
Quantum Bayesianism Explained by Its Founder - guerrilla
https://www.quantamagazine.org/quantum-bayesianism-explained-by-its-founder-20150604/
======
RobertoG
>>>"At the other end of the spectrum is Bruno de Finetti. He says there’s no
reason whatsoever for my probabilities and yours to match, because mine are
based on my experience and yours are based on your experience. "

That doesn't deny an objective reality out there, it's just saying that we
have different information about what it's out there and, therefore, we arrive
to different conclusions.

I don't understand how that perspective is different from just "objective
Bayesianism" with different priors.

~~~
lilgreenland
I agree. It feels like he is just combining normal Bayesianism with normal
quantum mechanics. Which is seems fine, but I'm annoyed he dodged the question
about nonconscious things also having a personalized set of priors.

I'd like to hear him explain the single particle double slit experiment from
this QBism perspective.

~~~
nonbel
Sounds like the wave model describes the statistical properties of a
population of photons.

Eg, a coin flip is deterministic if you know all the forces involved (airflow,
force of flip, exact distribution of mass of the coin, etc). But since we are
usually ignorant of all that, instead we model it as a bernoulli trial.

~~~
kgwgk
But that’s not what qbism is about: a wave function (pure state) doesn’t
represent ignorance about a true underlying physical state, it’s a maximally
sharp state of belief.

~~~
nonbel
> Regarding quantum states as degrees of belief implies that the event of a
> quantum state changing when a measurement occurs—the "collapse of the wave
> function"—is simply the agent updating her beliefs in response to a new
> experience.
> [https://en.m.wikipedia.org/wiki/Quantum_Bayesianism](https://en.m.wikipedia.org/wiki/Quantum_Bayesianism)

You could be right, That is what this sounds like to me though. According to
the model there is a 50% chance the coin will land on heads, until you flip
it.

~~~
kgwgk
For a pure state a measurement doesn’t improve our knowledge about the state
of the physical system, it changes it (and we get information about the new
state). The Bayesian updating applies to mixed states, where there exists
“classical” uncertainty while for a pure state the uncertainty is purely
“quantum”.

"Quantum measurement is nothing more, and nothing less, than a refinement and
a readjustment of one’s initial state of belief. [...] Let us look at two
limiting cases of efficient measurements. In the first, we imagine an observer
whose initial belief structure ρ = |ψ⟩⟨ψ| is a maximally sharp state of
belief. By this account, no measurement whatsoever can refine it. [...] The
only state change that can come about from a measurement must be purely of the
mental-readjustment sort: We learn nothing new; we just change what we can
predict as a consequence of the side effects of our experimental intervention.
That is to say, there is a sense in which the measurement is solely
disturbance."

[https://www.perimeterinstitute.ca/personal/cfuchs/Oviedo.pdf](https://www.perimeterinstitute.ca/personal/cfuchs/Oviedo.pdf)

~~~
nonbel
Thanks, not what I was thinking it sounds like.

------
jbay808
> Eventually my colleague Rüdiger Schack and I felt that to be consistent we
> had to break the ties with Jaynes and move more in the direction of de
> Finetti.

For those who are curious, here's Jaynes' view on the subject:

[https://bayes.wustl.edu/etj/articles/prob.in.qm.pdf](https://bayes.wustl.edu/etj/articles/prob.in.qm.pdf)

~~~
kgwgk
The entropic dynamics approach from Caticha is closer to Jaynes’ ideas (and
more interesting than qbism in my opinion, as it’s quite different from
standard QM instead of just giving a Bayesian interpretation to it):
[https://arxiv.org/abs/1908.04693](https://arxiv.org/abs/1908.04693)

------
hexxiiiz
Am I missing something here? A wave function is NOT a probability
distribution! Wave functions have phase! The double slit experiment and bells
paradox both clearly show that quantum effects can be created that cannot be
explained in terms of classical trajectories that are simply unknown. Phase
can cause wave functions to interfere with themselves, and so forth ... I can
only imagine that Fuchs must deal with this in his thousands of pages, but the
article makes it seem like his interpretation was one dismissed at the
beginning because quantum mechanics demonstrably involves effects that cannot
just be a due to a lack of classical state information.

------
brummm
From the post 5 years ago, this blog post summarizes the article well:
[https://motls.blogspot.com/2015/06/is-quantum-reality-
person...](https://motls.blogspot.com/2015/06/is-quantum-reality-
personal.html)

Edit: Going through recent posts of this guy, I would also like to state that
anything non physics related on his blog is crazy right-wing babble.

------
MindGods
See the discussion from 2015:

[https://news.ycombinator.com/item?id=9714618](https://news.ycombinator.com/item?id=9714618)

Same article but published by Wired.

------
gsliepen
Too many people, including physicists, confuse theories with reality. Quantum
mechanics is a very powerful theory that correctly predicts a lot of observed
phenomena. But while QM talks about a wavefunction, it doesn't mean the
wavefunction actually exists. I myself particularly like the Everett
interpretation, which some believe implies a multiverse, but I don't think the
actual existence of a multiverse is necessary.

~~~
lmm
What other definition is there for saying that something exists, than that it
correctly predicts observed phenomena? If you say that the table in front of
me exists, surely that's nothing more or less than saying that the concept of
a table correctly predicts the table-like sensations you see and feel.

~~~
someguyorother
To me, "exists" is stronger than "predicts observation", although I can't
entirely describe how, except by example.

Let's say you're in a game. You can create a physics of the game world based
on how that game behaves. However, the game world doesn't exist, as such. It's
the result of a pattern produced by a computer, interpreted by our minds,
which fills in the blanks and makes the game world feel more real than it is.

It makes sense that an account of existence should either include the
substructure (in this case, the computer) or the observer (in this case, the
mind doing the filling-in), or possibly both. On the other hand, a description
of the behavior of the system is much simpler.

Another example: simulation hypothesis people sometimes talk about the
possibility of "breaking out" of the simulation, as if the simulation is less
somehow less true than whatever is behind it.

But from a predicting-observed-phenomenon perspective, what you describe by
physics exists. So the distinction between the rules of physics that apply in
a certain domain because you're in a simulation, and the rules of physics that
apply in another domain once you find a way to break out, is entirely
artificial. That people don't see it that way seems to indicate that they mean
something else by existence.

~~~
GoblinSlayer
Without relying on a thing that actually exists - a computer running the game
- you can't properly describe the behavior of the game.

------
GoblinSlayer
>As QBism understands a quantum measurement outcome, it’s personal. No one
else can see it. I see it or you see it. There’s no transformation that takes
the one personal experience to the other personal experience. William James
was just wrong when he tried to argue that “two minds can know one thing.”

EPR paradox demonstrates that observers can know each other experience.

------
wcoenen
If the wavefunction is assumed to only encode epistemic uncertainty of the
observer, then how are phenomena like tunneling explained?

~~~
nonbel
From my reading, they would say some particles are just much more energetic
than usual (for some reason we are ignorant of). See the post above about
flipping a coin. According to the statistical model we use for coin flips, it
is really unlikely to flip 100 heads in a row but not impossible.

~~~
wcoenen
Wouldn't it be possible to see this experimentally? You'd find particles
jumping out of a "potential energy well" which they shouldn't be able to
escape via tunneling.

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Koshkin
> _PROBABILITY DOES NOT EXIST_

That's what Bayesianists think, but not frequentists. (As a frequency,
probability is very much real. And, what would be the point of talking about
something that "does not exist," anyway?)

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mellosouls
(2015)

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usgroup
Yes ... you add Bayesian to the thing and it becomes new, principled and
superior. It’s been a trend for about 3 decades now.

~~~
faaabio1618
Read the article, you can learn something :)

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shellac
I'm struggling to see how this anything more than Bohm's theory with a dubious
interpretation of the guiding wave (which, to be honest, Bohm himself was
guilty of). Like Bell without the 'Beables'.

