 2, these probabilities are not part of the world as it would be in our absence. Frequencies are counts of definite outcomes, but such things are not unique or exclusive facts in Everettian quantum theory, because given any possible counting of outcomes of a repeated experiment, there are branches which have that count. There are branches in which those counts agree with the predictions of quantum mechanics (with Rule 2) and branches in which they don’t. We cannot say the former are more probable than the latter, because in Everettian quantum theory there are no objective probabilities. We cannot even say that the former are more numerous than the latter because in realistic cases there will be infinite numbers of each.

You read this right: Everettian quantum mechanics predicts that an infinite number of observers will observe experimental results that disagree with the predictions of quantum mechanics! That is the fate of the infinite number of observers whose ill fortune takes them along malevolent branches. It is also the case that an infinite number of observers on benevolent branches see experimental results consistent with quantum theory’s predictions. But that is small consolation, because a benevolent branch can turn malevolent at any moment.

What it seems we cannot say, in Everettian quantum mechanics, is that quantum theory predicts objective probabilities, which are inherent features of nature that exist in our absence. And, unless we find another way to introduce probabilities, we cannot say that the theory can be tested by doing the experiment and counting the different outcomes, because the failure of any such test can be dismissed by supposing that we are just on a malevolent branch—and those are not any less probable or any less numerous than the benevolent branches which confirm the probabilistic predictions of quantum mechanics.

To address this situation, David Deutsch made an interesting proposal, which was to ask

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