How many interpretations of QM do you speak?

[Sambuco]

To violate SI (NSD) there has to be a correlation between measurement settings and the hidden variables of the quantum system. If you don't invoke hidden variables (and we don't), then what is correlated with the measurement settings to violate SI?

In these extended Wigner's friend scenarios, that role is played by the outcomes of the measurements performed by the friends located inside the laboratories. This is stated explicitly in Bong's paper:

"Within the definitions of L and NSD, c, d play the formal role of the hidden variables λ in the usual derivation of Bell inequalities. However, we emphasize again that those correspond to observed events, and note that we make no assumption about hidden variables predetermining all measurement outcomes."

In that excerpt, c, d are the results of the measurements made by Charlie and Debbie, respectively. They are Alice and Bob's friends.

By L we mean "no spooky actions at a distance," i.e., no superluminal influence acting between the wings of the experiment. That is causal by definition. Perhaps you also include separability in L?

No, L is simply the locality assumption in Bell's original formulation (1964). This does not include non-separability. From Bong's paper:

"The probability of an observable event e is unchanged by conditioning on a space-like-separated free
choice z, even if it is already conditioned on other events not in the future light-cone of z."

Lucas.

 

[Sambuco]

Then again I guess there’s a reason why it isn’t more populær.

I think it is because, ultimately, it is not really an "interpretation," since it proposes that the evolution is non-unitary as a consequence of limitations in measuring time with real clocks.

Lucas.

 

[Matterwave]

Regardless of how successfully it does so, I believe the article aims to formulate the measurement problem in a way that is: (i) clearer than current formulations, by avoiding the introduction of the notion of measurement, and (ii) allows for a more fruitful dialogue between different interpretations/schools.

Lucas.

Ok, I will add the paper to my reading list! It just keeps getting longer~

 

[sbrothy]

I think it is because, ultimately, it is not really an "interpretation," since it proposes that the evolution is non-unitary as a consequence of limitations in measuring time with real clocks.

Lucas.

That may well be the reason why I have difficulty regarding it as an “interpretation”.

 

[iste]

I would call it non-probabilistic.

By the way, a joint probability of position and momentum can be defined with the Husimi distribution. It satisfies all the Kolmogorov axioms. But the resulting marginal probability of position or momentum is not equal to the usual quantum mechanical probability of position or momentum, because this corresponds to a different type of measurement. The Husimi distribution is measured by a POVM measurement, which is not a projective measurement.

I think I change my mind, thinking about it; I think more assumptions and interpretation of what the probabilities are or are not about is needed to say they violate Kolmogorovan probability axioms. But they look like they do at face-value.

 

[Demystifier]

I think quantum theory is not so difficult to understand once you accept this: Quantum theory, in its usual textbook form, is not about the microscopic world itself. It is about the response of our macroscopic probes of the microscopic world. But it turns out that this response does not depend on details of the macroscopic probes, creating an illusion that the probes are not really important and that quantum theory is about the microscopic world itself, which creates a lot of confusion and apparent paradoxes.

 

[A. Neumaier]

Regardless of how successfully it does so, I believe the article aims to formulate the measurement problem in a way that is: (i) clearer than current formulations, by avoiding the introduction of the notion of measurement, and (ii) allows for a more fruitful dialogue between different interpretations/schools.

The best paper that formulates the measurement problem in a way that is clearer than current formulations is

• Muller, F. A. (2023). Six measurement problems of quantum mechanics. In Non-reflexive logics, non-individuals, and the philosophy of quantum mechanics: Essays in honour of the philosophy of Décio Krause (pp. 225-259). Cham: Springer International Publishing. https://arxiv.org/pdf/2305.10206

 

[sbrothy]

That may well be the reason why I have difficulty regarding it as an “interpretation”.

Six Measurements Problems of Quantum Mechanics.

 

[RUTA]

In these extended Wigner's friend scenarios, that role is played by the outcomes of the measurements performed by the friends located inside the laboratories. This is stated explicitly in Bong's paper:

"Within the definitions of L and NSD, c, d play the formal role of the hidden variables λ in the usual derivation of Bell inequalities. However, we emphasize again that those correspond to observed events, and note that we make no assumption about hidden variables predetermining all measurement outcomes."

In that excerpt, c, d are the results of the measurements made by Charlie and Debbie, respectively. They are Alice and Bob's friends.

The exact definition is:

“Assumption 2 (No-Superdeterminism (NSD)): Any set of events on a spacelike hypersurface is uncorrelated with any set of freely chosen actions subsequent to that space-like hypersurface”

So, it's not just c and d but "any set of events on a spacelike hypersurface," which would cover any hidden variables. That's why we wrote:

"NSD and L are just the assumptions for the Bell inequality, so other than an new inequality nothing new has been introduced in this paper regarding QM."

and concluded:

 

[sbrothy]

The exact definition is:

So, it's not just c and d but "any set of events on a spacelike hypersurface," which would cover any hidden variables. That's why we wrote:

and concluded:
View attachment 374754

Hah, i can’t hold a candle to you. I wish I knew half of what you do.

EDIT: I can’t even “like” your post as I have no idea what you’re talking about .

 

[RUTA]

Hah, i can’t hold a candle to you. I wish I knew half of what you do.

EDIT: I can’t even “like” your post as I have no idea what you’re talking about .

I started research in quantum foundations in 1994, so I sometimes use the specialized terminology that comes with the field. Sorry.

 

[sbrothy]

I started research in quantum foundations in 1994, so I sometimes use the specialized terminology that comes with the field. Sorry.

Nah it’s OK. I get the gist but the deeper meaning..? Nah…

 

[RUTA]

Nah it’s OK. I get the gist but the deeper meaning..? Nah…

Here is a very simple explanation of entanglement using polarization-entangled photons. You can see how entanglement is all about conservation per the equality of perspectives:

 

[sbrothy]

I’ll take a look although I think I’ve read enough to understand the bare basics of entanglement. It’s your mathematical understanding I envy.

 

[Sambuco]

So, it's not just c and d but "any set of events on a spacelike hypersurface," which would cover any hidden variables.

Immediately after the definition of NSD, the authors say:

"In the EWFS, this implies that c and d are independent of the choices x and y:
• NSD (in the EWFS and under Assumption 1): P(cd∣xy) = P(cd) ∀ c, d, x, y"

which means there are no hidden variables involved in this scenario.

That's why we wrote:
"NSD and L are just the assumptions for the Bell inequality, so other than an new inequality nothing new has been introduced in this paper regarding QM."

As I mentioned in post #145, I don’t quite follow what you are trying to say here, given that the assumptions of the LF theorem are less restrictive than those of Bell’s theorem. This is discussed in Bong's paper, where the authors point out that, in addition to AOE, NSD, and L, Bell's theorem requires "outcome independence" as an additional assumption. Consequently, there are scenarios in which Bell's inequalities are violated, but LF is not. One well-known example is the EPRB experiment.

and concluded:

In the excerpt you cite from your paper, you discuss the experiment presented in Bong’s paper. It is important to distinguish between the theorem and the experiment. While the latter may be of interest in its own right, it is not what I am referring to, which is the theorem. In fact, the authors themselves describe it as a "proof-of-principle experiment."

Lucas.

 

[RUTA]

Immediately after the definition of NSD, the authors say:

"In the EWFS, this implies that c and d are independent of the choices x and y:
• NSD (in the EWFS and under Assumption 1): P(cd∣xy) = P(cd) ∀ c, d, x, y"

which means there are no hidden variables involved in this scenario.


As I mentioned in post #145, I don’t quite follow what you are trying to say here, given that the assumptions of the LF theorem are less restrictive than those of Bell’s theorem. This is discussed in Bong's paper, where the authors point out that, in addition to AOE, NSD, and L, Bell's theorem requires "outcome independence" as an additional assumption. Consequently, there are scenarios in which Bell's inequalities are violated, but LF is not. One well-known example is the EPRB experiment.


In the excerpt you cite from your paper, you discuss the experiment presented in Bong’s paper. It is important to distinguish between the theorem and the experiment. While the latter may be of interest in its own right, it is not what I am referring to, which is the theorem. In fact, the authors themselves describe it as a "proof-of-principle experiment."

Lucas.

We explain the LF inequality in the three equations just before the conclusion I sent you:

The reason we talk about Bong et al. is because it's an actual experiment. The gedanken version would require an actual observer be screened off (be kept from interacting with the environment of self-consistently shared classical information defining the experiment from Alice and Bob's perspectives), which introduces self-inconsistencies as we explained at the outset; essentially they want to have their cake and eat it too. The 'mystery' is that Alice and Bob's experimental outcomes dictate no definite path through the experiment in violation of the fact that Charlie and Debbie's experiments dictate a definite path through the experiment. But if Charlie and Debbie are screened off, how do we localize Charlie or Debbie to a definite spatial location along a path through the experiment? That requires they have commuting position and momentum meaning they weren't actually quantum (screened off) to begin with.

So, the bottom line is that what was actually done (and what can actually be done consistently with QM) violates the inequalities as we explained per QM, which violates L or SI for causal explanations. Ours is not a causal explanation, so we account for the violations of the inequalities without violating L or SI.

 

[Sambuco]

The reason we talk about Bong et al. is because it's an actual experiment. The gedanken version would require an actual observer be screened off (be kept from interacting with the environment of self-consistently shared classical information defining the experiment from Alice and Bob's perspectives), which introduces self-inconsistencies as we explained at the outset; essentially they want to have their cake and eat it too. The 'mystery' is that Alice and Bob's experimental outcomes dictate no definite path through the experiment in violation of the fact that Charlie and Debbie's experiments dictate a definite path through the experiment. But if Charlie and Debbie are screened off, how do we localize Charlie or Debbie to a definite spatial location along a path through the experiment? That requires they have commuting position and momentum meaning they weren't actually quantum (screened off) to begin with.

So, the bottom line is that what was actually done (and what can actually be done consistently with QM) violates the inequalities as we explained per QM, which violates L or SI for causal explanations. Ours is not a causal explanation, so we account for the violations of the inequalities without violating L or SI.

Okay, it's clear to me that your proposal/interpretation doesn't violate NSD or L. If you don't mind, I have another question regarding the gedanken version. In the paper, you say:

"Technically, one would have to keep Wigner's friend and his entire lab from interacting with the universe, e.g., no exchange of photons. That's certainly beyond anything we can do now, but more importantly, this means the classical information possessed by the friend with lab while screened off is not accessible to Wigner or anyone else in the universe. Since the universe is precisely all shared, self-consistent classical information, the friend's classical information while screened off, being unshared/unaccessible to everyone else, is not even part of the universe and therefore not a part of objective reality."

Upon reading this, I see you claim that there is "classical information" within the friend's laboratory, associated with a measurement he/she might have performed, but that, given the laboratory is screened off, this information is not part of an "objective reality". I am fully on board with that statement. The thing is, as I see it, that is precisely the violation of the AOE assumption, as defined in the Bong's paper and in subsequent works. On the one hand, that would be perfect for me, since I would then finally understand which assumption of the LF theorem your proposal/interpretation violates. However, I believe you hold a different view. Is that correct?

Lucas.

 

[RUTA]

Okay, it's clear to me that your proposal/interpretation doesn't violate NSD or L. If you don't mind, I have another question regarding the gedanken version. In the paper, you say:

"Technically, one would have to keep Wigner's friend and his entire lab from interacting with the universe, e.g., no exchange of photons. That's certainly beyond anything we can do now, but more importantly, this means the classical information possessed by the friend with lab while screened off is not accessible to Wigner or anyone else in the universe. Since the universe is precisely all shared, self-consistent classical information, the friend's classical information while screened off, being unshared/unaccessible to everyone else, is not even part of the universe and therefore not a part of objective reality."

Upon reading this, I see you claim that there is "classical information" within the friend's laboratory, associated with a measurement he/she might have performed, but that, given the laboratory is screened off, this information is not part of an "objective reality". I am fully on board with that statement. The thing is, as I see it, that is precisely the violation of the AOE assumption, as defined in the Bong's paper and in subsequent works. On the one hand, that would be perfect for me, since I would then finally understand which assumption of the LF theorem your proposal/interpretation violates. However, I believe you hold a different view. Is that correct?

Lucas.

We (actually, Bub's ref 49) have a specific computation starting with:

at the bottom of p. 18 and ending with:

at the bottom of p. 19. Again, the contradictions that arise in EWFS are due to the inconsistent use of the physics formalism. In this case, quanta that obey Hilbert space kinematics are physically/phenomenologically distinct from bodily objects that obey spacetime kinematics, e.g., position and momentum commute for the latter but not the former. In general, when you ascribe phenomenological characteristics of bodily objects to quanta and then use Hilbert space kinematics to predict outcomes, you will get contradictions and/or absurdities.

Another example of this mistake is Schrodinger's Cat (essentially a purported WF scenario). If the cat is in a quantum superposition of Live + Dead (it is a qubit, not a classical bit), then you must be able to specify the measurement complementary to the 'open-the-box' measurement whose outcome is Live + Dead with 100% probability. See my Insight on that: https://www.physicsforums.com/insights/schrodingers-cat-and-the-qbit/.

 

[PeterDonis]

quanta that obey Hilbert space kinematics are physically/phenomenologically distinct from bodily objects that obey spacetime kinematics

I think this gets at a key point in the QM interpretation debate: I don't think all QM interpretations would agree with this statement, but would instead say that the spacetime kinematics are emergent (somehow--of course nobody knows how and any interpretation that says this has to handwave about it) from the Hilbert space kinematics, and that there are not two physically distinct kinds of objects, but only one.

 

[RUTA]

I think this gets at a key point in the QM interpretation debate: I don't think all QM interpretations would agree with this statement, but would instead say that the spacetime kinematics are emergent (somehow--of course nobody knows how and any interpretation that says this has to handwave about it) from the Hilbert space kinematics, and that there are not two physically distinct kinds of objects, but only one.

I'm simply stating a phenomenological fact. If one imagines otherwise and uses that model to create a new formalism for new phenomena, that's a different story. As of now, we know which phenomena are described by Hilbert space kinematics and which are described by spacetime kinematics, and if you try to mix them, you get contradictions and absurdities.

 

[PeterDonis]

As of now, we know which phenomena are described by Hilbert space kinematics and which are described by spacetime kinematics

I'm not sure I agree. We don't make measurements in Hilbert space; we make them in spacetime. So every piece of evidence we have is evidence about events in spacetime, not somethings in Hilbert space. There's no way to distill out the spacetime and just have "Hilbert space kinematics" if you're going to look at the evidence, which you have to to make any claim about what is a "phenomenological fact".

 

[RUTA]

I'm not sure I agree. We don't make measurements in Hilbert space; we make them in spacetime. So every piece of evidence we have is evidence about events in spacetime, not somethings in Hilbert space. There's no way to distill out the spacetime and just have "Hilbert space kinematics" if you're going to look at the evidence, which you have to to make any claim about what is a "phenomenological fact".

Hilbert space kinematics describes quantum phenomena in spacetime. If you apply Hilbert space kinematics to what you otherwise consider to be a bodily object, e.g., a cat in a box or Debbie in her lab, then you get contradictions and/or absurdities. I call that a "phenomenological fact" because I'm not making any ontological assumptions as with an "interpretative fact."

 

[PeterDonis]

If you apply Hilbert space kinematics to what you otherwise consider to be a bodily object, e.g., a cat in a box or Debbie in her lab, then you get contradictions and/or absurdities.

I'm still not sure I agree with the phenomenological claim you are making here.

What about a buckyball going through a double slit? That shows quantum interference, but we can take a picture of the buckyball with an electron microscope. Is the buckyball a "bodily object"?

How about a superconducting magnet? That certainly seems to be a "bodily object", but superconductivity is an inherently quantum phenomenon. A SQUID, which also certainly seems to be a "bodily object", can be put in a state that is the exact analogue, in terms of magnetic flux, of the "Live + Dead with 100% probability" state you describe in connection with Schrodinger's Cat. [1]

Superfluidity is also an inherently quantum phenomenon. Is a beaker of superfluid liquid helium not a "bodily object"?

[1] https://arxiv.org/html/cond-mat/0004293v2

 

[RUTA]

I'm still not sure I agree with the phenomenological claim you are making here.

What about a buckyball going through a double slit? That shows quantum interference, but we can take a picture of the buckyball with an electron microscope. Is the buckyball a "bodily object"?

How about a superconducting magnet? That certainly seems to be a "bodily object", but superconductivity is an inherently quantum phenomenon. A SQUID, which also certainly seems to be a "bodily object", can be put in a state that is the exact analogue, in terms of magnetic flux, of the "Live + Dead with 100% probability" state you describe in connection with Schrodinger's Cat. [1]

Superfluidity is also an inherently quantum phenomenon. Is a beaker of superfluid liquid helium not a "bodily object"?

[1] https://arxiv.org/html/cond-mat/0004293v2

Whether something is a bodily object or a quantum is established by its context. A silver atom behaves as a qubit in the Stern-Gerlach experiment while a rubidium atom exchanging 100,000 photons per second with its environment of an atomic trap is a bodily object.

 

[PeterDonis]

Whether something is a bodily object or a quantum is established by its context.

So you would say that a SQUID in the experiment described in the paper I referenced is not a "bodily object" because of the context? But if I do a different experiment on it that doesn't involve the same kind of magnetic flux state, then it is a bodily object?

a rubidium atom exchanging 100,000 photons per second with its environment of an atomic trap is a bodily object.

But a collection of atoms with integral spin that are put into a trap and then cooled to form a Bose-Einstein condensate would not be bodily objects?

Of course you can define the term "bodily object" however you like. I'm just having trouble seeing the usefulness of the definition you appear to be using. The SQUID and the atoms don't know whether they're "bodily objects" or not by your definition, yet they are still able to figure out what to do.

 

[PeterDonis]

Whether something is a bodily object or a quantum is established by its context.

So photons going one at a time through a standard double slit apparatus would not be bodily objects, but if I put detectors at each slit that register the photons' passage, destroying the interference, then they would be bodily objects?

What if I put polarizers at each slit and then rotate them to tune how much interference there is? This experiment has been done, and you can continuously vary the interference from 100% to zero by changing the relative angle of the polarizers. If I set things up so there is 50% interference, are the photons 50% "bodily objects"?

 

[Sambuco]

Again, the contradictions that arise in EWFS are due to the inconsistent use of the physics formalism. In this case, quanta that obey Hilbert space kinematics are physically/phenomenologically distinct from bodily objects that obey spacetime kinematics, e.g., position and momentum commute for the latter but not the former.

Okay, now I think I understand your proposal. However, there is something I find problematic, so to speak. You argue that, since the friend's laboratory is screened off from the outside (the rest of the universe), the systems within the laboratory (including the friend) must be treated as quantum systems. I agree that this is the case from the perspective of Wigner, who is outside, but I do not see what physical principle can be invoked to justify the claim that Wigner's perspective is more fundamental than his friend's.

One possibility is to take the whole context into account, that is, whether Wigner applies a unitary operation to the laboratory that reverses the measurement made by the friend. Naturally, in doing so, the friend’s memories will be erased and, ultimately, when they meet, the friend will share the same perspective as Wigner. However, this does not mean that the friend could not have held a distinct perspective before that operation was carried out. As I stated above, I do not see what physical principle can be invoked to adopt such a position.

In general, when you ascribe phenomenological characteristics of bodily objects to quanta and then use Hilbert space kinematics to predict outcomes, you will get contradictions and/or absurdities.

Well, precisely, what allows one to avoid any contradiction is to reject AOE.

Lucas.

 

[RUTA]

Good questions! Here is my response. Careful, in order to get this posted today (because of other commitments) I've been working on this since 3am and it's about 6am now, so I'm on very little sleep.

Part 1: Paradox of Length Contraction
Physics tells me the following two facts are true when Alice and Bob are moving relative to each other:
1. Alice's measurements will show that Bob's meter sticks are shorter than hers.
2. Bob's measurements will show that Alice's meter sticks are shorter than his.
Those facts are contradictory per Galilean kinematics where spatial length is invariant, but they are both true in Minkowski kinematics because length is not invariant per the relativity of simultaneity. That means I have reduced my conundrum (unresolved contradiction) to a paradox (resolved contradiction). Physics does not predict, nor does reality instantiate, a contradiction in this case.

Part 2. Grandfather Paradox
GR has vacuum solutions with closed timelike curves. Suppose I go into my past on such a curve and kill my grandfather before my father was born. How can I have been born to kill my grandfather when I could not have been born because I killed my grandfather? Do I believe that physics really predicts a contradiction in this case? My response: Show me the stress-energy tensor for this GR solution. Einstein's equations have a stress-energy tensor on the RHS and when you are having me "suppose" a person is being sent along the curve of a vacuum solution, even if that person doesn't change the metric to within experimental limits, you have introduced a non-zero stress-energy tensor. The Schwarzschild vacuum solution predicts clocks in orbit around Earth will run faster or slower than clocks on Earth based on their altitude. We can (and have) checked this by placing clocks in orbit on the geodesics of the Schwarzschild vacuum solution, i.e., it is possible to construct the stress-energy tensor for the clock in that otherwise vacuum solution that doesn't alter the metric so as to confirm the GR prediction. Sorry, call me stupid, but I can't "suppose" someone being sent back in time on the curve of a vacuum solution leading to the grandfather paradox. In order to resolve the paradox (showing that physics doesn't predict a contradiction), I would need to see that stress-energy tensor.

Part 3. Entanglement Paradox
Mermin published a famous paper (referenced in video below) using his hypothetical "device" showing that QM predicts the following two facts about a pair of spin-entangled particles when their spins are measured in one of three directions in the plane perpendicular to the path from the source to the detector:
1. When the spins are measured in the same direction, the outcomes are always the same (1/2 up and 1/2 down).
2. When the spins are measured in different directions, the outcomes are the same in 1/4 of the trials (1/8 up and 1/8 down).
His paper then shows you why these two facts should strike you as contradictory per local causality and statistical independence. Nonetheless, QM predicts, and experiments show, that these two facts are both actually true. Does that mean physics predicts, and reality instantiates, a contradiction in this case? No, this conundrum (per Mermin) can be reduced to a paradox in total analogy to the paradox of length contraction, as I explain in this talk (references therein):

To reduce this conundrum to a paradox, we abandoned neither locality nor SI, but causality. Physics does not predict, nor does reality instantiate, a contradiction in this case.

Part 4. EWFS Paradox
Suppose Charlie and Debbie each conduct a quantum experiment in their labs on a qubit and we screen off them and their labs so as to treat them as qubits for Alice and Bob. QM predicts that it is possible for Alice and Bob's measurements of Charlie and Debbie's outcomes to contradict Charlie and Debbie's actual outcomes. Do I believe that physics really predicts a contradiction in this case? My response: Show me Alice's (or Bob's) experimental measurement configuration on Charlie (or Debbie) so I can see how it maps to the Hilbert space kinematics for a qubit rather than the spacetime kinematics for a classical bit. This is totally analogous to the claim that Schrodinger's Cat is in a quantum superposition of Live + Dead, i.e., that it's a qubit. Yes, it's possible to make very complex systems behave as qubits, e.g., silver atoms with their 47 electrons, 47 protons, and 60/62 neutrons in the SG experiment. There, if the atoms behave as bodily objects per Newtonian mechanics with its spacetime kinematics under the force of the SG magnetic field and I orient the SG magnets at angle theta relative to their +1 state in some spatial direction, I will obtain +1cos(theta) per the Newtonian projection of angular momentum. Instead, I get +1 and -1 outcomes that average +1cos(theta) per Hilbert space kinematics. According to Hilbert space kinematics for a qubit, as theta varies continuously in space I get a continously varying distribution of +1 and -1 outcomes at my detector from 100% +1 at theta = 0 to 50% +1 and 50% -1 outcomes at theta = 90 deg (complementary measurements). According to Newtonian mechanics for bodily objects in spacetime, there is only ever one outcome of +1cos(theta) that varies continuously from +1 at theta = 0 to 0 at theta = 90 deg. Sorry, call me stupid, but I can't "suppose" a cat (or Charlie or Debbie) being in a quantum superposition of Live + Dead (likewise for Charlie or Debbie's situation). In order to resolve the paradox (showing that physics doesn't predict a contradiction), I would need to see, at minimum, the complementary measurement to 'open the box' and its outcome of Live + Dead. Again, call me stupid, but simply referring to phenomena where QM works (and doesn't predict any contradictions) then telling me to "suppose" the cat or Charlie or Debbie are qubits doesn't help.

That's where we're at in this discussion.