(Nature) Quantum theory has a long history of thought experiments. In the most famous thought experiment, physicist Erwin Schrödinger described how a cat in a box could be in an uncertain predicament. The peculiar rules of quantum theory meant that it could be both dead and alive, until the box was opened and the cat’s state measured. Now, physicists Daniela Frauchiger and Renato Renner of the Swiss Federal Institute of Technology (ETH) in Zurich, have devised a modern version of the paradox by challenging that riddle with their own version of the paradox. They replace the kitty with physicists conducting experiments. The result of their new theory has stumped other physicists.
The new “multiplayer” version of a classic is unusual. It shows that if current interpretations of quantum mechanics hold true, then different experimenters can get opposing results about the physicists in the box. In short, it’s quantum theory contradicting itself.

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