
Quantum theory cannot consistently describe the use of itself - lisper
https://www.nature.com/articles/s41467-018-05739-8
======
whatshisface
"Quantum theory" does not contain any instructions for how to give "quantum
behavior" to observers. Either you imagine there being observers outside of
the system (in which case there are some rules for calculating the probability
distributions of what they see), or you imagine one big system with no special
role of observer (in which case you can "count" the dominance of some states
over others in the great big universal superposition).

In the first case there is such a thing as an observer but they aren't
quantum, in the second case there is no distinguished role of observer.

~~~
fspeech
Very good point!

Is it non-controversial to assume a randomness generator in a fundamental
thought experiment? Also macro state. It is hard to see how the deterministic
part of QM could be inconsistent. However the interpretation of its
application to accommodate classical concepts had never seemed self-consistent
even before this.

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mlthoughts2018
Scott Aaronson discussed this on his blog,

[https://www.scottaaronson.com/blog/?p=3975](https://www.scottaaronson.com/blog/?p=3975)

The discussion there suggests there might be some semantic disagreement over
exactly what is meant by certain measurements and observer states in the
paradox, and so Aaronson at least does not agree that the paper creates the
trilemma situation that it primarily focuses on.

This discussion also breaks down the paradox into terms that describe it as a
combination of both the previously known thought experiments for Wigner’s
friend and Hardy’s paradox.

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johncolanduoni
The first requirement of their no-go theorem ("an agent can be certain that a
given proposition holds whenever the quantum-mechanical Born rule assigns
probability-1 to it") is interesting, considering the fact that "happens with
probability one" is no longer synonymous with "always happens" when you have a
probability space with infinite cardinality. Hence the distinction between the
technical terms "almost always" and "always" in probability theory.

I haven't finished reading the paper, but does anyone who has already
encountered it know if this objection is covered (either in the paper itself
or in later discussion)?

~~~
mlthoughts2018
I don’t believe it was raised anywhere, also just one harp on what you said:
it’s not just having infinite cardinality but also having an appropriate
probability measure on the space. For example, the positive integers have
infinite cardinality, but you can put probability distributions on that space
for which there are no non-empty subspaces with probability zero, like
(1/2)^k.

However I don’t think this issue would actually matter, because from the
observer’s point of view in such a situation the probability of discovering
you are in one of the Everett branches where the probability-1 event did not
occur is exactly 0, so in terms of forming beliefs, you’d assign zero
probability to that outcome.

Think of it this way, if I create a random continuous function on [0, 1], the
probability that it is also differentiable is 0, even though there are tons of
familiar examples like x^2 or sin(x).

So if I offer you a wager of whether the randomly generated function is
differentiable, your belief ought to be that it _always_ is not, even though
the technical term would be “almost always” in formal measure theoretic
probability theory.

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joe_the_user
This is four months old and much heralded when it was published. Has there
been any response to it since then?

~~~
joe_the_user
Just found this refutation:

[https://dustinlazarovici.com/wp-
content/uploads/comment_renn...](https://dustinlazarovici.com/wp-
content/uploads/comment_renner_new.pdf)

~~~
bordercases
Too bad it uses Bohmian Mechanics.

~~~
arunix
Why is that bad?

~~~
bordercases
Bohmian mecahnics isn't identical to Quantum mechanics empirically. I used to
think that but there are counterexamples. Additionally, the metaphysical
strains it takes are philosophically appealing but not experimentally
productive.

------
dang
Discussed a few months ago via a couple of popular articles:
[https://news.ycombinator.com/item?id=18023452](https://news.ycombinator.com/item?id=18023452)

------
asimpletune
Could the conclusion of this paper point towards a useful quantum computer
being an impossible result?

~~~
Myrth
It's purely an opinion and interpretation.

The other interpretation is that the experiment shows that collapse of
potential fields into manifest reality is individual for each observer.

~~~
kakarot
> The other interpretation is that the experiment shows that collapse of
> potential fields into manifest reality is individual for each observer

General Relativity makes sense because our interpretation of a phenomenon is
distorted in a mathematically reversible way which is localized to the
observer.

If it were to be proven that a given wave function could collapse into two
distinct quantum states simultaneously depending on the state of the observer,
it would basically throw a wrench into the entire system.

------
deytempo
Nothing is capable of describing itself and the system in which it exists.
It’s a much more complicated version of a file that contains its own hashsum

~~~
earthicus
It's more subtle than that, for example:

[https://en.wikipedia.org/wiki/Self-
verifying_theories](https://en.wikipedia.org/wiki/Self-verifying_theories)

~~~
okintheory
An inconsistent theory can prove its own consistency, so in what sense does a
system proving its own consistency provide evidence that it is consistent?

~~~
earthicus
This issue is addressed informally in the introduction to the authors paper
available here [1]. The pivotal move that addresses your complaint head on is
Theorem 4.3.

[1]
[https://www.jstor.org/stable/2695030](https://www.jstor.org/stable/2695030)

------
mirimir
It's mind-boggling that they don't cite Gödel.

Edit: This is a Quora gem: [https://www.quora.com/Did-Russell-understand-
Godels-incomple...](https://www.quora.com/Did-Russell-understand-Godels-
incompleteness-theorems-Is-there-any-writing-of-Russells-thoughts-on-Godels-
incompleteness-theorem-Is-there-any-reliable-historic-biographic-source-on-
Russells-understanding-of-Godel)

