
Quantum Bayesianism - abrax3141
http://www.wm.edu/news/stories/2017/qbism-q--a-does-quantum-bayesianism-hold-the-keys-to-the-future-of-physics-hans-von-baeyer-believes-so.-.php
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Strilanc
Can someone explain what the deal with qbism even is?

Like, I get that when you do weak measurements the random walk the qubit takes
[2] looks basically identical to inference. That's not too surprising, given
how similar the definition of quantum states is to the definition of
probability distributions. But superpositions aren't _literally_ probability
distributions. In particular, unlike with probabilities, you can prove that
super-logical agents can't rationally disagree about the superposition that a
state is in. Superpositions _can 't_ be observer-dependent in the way that
probability distributions can, because you can create situations where agents
who are disagreeing about a superposition must be risking the highest of
Bayesian sins: seeing something happen to which they assigned a 0% probability
[1]. To me, this directly disagrees with Qbism's tendency to think about
quantum states as subjective instead of objective.

1:
[http://www.scottaaronson.com/blog/?p=822](http://www.scottaaronson.com/blog/?p=822)

2: [http://algorithmicassertions.com/2016/04/24/eves-quantum-
clo...](http://algorithmicassertions.com/2016/04/24/eves-quantum-clone-
computer.html)

~~~
n4r9
I can try to offer something about the tension between the PBR theorem and
QBism. I sat through a conference talk by Chris Fuchs not long after PBR came
out, although some of the detail will probably be lost in translation.

PBR assumes that the quantum state of a system represents a state of ignorance
or partial information concerning an objective, underlying (hidden variable)
reality. In QBism the quantum state of a system instead represents an agent's
beliefs about what will happen in future interactions with the system. There's
no mention of an underlying observer-independent reality. It's not assumed
that the physical state of a system can be portrayed with objective, observer-
independent variables, nor that a given quantum state assigns probabilities to
those variables.

A cynical summary might be that QBism weasles out of PBR type issues by
sticking to radical/subjective Bayesianism, rather than objective Bayesianism.

~~~
Strilanc
Maybe I should put it this way... what do we _gain_ by accepting qbism?

For example, accepting many-worlds gives parsimony of postulates. The number
of postulates goes down, because the Born rule is derived from decoherence and
unitarity instead of being postulated from directly.

Accepting a collapse interpretation gives easy translation between theory and
practice. You don't have to justify squaring the amplitudes, you just do it
and it works.

Accepting a pilot-wave interpretation gives you deterministic rules and an
exact single state.

Accepting a qbism interpretation gives you...?

~~~
n4r9
Well, one answer is that it resolves the measurement problem, and variants
like Wigners friend, in a coherent way and without the additional issues posed
by those other interpretations (although collapse interpretations aren't quite
"interpretations" in the same sense). As I understand it - being neither an
expert or advocate, just interested in quantum foundations - the QBist
attitude is that all those no-go theorems point to quantum theory being little
more than a dutch book of the universe. The whole idea is you're barking up
the wrong tree if you want to give a direct physical meaning to the quantum
state. If you're already welded to the notion that a quantum state corresponds
to an external reality then you'll probably never find QBism very satisfying.
And that's fair enough.

Don't get me wrong here - what you're saying is a very valid criticism and
probably what most people have against QBism. I myself find it difficult to
see what value it adds, unless you already have a certain approach to
scientific knowledge. I was just trying trying to clarify some of the points
your original post made. There is a lot of literature about how QBism relates
to various results and other interpretations. Chris Fuchs' own writings are
very entertaining and worth a read.

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jerf
To those who check the comments before reading, note this article says nothing
about what Quantum Bayesianism is. It's an announcement of a presentation and
apparently a book, and a sales job on how important the idea is, but has no
trace of the idea. I'm not saying there's anything wrong with that per se, but
it's probably not what an HN reader is expecting.

~~~
WhitneyLand
Yes but is has some interesting talk about theory vs. meaning, science vs.
philosophy, and other aspects of QM interpretations.

Btw to address your concern:

[https://en.m.wikipedia.org/wiki/Quantum_Bayesianism](https://en.m.wikipedia.org/wiki/Quantum_Bayesianism)

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philipov
The article is an announcement of a lecture. I couldn't find a recording of it
or any previous physics lectures by him. It looks the only recordings are of
him giving religious sermons.

~~~
zeristor
Is there any other type of sermon?

~~~
philipov
2\. informal

a long or tedious piece of admonition or reproof; a lecture.

synonyms: lecture, tirade, harangue, diatribe; More

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abrax3141
A book just came out.

