Evolution of the Renner et al Wigner-like paradox

[stevendaryl]

Why not go with a simple explanation that agrees with all experiments--when a person measures a qubit it's collapsed? Even if that person wasn't me, or conducted the experiment in a different room/lab. If I don't know the measurement result, then I'd consider it a mixed state.

Practically speaking, we can always act for all intents and purposes as if all macroscopic quantities have definite values at all times. So if a person makes a measurement of spin, then whether he measured up or down is a macroscopic fact, and so it's either one or the other.

However, this practical approach seems inconsistent with the laws of quantum mechanics, which make no distinction between macroscopic and microscopic.

 

[Sunil]

I guess it depends on how you interpret the laws of QM and "universal validity," but to me the easiest way to read rule 7 is that the state is collapsed after any measurement, not just my measurements. Also seems more universal (to me). (Despite the ambiguity in "measurement," I think we all agree that a human measuring something and getting a definite result qualifies.)

What is IMHO instructive for intuition is the Bohmian picture. It tells that everything behaves approximately as if positions (better the configuration $q \in Q$) is measured. But there are exceptions. Those exceptions are measurements different from the configuration. Even if it is done only for some part of the system, say, one particle out of two in an entangled state, such a measurement influence the whole configuration in a non-classical way. This gives "surrealistic trajectories", which show something different from classical expectations caused by something happening far away (violating Einstein causality).

 

[msumm21]

Then the collapse of the the friend + lab, must have a unitary description from the other agent when isolated.

This is a "many worlds" interpretation, right? If the state of the qubit is collapsed according to someone in the lab, but in a superposition according to someone outside the lab so they "really" have different wavefunctions, not just different knowledge?

If so, I still wouldn't get the paper--this would mean they used the assumption of many worlds from the get go, then used it to show many worlds is the only interpretation that works.

 

[Demystifier]

Those exceptions are measurements different from the configuration.

Can you give an example?

 

[Sunil]

Can you give an example?

Consider the delayed which way measurement. The which way information measured initially is stored as an excited state of some atom if one of the ways is taken. The first excited state is usually an antisymmetric wave function, with the ground state being symmetric. But the Bohmian position is left or right. So the Bohmian trajectory will be the one as if it was not the way which was measured, so it gives one interference picture if the particle is on the left and the opposite one if it is on the right. As long as you do nothing, as well as when the final measurement is if the atom is located left or right, nothing strange and surrealistic happens beyond what one knows from the one-particle situation.

But if finally the energy of the atom is measured, so that some measurement device (which may be also only a single atom, becoming macroscopic does not change anything in dBB theory) will be positioned left if the atom was excited and right if it was in the ground state, this interaction will influence the far away particle. It will behave, after this, as if the initial measurement has really measured its way, and the interference is gone.

 

[Fra]

This is a "many worlds" interpretation, right? If the state of the qubit is collapsed according to someone in the lab, but in a superposition according to someone outside the lab so they "really" have different wavefunctions, not just different knowledge?

If so, I still wouldn't get the paper--this would mean they used the assumption of many worlds from the get go, then used it to show many worlds is the only interpretation that works.

I don't think I understnad your question. In any way, I don't see how this presumes MWI. I don't subscrive to mwi myself. But it's true that different interpretations see it differently.

As I see it: The premise of QM beeing "universally valid" supposedly means that "any Heisenberg cut" should be allowed - even those where a classical agent using QM, is on the quantum side of the cut. This becomes problematic as even the communication between two classical agents becomes constrained by QM, which makes it hard to formulate QM in the first place, becase it depends on the cut.

Even within Copenhagen there is a tension at play, as far as I recall Heisenberg emphasised the cut, while Bohr emphasized an more observer independent view, based on irreversible measurements, and the resolution between the views, at least as i understand it DEPENDS on that set of all classical observers are AGAIN not subject to QM when they communicate. If they do, we have a problem.

I think we do have a problem, in principle, and the theory needs modification and the measurement problem seems to be at the heart of this problem as well.

/Fredrik

 

[*now*]

In the first version of their arxiv preprint, Frauchiger and Renner argued that only the many word interpretation of QM was consistent.
https://arxiv.org/abs/1604.07422v1

In the second version, the famous one published in Nature Communications, they radically changed their conclusions. They argued that there are 3 categories of consistent QM interpretations, depending on which of the 3 natural assumptions is denied. One of those assumptions is the existence of a single outcome, so the many world interpretation was interpreted as the interpretation that denies that assumption.
https://arxiv.org/abs/1604.07422v2

Now in the most recent arxiv paper (published in Contemporary Physics), Renner (together with Nurgalieva) changes his opinion again. This time they argue that in fact many worlds do not deny the existence of a single outcome, so there are only two types of quantum interpretations, depending on whether they deny the assumption Q or C. Denying Q is denial that the quantum Born rule can be applied to any system, even if that system is a macroscopic observer. Denying C is denial that conclusions by different agents must be mutually consistent. It seems to me that those two types of interpretations roughly coincide with ontological interpretations (which propose existence of an objective reality) and non-ontological interpretations.
https://arxiv.org/abs/2106.05314

Given that Renner et al change their opinion so often, should we take them seriously? Does their change of opinion converge to something? If yes, to what?

“The distinction between relative and stable facts resolves the difficulties pointed out by the no-go theorem of Frauchiger and Renner, and is consistent with the experimental violation of the Local Friendliness inequalities of Bong et al..”

Published: 27 February 2021
Stable Facts, Relative Facts
Andrea Di Bargio & Carlo Rovelli
Foundations of Physics volume 51, Article number: 30 (2021)

Abstract
Facts happen at every interaction, but they are not absolute: they are relative to the systems involved in the interaction. Stable facts are those whose relativity can effectively be ignored. In this work, we describe how stable facts emerge in a world of relative facts and discuss their respective roles in connecting quantum theory and the world. The distinction between relative and stable facts resolves the difficulties pointed out by the no-go theorem of Frauchiger and Renner, and is consistent with the experimental violation of the Local Friendliness inequalities of Bong et al.. Basing the ontology of the theory on relative facts clarifies the role of decoherence in bringing about the classical world and solves the apparent incompatibility between the ‘linear evolution’ and ‘projection’ postulates.

 

[msumm21]

I don't think I understnad your question. In any way, I don't see how this presumes MWI.

I'm thinking that, if one person measures a qubit and collapses it to say, spin up, and another person is OK to consider the same system to be in a pure superposition including both up and down, then there are already 2 different, inconsistent realities (not 1 consistent world). So I'm labeling this as MWI - multiple "realities" happening at the same time: one with a collapsed qubit and the other with a pure superposition of up and down (entangled with an apparatus/environment).