So, they can co-exist as long as there is something that can be agreed about between observers. So, we might be talking about the same thing despite having differing views about it. — Wallows
I don't believe in objective reality so this is a bit moot for me. But I'm very interested in QM so I feel the need to comment.Physicists have long suspected that quantum mechanics allows two observers to observe different, conflicting realities. — Wayfarer
Wigner knows that his friend knows which way the spin goes, but Wigner doesn't know which way. So Wigner models the lab as a superposition while the friend does not. — andrewk
Is that what the paper does - create a situation where both observers have the same level of detail in their knowledge, and the details contradict each other? That would be a much stronger result than anything contemplated in Wigner's thought experiment, which seems to rely on a belief in 'objective collapse' to have any interest at all..I think to resort to Schodinger’s famous simile, it’s as if Bob observes a live cat, and Alice a dead one - and they’re both right. — Wayfarer
Physicists have long suspected that quantum mechanics allows two observers to observe different, conflicting realities. Now they’ve performed the first experiment that proves it by experimental realisation of what was previously a thought-experiment called ‘Wigner’s Friend’. — Wayfarer
I think it's overstating to say that Wigner and his friend experience conflicting realities. Rather, the friend just has more information than Wigner. The difference can be interpreted as purely epistemological. Wigner knows that his friend knows which way the spin goes, but Wigner doesn't know which way. So Wigner models the lab as a superposition while the friend does not. — andrewk
Is that what the paper does - create a situation where both observers have the same level of detail in their knowledge, and the details contradict each other? — andrewk
——Wigner can...perform an experiment to determine whether this superposition [in respect of a particular particle] exists or not. This is a kind of interference experiment showing that the photon and the measurement are indeed in a superposition.
From Wigner’s point of view, this is a fact— the superposition exists. And this fact suggests that a measurement cannot have taken place.
But this is in stark contrast to the point of view of the friend, who has indeed measured the photon’s polarization and recorded it. The friend can even call Wigner and say the measurement has been done (provided the outcome is not revealed).
So the two realities are at odds with each other. “This calls into question the objective status of the facts established by the two observers,” say Proietti and co.
Many Worlds has those observers in different world branches, — Andrew M
But Proietti and co’s result suggests that objective reality does not exist. In other words, the experiment suggests that one or more of the assumptions—the idea that there is a reality we can agree on, the idea that we have freedom of choice, or the idea of locality—must be wrong.
Many Worlds has those observers in different world branches,
— Andrew M
I don’t regard that as an explanation so much as a cop-out. — Wayfarer
But Proietti and co’s result suggests that objective reality does not exist. In other words, the experiment suggests that one or more of the assumptions—the idea that there is a reality we can agree on, the idea that we have freedom of choice, or the idea of locality—must be wrong.
So much for freedom of choice. — Banno
It isn't a live and dead cat, a blatant contradiction which cannot arise. Bob observes the cat and knows if it is dead or alive. Alice measures the cat still in superposition. That's very different than Alice measuring a dead cat and Bob a live one.Wigner knows that his friend knows which way the spin goes, but Wigner doesn't know which way. So Wigner models the lab as a superposition while the friend does not.
— andrewk
As I said to W., if it were that simple then it wouldn’t rate a comment.
I think to resort to Schodinger’s famous simile, it’s as if Bob observes a live cat, and Alice a dead one - and they’re both right. — Wayfarer
I don't believe in objective reality so this is a bit moot for me. — andrewk
The wording of the paper seems to be a argument against counterfactual definiteness (an objective reality). I'm all for that since I don't think there is such a thing, and Bell's theorem demonstrated long ago that you can't have both that and locality. — noAxioms
That the photon's state is in superposition. The other measurement is not in superposition with the photon. I suppose you can word it that the result of that known measurement is in superposition.That is what the MIT abstract says that it does:
Wigner can...perform an experiment to determine whether this superposition [in respect of a particular particle] exists or not. This is a kind of interference experiment showing that the photon and the measurement are indeed in a superposition. — Wayfarer
Yes, the superposition exists. The suggestion that a measurement cannot have taken place is false. The article does suggest this, but QM rules do not under any interpretation. From the beginning, Schrodinger's cat is in superposition despite the measurement obviously having taken place.From Wigner’s point of view, this is a fact— the superposition exists. And this fact suggests that a measurement cannot have taken place.
Are they at odds? Schrodinger's reality is not at odds with that of the cat, and never has been. You can put a human in the box watching the cat if your interpretation insists that humans are special, but I assure you that none of the measurements mentioned by that article were made by humans. Humans learn of the results (of probably thousands of runs) only well after the fact.But this is in stark contrast to the point of view of the friend, who has indeed measured the photon’s polarization and recorded it. The friend can even call Wigner and say the measurement has been done (provided the outcome is not revealed).
So the two realities are at odds with each other. “This calls into question the objective status of the facts established by the two observers,” say Proietti and co.
No interpretation is a cop out, but MWI cannot have those observers in different world branches since they communicate. Alice knows the polarity and tells Bob that she does. Bob knows that the particle is still in superposition and tells Alice so. That cannot happen if the two are in different branches.Many Worlds has those observers in different world branches,
— Andrew M
I don’t regard that as an explanation so much as a cop-out.
Most interpretations reject it. You take away Bohmian mechanics and Stochastic and Transactional interpretations, the latter two being interpretations with which I am not familiar. But all the ones you hear about (Copenhagen, MWI, Consistent histories, objective collapse, Wigner, QBism and Relational) all reject an objective reality. I have a personal preference for Relational, but I don't assert the other ones must be wrong.If you reject "objective reality", is there any interpretation other than Many Worlds which is acceptable? — Metaphysician Undercover
They're not. They're spinning it as something new. But if they've actually disproven the principle of counterfactual definiteness like the wording of the article implies (but does not actually state), then I'd like to hear from the side of those that assert it, like a Bohmian guy interpreting the results. I don't know enough about the interpretation to know how they interpret a superposition state.If most interpretations reject objective reality, then how is the article referred to in the op saying anything new? — Metaphysician Undercover
Let us begin by saying what quantum entanglement is not. It is not any kind of causality or spooky, instantaneous action at a distance. How do we know this? Because, theoretically, entanglement is a consequence of relativistic quantum theory, and relativity precludes this sort of interaction.
Yes, I know that there is no information transmitted faster than the speed of light in entanglement experiments, but that is not way relativity precludes spooky action at a distance. Imagine a EPRB-type experiment with two observers, A and B, equidistant from the entangling event. In our frame of reference A and B detect the spin simultaneously, so, if action at a distance were involved, it is indeterminate whether A's detection event is acting on B's, or B's on A's.
However, that is not the worst of it -- for if we consider the problem in a frame of reference in which A is moving toward the initial event, then A will detect the spin first and, if action at a distance were involved, necessarily, the detection event at A would have to act on that at B. If we consider the experiment in a frame in which B is moving toward the origin, the reverse is true. Thus, neither can be acting on the other and there is no sort of action at a distance.
So, what is going on here? Two factors are neglected by the usual analysis: (1) Detection dynamics and (2) transtemporal symmetry.
First, the result of a spin observation is not the spin of the quantum prior to observation. Consider a spin-0 quantum that decays into two quanta with spin. Let the EPRB detectors be set at right angles. Then, no matter what spins are detected, the sum of the detected spins cannot be zero! So, the detected spins are not initial spin (which was zero). This would seem to violate conservation of angular momentum, but not if we consider the detectors as well as the observed system. Obviously, the extra spin comes from the detectors. Thus, the detectors must be considered as well as the observed system, and the observed spin is not the the prior spin of the system, but the result of the interaction of the system with the detectors.
There is no time limit on quantum entanglement, so, we must acknowledge that EPRB detectors are not isolated and independent, but synchronized and entangled -- and the material in them has been entangled since the Big Bang. Thus, part of the answer Aspect-type experiments is to apply the idea of quantum entanglement on a cosmic, rather than a local, scale.
Second, none of the analyses I've seen consider transtemporal symmetry. Every case of entanglement involves some conservation law. The original EPR paper involved conservation of momentum. EPRB and Aspect-type experiments involve conservation of angular momentum. By Noether's theorem, all conservation laws reflect dynamic symmetries. Conservation of momentum reflects translational invariance and conservation of angular momentum reflects rotational symmetry. This suggests a deeper reflection on symmetry.
When we consider translational and rotational symmetry in different relativistic frames of reference, we wind up connecting points at different times, because the points that are symmetric in different frames have different times.
The most relevant application of transtemporal symmetry involves the Pauli exchange principle. In non-relativistic quantum theory, when we exchange the spatial coordinates of two Fermions (such as electrons), the multi-Fermion wave function changes sign. In the relativistic formulation, we must consider the Fermions not only at the same time, but each Fermion at its own time (this is Dirac's multi-time formulation). That means that world wave function, the joint wave function of every similar Fermion, has symmetries that link it not only at a given time, but at all times since the Big Bang.
This confirms what I said earlier about the non-independence of detectors in Aspect-type experiments. The detector wave functions are related and constrained by a transtemporal symmetry extending through all space-time. So, entanglement does not involve action at a distance, but transtemporal symmetry.
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