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Is quantumness in the eye of the beholder?

Quantum Bayesianism, or QBism, is an interpretation of quantum mechanics in which probabilities in quantum theory are personal to the agent in quantum theory. Join QBism co-founder Chris Fuchs for cocktails and conversation with Adán Cabello, Marissa Giustina and Rüdiger Schack—discussing probability, chaos, quantum randomness and the meaning of life, whilst trying to convince them that QBism is the ultimate quantum mechanical interpretation.

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Keywords: QBism quantum bayesianism fuchs schack cabello mechanics interpretation

18 days later

As usual, thought-provoking stuff from FQxI. I've watched the video; I also read the article by M. Mitchell Waldrop, which gave me a slightly clearer sense of what QBism is.

I think I'd need to read some papers on it to get a more complete understanding, but the basic principle as expressed in the article seems a very reasonable one to me: that reality is different for different observers.

For me, this principle can be seen by considering the Schrodinger's cat thought experiment (though I don't know whether the following considerations are in line with the QBism interpretation). In this experiment, the cat exists in a superposition of alive and dead states until the physicist opens the box. The wavefunction then collapses into one of the two states. At least, it does for the physicist. That is the physicist's reality.

There is another perspective, though - that of the cat. The cat knows whether it's alive or dead before the physicist opens the box. (The pedant in me is demanding a correction to the last sentence: the cat knows if it's alive. If it's dead, it obviously can't know anything - unless there is a feline afterlife!) Reality for the cat is therefore different from reality for the physicist.

To understand this in a more systematic way, it may help to borrow a notion from thermodynamics. Before the physicist opens the box, its contents - cat, poison gas and sample of radioactive material - form a closed system. This has one reality, in which the cat has a definite state. The physicist, being outside the closed system, has a different reality, in which the cat is a superposition of states. By opening the box, the physicist is being admitted to this system in which the cat has a definite state, and this is manifested as a collapse of the cat's wavefunction in the physicist's reality.

Taking a step back, though, it could be argued that one doesn't need quantum mechanics for this notion of observer-dependent reality to be realised - it can happen in classical mechanics too. Consider two observers occupying the same location. One is revolving about their axis very fast, while the other is at rest. They experience two different realities: if the one at rest experiences no forces, the revolving one will experience - and can measure - a centrifugal force.

What we are talking about here is a difference between reference frames. This use of multiple frames of reference underlies the physics of both special and general relativity.

However, the use of multiple reference frames in quantum mechanics has a different character. Some authors have argued that in quantum mechanics, a reference frame itself should be seen as a superposition. This superposition is described as a "Quantum Reference Frame" (QRF). I came across this idea in a fascinating 2019 paper by Giacomini et al Quantum mechanics and the covariance of physical laws in quantum reference frames. I started reading it a few years ago, then mislaid it and have just rediscovered it. The paper actually starts with a statement of the principle in my second paragraph above: "The state of a physical system has no absolute meaning, but is only defined relative to the observer’s reference frame in the laboratory". It describes how there is a long history of research into QRFs - once again, apparently kicked off by FQxI member Yuri Aharonov and collaborators - but that it treats these in a different way from the existing literature. I think FQxI readers might find this paper and maybe the wider body of work very interesting - as might researchers on QBism, if they're not already aware of it.

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