I Problems with Paper on QM Foundations

[Sunil]

There's no need for $A^{\mu}$ to be "causal", because it's not observable. What must be causal is the gauge-invariant electromagnetic field $F_{\mu \nu}$ which always is causal ("retarded solutions"), no matter in which gauge you calculate the potentials $A^{\mu}$.

Fine that you accept this.

First of all, it follows that you accept that such non-observable things like potentials have a place in physics, even if they are non-observable. Field theory with $A^{\mu}$ is much much simpler than field theory with $F_{\mu \nu}$ given that there is not even a reasonable Lagrangian for the interaction with a Dirac field using the $F_{\mu \nu}$.

The more problematic question is about their reality. The $F_{\mu \nu}$, or whatever defines them, are real, given that they have observable consequences. But are the $A^{\mu}$ real? Positivists have problems with this. Realists not. They know and accept that realist theories are hypotheses and will remain hypotheses forever, that we cannot prove them, only falsify them. So, what is real is, as part of a realist theory, hypothetical too. Some criteria for preference are obvious: Simplicity, Ockham's razor. Giving reality to the $A^{\mu}$ gives much simpler realist theories than accepting only the $F_{\mu \nu}$ as real.

What are then the equations for the $A^{\mu}$? They have to explain the equations for the $F_{\mu \nu}$ which we can test in a much better way given observations. So we have simplicity and explanatory power as unproblematic guiding principles. If the $F_{\mu \nu}$ follow well-defined evolution equations with Lorentz symmetry, the simplest explanation would be that the $A^{\mu}$ follow such equations too. This would be $\square A^{\mu}=0$. Why would a realist consider some more complicate realist theory? Don't complicate things without necessity.

What is, therefore, behind the "no need for $A^{\mu}$ to be "causal"? It is good old empiricism - the wish to derive the theories of physics from observations. We obviously cannot derive the equation $\square A^{\mu}=0$ from observation, given that there are a lot of other imaginable equations for the $A^{\mu}$ which give the same Maxwell equations for $F_{\mu \nu}$. So, $\square A^{\mu}=0$ cannot be part of a theory based on empiricism.

 

[vanhees71]

Fine that you accept this.

Why shouldn't I accept the electromagnetic potentials?

First of all, it follows that you accept that such non-observable things like potentials have a place in physics, even if they are non-observable. Field theory with $A^{\mu}$ is much much simpler than field theory with $F_{\mu \nu}$ given that there is not even a reasonable Lagrangian for the interaction with a Dirac field using the $F_{\mu \nu}$.

I've never denied this.

The more problematic question is about their reality. The $F_{\mu \nu}$, or whatever defines them, are real, given that they have observable consequences. But are the $A^{\mu}$ real? Positivists have problems with this. Realists not. They know and accept that realist theories are hypotheses and will remain hypotheses forever, that we cannot prove them, only falsify them. So, what is real is, as part of a realist theory, hypothetical too. Some criteria for preference are obvious: Simplicity, Ockham's razor. Giving reality to the $A^{\mu}$ gives much simpler realist theories than accepting only the $F_{\mu \nu}$ as real.

I don't use the word "real" anymore in discussions about Q(F)T, because I don't know its meaning anymore.

For sure $A^{\mu}(x)$ do NOT represent observables. They don't obey the microcausality principle and are not gauge invariant. Even within classical electrodynamics they don't represent observables directly. All observables must be gauge invariant in both the classical as well as in the quantized theory.

What are then the equations for the $A^{\mu}$? They have to explain the equations for the $F_{\mu \nu}$ which we can test in a much better way given observations. So we have simplicity and explanatory power as unproblematic guiding principles. If the $F_{\mu \nu}$ follow well-defined evolution equations with Lorentz symmetry, the simplest explanation would be that the $A^{\mu}$ follow such equations too. This would be $\square A^{\mu}=0$. Why would a realist consider some more complicate realist theory? Don't complicate things without necessity.

The equations of motion for the $A^{\mu}$ are of course gauge-dependent too. The $F_{\mu \nu}$ as gauge invariant fields are indeed the natural building blocks for local observables (fulfilling also the microcausality condition).

What is, therefore, behind the "no need for $A^{\mu}$ to be "causal"? It is good old empiricism - the wish to derive the theories of physics from observations. We obviously cannot derive the equation $\square A^{\mu}=0$ from observation, given that there are a lot of other imaginable equations for the $A^{\mu}$ which give the same Maxwell equations for $F_{\mu \nu}$. So, $\square A^{\mu}=0$ cannot be part of a theory based on empiricism.

You need the $A^{\mu}$ to formulate QED as a manifestly local QFT. Particularly you need a four-vector to have the usual transformation properties under Poincare transformations.

 

[Sunil]

Why shouldn't I accept the electromagnetic potentials?

Read your own quotations given below.

I don't use the word "real" anymore in discussions about Q(F)T, because I don't know its meaning anymore.

It is quite easy: It is what a realist theory defines as the ontology. If it does not define an ontology, it is not a realist theory.

For sure $A^{\mu}(x)$ do NOT represent observables. They don't obey the microcausality principle and are not gauge invariant.

Hm. They are not observables, about reality you refuse to speak, so what they are? Sort of something magical like "imaginary numbers" in the mind of Musil's Törless? Not real, not observable, only imaginable or so?

In the Lorenz gauge they would follow a local equation $\square A^{\mu}=j^{\mu}$ classically, so I would expect that microcausality would not be a problem too, not?

The other problem is why are you so sure? Our QFTs are effective theories, and it follows essentially automatically that their symmetries may appear approximate symmetries. Approximate symmetries play an important role in QFT too, what is the problem of only approximately gauge invariant gauge fields?

The $F_{\mu \nu}$ as gauge invariant fields are indeed the natural building blocks for local observables (fulfilling also the microcausality condition).

Except that Bohm Aharonov shows that not all observables can be computed locally from the $F_{\mu \nu}$.

You need the $A^{\mu}$ to formulate QED as a manifestly local QFT. Particularly you need a four-vector to have the usual transformation properties under Poincare transformations.

Except that the theory with the $A^{\mu}$ in the standard BRST approach is not even a quantum theory, but a physically meaningless mathematical exercise in an indefinite Hilbert space.

 

[martinbn]

@Sunil What does it mean for $A^\mu$(or anything else) to be real?

 

[vanhees71]

Read your own quotations given below.

It is quite easy: It is what a realist theory defines as the ontology. If it does not define an ontology, it is not a realist theory.

I know, but all these words have no clear meaning. I want to discuss physics not gibberish.

Hm. They are not observables, about reality you refuse to speak, so what they are? Sort of something magical like "imaginary numbers" in the mind of Musil's Törless? Not real, not observable, only imaginable or so?

The electromagnetic potentials are calculational tools. They don't represent observables directly but enable you to define operators that represent observables. That's standard textbook QFT.

In the Lorenz gauge they would follow a local equation $\square A^{\mu}=j^{\mu}$ classically, so I would expect that microcausality would not be a problem too, not?

In classical physics microcausality doesn't make sense. Of course in the Lorenz gauge you get the wave equation for the potentials and you can choose the retarded potentials, and you can choose the retarded solutions. This alone doesn't imply that they are directly observable. They are not but the electromagnetic field $F_{\mu \nu}=\partial_{\mu} A_{\nu}-\partial_{\nu} A_{\mu}$ is.

The other problem is why are you so sure? Our QFTs are effective theories, and it follows essentially automatically that their symmetries may appear approximate symmetries. Approximate symmetries play an important role in QFT too, what is the problem of only approximately gauge invariant gauge fields?

If you break gauge invariance, no matter how weakly, the entire theory becomes inconsistent. A gauge symmetry (in contradistinction to global symmetries) means that you use more degrees of freedom in your mathematical description than physical degrees of freedom. If you break gauge invariance the non-physical degrees of freedom become interacting, and fundamental features of QFT as a physical theory are violated, including the unitarity of the S-matrix, microcausality, Poincare invariance etc.

Except that Bohm Aharonov shows that not all observables can be computed locally from the $F_{\mu \nu}$.Except that the theory with the $A^{\mu}$ in the standard BRST approach is not even a quantum theory, but a physically meaningless mathematical exercise in an indefinite Hilbert space.

I have no clue what you want to say with this statement.

 

[Fra]

Here you assume that relativity must be a fundamental principle obeyed by everything. But we don't know that with certainty. Maybe it's true, maybe it's not. So far experiments found no counterexample, but it doesn't imply that they never will. In principle it's possible that relativity is emergent rather than fundamental, but you don't accept that even as a possibility.

I am not advocating Bohmian mechanicsm in any way, but I do agree with this point.

This is an important point and a special case of the question of observer equivalence vs observer democracy. What may be fundamental, could be democracy - not equivalence. The equivalence may well be emergent only, is the sense of an evolutionary steady state. Allowing oneself to relax fundamental things, and instead focus on observer democracy may add explanatory value to why the effective equivalence is observed, but also why they aren't perfect. It's like a "noise" at the level of physical law, but it need not be a bad thing.

I even think that anyone that thinks about the logical arguements of which relativity builds, may find that the observer democracy condition is most certainly more obvious, but a weaker conditions than equivalence. The equivalence condition is handy, but may simply be wrong. (I obviously think it is, but one does not have to buy into something I think to at least see that it's a logical possibility that is perfectly rational and sound)

/Fredrik

 

[Sunil]

I know, but all these words have no clear meaning. I want to discuss physics not gibberish.

No, the meaning is very clear. A realistic theory has to define an ontology. If it has a Lagrange formalism, the ontology of the system is defined by the trajectory in the configuration space $q(t)\in Q$ of the theory, in the Hamilton formalism by the trajectory in the phase space. And so on. These are quite well-defined things. If philosophers talk about realism and reality in a theory-independent sense, the result may be confusion, but the same holds for everything they talk about.

The electromagnetic potentials are calculational tools.

In other words, they have no status at all. A "calculational tool" can be things with no relation to the theory at all.

If you break gauge invariance, no matter how weakly, the entire theory becomes inconsistent. A gauge symmetry (in contradistinction to global symmetries) means that you use more degrees of freedom in your mathematical description than physical degrees of freedom. If you break gauge invariance the non-physical degrees of freedom become interacting, and fundamental features of QFT as a physical theory are violated, including the unitarity of the S-matrix, microcausality, Poincare invariance etc.

No. The Gupta-Bleuler formalism, resp. BRST quantization, which starts with a physically nonsensical indefinite Hilbert space for the $A^\mu$ fields, breaks down. One would have to start with real fields $A^\mu$, and without exact gauge symmetry they would plausibly be observable. So, they should be handled in the mathematical formalism like usual observables. That means, they have to live in a well-defined (not indefinite) Hilbert space. This automatically clarifies problems with unitarity. You don't have to consider a factorization of some indefinite Hilbert space which may fail, but have to start from a definite Hilbert space. This gives some QFT. If gauge symmetry is approximate, then plausibly relativistic symmetry is approximate too, thus, causality will go back to classical causality in a preferred frame (which will be only approximately unobservable), and Poincare invariance will be only approximate too. No problem with QFT as a consistent physical theory.

I have no clue what you want to say with this statement.

What I mean with the reference to the Bohm-Aharonov effect should be clear even from Wiki-Level information https://en.wikipedia.org/wiki/Aharonov–Bohm_effect which claims "The Aharonov–Bohm effect shows that the local E and B fields do not contain full information about the electromagnetic field, and the electromagnetic four-potential, (Φ, A), must be used instead. By Stokes' theorem, the magnitude of the Aharonov–Bohm effect can be calculated using the electromagnetic fields alone, or using the four-potential alone. But when using just the electromagnetic fields, the effect depends on the field values in a region from which the test particle is excluded."

Where do you disagree with "Except that the theory with the $A^\mu$ in the standard BRST approach is not even a quantum theory, but a physically meaningless mathematical exercise in an indefinite Hilbert space."? In BRST, you start with an indefinite Hilbert space where the $A^\mu$ are defined, not? Together with the $A^\mu$, which is IYO only a "calculational tool", this part of the BRST approach can be nothing more serious than a "calculational tool" too. In particular, it cannot be a quantum theory, because Hilbert spaces in QT are always Hilbert spaces and have a definite scalar product, and they have to have it because negative probabilities make no sense.

 

[Sunil]

@Sunil What does it mean for $A^\mu$(or anything else) to be real?

To be described by a realistic theory which we can reasonably believe that it is true (that means, in particular, not falsified yet, but other criteria count here too) as part of the ontology of that theory.

In a theory with Lagrange formalism the ontology of the system is described by the trajectory in the configuration space Q $q(t)\in Q$. What actually exists is, then, $q(t_0)$ together with some (usually one is sufficient) derivatives $\dot{q}(t_0)$, the other parts of the trajectory define what existed at some time in the past and what will exist in some time in the future.

So, for the classical field theory, the configuration space trajectory is defined by $A^\mu(x,t)$. In a realist interpretation of quantum theory, like Bohmian field theory, it means that the Bohmian trajectory will be $A^\mu(x,t)$.

It is not necessary that the $A^\mu(x,t)$ are fundamental objects. Chairs are also real, that means only that they consist of things which are real.

 

[Fra]

For me the issue is solved with the results of all the high-precision Bell tests.

We have different perspectives, but IMO, Bells inequality is not the key anymore I think. Bells theorem is clear, but the ansatz is makes on equipartitioning has no place in how I understand things these days anway. It's good to kill of the original, old times objections to QM. But I see more modernt objections, where Bells theorem does not help.

But as Neumaier said, the main issue seems to be the point of view here. But I think from the perspectives of multiple observers, both perspective makes sense, but the confusion arises when one simply assymes that the perspectives ought to be equivalent, or that one is right and the other one is wrong.

If one instead embraces that different observers (agents, not humans!) makes their own independent inferences(given their respective resources of information processing), and that the difference inferences are correlated or dependeng depends to the extent of mutual interaction, then one can merge the two views. Ie. there can be, from one agents view, hidden information that one could call "real". But it is also true that this hidden information is not inferrable to all other agents, this is exactly what the external agent can NOT make the equipartion ansatz in Bells theorem, and make predictions based on averaging the hidden variable. This is because the hidden variable is not intrinsic to the external observer!

For me this is not a contradiction! This is paradoxally why I, while superficially at the opposite camp of Bohmian mechanics, connect and symphatize with Demystifiers solipsist HV idea. I keep bringing this up, as I think it's more fruitful to find commmon denominators where we can agree, than to focus on all the things we can argue forever in circles about.

So in a nutshell, I think what some call solipsist views, is paradoxally the best way to save "reality", while escaping the Bell argument, and without requiring other pathological mechanism that violates locality etc. The soluton is that reality is real, but hidden, but not in the naive sense of "ignorance" because this assume the interacting nearby agents are informed about each other, but in the sense of beeeing non-inferrable. Ie. it's not possible to construct the measurements required to get the answers. And in an agent view, agents actions are independent of such things, this is why the ansatz in bells theorem does not hold.

1A - is then wrong, just like the future is never COMPLETELY determined by the past. It never gets better than a best inference.

1B - This is problematic as one first has to by the same standards, analyse how the hamiltonian is inferred! We know how it's done for atomic physics, but try to do it for cosmology; or scale the theory of the lab down to the agent-perspective of a proton, and you should find the same principal problem.

1C - if one, as Demystifiers suggests, takes determiate to mean ontic, and we allow the ontolgoy to be agent-relative, then not all agents at all scales, CAN share the same ontology. But I take it to mean that the say, local or "nearby observer" is the ontologt that would represent some sortof naked ontologt. All other ontolgoies are screened with layers of more or less complex inferences.

/Fredrik

 

[vanhees71]

No, the meaning is very clear. A realistic theory has to define an ontology. If it has a Lagrange formalism, the ontology of the system is defined by the trajectory in the configuration space $q(t)\in Q$ of the theory, in the Hamilton formalism by the trajectory in the phase space. And so on. These are quite well-defined things. If philosophers talk about realism and reality in a theory-independent sense, the result may be confusion, but the same holds for everything they talk about.

Nobody has yet made clear to me, what a clear definition of "ontology" is. I don't care for philosophy. Physics is about well defined observable objective facts. There are no trajectories in phase space in quantum mechanics or quantum field theory.

In other words, they have no status at all. A "calculational tool" can be things with no relation to the theory at all.

No. The Gupta-Bleuler formalism, resp. BRST quantization, which starts with a physically nonsensical indefinite Hilbert space for the $A^\mu$ fields, breaks down. One would have to start with real fields $A^\mu$, and without exact gauge symmetry they would plausibly be observable. So, they should be handled in the mathematical formalism like usual observables. That means, they have to live in a well-defined (not indefinite) Hilbert space. This automatically clarifies problems with unitarity. You don't have to consider a factorization of some indefinite Hilbert space which may fail, but have to start from a definite Hilbert space. This gives some QFT. If gauge symmetry is approximate, then plausibly relativistic symmetry is approximate too, thus, causality will go back to classical causality in a preferred frame (which will be only approximately unobservable), and Poincare invariance will be only approximate too. No problem with QFT as a consistent physical theory.

These are all mathematical formulations of the theory, which also clearly defines what is representing observables, e.g., S-matrix elements in "vacuum QFT", thermodynamical quantities for different kinds of media with there different phases in many-body situations (quark-gluon plasma, neutron stars, usual matter around us), transport coefficients, etc.

If gauge symmetry is not exact, the model doesn't make physical sense. There can be approximate global symmetries like chiral symmetry of QCD in the light-quark sector, which make physical sense, but if a local gauge symmetry is broken in any way, it doesn't make any sense anymore. This has nothing to do with Poincare invariance. You can have, e.g., non-relativistic QED or QCD, which is an approximation to the relativistic theory which is consistent and provides well-defined descriptions of observables, but as soon as the underlying local gauge symmetry is broken, it doesn't make any sense anymore. If you know the BRST formalism or Gupta Bleuler for QED, then this should be immediately clear!

What I mean with the reference to the Bohm-Aharonov effect should be clear even from Wiki-Level information https://en.wikipedia.org/wiki/Aharonov–Bohm_effect which claims "The Aharonov–Bohm effect shows that the local E and B fields do not contain full information about the electromagnetic field, and the electromagnetic four-potential, (Φ, A), must be used instead. By Stokes' theorem, the magnitude of the Aharonov–Bohm effect can be calculated using the electromagnetic fields alone, or using the four-potential alone. But when using just the electromagnetic fields, the effect depends on the field values in a region from which the test particle is excluded."

That's wrong. What's observed in the AB effect is a non-integrable phase factor, which depends however on a gauge-invariant quantity only, i.e., the flux of the magnetic field through a surface intersecting the solenoid,
$$\Phi_B=\int_{A} \mathrm{d}^2 \vec{f} \cdot \vec{B} = \int_{\partial A} \mathrm{d} \vec{x} \cdot \vec{A}.$$
It's an integral over the vector potential along a closed path, and this is gauge invariant. There are no gauge-dependent observables. This was a contractio in adjecto!

Where do you disagree with "Except that the theory with the $A^\mu$ in the standard BRST approach is not even a quantum theory, but a physically meaningless mathematical exercise in an indefinite Hilbert space."? In BRST, you start with an indefinite Hilbert space where the $A^\mu$ are defined, not? Together with the $A^\mu$, which is IYO only a "calculational tool", this part of the BRST approach can be nothing more serious than a "calculational tool" too. In particular, it cannot be a quantum theory, because Hilbert spaces in QT are always Hilbert spaces and have a definite scalar product, and they have to have it because negative probabilities make no sense.

The entire point of the BRST approach is to evaluate well-defined observables in a well-defined Hilbert space from a gauge theory, which by definition deals with a description with contains non-physical unobservable elements in the description. You introduce ghost fields. For an un-higgsed gauge symmetry these are the Faddeev-Popov ghost fields taylored such that in a given gauge-fixing the contributions of unphysical degrees of freedom of the gauge potentials $A_{\mu}^a$ to observables quantities are always precisely canceled by the contributions from the ghost fields. That makes that each "gluon" is described by 4 vector-field degrees of freedom, of which two are unphysical, and the contributions of the two unphysical fields to observable quantities is canceled by a corresponding contribution from the correponding two also unphysical degrees of freedom of the ghost fields.

For a higgsed gauge symmetry in non-unitary (e.g., the manifestly renormalizable $R_{\xi}$ gauges invented by 't Hooft) you have in addition also unphysical "would-be-Goldstone fields". For each Higgsed subgroup of the gauge group you have in addition to the Faddev-Popov ghosts a corresponding would-be-Goldstone ghost which together with the unphysical degrees of freedom of the corresponding gauge potential conspire such that only the physical degrees of freedom of the gauge field contribute to the S-matrix, and these are three field-degrees of freedom corresponding to the three polarization states of a massive vector boson. Since the S-matrix is gauge invariant you can take the limit to the unitary gauge, which shows that there are no Goldstone modes for any higgsed local gauge symmetry but massive gauge bosons instead.

 

[Demystifier]

I am not advocating Bohmian mechanicsm in any way, but I do agree with this point.

This is an important point and a special case of the question of observer equivalence vs observer democracy. What may be fundamental, could be democracy - not equivalence. The equivalence may well be emergent only, is the sense of an evolutionary steady state. Allowing oneself to relax fundamental things, and instead focus on observer democracy may add explanatory value to why the effective equivalence is observed, but also why they aren't perfect. It's like a "noise" at the level of physical law, but it need not be a bad thing.

I even think that anyone that thinks about the logical arguements of which relativity builds, may find that the observer democracy condition is most certainly more obvious, but a weaker conditions than equivalence. The equivalence condition is handy, but may simply be wrong. (I obviously think it is, but one does not have to buy into something I think to at least see that it's a logical possibility that is perfectly rational and sound)

/Fredrik

What is observer democracy? That each observer has his own reality, not equivalent to the reality of other observers?

 

[Fra]

In other words, they have no status at all. A "calculational tool" can be things with no relation to the theory at all.

I think this is because we still do not understnad the theories deep enough. In a sense, we lack the physical understanding of the various calculations tools, because they tools are constructed in an extrinsic way, and they appear ad hoc; no matter how well corroborated. This gets more pressing one one tries to expand they theory an solve open questions. But I guess the ambition is that we could improve here.

But what if, we can translate the "calculations" down to the level of computations made by the observer/agent itself? Then the mathematics, should have a physical correspondence, which is automatically always "normalized" to the agents perpective. And unlike current theories, this renormalization must involves more variables than just spatial and energy scales, and it is not simply a statistical averaging to get macro variables.

This gets us a picture where the "theory" itself is not static, but itself evolving. Then one would have an ontology of agents, and a "inferred picture" of the environment which is regulated by the agents instrisic information processing capabilities. This would as i see it, unify the ontic and epistemological views. But "solving" the mathematical problem we get when formulating it like this, is not trivial, and it will in particular not take on the simple form of a differential equation, with a given state space and initial conditions, because forcing such a model would lead it seems to an infinitely complex model with a absurd fine tuning problem; that would render the theory imposible or any agent to actually implement. This "agent constraint" is what is the "natural regulator" when constructing the model.

/Fredrik

 

[Demystifier]

@Sunil What does it mean for $A^\mu$(or anything else) to be real?

Being real is a primitive notion, it cannot be defined in terms of something else. Somewhat like the notion of set in set theory. Which gives me an idea! The notion of set is not defined, but there are some axioms (ZF) that sets obey. Analogously, perhaps it's possible to make some list of axioms that any real object must satisfy, given a collection of measurable predictions. And then we can have various models that satisfy those axioms, which are nothing but various interpretations of the collection of measurable predictions. In particular, we can have standard and nonstandard interpretation of QM, just like we have standard and nonstandard interpretation of axioms for real numbers.

 

[Fra]

What is observer democracy? That each observer has his own reality, not equivalent to the reality of other observers?

As I always seen it the general constructing principle between not only relativity but also any gauge interaction, is that what any observer actually observes is just as valid description of reality as that of another one. This also includes that any inference of the apparent laws made by an observer, must be as valid as that of another observer. This is what i call the observer democracy.

Now, assuming that the various observers does not interact in a way that they significnatly influence each other or significantly modify the common environment, the natural next requirement is to expect that any observer should infer the SAME laws of physics, from their observations.

Then the invariants are formed from the equivalence class of all possible views. And one can talk about the equivalencec class or the invariants as the objective reality, all other things are gauged away as irrelevant observer choices.

This is a short description of key constructing principles of most of modern physics. And the arguments are hard to argue against, they are plausible and sound.

But if one allows for interaction of the observers, the game changes. We still have observer democracy, in the sense that no observer has priority of any other, but they may start to influence and interact with each other, and it's logically possible that two observers arrive at different views, where they can not even agree on their relations (which the equivalence implicit in symmetry transformations implies). In this case, instead of a timelesse symmetry transformation, we face a negotiation process where observers can literally and physically fight each other, and the result (after evolution time) may be an equilibiruym with a modified population of observers, that are simply TUNED for mutual equivalence.

I am just lifting the possibility, that this negotiation process, may be a KEY to understanding the laws and the hierarchy of interactions in nature. And in this view, one may need to see thgat elements of reality or ontologies may be subjective to the perspective, and at least in the transient sense, not necessarily related by perfect symmetry transformations as they don't exist on beforehand, they are the result of evolution.

/Fredrik

 

[Fra]

What is observer democracy? That each observer has his own reality, not equivalent to the reality of other observers?

Just to clarify. All observers are in the same world, there is only one universe in my view. (I'm not talking about MWI (which i never really got the real point of to be honest, it just seems weird and not solve any problems).

But each observer is it's own "inference-machinery" and has their own inferred, but real view of what the common world is like, that are inferred from real observations of what actually happens - from its perspetive.

But the differerent observers are not a priori assume to be related by symmetry transformations they can agree upon. It means as evolution forces agent-population changes, the symmetries are always approximate and not perfect, as they are evovling. There is also no non-physical "gods perspective" in thi, there are only the views of real physical material agents.

/Fredrik

 

[Sunil]

Nobody has yet made clear to me, what a clear definition of "ontology" is.

What's the problem? Ontology - that's the definition what, according to that

Physics is about well defined observable objective facts. There are no trajectories in phase space in quantum mechanics or quantum field theory.

Quantum theory is not a realist theory, but there exist realist interpretations, like Bohmian mechanics. These interpretations have trajectories in configuration space.

That's wrong. What's observed in the AB effect is a non-integrable phase factor, which depends however on a gauge-invariant quantity only, i.e., the flux of the magnetic field through a surface intersecting the solenoid,
$$\Phi_B=\int_{A} \mathrm{d}^2 \vec{f} \cdot \vec{B} = \int_{\partial A} \mathrm{d} \vec{x} \cdot \vec{A}.$$
It's an integral over the vector potential along a closed path, and this is gauge invariant. There are no gauge-dependent observables. This was a contractio in adjecto!

Do you read the text before you answer? Where I have claimed that there are gauge-dependent observables? The Wiki quote mentions the same thing as your formula.

The claim is different. You cannot compute this number using the $F_{\mu\nu}$ along the trajectory of the electrons.

If gauge symmetry is not exact, the model doesn't make physical sense. There can be approximate global symmetries like chiral symmetry of QCD in the light-quark sector, which make physical sense, but if a local gauge symmetry is broken in any way, it doesn't make any sense anymore.
This has nothing to do with Poincare invariance. ... but as soon as the underlying local gauge symmetry is broken, it doesn't make any sense anymore. If you know the BRST formalism or Gupta Bleuler for QED, then this should be immediately clear!

Again, do you read the text before you answer? Is it difficult to understand my "the Gupta-Bleuler formalism, resp. BRST quantization, which starts with a physically nonsensical indefinite Hilbert space for the fields, breaks down" which you have quoted.

The entire point of the BRST approach is to evaluate well-defined observables in a well-defined Hilbert space from a gauge theory, which by definition deals with a description with contains non-physical unobservable elements in the description.

Yes. Which obviously breaks down once the unobservable "calculational tools" become observable. Not because there is anything wrong with approximate gauge invariance, but because BRST uses an artificial indefinite metric for the $A^\mu$ which cannot make sense for observables.

 

[vanhees71]

I meant what's wrong is the claim "The Aharonov–Bohm effect shows that the local E and B fields do not contain full information about the electromagnetic field." Indeed in the AB experiment the particles don't have trajectories, and it's crucial that they don't have since the observable is a phase difference of the corresponding partial waves going the one or other way beyond the solenoid. The phase difference CAN be calculated with the observable field $\vec{B}$, and that's the important point. The observable (the shift of interference fringes when observing an ensemble of many particles) IS a gauge-invariant quantity although the "local formulation" needs the use of the gauge-dependent potential.

The BRST formalism does NOT use an ill-defined Hilbert space but constructs an adequate well-defined Hilbert space to quantize a gauge theory. Again: A gauge theory, for which gauge invariance is only "approximate" (a contradiction in itself) is not a well-defined theory at all. The entire BRST construction of the adequate Hilbert space, leading to a unitary S-matrix breaks down then.

 

[Sunil]

I meant what's wrong is the claim "The Aharonov–Bohm effect shows that the local E and B fields do not contain full information about the electromagnetic field." Indeed in the AB experiment the particles don't have trajectories, and it's crucial that they don't have since the observable is a phase difference of the corresponding partial waves going the one or other way beyond the solenoid. The phase difference CAN be calculated with the observable field $\vec{B}$, and that's the important point.

But the point was that this requires information about the $\vec{B}$ field inside the solenoid, where the electron is not. It is either on this trajectory or the other one - or nearby. Because of this point I have used the word "local".

The observable (the shift of interference fringes when observing an ensemble of many particles) IS a gauge-invariant quantity although the "local formulation" needs the use of the gauge-dependent potential.

No necessity to cry given that I have not questioned this.

The BRST formalism does NOT use an ill-defined Hilbert space but constructs an adequate well-defined Hilbert space to quantize a gauge theory.

Please read carefully what I write. Are you able to understand the difference between "physically nonsensical indefinite Hilbert space" and "ill-defined Hilbert space"? Of course, the indefinite Hilbert space is a well-defined mathematical object. It is physically nonsensical, because negative probabilities (however well-defined) are physically meaningless.

Again: A gauge theory, for which gauge invariance is only "approximate" (a contradiction in itself) is not a well-defined theory at all. The entire BRST construction of the adequate Hilbert space, leading to a unitary S-matrix breaks down then.

Again: There is no doubt that the BRST construction breaks down, no need to repeat this.

But this does not make a theory with approximate gauge invariance not well-defined. One has to use another approach to the quantization of such a theory, one which handles the $A^\mu(x)$ as observable quantum fields.

 

[vanhees71]

But the point was that this requires information about the $\vec{B}$ field inside the solenoid, where the electron is not. It is either on this trajectory or the other one - or nearby. Because of this point I have used the word "local".

It is indeed an astonishing topological feature of fields that you have information about them when knowing them only along a much smaller subset of their domain. One of the most simple cases are holomorphic functions, whose values you know everywhere in an area given the values along its boundary (Cauchy's theorem), but this indeed saves both, "locality", i.e., a local description of the interaction between charged particles and the electromagnetic field in terms of the gauge potential (and this must be so, because the em. field as a massless vector field must necessarily be described as a gauge field, as can be inferred from the representation theory of the proper orthochronous Poincare group a la Wigner 1939).

No necessity to cry given that I have not questioned this.

Please read carefully what I write. Are you able to understand the difference between "physically nonsensical indefinite Hilbert space" and "ill-defined Hilbert space"? Of course, the indefinite Hilbert space is a well-defined mathematical object. It is physically nonsensical, because negative probabilities (however well-defined) are physically meaningless.

I only wanted to emphasize that the BRST quantization formalism is necessary precisely because you need a well-defined Hilbert space and a unitary S-matrix to make physical sense of the entire construction.

Again: There is no doubt that the BRST construction breaks down, no need to repeat this.

But this does not make a theory with approximate gauge invariance not well-defined. One has to use another approach to the quantization of such a theory, one which handles the $A^\mu(x)$ as observable quantum fields.

Why are you then claiming again and again that a violation of gauge invariance. You are contradicting yourself with these two sentences, or can you quote a scientific paper where they successfully can interpret the gauge potentials as observable quantum fields?

 

[Sunil]

It is indeed an astonishing topological feature of fields that you have information about them when knowing them only along a much smaller subset of their domain. One of the most simple cases are holomorphic functions,

There are Hilbert spaces of holomorphic functions, but they have nothing to do with field theory.

I only wanted to emphasize that the BRST quantization formalism is necessary precisely because you need a well-defined Hilbert space and a unitary S-matrix to make physical sense of the entire construction.

And that's wrong. You can start from a vector field $A^\mu$ too.

There will be some problems with the implementation of the Lorenz gauge, but similar problems appear in condensed matter theories too if you have a continuity equation: If, say, the fundamental theory has exact particle conservation, and you want to have a field theory based on the density $\rho$ together with the continuity equation, it is not easy to reach that $\hat{\rho} \ge 0$ and that the conservation law is not even violated even by vacuum oscillations. Quantum condensed matter theory nonetheless works nicely.

Why are you then claiming again and again that a violation of gauge invariance. You are contradicting yourself with these two sentences, or can you quote a scientific paper where they successfully can interpret the gauge potentials as observable quantum fields?

Please quote me in a meaningful way. I do not claim "a violation of gauge invariance" but usually write complete sentences. Like the following: Theories with vector fields $A^\mu$ with some approximate gauge invariance are possible. They may be non-renormalizable, but this does not make them invalid as effective field theories. But these theories will certainly not use the Gupta-Bleuler resp. BRST approach, but start with a definite Hilbert space.

I only wanted to emphasize that the BRST quantization formalism is necessary precisely because you need a well-defined Hilbert space and a unitary S-matrix to make physical sense of the entire construction.

And that's simply wrong. You can make physical sense of vector fields if you start with a definite Hilbert space. Of course, you need exact gauge invariance to be able to factorize, and without factorization you cannot make physical sense of the whole construction build on the indefinite Hilbert space. But you are not at all obliged to start with some indefinite Hilbert space.

The original approach to QED did not use an indefinite Hilbert space. The references to the original approach:
Dirac P.A.M. (1927). The Quantum Theory of the Emission and Absorption of Radiation. Proc Roy Soc A114, 243-265
Fermi, E. (1932). Quantum Theory of Radiation. Rev Mod Phys 4(1), 87-132
But I would nonetheless recommend instead
Akhiezer, A.I., Berestetskii , V.B. (1965). Quantum Electrodynamics.
which give also the formulas for the original approach.

 

[vanhees71]

There are Hilbert spaces of holomorphic functions, but they have nothing to do with field theory.

You can take holomorphic functions as scalar fields (or their real and imaginary parts) in 2D Euclidean space. These are the "harmonic functions" with the properties described. It was only an example. Of course this mathematical phenomenon is more generally just Stokes's theorem for differential forms.

And that's wrong. You can start from a vector field $A^\mu$ too.

There will be some problems with the implementation of the Lorenz gauge, but similar problems appear in condensed matter theories too if you have a continuity equation: If, say, the fundamental theory has exact particle conservation, and you want to have a field theory based on the density $\rho$ together with the continuity equation, it is not easy to reach that $\hat{\rho} \ge 0$ and that the conservation law is not even violated even by vacuum oscillations. Quantum condensed matter theory nonetheless works nicely.

I'm talking about massless spin-1 fields in relativistic field theories. They are necessarily gauge fields, as can be derived from the representation theory of the Poincare group.

Please quote me in a meaningful way. I do not claim "a violation of gauge invariance" but usually write complete sentences. Like the following: Theories with vector fields $A^\mu$ with some approximate gauge invariance are possible. They may be non-renormalizable, but this does not make them invalid as effective field theories. But these theories will certainly not use the Gupta-Bleuler resp. BRST approach, but start with a definite Hilbert space.

You said repeatedly that local gauge symmetries can be approximate, but that's not true, because then they loose their physical meaning. This has nothing to do with (Dyson-) renormalizability or non-renormalizable effective theories.

And that's simply wrong. You can make physical sense of vector fields if you start with a definite Hilbert space. Of course, you need exact gauge invariance to be able to factorize, and without factorization you cannot make physical sense of the whole construction build on the indefinite Hilbert space. But you are not at all obliged to start with some indefinite Hilbert space.

No, massless vector fields must be necessarily quantized as gauge fields. For massive vector fields you are right.

The original approach to QED did not use an indefinite Hilbert space. The references to the original approach:
Dirac P.A.M. (1927). The Quantum Theory of the Emission and Absorption of Radiation. Proc Roy Soc A114, 243-265
Fermi, E. (1932). Quantum Theory of Radiation. Rev Mod Phys 4(1), 87-132
But I would nonetheless recommend instead
Akhiezer, A.I., Berestetskii , V.B. (1965). Quantum Electrodynamics.
which give also the formulas for the original approach.

Of course, in the early days they completely fixed the gauge before quantizing. That's another equivalent way to quantize the em. fieeld, which is even preferrable if you learn the subject for the first time. It's only disadvantage is that it is not manifestly covariant, which makes calculations of higher-order perturbative corrections a nightmare.

Nevertheless also there gauge invariance is needed to make physical sense of the theory. There's no way out: The math of the Poincare group tells you that a massless vector field must be a gauge field.

 

[Sunil]

I'm talking about massless spin-1 fields in relativistic field theories. They are necessarily gauge fields, as can be derived from the representation theory of the Poincare group.

Given that I do not claim that the result of the limit $m\to 0$ of massive vector fields will be something different in its observable predictions, I see no reason to care about such results. If that result tells us that both methods give, in the result, the same theory, fine. If you name them gauge fields or not does not matter.

No, massless vector fields must be necessarily quantized as gauge fields. For massive vector fields you are right.
...
You said repeatedly that local gauge symmetries can be approximate, but that's not true, because then they loose their physical meaning.

And here I simply disagree. AFAIU this is a misunderstanding based on the memories of time when people thought that non-renormalizable theories make no physical sense.

Take a naive discretization of a chiral lattice theory. It will not have exact gauge invariance on the lattice, but will be a well-defined theory. A finite number of degrees of freedom on a finite lattice (say on a large cube) and no infinities. Quantized in a straightforward canonical way. What will be the large distance limit?

Of course, in the early days they completely fixed the gauge before quantizing. That's another equivalent way to quantize the em. fieeld, which is even preferrable if you learn the subject for the first time. It's only disadvantage is that it is not manifestly covariant, which makes calculations of higher-order perturbative corrections a nightmare.

Ok, that's already much better.

I think that I have already said that I don't care about the mathematical tricks for approximate computations. You use "dimensional regularization" with physically completely meaningsless "dimensions" $4-\varepsilon$ in your renormalization? Fine, as long as you don't sell this as being something physically meaningful. You use an indefinite Hilbert space for computing your scattering coefficients? Fine, as long as you don't sell this as being something physically meaningful.

If this particular trick needs gauge invariance, it follows that you cannot apply this trick if there is no such exact gauge invariance, as for a naive chiral gauge field on the lattice. But this is a problem for computations, not for the meaningful definition of the theory itself.

There's no way out: The math of the Poincare group tells you that a massless vector field must be a gauge field.

And what makes the difference for me if I have a well defined theory with massive gauge fields with observable potentials and take the limit $m\to 0$? The result will be a gauge theory? Fine. What's the problem?
(BTW, the math of the Poincare group may be also irrelevant on the fundamental lattice level.)

 

[vanhees71]

Again. This has nothing to do with Dyson renormalizability or effective field theories with an infinite number of parameters ("low-energy coupling constants"). A gauge theory, for which the local gauge symmetry is broken has no physical interpretation, because it leads to acausality and a non-unitary S-matrix.

The limit $m \rightarrow 0$ for massive vector fields is non-trivial. You can describe the massive vector field as a Proca field. Then the limit is not well defined (as you can already see looking at the free propagator of this field, containing a piece $\propto p_{\mu} p_{\nu}/m^2$. Another possibility is to describe it as an Abelian gauge field, which is the Stueckelberg approach. Then the limit $m \rightarrow 0$ can be taken and leads to the usual gauge theory for a massless vector field. Again, whenever you deal with massless vector fields you end up with a gauge theory!

 

[Sunil]

You have not got the point that I'm not afraid of ending up with a gauge theory? My point is that this does not require that the $A^\mu$ should be handled as completely unphysical fields living in an indefinite Hilbert space of the Gupta-Bleuler resp. BRST approach.

One possibility would be the one you have accepted with your "in the early days they completely fixed the gauge before quantizing". Another one, canonical quantization on a lattice of a chiral gauge field which does not have exact lattice gauge symmetry on the lattice. It is nonetheless well-defined, unitary, and has some large distance continuous limit.

I'm quite happy if these examples all lead to the same gauge theory. In this case, the gauge potentials would be normal physical fields living in physical definite Hilbert space. Even if the operators measuring their values would not be observables because of gauge symmetry in the continuous limit, they would be handled with the same mathematics as usual observables. And there would be the BRST approach which would be preferable for computations because some manifestly covariant integrals are easier to compute.

 

[vanhees71]

You have not got the point that I'm not afraid of ending up with a gauge theory? My point is that this does not require that the $A^\mu$ should be handled as completely unphysical fields living in an indefinite Hilbert space of the Gupta-Bleuler resp. BRST approach.

I try one last time: There is no indefinite Hilbert space. The entire point of the covariant operator quantization of gauge theories (BRST) is that there is no such thing!

One possibility would be the one you have accepted with your "in the early days they completely fixed the gauge before quantizing". Another one, canonical quantization on a lattice of a chiral gauge field which does not have exact lattice gauge symmetry on the lattice. It is nonetheless well-defined, unitary, and has some large distance continuous limit.

If you quantize on a lattice it's by construction clear that there's nothing observable which is not gauge invariant.

I'm quite happy if these examples all lead to the same gauge theory. In this case, the gauge potentials would be normal physical fields living in physical definite Hilbert space. Even if the operators measuring their values would not be observables because of gauge symmetry in the continuous limit, they would be handled with the same mathematics as usual observables. And there would be the BRST approach which would be preferable for computations because some manifestly covariant integrals are easier to compute.

The values of the gauge fields cannot be observables, because the operators do not obey the microcausality condition.

 

[Sunil]

I try one last time: There is no indefinite Hilbert space. The entire point of the covariant operator quantization of gauge theories (BRST) is that there is no such thing!

Ok, then I give up.

"The scalar photons are treated by using an indefinite metric"
Gupta, S.N. (1950). Theory of longitudinal photons in quantum electrodynamics, Proc Phys Soc A 63(7), 681-691

"Gupta has introduced an alternative method of quantization for the Maxwell field which differs from the usual one in that the scalar part of the field is quantized by means of the indefinite metric of Dirac. It is shown that this method can be extended into a general and consistent theory, including the case of interaction with electrons."
Bleuler, K. (1950). Eine neue Methode zur Behandlung der longitudinalen und skalaren Photonen. Helvetica Physica Acta 23(V), 567-586

Of course, as I have written already many many times, at the end of the construction some definite Hilbert space is constructed. But this does not make the Hilbert space where the potential operators $A^\mu$ live definite.

If you quantize on a lattice it's by construction clear that there's nothing observable which is not gauge invariant.

No. There are gauge-invariant lattice theories, namely Wilson lattice gauge fields. But it works only for vector gauge fields. If you use a naive lattice approximation for chiral gauge fields then there will be no gauge invariance in the lattice theory. But this does not forbid you to define the lattice theory in a reasonable canonical way (without indefinite Hilbert spaces).

The values of the gauge fields cannot be observables, because the operators do not obey the microcausality condition.

In the Lorenz gauge the equations are Lorentz-covariant and classically Einstein-causality holds. So there is no base to assume that the corresponding quantum operators will not fulfill the microcausality condition.

Then, there is no base for assuming that a lattice regularization fulfills relativistic symmetry requirements. A lattice regularization is the most straightforward way to regularize quantum gravity at Planck scale and to obtain a regularized theory with a finite number of degrees of freedom which one can quantize canonically. To expect from such a lattice regularization relativistic symmetry would be strange.

 

[A. Neumaier]

In the Lorenz gauge the equations are Lorentz-covariant and classically Einstein-causality holds.

No. Without gauge fixing, the coupled Maxwell-Klein Gordon equations (the nearest classically to QED) are not hyperbolic, hence do not satisfy Einstein-causality.

 

[Sunil]

No. Without gauge fixing, the coupled Maxwell-Klein Gordon equations (the nearest classically to QED) are not hyperbolic, hence do not satisfy Einstein-causality.

$\square A^\mu = j^\mu$ not hyperbolic?

Then, what has your "without gauge fixing" to do with my "in the Lorenz gauge"? Are you about the remaining gauge freedom after the Lorenz gauge?

 

[A. Neumaier]

$\square A^\mu = j^\mu$ not hyperbolic?

The coupled equations must be hyperbolic. Without coupling you only have a free field.

 

[Sunil]

The coupled equations must be hyperbolic. Without coupling you only have a free field.

The point being? I have included a source term. I would ask you to show me which part of the coupled system makes it non-hyperbolic. Is the equation for the matter fields not hyperbolic?

 

[A. Neumaier]

The point being? I have included a source term. I would ask you to show me which part of the coupled system makes it non-hyperbolic. Is the equation for the matter fields not hyperbolic?

I think it is not the coupling to the scalar field (as I first thought) but the constraint $\partial_\mu A^\mu =0$ that spoils hyperbolicity. I don't remember the details and don't have the time now to check; thus maybe I am wrong.

 

[Sunil]

I think it is not the coupling to the scalar field (as I first thought) but the constraint $\partial_\mu A^\mu =0$ that spoils hyperbolicity.

Ok, this has at least some plausibility. To introduce conservation laws into a field theory is known to be problematic.

I know (memory, without good sources, sorry) that this creates some problems in condensed matter theory too. Take a particle theory with exact conservation law. Then you can use, in a large distance approximation, a classical field theory with fields $\rho(x,t)$ and the corresponding momentum $\pi^i(x,t) = \rho(x,t) v^i(x,t)$. Try to define a quantum field theory for these four fields such that it is the large distance limit of the fundamental theory. How to reach that $\hat{\rho}(x,t) \ge 0$ given that particle number in the fundamental theory is always positive? How to reach that $\partial_t \hat{\rho}(x,t) + \partial_i \hat{\pi}^i(x,t) = 0$ exactly, without even vacuum oscillations around zero, given that particle number is exactly conserved in the fundamental theory? AFAIR there are no good answers for this.

So it seems quite plausible that there are similarly no good answers how to preserve Einstein-covariance of a classical theory with such an exact conservation law during quantization.

 

[Demystifier]

I know (memory, without good sources, sorry)

Now @PeterDonis will come at you.

 

[zonde]

I was on you-tube and saw a video from Oxford on QM foundations. I didn't agree with it, but that is not an issue - I disagree with a lot of interpretational stuff. The video mentioned a paper they thought essential reading:
https://www.mathematik.uni-muenchen.de/~bohmmech/BohmHome/files/three_measurement_problems.pdf

Really nice idea in this paper. If you want to understand what are the problems with QM you have to investigate the areas where QM is weak and that is individual events.

Is it me, or has the author not shown the appropriate care? In particular, they claim theories that violate 1a are hidden variable theories. I thought - what - how does that follow. It may simply mean nature is fundamentally probabilistic. Or, in other words, the three assumptions are not inconsistent.

Specifically, I do not think the following is logically justified:
'And since we are interested in individual cats and detectors and electrons since it is a plain physical fact that some individual cats are alive and some dead, some individual detectors point to "UP" and some to "DOWN", a complete physics, which is able at least to describe and represent these physical facts, must have more to it than ensemble wave-functions.'

My response is - that might be your idea of what compete physics is, but it might be best not to assume that is everyone's idea of complete physics. Einstein, of course, thought QM incomplete, but I am not sure that is necessarily why.

Hmm, to me it seems rather clear just by reading 1A statement: "1.A The wave-function of a system is complete, i.e. the wave-function specifies (directly or indirectly) all of the physical properties of a system."
Complete/incomplete means whether wave-function is or is not best possible description of individual system. And if it's not then additional variables (HV) can improve description of individual system.
It's just a definition of a term. And it is related to history of QM so it does not appear out of the blue.

Anyways, I thought of slightly modified three statements after reading Maudlin's paper.
Given single preparation of quantum state and measurement of the state that gives two easily distinguishable groups of events, we can say that considering each event and taking into account that there are two possible outcomes at least one of the options is true:
1. Measurement event revealed something physical.
2. Measurement event created something physical.
3. Measurement event is not physical.

 

[stevendaryl]

The ensemble interpretation, analogous to the frequentist interpretation of probability (which has the issue that the law of large numbers is only valid for infinite 'experiments'), is a 'practical' interpretation that can't answer fundamental matters. However, it is used all the time in applications.

I don’t understand how the ensemble interpretation of QM is an interpretation.

In classical statistical mechanics, the meaning of an ensemble of systems is a collection of systems that are macroscopically identical but microscopically different.

But in quantum mechanics, an electron that is in a superposition of spin-up in the z-direction and spin-down in the z- direction cannot be interpreted in that way. If you have an ensemble of electrons in that state, it cannot be interpreted as some fraction of them being spin-up and some fraction being spin-down.

Maybe after a measurement of the spin in the z-direction, you can interpret it that way. But that seems to me equivalent to a “collapse” interpretation of measurement.

 

[A. Neumaier]

But in quantum mechanics, an electron that is in a superposition of spin-up in the z-direction and spin-down in the z- direction cannot be interpreted in that way. If you have an ensemble of electrons in that state, it cannot be interpreted as some fraction of them being spin-up and some fraction being spin-down.

In the ensemble interpretation, a superposition is supposed to describe an ensemble of electrons in the superposition, not an ensemble of electrons in one of two particular states.

 

[PeterDonis]

In classical statistical mechanics, the meaning of an ensemble of systems is a collection of systems that are macroscopically identical but microscopically different.

But in quantum mechanics, an electron that is in a superposition of spin-up in the z-direction and spin-down in the z- direction cannot be interpreted in that way. If you have an ensemble of electrons in that state, it cannot be interpreted as some fraction of them being spin-up and some fraction being spin-down.

In the QM ensemble interpretation as compared with classical statistical mechanics, "macroscopically the same" corresponds to "all prepared by the same process", and "microscopically different" corresponds to "will not all give the same result when measured in the same way". Whether or not this is a good enough correspondence to justify the term "ensemble interpretation" is a matter of choice of words; the interpretation itself is clear about what it's saying, and does not claim to be in exact correspondence with the classical statistical mechanics definition of an ensemble.

 

[zonde]

Anyways, I thought of slightly modified three statements after reading Maudlin's paper.
Given single preparation of quantum state and measurement of the state that gives two easily distinguishable groups of events, we can say that considering each event and taking into account that there are two possible outcomes at least one of the options is true:
1. Measurement event revealed something physical.
2. Measurement event created something physical.
3. Measurement event is not physical.

Speaking about option 3. which basically is about MWI. In Maudlin's paper objections to MWI are related to Born's rule. I would say that there is more serious philosophical problem.
MWI denies that my perception of certain consistent observations as objective physical facts is justified.

I will explain. I make an observation, say I look at a track in Wilson cloud chamber. Now I ask: am I seeing things or is this objective physical fact that there is a track in that cloud chamber?
So I ask somebody else to look at this track and ask him if he is seeing the same. But even if he agrees with me he could be influenced by unconscious bias say because he tends to consider more seriously the option that I am right. Or he just lies because it benefits him somehow.
So I ask him to do the experiment according to given instructions and to write down what he is seeing. Then to avoid possibility that my memory is cheating me I make a record myself and after he has made the experiment we compare our records.
If we do that and the records are consistent with each other I can say with great confidence that I am not seeing things and I should consider the track in cloud chamber as a objective physical fact.

So, what does MWI say about all that? It says I can't relay on all these things as a method of getting objective physical facts. Measurement records are not facts at all, they are just illusions of facts because some QM aspect of me is aligned with some QM aspect of measurement record. And communication with other person gives me consistent experience because some QM aspect of me is aligned with certain QM aspect of other person including this QM aspect of all the communication media between us.

So what I can do instead? Well, I should pretend that measurement records are objective facts and repeat to myself the magic word "decoherence" while remembering at the back of my mind that deep down it's a fake certainty.

How this approach is different from superdeterminism?

 

[zonde]

Speaking about option 1. it is actually quite simple - it contradicts observations.
It is obvious with quite simple three polarizers experiment:

 

[Morbert]

Speaking about option 1. it is actually quite simple - it contradicts observations.
It is obvious with quite simple three polarizers experiment:

For posterity:

Consistent Histories as described by Robert Griffiths would assert option 1, and would do so without recourse to hidden variables or similar extension to QM.

When I get a chance I can lay out how CH would approach this experiment.

 

[Morbert]

Call the first, middle, and last filters E G and F respectively.

A conventional description of the photon as is passes through the three filters would present these 4 possibilities*:

i) A photon is absorbed by filter E
ii) A photon passes through filter E and is forced into a vertical polarisation, and is absorbed by filter F
iii) A photon passes through the filter E and is forced into a vertical polarisation, passes through the filter F and is forced into a 45 degree polarisation, and is absorbed by the filter G
iv) A photon passes through the filter E and is forced into a vertical polarisation, passes through the filter F and is forced into a 45 degree polarisation, and passes through the filter G and is forced into a horizontal polarisation

We can see that, in these descriptions, the interaction with a filter is what creates the corresponding polarisation property.

QM let's us write down each of these possibilities as a string of time-ordered projectors. If the time of interaction for E G and F are t1, t2, and t3 then the possibilities can be written down as

i) $\left[E',F_0,G_0\right]_{t_1+\delta t}\odot I_{t_2 + \delta t}\odot I_{t_3+\delta t}$
ii) $\left[\uparrow,E,F_0,G_0\right]_{t_1+\delta t}\odot\left[E,F',G_0\right]_{t_2 + \delta t}\odot I_{t_3 + \delta t}$
iii) $\left[\uparrow,E,F_0,G_0\right]_{t_1+\delta t}\odot\left[\nearrow,E,F,G_0\right]_{t_2 + \delta t}\odot\left[E,F,G'\right]_{t_3+\delta t}$
iv)$\left[\uparrow,E,F_0,G_0\right]_{t_1+\delta t}\odot\left[\nearrow,E,F,G_0\right]_{t_2 + \delta t}\odot\left[\rightarrow,E,F,G\right]_{t_3+\delta t}$

QM will return probabilities for these possibilities no problem. But a consistent historian** would say you can move the polarisation projectors to before the filter interactions like so:
i) $\left[E',F_0,G_0\right]_{t_1+\delta t}\odot I_{t_2 + \delta t}\odot I_{t_3+\delta t}$
ii) $\left[\uparrow\right]_{t_1-\delta t}\odot\left[E,F_0,G_0\right]_{t_1+\delta t}\odot\left[E,F',G_0\right]_{t_2 + \delta t}\odot I_{t_3 + \delta t}$
iii) $\left[\uparrow\right]_{t_1-\delta t}\odot\left[E,F_0,G_0\right]_{t_1+\delta t}\odot\left[\nearrow\right]_{t_2-\delta t}\odot\left[E,F,G_0\right]_{t_2 + \delta t}\odot\left[E,F,G'\right]_{t_3+\delta t}$
iv)$\left[\uparrow\right]_{t_1-\delta t}\odot\left[E,F_0,G_0\right]_{t_1+\delta t}\odot\left[\nearrow\right]_{t_2-\delta t}\odot\left[E,F,G_0\right]_{t_2 + \delta t}\odot\left[\rightarrow\right]_{t_3-\delta t}\odot\left[ E,F,G\right]_{t_3+\delta t}$

These, according to CH, would correspond to the possibilities:

i) A photon is absorbed by filter E
ii) A photon passes through filter E already having a vertical polarisation and is absorbed by F.
iii) A photon passes through the filter E already having a vertical polarisation, passes through F already having a 45 degree polarisation, and is absorbed by the filter G
iv) A photon passes through the filter E already having a vertical polarisation, passes through F already having a 45 degree polarisation, and passes through G already having a horizontal polarisation.

and QM will still return consistent probabilities for these possibilities.

So what happens when you remove the filter F? You lose the ability to infer anything about 45 degree polarisation between filters E and G because the possibilities will no longer decohere, and QM will refuse to return consistent probabilities, analogous to the way that, in the double-slit experiment, paths that specify one slit or the other will not decohere when a detector is not present. It's not that removing the filter retroactively changes the polarisation of photons. It's that you lose the ability to address polarisations at certain times without certain filters in place.

* For expediency I've omitted possibilities that are consistent but return probability 0

** I should make it clear that Griffiths's CH account of measurements revealing pre-existing properties is not standard among all CH proponents.