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Cats, all the way down?

December 5, 2018

Ever since Schrödinger first boxed his cat, it has been clear that quantum mechanics implies a weird universe, if it applies also to macroscopic entities like us. A new gedanken, by Frauchiger and Renner at ETF, shows that communicating observers cannot always agree on macroscopic facts, like the toss of a coin. (Cite.) That doesn’t necessarily imply MWI over all other interpretations of QM, as the article suggests. But it does create a divide between collapse interpretations and interpretations that are “quantum all the way down.” (Whether the Copenhagen interpretation is the latter depends to some extent on how one interprets intermediate observers.)

It is always fun to get a new gedanken that exposes the weirdness of QM. The importance of that lies in two things. First, it helps sharpen thinking. There is significant hand waving in QM over observers, buried in the Born rule. That gets ever more exposed. Leading perhaps to better explanation. Second, and more important, a gedanken such as this can lead to interesting empirical probes of how our universe behaves. Just as the delayed-choice scenario, that John Wheeler invented and so liked, was at first “just” a gedanken, and then led to a variety of experiments, this “observers of observers” scenario might at some point move gradually into the lab. We’ll see.

If the upshot of the delayed-choice scenario was that nothing is a phenomenon until it is observed, the upshot of this scenario is: and not always then. It seems to me that what Frauchiger and Renner show is that, in a fully quantum world, not even an observer making an observation is always enough to pin things down. But who knows? When this moves into the lab, maybe a collapse theory will at some point prevail. Though I wouldn’t bet that way.

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