PeterDonis
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Thanks for spotting this, this paper is another key reference in the paper linked in the OP.can also suggest reading https://arxiv.org/abs/1804.00749
Reading https://www.newscientist.com/article/2194747-quantum-experiment-suggests-there-really-are-alternative-facts/ is how I found out about the article.... I was alerted to a few days ago and haven't fully yet read in detail ...
I haven’t reproduced all the calculations yet, I’ve been too busy, but I will definitely do so and report back. Having read the paper, I suspect DarMM is correct, though.So essentially by varying these ##\mathcal{X}## and ##\mathcal{Z}## measurements Zeus and Wigner should be able to detect violations of the CHSH inequality.
The contradiction is that from their own perspective Alice and Bob have obtained some definite outcome for their spin measurements, say ##z_{1}## and ##z_{2}##. So there is a fact about what Alice and Bob have measured, meaning there is a a definite value Zeus for example will obtain if he measures in the ##\mathcal{Z}## basis. Now if we assume that Zeus and Wigner both measure in the ##\mathcal{X}## basis, we have a set of two ##\mathcal{X}## outcomes and the definite ##\mathcal{Z}## outcomes they would have obtained has they measured in that basis. That's essentially four "elements of reality" or values in each run of the experiment:
$$\left\{\mathcal{Z}_{A},\mathcal{Z}_{B},\mathcal{X}_{A},\mathcal{X}_{B}\right\}$$
Then you'd build a probability distirubition for repeated runs of the experiment ##p\left(\mathcal{Z}_{A},\mathcal{Z}_{B},\mathcal{X}_{A},\mathcal{X}_{B}\right)##. However the existence of such a common probability distribution means that one cannot obtain a violation of the Bell inequalities. Hence the contradiction.
This is very similar to Masanes's version of the Frauchiger-Renner argument. Creating a situation where the superobservers should see CHSH violations, but the existence of definite objective outcomes for the observers implies a common probability distribution in contradiction to the CHSH inequalities. This common probability distribution is created by using a "trick" that allows outcomes for all four Bell measurements in a single run. Masanes uses reversibility to obtain this, Brukner uses counterfactuals to include the values ##\mathcal{Z}## would have definitely had had it been measured.
I don't think this criticism of the experiment is warranted, from how I take the paper, at least. IMO it spawns from too literal an expectation for the Wigner's friend gedanken experiment aspect of it. It's more like a formal proof of concept, using single photons and a setup where everything can be kept coherent up until the final macroscopic measurement result of each trial is produced. I believe this is separate from any issues it has with assuming counterfactuals.First, this cannot possibly be a true Wigner’s friend experiment, since the friends are not screened off, i.e., they and their labs are interacting extensively with Wigner and Zeus’s labs, so decoherence will certainly render them “classical.” QM behavior does not follow from mere ignorance. Imagine for example that I scatter photons off electrons in a twin-slit experiment and use those photons to create “which-slit” information. If I merely hide the electron detector screen from my view and let my friend watch it, do you believe my friend will see an interference pattern? Of course not, but that’s exactly what they’re doing here.
The criticism is very fair. The experiment is missing the essential point of Wigner's friend, i.e., the friend (measurement and recording device) must be screened off. Indeed, the entire collection of outcomes is compatible with a single, self-consistent (per QM) observer independent reality contrary to their claim. So, this is not even a proof of principle. See the two slides attached from Brukner and Renner's talks, respectively, at the APS March Meeting two weeks ago. Until someone can screen off a measurement, Wigner's friend has not been instantiated in fact or principle.I don't think this criticism of the experiment is warranted, from how I take the paper, at least. IMO it spawns from too literal an expectation for the Wigner's friend gedanken experiment aspect of it. It's more like a formal proof of concept, using single photons and a setup where everything can be kept coherent up until the final macroscopic measurement result of each trial is produced. I believe this is separate from any issues it has with assuming counterfactuals.
I think the resolution is easy to see in your view.I haven’t reproduced all the calculations yet, I’ve been too busy, but I will definitely do so and report back. Having read the paper, I suspect DarMM is correct, though.
First, this cannot possibly be a true Wigner’s friend experiment, since the friends are not screened off, i.e., they and their labs are interacting extensively with Wigner and Zeus’s labs, so decoherence will certainly render them “classical.” QM behavior does not follow from mere ignorance. Imagine for example that I scatter photons off electrons in a twin-slit experiment and use those photons to create “which-slit” information. If I merely hide the electron detector screen from my view and let my friend watch it, do you believe my friend will see an interference pattern? Of course not, but that’s exactly what they’re doing here.
Second, if DarMM is correct in his analysis I quoted here (again, I suspect he is), then this is really an instantiation of the Quantum Liar Experiment. Read my Insight on that and you’ll see what I mean.
I have a student working with me on this for his senior project. Once we have it all analyzed and properly critiqued, I’ll post something here.
To be fair to the experimenters Brukner and the experiment don't intend to include Bohmian Mechanics or Many-Worlds which are discussed a good bit in that article. Although those interpretations do provide a good refutation of the article title and media headlines. The paper should have been called something like:
Yes. But then nobody would care.The paper should have been called something like:
"Experimental investigation of Objective Reality in Single World Local Interpretations with Global Unitarity"
Hm, involving photons, I'd be very careful with statements about Bohmian interpretations. Particularly for photons at least I've seen no convincing Bohmian setup yet.All these experiments and no-go theorems are compatible with Bohmian mechanics in which an observer independent reality exists.
How about the version of Bohmian mechanics (linked in my signature below) in which photon does not have a trajectory?Hm, involving photons, I'd be very careful with statements about Bohmian interpretations. Particularly for photons at least I've seen no convincing Bohmian setup yet.
It directly implies multiple worlds.This is actually the same kind of thinking that leads to Many Worlds type observations that my measurement of definite outcomes is in some contradiction with a superobserver's assigning a superposed state to me. All the Frauchiger-Renner type arguments are basically trying to push a contradiction out of this. However there really seems there isn't a contradiction. Somebody can assign a superposed state to me despite my seeing a definitive outcome and it neither implies multiple worlds nor that my outcomes are private/subjective. It's just that the superobserver's state assignment refers to the statistics of superobservables that have yet to be measured.
No it isn't. It's only the case if you ascribe an ontic status to the quantum state and add no additional variables. In an epistemic view one observer can have another in superposition without multiple worlds being involved.As long as you have a quantum mechanical system that behaves in the above way, the many worlds interpretation is intrinsic to that system's description.
The other observer in a superposition is experiencing multiple worlds by definition. That's what the term means.No it isn't. It's only the case if you ascribe an ontic status to the quantum state and add no additional variables. In an epistemic view one observer can have another in superposition without multiple worlds being involved.
Superposition does not mean "experiencing multiple worlds", it's a form of statistics. What textbook are you getting that definition from?The other observer in a superposition is experiencing multiple worlds by definition. That's what the term means.
The observer in a superposition sees a single outcome of their observation, and does not interact with the part of the wavefunction where they see the alternative outcome.Superposition does not mean "experiencing multiple worlds", it's a form of statistics. What textbook are you getting that definition from?
Superposition doesn't have to mean "being in both states at once". In an epistemic view it simply means you have a chance of being found in either, just as in a state from Kolmogorov probability.
Well it's the Many World's view of observer superposition, but nothing about observer superposition requires a Many Worlds view, i.eThe observer in a superposition sees a single outcome of their observation, and does not interact with the part of the wavefunction where they see the alternative outcome.
That's textbook multiple worlds. That's all it is.
How would you interpret the fact that a quantum computer uses the principle of superposition of quantum mechanics to perform an immense number of calculations in parallel ?Superposition doesn't have to mean "being in both states at once". In an epistemic view it simply means you have a chance of being found in either, just as in a state from Kolmogorov probability.