
A Private View of Quantum Reality - pskotarczak
http://www.wired.com/2015/06/private-view-quantum-reality/
======
DennisP
I guess I don't get it. How is this different from a hidden-variable theory?
How does it explain the two-slit experiment?

~~~
altrego99
Yep this is hidden value theorem.

I think either the author messed up the concept badly, or what is more likely,
the proponent of this 'QBism' doesn't get it.

He appears to think spooky action is just lack of information - something like
"if I open the box and see white marble, you must have the black one".
Possibly haven't read or doesn't understand EPR thought experiment (which
shows how by choosing to make a certain kind of observation, what he and thus
the other guy sees).

Looks like pseudoscience to me.

~~~
gsteinb88
Yeah, no, Chris Fuchs is definitely not doing pseudoscience. Please don't make
judgements like that based on popular science writeups (though, this is
actually one of the better ones I've seen on interpretations of QM, even if
the figure is horribly misleading/wrong).

More generally, QBism is not a hidden variable theorem in any sense. What the
article glosses over is that QBism does still require a modification to
standard probabilities that (when combined with Baysian/information theoretic
reasoning) gives you the measurement probabilities you actually see in the
lab.

Chris Fuchs' writeups are pretty fantastic:
[http://perimeterinstitute.ca/personal/cfuchs/](http://perimeterinstitute.ca/personal/cfuchs/)

Specifically, "Quantum Mechanics as Quantum Information, Mostly" is a short
and fun introduction (okay, 32 pages, but pretty easy reading).

~~~
gus_massa
It's very difficult to read "Quantum Mechanics as Quantum Information,
Mostly", in particular because it mixes the equations with unrelated comments
like:

> _A grain of sand falls into the shell of an oyster and the result is a
> pearl. The oyster 's sensitivity to the touch is the source of a beautiful
> gem._

> _Last year, I watched my two-year old learn things at a fantastic rate, and
> though there were untold lessons for her, there were a sprinkling for me
> too._

A better reading material is the solution of a simple exercise, that explains
the difference between the usual approach and the QB approach. (Is there any
differences in the results?)

Someone has suggested the double slit experiment, because it's nice and easy
to explain with words, but the continuous distribution makes the calculations
difficult. I prefer the three Stern-Gerlach experiments because it's discrete
and the math is easier. I think I read that experiment in a Feynman book, but
I don't remember the exact citation. (The SG in the middle is the equivalent
to the double slit.)

I just found this PDF that explain clearly the situation:
[http://docslide.us/documents/spin-and-quantum-measurement-
da...](http://docslide.us/documents/spin-and-quantum-measurement-david-
mcintyre.html) . It's the "Experiment 4" (subsection 1.2.4, page 10). Can you
explain the differences between the usual and the QB approach in this
experiment?

~~~
gsteinb88
So I think you're confused between: (a) Experimental predictions of QM
calculations (b) Interpretations of why you're performing those calculations

In particular, things like collapse of the wave-function (for example) present
some difficulty for (b) -- not for (a) -- which is what QBism is trying to
address. It's also why you don't get problems to solve here, and why I
recommended "Quantum Mechanics as Quantum Information, Mostly". Yes, it's
written casually (have a look at Fuchs' and others' publications on the arXiv
that made it to scientific journals if you want fewer asides about children)
but the casual nature is because this is about how we view the problems in the
first place, and why we make the calculations we do, not how to carry out the
specific calculations.

Consider this (example stolen from Fuchs, somewhere): We knew the correct
equations of special relativity years before Einstein came along -- that's why
it's called the Lorentz transform, not the Einstein transform. But Einstein's
genius was to boil things down to two laws (within an inertial reference
frame, typical laws of motion hold, and the speed of light is the same in all
reference frames). From _there_ we moved from simple calculations that we
already knew how to do to a much deeper understanding of the subject. That's
what Fuchs' and others working on interpretations of QM are trying to do --
not change the way we make calculations, but understand why the laws are the
way they are in the hopes of extracting something new and different from that
knowledge.

------
stared
QM interpretation is a subtle thing and QBism makes a lot of sense. Yet (IMHO)
it trades practical interpretation for some (nomen omen) purity.

In QM you can describe a quantum state with a wavefunction (to incorporate
quantum ignorance) or a density matrix (to incorporate both quantum and
classical ignorance). (A for classical ignorance only, one would just use
probability vector).

States than can be described with just a wavefuctions are called pure states.
And many-world interpretation revolves around having _only_ pure states (so,
while being unsettling, does not add other assumptions and is, well... more
pure). In QBsim pure states are not singled out.

Having said that, if you are coming to QM, QBism will protect you against a
lot of BS. :)

And BTW: I remember my meeting with Chris Fuchs. It was a very illuminating
story how did he turned from a (sinful) frequentist to an (enlightened)
bayesianist. And how we should focus on knowing and not knowing things, rather
that consider reality as an abstract thing.

A --bit-- lot of shameless self-advertisement:

\- Hydrogen ion and classical vs quantum not-knowing:
[https://johncarlosbaez.wordpress.com/2015/03/13/quantum-
supe...](https://johncarlosbaez.wordpress.com/2015/03/13/quantum-
superposition/)

\- My quantum game - I want _show_ how does quantum mechanics work
[http://quantumgame.io/](http://quantumgame.io/) (just a sign-up list; I will
release alpha this Sept)

\- in my PhD thesis I coined qubism (for a plotting scheme for quantum
states), only to realize later that a similar word is already taken; chapter 3
of [http://arxiv.org/abs/1412.6796](http://arxiv.org/abs/1412.6796): "The name
qubism (inspired by Cubism, the art movement) should not be confused with
QBsim (quantum Bayesianism)"

~~~
platz
QB still seems like Berkely to me, but just with a dose of math.

~~~
stared
In QM math is essential. Most of things I had learnt about QM before learning
its basic, well, mechanics were actually building false intuitions.

Without math, many-world interpretation is a ridiculous absurd, and an
Ockham's nightmare. With - well, it's pure QM _without_ adding additional
assumptions.

~~~
amelius
> ...and an Ockham's nightmare...

Just never forget that Ockham's razor has absolutely no basis in science.
Also, what one person finds a simpler or more elegant theory, another person
might not; in that sense it is quite arbitrary.

~~~
nazgulnarsil
Has a basis in information theory. But it's funny how you would assert that so
confidently. What lead you to being that confident?
[https://en.wikipedia.org/wiki/Kolmogorov_complexity](https://en.wikipedia.org/wiki/Kolmogorov_complexity)

~~~
amelius
> What lead you to being that confident?

It was certainly not my intention to impress an opinion on anyone else.
Everybody here has the brains to think differently.

I'll be happy to see my comments proved wrong; please provide proper
argumentation.

To elaborate a little more on my original point: the razor has proved useful
as a guidance for thinking, and historically it has helped us to avoid going
into paths that are not worthwhile, but there is absolutely no guarantee. In
fact, the razor might even prevent us from going into paths that _are_
fruitful. So, I'd be cautious about it. That was all, basically.

------
Animats
This is one of those things where it's really hard to tell whether it's a
major step forward or utter bullshit. The question to ask, of course, is "how
could this be tested experimentally?". It it's testable, it's a lot more
important than if it isn't.

The proposed theory would seem to indicate that it's possible to get the real
world into a state similar to an out-of-sync network game. Some games can get
into a state where the player states have diverged, but the players are still
connected. Is this article claiming that the real world can do that? That
should be testable.

~~~
lisper
A good litmus test for an article like this is whether it talks about the Bell
inequalities. If it doesn't (and this one doesn't) it's very likely bullshit.

~~~
gus_massa
I agree with both sentences, but this articles at least say something about
the Bell inequalities:

> _My fellow QBists and I instead think that what Bell’s theorem really
> indicates is that the outcomes of measurements are experiences, not
> revelations of something that’s already there._

I studied the Bell inequalities at the university, but I have no cue about
what this sentence means.

~~~
lisper
Makes two of us. Sounds like BS to me.

------
DonGateley
Luboš Motl, arch conservative, thinks it's essentially correct and is exactly
what the founders tried to convey in the first place. As always, of course,
Lumo gets it and almost no one else does. :-)

[http://motls.blogspot.com/2015/06/is-quantum-reality-
persona...](http://motls.blogspot.com/2015/06/is-quantum-reality-
personal.html)

------
c3d
I think its a step in the right direction, but I believe one can go even
further. It is not necessary to assume an observer or a conscious entity (the
dog problem in the article), if one remarks that physical entities that have
the "collapse of the wave function" property make good measurement
instruments. In other words, knowledge is gained more rapidly with a good
measurement instrument, and that is what defines the "collapse", not
consciousness.

In that interpretation, QM is seen as the only way to do physics when you
represent your knowledge of a system using probabilities. Measurement
instruments are a choice among all the physical systems, and what we consider
good physical instruments lead to QM "axioms".

This is elaborated here:
[http://cc3d.free.fr/tim.pdf](http://cc3d.free.fr/tim.pdf).

~~~
darkmighty
This kind of argument works for classical thermodynamics too, where observing
a particle in a gas collapses it's velocity distribution; where if we take a
confined gas we can localize it (confining it within a tight space), only to
grow uncertain about the momentum of constituents, or expand it, growing
uncertain about the position.

In other words, for any probabilistic physical framework it seems to me you
are going to need to put a magical 'sampling' or 'observation' somewhere.

Making this explicit is one of the reasons I like Bohm's formulation:
[https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory...](https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory#Relation_to_the_Born_Rule)

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norea-armozel
It seems like an odd approach, but it could explain consciousness better since
it assumes that each measured outcome is private information of the observer
and each observer makes up the difference in regards to having differently
measured outcomes. But there's something troubling about that for me. Does it
mean when someone measured the health of Christopher Lee he didn't die? But
for another observer who watched his vitals he did die? Are they in the same
world? Or does this require the many world's interpretation to remain intact?
I hope I'm not stretching QBism beyond its intended goals, but that's the sort
of conclusion I can see being made from it.

------
beefman
Original source: [https://www.quantamagazine.org/20150604-quantum-
bayesianism-...](https://www.quantamagazine.org/20150604-quantum-bayesianism-
qbism/)

