The pioneering quantum physicist Niels Bohr was something of a quote machine, with numerous pithy comments about the philosophical foundations of quantum physics that are quoted whenever the theory comes up. Sadly, a lot of those quotes turn out to be kind of empty when you try to dig into them and attach them to any kind of concrete meaning. One of the few that's good, though, is "How wonderful that we have met with a paradox. Now we have some hope of making progress."
I say that largely because, as I say probably too often around here, I'm an experimentalist, and to my mind the emergence of a paradox is a good sign, because a paradox is by definition a situation where two seemingly equally valid lines of reasoning lead to mutually contradictory outcomes. And since as far as we know, there's only one actual physical reality, that suggests the possibility of doing an experiment that can determine which of them is actually correct, and then progress is possible. (It's possible that Bohr meant something else by that, since as noted above, he was (in)famously obscure. This is my interpretation, and it's my blog, so we're going to run with that.)
There's been a lot of buzz this week about a new paper featuring a thought experiment that "breaks quantum mechanics," in the breathless terms of the headline writers at Nature and Scientific American. Descriptions of this give the impression of a useful paradox in the sense above, so this might indicate some progress in the field.
The trigger for all this excitement is a paper by Daniela Frauchiger and Renato Renner of the Swiss Federal Institute of Technology (ETH) in Zurich (Einstein's alma mater) whose title is "Quantum theory cannot consistently describe the use of itself." Most of the news stories about this attain nearly Bohr-ian levels of obscurity regarding what's going on, so I went to the source (you can get a pdf preprint from ETH), which... didn't entirely clear things up as to the overall implications, but at least I have a better idea of what's going on in the experiment.
The core scenario is an extension of the famous "Wigner's friend" variant of the Schrödinger cat thought experiment. Wigner imagined a scientist doing the cat-in-a-box thing in a sealed lab, and asked what a friend of that scientist should take as the quantum state of the lab before hearing the result from the experiment. This was mostly in service of the slightly loopy idea that consciousness plays a role in quantum physics, but the question of what agents inside an experiment perceive remains somewhat interesting even though most physicists have moved past assigning a special role to human (or feline) observers.
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