Just when you thought you’d heard every quantum mystery that was possible, out pops another one. Jeff Tollaksen mentioned it in passing during his talk at the recent Foundation Questions Institute conference. Probably Tollaksen assumed we’d all heard it before. After all, his graduate advisor, Yakir Aharonov?—who has made an illustrious career of poking the Schrödinger equation to see what wild beasts come scurrying out—first discovered it in the 1990s and discussed it in chapter 17 of his 2005 book, Quantum Paradoxes. But it was new to me.
The situation is an elaboration of Schrödinger’s thought experiment. You have a cat. It is either purring or meowing. It is curled up in one of two boxes. As in Schrödinger’s scenario, you couple the cat to some quantum system, like a radioactive atom, to make its condition ambiguous—a superposition of all possibilities—until you examine one of the boxes. If you reach into box 2, you feel the cat. If you listen to the boxes, you hear purring. But when you listen more closely, you notice that the purring is coming from box 1. The cat is in one box, the purring in the other. Like a Cheshire Cat, the animal has become separated from the properties that constitute a cat. What a cat does and what a cat is no longer coincide.
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