Undergrad Are there signs that any Quantum Interpretation can be proved or disproved?

[gentzen]

Two questions:
1) ...
2) What would be a simple reasonable model for a screen? A 2D non-relativistic quantum field? That seems simple enough, but may be "too coherent".

My question for a simple model of a screen accidentally skipped an important intermediate step: The description of the "quantum-result" of the Stern-Gerlach experiment can be given without relying on any specific model of a screen: As a wavefunction (or a collection of wavefunctions with suitable positive weights summing to 1 if details of the thermal state of the source should be included in the model too) that still depends on spin and linear moment of the silver atoms in addition to their (x,y) position just above the surface of the screen. (The linear momentum includes both energy and direction. It will be strongly correlated with spin and (x,y) position as a result of the Stern-Gerlach "quantum-measurement setup".)

This description can be used as input for any model of a screen that finally turns it into a position measurement. Such a model could be a photographic plate with its finite grains corresponding to a discrete measurement, or some impenetrable barrier that stops the silver atoms near its surface so that they can be observed directly, corresponding to a continuous measurement.

 

[A. Neumaier]

What would be a simple reasonable model for a screen?

A 2D array of photosensitive pixels that can change their color through interaction with an incident particle.

 

[EPR]

Information is not real. The things the information is about are!

I am not sure I follow. Chasing this path leads to contradiction and confusion. The things you naively seem to define as real cannot withstand scrutiny... but it's okay to be a confused spirit. Most are anyway. I just don't see how you can maintain a coherent worldview like this. I have tried and at times get headaches.
Henry Stapp puts it nicely: “One generally finds that the evolved state of the system below the cut cannot be matched to any conceivable classical description of the properties visible to observers.”

 

[PeterDonis]

The things you naively seem to define as real cannot withstand scrutiny

As I noted in post #186, the term "real" is not a physics term, and should be avoided in this discussion.

I suspect that the "things information is about" that @A. Neumaier was referring to as "real" are things like clicks in Geiger counters, i.e., experimental data, and by "real" he simply meant that if we do not take the experimental data we have as given, we cannot do physics at all since we have no basis on which to test our models. Any attempt to go beyond that in defining what "real" means probably gets into philosophy and is hence off topic for this discussion.

 

[A. Neumaier]

I am not sure I follow. Chasing this path leads to contradiction and confusion.

No. Information about something that does not exist is fiction, not science.

 

[A. Neumaier]

I suspect that the "things information is about" that @A. Neumaier was referring to as "real" are things like clicks in Geiger counters, i.e., experimental data, and by "real" he simply meant that if we do not take the experimental data we have as given, we cannot do physics at all since we have no basis on which to test our models.

This is only part of what is real, i.e., of what exists. The Geiger counters, i.e., the quantum objects that make up the equipment used for experiments are as real. This uses 'real' in the everyday sense, without any sophisticated philosophy.

Those who claim reality of 'information about an electron' contained in a wave function are inconsistent if they don't treat the electron as being real and having real properties that we can sometimes have some information (know something) about.

 

[vanhees71]

I'm still not clear about, what your goal is to prove from the quantum formalism. You said it's not clear how there can be single well-defined outcomes when measuring something. If you want to prove something you have to say what you allow as assumptions (axioms) and what should be derived from these assumptions.

Since it seems to concern rather the interpretational part than the mathematical one I think there is not much to be done here since the fact that there are single well-defined outcomes when measuring some observable is simply taken as an empirical fact, and what the quantum-theoretical formalism provides is a theory to calculate the probability of those outcomes of measurements given a certain physical situation (encoded in the formalism by the notion of quantum states and by state preparation procedures in the lab). That QT only provides probabilities and not a deterministic description is also the result of observations, i.e., up to now we have not found any deterministic theory, which should be a hidden-variable theory of some kind such that the probabilities come in just as describing our ignorance as in classical statistical mechanics rather than being an irreducible fact of Nature as the orthodox interpretation of QT (particularly the minimal statistical interpretation) assumes. What Bell's theorem and it's empirical investigation shows is that such a determinsitic HV theory would not be one with local interactions (local in the sense of the standard relativistic QFTs), and so far there has no convincing nonlocal relativistic deterministic theory been found. There's also no empiricial hint whatsoever for the need of such a theory since all observations are consistent with standard QTs predictions.

So what do you think has to be "proven" with regard to definite outcomes when measuring, and which assumptions would you accept for such a proof?

 

[A. Neumaier]

the fact that there are single well-defined outcomes when measuring some observable is simply taken as an empirical fact

Since measurement takes a huge variety of concrete manifestations, this empirical fact about certain complex arrangements of atoms should (as all other empirical facts) be provable from the fact that measurement devices are quantum objects.

Instead, the notion of measurement is in the foundations of the statistical interpretation.

What Bell's theorem and it's empirical investigation shows is that such a determinsitic HV theory would not be one with local interactions (local in the sense of the standard relativistic QFTs),

This is neither implied by Bell's theorem nor by any empirical investigations. The results of Bell-type experiments exclude noncontextual local observables propagating according to classical dynamics of local hidden variables. Nonlocal hidden variables or nonlocal observables are not covered by Bell's assumptions.

So what do you think has to be "proven" with regard to definite outcomes when measuring, and which assumptions would you accept for such a proof?

Why a large quantum system interacting with a spin has definite outcomes even though the unitary dynamics produces a nonthermal state. More specifically:

Since the state is the maximum that can be known about a system, and since the definite outcome is definitely known, any objective macroscopic property of the quantum system (including the definite outcome) must be somehow encoded in the state of the system. Thus one has to be able to describe in mathematical detail a notion of state such that, and how,

1. the outcome is determined by the state of the system., and
2. the unitary dynamics of detector + spin (+ whatever else is needed) ensures that, from a well-defined initial state of this combined system, one arrives at this definite outcome.

One can assume some stochasticity to simplify the complexity of the system (with the same kind of arguments as in Brownian motion or classical statistical mechanics).

But one cannot assume anything about measurement, since the latter should be a consequence of the structure of the measurement device.

 

[Jarvis323]

How you can build solid philosphical foundations of QM without using philosophy is only a question that philosophy can address.

Using pseudo-philosophy in place or real philosophy is not the answer in my opinion.

 

[gentzen]

How you can build solid philosphical foundations of QM without using philosophy is only a question that philosophy can address.

A. Neumaier also uses philosophy. The main part of my review of his paper Foundations of quantum physics II. The thermal interpretation begins:

The presentation is easy to read, and contains many remarks and observations that are spot on both practically and philosophically.

However, this is a physics forum, so he focuses on the physics here.

 

[PeterDonis]

This is only part of what is real, i.e., of what exists.

Agreed. I was actually hoping you would expand on what I wrote, so thanks!

Those who claim reality of 'information about an electron' contained in a wave function are inconsistent if they don't treat the electron as being real and having real properties that we can sometimes have some information (know something) about.

Agreed. It would also be consistent to say that electrons themselves are not real, only observations like dots on a detector screen are, and that the information in the wave function is about those observations.

 

[A. Neumaier]

It would also be consistent to say that electrons themselves are not real, only observations like dots on a detector screen are, and that the information in the wave function is about those observations.

Yes. But something real goes somehow from the source to the detector. Nobody would be interested in the dots on a detector if they wouldn't give information about this something. Thus this something (whether called electrons or electron field) must be real and have real properties.

 

[PeterDonis]

something real goes somehow from the source to the detector

I'm not sure how one could justify this claim experimentally, except in the obvious special case where we continuously measure something in between. In any other case, this claim, however plausible it seems, cannot, I think, be taken as a necessary axiom that any interpretation of QM (or physical theories in general) must include. It can, of course, be taken as an axiom for a particular interpretation if the person building the interpretation wants to (I assume, for example, that you would take it as an axiom in your thermal interpretation).

 

[A. Neumaier]

I'm not sure how one could justify this claim experimentally

Just point the source in a direction different from the detector, and you don't get a response, while pointing it towards it gives a response.

Of course it is not a watertight proof since one can assume action on distance, but I think no physicist in his right senses would explain it that way.

In any case, it is what experimenters have to assume when they design their experiments; without this none of the Bell-type experiments would make sense.

 

[PeterDonis]

Just point the source in a direction different from the detector, and you don't get a response, while pointing it towards it gives a response.

To be clear, the claim I am saying can't be justified experimentally is not "there is some kind of physical effect between the source and the detector", but the narrower claim that "something real goes somehow from the source to the detector", which I am reading as claiming that something real, in the same sense as the source and the detector are real, has to travel continuously through the intervening space (or spacetime if we are using a relativistic interpretation).

 

[A. Neumaier]

"something real goes somehow from the source to the detector", which I am reading as claiming that something real, in the same sense as the source and the detector are real, has to travel continuously through the intervening space (or spacetime if we are using a relativistic interpretation).

It can be demonstrated experimentally that, even when source and detector are far away, the source can reliably send signals that the detector responds to. These signals are surely as real as anything we consider real, though they are not material. For everything we consider real is known to us only through such signals.

What cannot be demonstrated experimentally is that there is a medium carrying these signals. But physics has names for these media - particles and fields. It would be very strange to teach physics students mandatory courses whose subject matter are nonexisting things.

 

[PeterDonis]

What cannot be demonstrated experimentally is that there is a medium carrying these signals.

If this is equivalent to saying that it can't be demonstrated experimentally that the signals are there between the source and the detector (since by definition there are no measurements being made between them), then you are agreeing with me.

If you mean something else by this, please elucidate.

 

[A. Neumaier]

If this is equivalent to saying that it can't be demonstrated experimentally that the signals are there between the source and the detector

The signals can be measured everywhere we place a detector (in the direction of emission). Thus we know experimentally that what is emitted is detectable anywhere on its path of transmission.

How to interpret this is of course beyond experimental capabilities. But classically we know of bullets or magnetic fields or light only through their manifestations when measured, and we conclude without doubt that wherever something is measured or could be measured it is present, unless we have strong reasons to think otherwise.

Thus our notion of reality is based on assuming presence through the possibility of measuring it - no matter whether we actually do the measurement. Why this should not always count as a sufficient argument for reality needs justification.

 

[vanhees71]

Since measurement takes a huge variety of concrete manifestations, this empirical fact about certain complex arrangements of atoms should (as all other empirical facts) be provable from the fact that measurement devices are quantum objects.

Instead, the notion of measurement is in the foundations of the statistical interpretation.

This is neither implied by Bell's theorem nor by any empirical investigations. The results of Bell-type experiments exclude noncontextual local observables propagating according to classical dynamics of local hidden variables. Nonlocal hidden variables or nonlocal observables are not covered by Bell's assumptions.Why a large quantum system interacting with a spin has definite outcomes even though the unitary dynamics produces a nonthermal state. More specifically:

Since the state is the maximum that can be known about a system, and since the definite outcome is definitely known, any objective macroscopic property of the quantum system (including the definite outcome) must be somehow encoded in the state of the system. Thus one has to be able to describe in mathematical detail a notion of state such that, and how,

1. the outcome is determined by the state of the system., and
2. the unitary dynamics of detector + spin (+ whatever else is needed) ensures that, from a well-defined initial state of this combined system, one arrives at this definite outcome.

One can assume some stochasticity to simplify the complexity of the system (with the same kind of arguments as in Brownian motion or classical statistical mechanics).

But one cannot assume anything about measurement, since the latter should be a consequence of the structure of the measurement device.

Open quantum systems, and measurement devices are always open quantum systems, are never described by unitary time evolution but by a coarse-grained effective description of macroscopic observables. There are many approaches to this, among them the projection-operator formalism (Zwanzig et al), Lindblad equations for reduced stat. ops., quantum Langevin approaches, the influence functional, Kadanoff-Baym.

 

[A. Neumaier]

Open quantum systems, and measurement devices are always open quantum systems, are never described by unitary time evolution but by a coarse-grained effective description of macroscopic observables.

If one includes enough of the environment they can be treated as closed systems. Indeed, a very traditional way to do the coarse graining is to start from a unitary evolution and to derive the coarse-grained dynamics of a subsystem.

There are many approaches to this, among them the projection-operator formalism (Zwanzig et al), Lindblad equations for reduced stat. ops., quantum Langevin approaches, the influence functional, Kadanoff-Baym.

All of these start with a unitary dynamics of a more detailed description to obtain the coarse grained dynamics using suitable approximations.

So what do you think has to be "proven" with regard to definite outcomes when measuring, and which assumptions would you accept for such a proof?

In view of the above, the tasks are:

1. to produce a coarse-grained dynamics for spin+detector from the unitary evolution of a bigger system,
2. to identify in the resulting model for the open system spin+detector macroscopic observables describing the pointer,
3. to prove that the coarse-grained dynamics leads to a unique pointer result, a result depending upon the initial spin state in the way predicted by Born's rule.

 

[vanhees71]

It is a priori clear that you cannot understand the classical behavior of the measurement device using only unitary time evolution. Thus, what you aim at is well-known to be impossible to be achieved.

You cannot even in classical mechanics understand the thermodynamics of an ideal gas from solving the full microscopic equation of motion (Liouville equation).

Your points 1.-3. are indeed the true goal, and I think this has been achieved by all the work on "decoherence" in the past 2-3 decades.

 

[A. Neumaier]

Your points 1.-3. are indeed the true goal, and I think this has been achieved by all the work on "decoherence" in the past 2-3 decades.

No. Decoherence does not contribute to the unique outcome problem - it fails on point 3 of my list of requirements. Schlosshauer, the most competent physicist on decoherence, is very explicit about this, and I agree with him.

 

[vanhees71]

Ok, then what's wrong with the model treatments for position mesurements in

Joos et al, Decoherence and the Appearance of a Classical World in Quantum Theory ?

 

[EPR]

So, even though all 'stuff' is quantum mechanical in nature and there is nothing in the evolution of the quantum system that mandates to bring about single outcomes, there must be something non-quantum that destroys superpositions and renders them into definite outcomes.
Some assume that something to be the very thing that we are trying to explain(some other 'classical' stuff, macroscopic detectors, detector plates, etc). But this is circular reasoning.

If we rehash the 3 points:

1. Everything we have ever come across turns out to be quantum in nature.
2. Nothing in the formalism dictates that quantum systems must take on definite values under certain circumstances. Yet they do.
3. Whenever we act to observe, explore, see, know, inquire in any way about it, the quantum system acts as if it were in fact classical.
4. Based on point 2 and 3, we can draw the conclusion that quantumness cannot be destroyed and formatted into single outcomes on its own by other quantum systems.

So, the only conclusion left is that whatever takes place during measurement/observation is not due to anything quantum in nature. And since obviously everything is in fact quantum in nature, the next obvious conclusion is that the reason for the definite outcomes is not and cannot be quantum in nature. I.e. cannot be something from our familiar, observable and measurable environment.
There's very few options left to pursue from this point on. So a dedicated scientist might as well say the goal of science is to see what we can say about our observations(nature) and not to explain what and esp. why it happens.
I.e. this question cannot be solved in scientific terms. We are unable to explain our observations from within the realm of our observations because essentially it takes more than what we can identify within the realm of the observable for the same observable to work and even exist and be measurable.

 

[A. Neumaier]

Ok, then what's wrong with the model treatments for position mesurements in

Joos et al, Decoherence and the Appearance of a Classical World in Quantum Theory ?

Decoherence produces decay of phase information but does not explain how a single measurement results. It produces an improper diagonal density matrix instead of one where position has a fixed value. The improper mixture is not resolved into single results.

 

[vanhees71]

Ok, then the goal you have in mind is not to reach within standard quantum mechanics, i.e., one has to extent quantum mechanics, i.e., something like the GRW theory with "explicit collapse". I always considered it sufficient to prove precisely that you get what you call an "improper diagonal density matrix", i.e., the probability distribution for the position (taken as a "pointer observable"). Since this is anyway all we can expect to measure and to predict with our theories, for me that's sufficient.

 

[A. Neumaier]

then the goal you have in mind is not to reach within standard quantum mechanics

No; since nobody proved the impossibility of goal 3, it is still an open problem,.

It is only clear that this goal cannot be reached within the minimal statistical interpretation alone, since the only contact of the latter to reality is Born's rule, which presupposes unique outcomes for large quantum devices in nonthermal states.

I always considered it sufficient to prove precisely that you get what you call an "improper diagonal density matrix", i.e., the probability distribution for the position (taken as a "pointer observable"). Since this is anyway all we can expect to measure and to predict with our theories, for me that's sufficient.

This amounts to taking the existence of unique macroscopic outcomes as an additional axiom in addition to the statistical interpretation.

 

[gentzen]

Ok, then the goal you have in mind is not to reach within standard quantum mechanics, i.e., one has to extent quantum mechanics, i.e., something like the GRW theory with "explicit collapse".

Based on what the proponents of consistent histories wrote about related matters, my guess is that one cannot disprove Many-Worlds (within standard quantum mechanics), but that one cannot prove it either, nor can one prove the need for something like GRW.

Here is an extract from a quote from “Quantum Philosophy” by Roland Omnès:

And R. Omnès is also pretty clear that he is convinced that there is only a single world, even if the “consistent histories” doesn’t explain it, and cannot even disprove Many-Worlds. His defense is to admit that there is still a disagreement left between Reality and quantum theory, but that it would be hubris to expect otherwise. At least that is how I interpret the words on page 214:

… have reproached quantum physics for not explaining the existence of a unique state of events. It is true that quantum theory does not offer any mechanism or suggestion in that respect. This is, they say, the indelible sign of a flaw in the theory, … Those critics wish at all costs to see the universe conform to a mathematical law, down to the minutest details, and they certainly have reason to be frustrated.
…
I embrace, almost with prostration, the opposite thesis, the one proclaiming how marvelous, how wonderful it is to see the efforts of human beings to understand reality produce a theory fitting it so closely that they only disagree at ...

What I guess is possible for A. Neumaier to achieve is to prove that the existence of a single world with a single quantum state is consistent both with standard quantum mechanics and with the fact that there are single well-defined outcomes when measuring some observable.

 

[PeterDonis]

even though all 'stuff' is quantum mechanical in nature and there is nothing in the evolution of the quantum system that mandates to bring about single outcomes, there must be something non-quantum that destroys superpositions and renders them into definite outcomes.

Not if you adopt the many-worlds interpretation.

 

[EPR]

Not if you adopt the many-worlds interpretation.

Yes. The variant of MWI where wavefunctions are somehow real, yet they extend throughout spacetime.