**QBism: The Future of Quantum Physics**

I am a little disappointed with what I have learnt from QBism (Quantum Bayesianism). The subject that I’ve really got a clearer and better understanding from this book is the frequentist vs Bayesian interpretations of probability. I think I’ll probably never forget what their main difference is.

The QBism solution for wavefunction collapse is explained on the 2^{nd} paragraph on p.133: “In any experiment the calculated wavefunction furnishes the prior probabilities for empirical observations that may be made later. Once an observation has been made … new information becomes available to the agent performing the experiment. With this information the agent updates her probability and her wavefunction—instantaneously and without magic. The collapse sheds its mystery. Bayesian updating describes it and finally makes the missing step explicit.” My immediate reaction was “What ? … Is that it ?!” As of now, I can’t really appreciate the beauty of this so-called “solution”. It’s more like a “quip” or like a lawyer manipulating some legal interpretations to suit his/her needs. Maybe, as usual, quantum mechanical stuff has never been easily understood and/or appreciated, especially not quickly. Its beauty or extraordinariness may sink in for me in the future years …

The GHZ experiment described in p.162-169 is very interesting. In its simplicity, theoretically, one event can prove quantum mechanics right and the classical prediction wrong. Nevertheless, this is just “common” quantum mechanics, not really due (soley) to QBism.

The only potential contribution from QBism mentioned in this book is the “quantum law of total probability” along with QBists’ efforts to try to express the quantum rules in terms of probabilities rather than wavefunctions (p.226-230). Unfortunately, they have not found a proof yet even after involving “a small coterie of mathematicians and mathematical physicists” (4^{th}-5^{th} lines from the bottom of p.228) for a decade. There, the term that distinguishes the quantum version of the total probability from the classical counterpart is an integer called the “quantum dimension” of the system, which “has nothing to do with space or time but with the number of states a quantum system can occupy” (19^{th}-20^{th} lines on p.229). This dimension is said (p.230) to be more fundamental than Planck’s constant but it seems that QBists haven’t found a “practical meaning” of it.

The cost of adopting QBism seems mainly the requirement of an agent who sets a prior probability and then updates with a posterior probability. It is not a concern for me and even not really a “cost” to me, especially compared to the “Many-Worlds Interpretation”. Though I am not sure whether QBism can ultimately provide a bridge between psychology and physics, like what Marcus Appleby speculatively suggested (p. 230-231), essentially all quantum mechanical issues are related to certain observation and/or measurement which always requires at least one agent. … Ah ! … of course, writing up to here, the other issue of the Copenhagen interpretation of quantum mechanics as to what happened to the physical processes when there was no consciousness to observe or measure, like in early big bang epoch, is probably also or especially a difficulty for QBism 🙂

p.197 (17^{th }line): definitely, there is an omission of a full stop “.” in the sentences ” … begins to change The law starts …” after the word “change”.

Dear Kin, I happened to stumble across your blog this morning, but was saddened when I read your, “What? … Is that it?!” Maybe I could suggest these two easy-reading pieces to you: https://arxiv.org/abs/1003.5209 and https://arxiv.org/abs/1207.2141. I wonder if they might leave you with a little more excitement for QBism?

Thank you ! I hope I may find time and enough wisdom to understand your preprints …. Thank you again.