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StevieTNZ said:can also suggest reading https://arxiv.org/abs/1804.00749
StevieTNZ said:... I was alerted to a few days ago and haven't fully yet read in detail ...
DarMM said: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.RUTA said: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.
eloheim said: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.RUTA said: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:Demystifier said:Sober comments on the experiment: http://dailynous.com/2019/03/21/phi...ZedZiEogcshXf1R-aPW4WEA-Nmp6vzS0MWJgKWiC5qJxc
Yes. But then nobody would care.DarMM said: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.Demystifier said: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?vanhees71 said: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.DarMM said: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.kimbyd said: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.DarMM said: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?kimbyd said: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.DarMM said: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.ekimbyd said: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.
That's textbook multiple worlds. That's all it is.
DarMM said: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.
By the simple fact that it doesn't. If quantum computers truly made use of all components of the wavefunction as a physical resource, then you'd expect them to be capable of NP-complete problems like the traveling salesman problem, see here for a brief comment on this: https://arxiv.org/abs/1409.1570microsansfil said: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 ?
/Patrick
In relation to the post above, the mistake you're making here is viewing the components of the wavefunction as physical. Not all interpretations agree on this and thus there would be no physical alternate outcome part of the wavefunction. The other "branches" of the wavefunction in an epistemic view are as physical as the probability ##p(2)## of getting a roll of ##2## on a dice after you observe a roll of ##1##. The ##p(2)## isn't something physical that has "gone somewhere" with its alternate outcome, it's just an epistemic quantity removed after Bayesian updating.kimbyd said: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.
That's textbook multiple worlds. That's all it is.
Sure. But what does physical mean? Both ##|A\rangle## and ##|B\rangle## components of the observer's wavefunction experience those observations. There's no way to distinguish the two anywhere in the math. You can, if you want, say that ##|A\rangle## is physical while ##|B\rangle## is not. Or that the "true" observer is chosen randomly. But that doesn't change the fact that ##|B\rangle## still goes on and has a life and sits down to dinner with her kids. She may be unphysical, but she still has a life.DarMM said:In relation to the post above, the mistake you're making here is viewing the components of the wavefunction as physical. Not all interpretations agree on this and thus there would be no physical alternate outcome part of the wavefunction. The other "branches" of the wavefunction in an epistemic view are as physical as the probability ##p(2)## of getting a roll of ##2## on a dice after you observe a roll of ##1##. The ##p(2)## isn't something physical that has "gone somewhere" with its alternate outcome, it's just an epistemic quantity removed after Bayesian updating.
There's no way to distinguish which one is real between ##p(1)## and ##p(2)## in the probability density function for a dice roll, but still only one of ##1## and ##2## occurs.kimbyd said:Sure. But what does physical mean? Both ##|A\rangle## and ##|B\rangle## components of the observer's wavefunction experience those observations. There's no way to distinguish the two anywhere in the math. You can, if you want, say that ##|A\rangle## is physical while ##|B\rangle## is not. Or that the "true" observer is chosen randomly. But that doesn't change the fact that ##|B\rangle## still goes on and has a life and sits down to dinner with her kids. She may be unphysical, but she still has a life.
Well there are many more views than this. Bohmian Mechanics for example has all components of the wavefunction as real, but isn't Many Worlds. Also Epistemic views view all of the wavefunction as not real as I explained above.kimbyd said:The interpretations other than many worlds argue one of two things:
1) Only one component is real, and the others are not.
2) The "not real" components cease to exist.
Interpretations like (1) are nonsensical. Interpretations like (2) are what this kind of thing is targeted at.
Thank you for your answer. It's a question that concerns me as a qubist. I read this viewpoint https://www.aps.org/publications/apsnews/199806/quantumcomputing.cfmDarMM said:By the simple fact that it doesn't.
...
Fundamentally this is a question of whether or not quantum mechanics describes the macro world we inhabit. It very obviously describes the outcomes of a great many experiments to a tremendous degree of accuracy. And the theory predicts that in the macro world we inhabit, the peculiarities of quantum mechanics such as entanglement and superposition just won't be apparent.DarMM said:There's no way to distinguish which one is real between ##p(1)## and ##p(2)## in the probability density function for a dice roll, but still only one of ##1## and ##2## occurs.
You're still implicitly understanding the wavefunction as ontic with statements like ##|B\rangle## goes onto live their life. The point is that in Epistemic views all of ##|\psi\rangle## is not real, it just encodes expectations.
Let me make this very simple. In a dice roll, when I roll a ##1## what "happens" to ##p(2)## in your opinion?
Bohmian mechanics labels one branch as "real" by saying that the particles live there.DarMM said:Well there are many more views than this. Bohmian Mechanics for example has all components of the wavefunction as real, but isn't Many Worlds. Also Epistemic views view all of the wavefunction as not real as I explained above.
No it's a description of what the epistemic view of the wavefunction is like. No relation to invoking a macro/micro distinction.kimbyd said:Fundamentally this is a question of whether or not quantum mechanics describes the macro world we inhabit.
Not at all. Consider the macrostate in Statistical Mechanics. That is epistemic and yet supposes no hard micro/macro separation.kimbyd said:By trying to make this real/not real distinction, you're effectively saying that there is a hard separation between the macro world we inhabit and these small-scale experiments.
It's fairly trivial. What happens to components of a probability distribution upon observation of one outcome? The usual answer is they are removed by Bayesian conditioning. Basic probability theory.kimbyd said:As to your question, it's just too ill-defined for me to answer, and I see no point trying to pull it together to make it make any sense.
No it isn't, for exactly the reasons I mentioned in post #6. The observer being in superposition can simply be seen as reflecting the epistemic condition of the superobserver regarding superobservables.kimbyd said:But again, the Wigner's Friend experiment is a demonstration that observers can be in a superposition of states. The states are "observed outcome A" and "observed outcome B".
The demonstration of that superposition is evidence that there are different components of the wavefunction who have different experiences.
kimbyd said:The demonstration of that superposition is evidence that there are different components of the wavefunction who have different experiences.