
Wigner's Friend - monort
https://en.wikipedia.org/wiki/Wigner%27s_friend
======
andreareina
Scott Aaronson on the Extended Wigner's Friend thought experiment:
[https://www.scottaaronson.com/blog/?p=3975](https://www.scottaaronson.com/blog/?p=3975)

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beautifulfreak
A recent experiment seems to confirm that different observers see conflicting
realities: [https://www.technologyreview.com/s/613092/a-quantum-
experime...](https://www.technologyreview.com/s/613092/a-quantum-experiment-
suggests-theres-no-such-thing-as-objective-reality/)

~~~
atq2119
Honest question: Do physicists studying these problems genuinely believe that
these experiments show some kind of conflict or paradox, or is this just a
case of people writing clickbait articles?

I'm not a physicist and only dug deeply enough into the topic to understand
the basic quantum algorithms for factoring and discrete logarithm, but it
seems to me that there is very obviously no conflict. It's just that the
"observer of the observer" observes a meta-reality that contains all possible
realities observed by the "observer". There doesn't seem anything strange
about this if you just don't insist on "measurement" having a special physical
meaning and accept the universal wave function. But maybe there's a twist I'm
missing.

~~~
Viliam1234
Not a physicist either, but seems to me that the whole issue is people
refusing to admit that they are themselves composed of atoms that follow the
laws of quantum physics.

If a set of atoms can be in a superposition, so can be you. Because you are a
set of atoms, duh.

~~~
millstone
All physicists acknowledge they are composed of atoms, duh!

It does not follow that "if a set of atoms can be in a superposition, so can
you." Here's some ways it might fail:

1\. In the objective collapse theories, quantum systems undergo collapse when
they reach a certain size. I cannot be in a superposition because I am
physically too big.

2\. In the ensemble interpretations, a quantum state applies to a set of many
identically prepared systems. Since there is only one of me, we cannot speak
meaningfully of my quantum properties.

Admittedly neither of these are satisfying interpretations, but they have
their proponents.

The deeper objection is that "you can be in a superposition" risks detachment
from experiments. If we cannot talk about the result of an experiment, that we
cannot do science.

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yarg
I've been thinking about this sort of thing.

It seems more reasonable to me that the act of measurement actually does not
change the state of the quantum system, but entangles the observer with the
system.

It is the observer and not the pre-existing entanglement that is subject to
change as a result of measurement.

If a man in a box at some stage measures the state of a cat in a box, then my
perspective of the box is a superposition of: a man who has not yet measured
the state of the cat; a man who measured the state of the cat, and found it
alive; and a man who measured the cat and found it dead.

I can expand this superposition with arbitrary possibilities - such as whether
or not the cat is yet to enter rigor mortis.

If without seeing the cat myself - or the man's reaction to seeing the cat - I
watch as the man looks at the cat, my superposition is reduced to the
possibilities in which the man has measured the cat.

This is not about the state of reality - it's about the localisation of
information regarding the state of reality.

(Additionally, if I saw the man's reaction to his observation of the cat it
would skew my superposition based on the likelihoods that he would react that
way if the cat were dead/alive).

Spooky action at a distance is another one where I don't think the common
interpretation holds up - perspectives on quantum systems should be absolutely
free to contradict each other (in terms of absolute measurement) it is only
when the light cones of the quantum systems meet that contradictions are
excluded.

~~~
lonelappde
I believe you are describe quantum information theory. Paging user 'lipser!

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Jugurtha
I thought it was related to an anecdote: Paul Dirac married "Wigner's sister".
It appears a friend asked him about who the woman was and Dirac replied "This
is Wigner's sister, who is now my wife."

Wigner apparently called Dirac his "famous brother-in-law".

Other anecdote: Oppenheimer, when recommending Feynman included a quote from
Wigner on Feynman: "He is a second Dirac, only this time human."

I enjoy these bits. I also enjoyed going through a graph of people as a kid in
order to sleep. People as vertices and relations as edges.

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golergka
Doesn't it just mean that existence or non-existence of superposition is
simply relative to the observer?

~~~
lisper
More or less. It's not true in general, only when one of the observers is
considered as a system that is isolated from a second observer. Two observers
who are not isolated from each other, i.e. who are mutually entangled (which
is to say, all real-world observers) will always agree on the state of the
world.

