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

[A. Neumaier]

What above and similar types of reasoning miss completely is that pointer states are ensembles with huge numbers of states that are exponential in $N=10^{20}$.

Nonsense. A pointer state is a single state, not an ensemble of states. For the latter you need an ensemble of pointers. But experiments are made with single pointers.

This is the source of exponentially small overlap of classical histories of collective coordinates. Now, Banks's book (Chapter 10) has not one but three different arguments to show the exponentially small overlap.

This misses the point. The tiny overlap of nearly classical macroscopic states of a classical pointer has nothing to do with the huge uncertainty of the nonclassical macroscopic states of the pointer that arise from an assumed unitary dynamics of a detector for a spin variable. Thus Banks arguments in Chapter 10 contribute nothing to the explanation of the measurement process.

The sloppiness of Bank's arguments in general, and the resulting low quality, can also be seen from other strange assertions that he makes, full of confidence in his magical powers of reasoning by pure assertion:

This violates only one rule of classical logic: The Law of the Excluded Middle. That law takes as the definition a state that one cannot be in two states simultaneously. Ultimately, like any other law in a scientific theory, the Law of the Excluded Middle must be tested by experiment, and it fails decisively for experiments performed on microscopic systems.

A microscopic system is never in two different states simultaneously. Being in a superposition of classical states is a very different property, which does not contradict the law of excluded middle. The law of excluded middle (not not A =A) is perfectly valid in quantum logic, with a trivial proof: $1-(1-P_A)=P_A$.

Instead, what fails decisively in quantum logic is that one cannot define a meaningful notion of implication with the properties needed for logical reasoning. Indeed, physically relevant reasoning in quantum physics has always been done exclusively with classical logic (including the law of excluded middle), not with quantum logic.

experimental results are quoted with “error bars.” This means that the results of any experiment are themselves given by a probability
distribution.

Not at all. Error bars only mean that the results of experiments are given with an indication of uncertainty. This is far from giving a probability distribution, which cannot be reliably given in many circumstances. It is usually non-Gaussian and hence unknown, unless a huge number of experiments are evaluated together.

Our insistence that there are only a finite number of states means that we can only contemplate discrete time evolution

Banks admits here that his powers of contemplation are far too little developed to cope correctly with quantum phenomena. Clearly, Banks has never heard of shot noise, a 2-state stochastic process in continuous time common in quantum experiments.

A source that makes without hesitation several such strange assertions in the first few pages of the book - a chapter titled ''What you will learn from this book'' - cannot be taken seriously. One can be sure that one learns a lot of wrong things, alongside the standard material - without any guidance of how to separate the wheat from the chaff.

 

[PeterDonis]

you used the word "reality" which is a heavy loaded term. Given that we are in the "quantum interpretation" branch of the forum, you may want to define what you mean by that, before making conclusions about other people's positions.

I didn't mean to attribute my own personal meaning to the term "reality"; as I have already explained, I meant it to mean "whatever Banks is asserting in the given quote". Again, if you don't like the word "reality" to describe what Banks is asserting, feel free to substitute some other word; my argument was based on the substance of what Banks said in the quote, not on the words I used to describe it.

I suggest you read Banks's paper itself first

Doing that now (thanks for the reference).

 

[PeterDonis]

I suggest you read Banks's paper itself first, so you know in what context that quote was given and what are actual conclusions that he draws from the smallness of the above discussed overlap

Ok, having read the paper, the conclusion he draws is that it's ok to just use the classical calculation since the exponentially small interference terms in the quantum calculation make it "in principle" (his phrase) the same as the classical calculation.

That conclusion is wrong. The quantum calculation does not just give interference terms that are exponentially small; even if we ignore the interference terms, it gives a superposition of multiple classical results. But the classical calculation only gives one classical result. So the two are not the same even if we ignore the exponentially small interference terms.

(Btw, I'm assuming for the sake of argument that Banks is correct that the interference terms are actually exponentially small. I have not checked his argument for that since it doesn't really matter for what I have said above.)

I will agree that my previous post did not correctly capture what Banks was actually saying in the quote I referenced earlier. However, that doesn't really help you here, since, when correctly captured, as above, Banks's claim is wrong.

 

[physicsworks]

Nonsense. A pointer state is a single state, not an ensemble of states. For the latter you need an ensemble of pointers. But experiments are made with single pointers.

Nonsense. There is a huge number of linearly independent states that have the same expectation value for the position of the needle on a macroscopic apparatus. Also, you grossly misused the word ensemble in the quote. Furthermore, you don't control microscopic changes in the apparatus, so there is an exponentially vanishing chance for you to start with the same pointer state every time you measure something in a long string of repeated measurements of "identically" prepared systems, which is the only way to check QM predictions.

the conclusion he draws is that it's ok to just use the classical calculation

I don't agree that this is his conclusion, but I'm afraid I'm tired of arguing about what other people say.

 

[A. Neumaier]

There is a huge number of linearly independent states that have the same expectation value for the position of the needle on a macroscopic apparatus.

This does not matter. A single pointer is at any time in a single of these states. The collective position operator is an average over microscopic position operators. But operators are not states, so this average does not make the pointer an ensemble.

If you calculate the expectation value of the pointer position in a nonclassical superposition of classical states you get a value far from the measured values, though consistent with the huge uncertainty.

 

[A. Neumaier]

I'm afraid I'm tired of arguing about what other people say.

You made it your argument by quoting it. So you are tired of defending your own arguments...

 

[PeterDonis]

I don't agree that this his conclusion

From p. 6 of the paper:

For me, these considerations resolve all the angst associated with the Schrodinger's cat paradox. Figurative superpositions of live and dead cats occur every day, whenever a macroscopic event is triggered by a micro-event. We see nothing remarkable about them because quantum mechanics makes no remarkable predictions about them. It never says "the cat is both alive and dead", but rather, "I can't predict whether the cat is alive or dead, only the probability that you will find it alive or dead if you do the same experiment over and over."

It is the classical calculation that says "I can't predict whether the cat is alive or dead, only the probability that you will find it alive or dead if you do the same experiment over and over." The quantum calculation, as Banks himself shows on p. 5 of the paper, does say "the cat is both alive and dead", or more precisely, "the cat is in a superposition of alive and dead, with amplitudes $\alpha$ and $\beta$ that are derived from the initial state". That is what the equations at the top of p. 5 are saying. Note that there are no interference terms in the second equation there, yet that equation still describes a superposition of macroscopically different states. So unless he is claiming that ignoring the interference terms allows you to replace the quantum calculation with the classical calculation, what he says in the quote I gave earlier in this post makes no sense.

 

[vanhees71]

This does not matter. A single pointer is at any time in a single of these states. The collective position operator is an average over microscopic position operators. But operators are not states, so this average does not make the pointer an ensemble.

If you calculate the expectation value of the pointer position in a nonclassical superposition of classical states you get a value far from the measured values, though consistent with the huge uncertainty.

A macroscopic "pointer state" is not described by a single pure state but by a statistical operator close to thermal equilibrium. You have of course thermal as well as quantum fluctuations (quantified as the standard deviations from the mean) which are however very small compared to the accuracy you use to define the macroscopic pointer reading.

It's of course true that nowadays experimentalists are able to prepare also macroscopic (or mesoscopic) systems showing quantum behavior, but that's underlining the point of view given above: As soon as I'm able to resolve quantum behavior, I'll observe it. If I don't I observe classical behavior modulo FAPP negligible fluctuations.

Whether or not you accept this as an explanation for the (apparent) classical behavior of macroscopic systems or not, is of course your personal opinion, and it's hard to argue about it. Of course, if you don't accept probabilistic arguments, it's hard to find a satisfactory argument, but then you also are back to the hitherto unsuccessful attempts to find a comprehensive unified determinstic description of the world like Einstein and Schrödinger in their more mature years. One should note that such attempts seem to be even more hopeless than they seemed to be in those days, because after all the decades of Bell's et al work, making the vague, philosophical EPR ideas scientific and the very successful experimental tests, confirming quantum theory in comparison to local deterministic HV models.

I'm not so sure about Banks's paper (I don't have the book). It seems to make many words with little mathematical analysis. For this topic, I'd rather refer to the book by Joos, Zeh, et al, Decoherence and the Appearance of a Classical World in Quantum Theory, Springer (2003).

 

[A. Neumaier]

A macroscopic "pointer state" is not described by a single pure state but by a statistical operator close to thermal equilibrium.

According to the standard view, the true state of the pointer is pure, and only our ignorance turns it into a density operator.

But when coupled for measurement with a spin even a pointer described statistically by a density operator close to equilibrium turns under the now appropriate von-Neumann dynamics into a nonclassical very out-of equilibrium mixture with huge uncertainty for the pointer position.

This is because in repeated experiments (which are necessary to apply the statistical interpretation), the pointers point in 50% of the experiments to the left, and in the other 50% to the right, so that their expectation is in the middle and the uncertainty is macroscopic.

Unexplained (and simply assumed in the statistical interpretation) is why nevertheless the pointer has a nearly definite position in each single experiment. This is the ''problem of definite outcomes'' - the part of the measurement problem unsolved by decoherence.

 

[A. Neumaier]

I'm not so sure about Banks's paper (I don't have the book). It seems to make many words with little mathematical analysis.

Banks adheres to the many words interpretation of quantum mechanics. It is immune against criticism since (due to the lack of a conservation law for words) one can always create more words out of nothing.

 

[vanhees71]

According to the standard view, the true state of the pointer is pure, and only our ignorance turns it into a density operator.

But when coupled for measurement with a spin even a pointer described statistically by a density operator close to equilibrium turns under the now appropriate von-Neumann dynamics into a nonclassical very out-of equilibrium mixture with huge uncertainty for the pointer position.

This is because in repeated experiments (which are necessary to apply the statistical interpretation), the pointers point in 50% of the experiments to the left, and in the other 50% to the right, so that their expectation is in the middle and the uncertainty is macroscopic.

Unexplained (and simply assumed in the statistical interpretation) is why nevertheless the pointer has a nearly definite position in each single experiment. This is the ''problem of definite outcomes'' - the part of the measurement problem unsolved by decoherence.

I don't know any "standard view", which describes a macroscopic observable as a pure state. You cannot even write it down, because of the many microscopic degrees of freedom you'd have to consider.

Your description of a spin measurement contradicts the observations. A silver atom, being prepared in a thermal state (coming out of the hole of an oven kept at some temperature) running through a Stern-Gerlach magnet leaves one spot on a photo plate defining its position within the position resolution of the plate. There are two spots after a large number of silver atoms has hit the screen, with 1/2 probability for ending up in each of the spots due to the entanglement between the spin component determined by the direction of the magnetic field. It's pretty easy to describe this measured distribution within, and that's all what QT promises to predict. What is unsolved is not clear to me.

 

[Lord Jestocost]

This is the ''problem of definite outcomes'' - the part of the measurement problem unsolved by decoherence.

What is unsolved is not clear to me.

Maximilian Schlosshauer in „Quantum Decoherence“ (Phys. Rep. 831, 1-57 (2019)):

„Application of the unitary Schrödinger evolution to a measuring apparatus interacting with a system prepared in a quantum superposition state cannot dynamically describe the stochastic selection of a particular term in the superposition as the measurement outcome (the ‚collapse of the wave function‘); rather, system and apparatus end up in an entangled state, with all terms of the original superposition still present and quantum-correlated with different apparatus states. This is the measurement problem: the question of how to reconcile the linear, deterministic evolution described by the Schrödinger equation with our observation of the occurrence of random measurement outcomes….

…. The measurement problem as just defined cannot be solved by decoherence.“

 

[A. Neumaier]

I don't know any "standard view", which describes a macroscopic observable as a pure state. You cannot even write it down, because of the many microscopic degrees of freedom you'd have to consider.

The standard view (e.g., Landau and Lifschitz) is that all quantum systems (hence also macroscopic ones) are described by a wave function, which is the most complete description of the state. In your lecture notes on quantum physics you also introduce this standard view when you discuss the postulates of quantum mechanics.

Getting from the standard view a density operator is an additional step involving incomplete knowledge.

 

[dextercioby]

The distinction between „macroscopic observable/state” and „microscopic observable/state” doesn't pertain to axiomatic Quantum Mechanics, but to those foundations of classical/quantum statistical mechanics which attempt to explain from statistical principles the phenomenological principles of quasistatic/equilibrium thermodynamics.
So in the Copenhagen/orthodox/textbook formulation, "the state of a quantum system" is always a clear mathematically well-defined object. "microscopic"/"macroscopic" are not words in a QM axiom, i.e. QM is universal.

 

[gentzen]

Your description of a spin measurement contradicts the observations.

A pointer is not a good picture for a screen. So the mismatch between description and observations is no surprise. Two questions:
1) Is there an experiment (preferably related to spin measurement) where a pointer is a good picture for the physical mechanism of the measurement device?
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".

A silver atom, being prepared in a thermal state (coming out of the hole of an oven kept at some temperature) running through a Stern-Gerlach magnet leaves one spot on a photo plate defining its position within the position resolution of the plate. There are two spots after a large number of silver atoms has hit the screen, with 1/2 probability for ending up in each of the spots due to the entanglement between the spin component determined by the direction of the magnetic field. It's pretty easy to describe this measured distribution within, and that's all what QT promises to predict. What is unsolved is not clear to me.

I am not sure that this is really "all what QT promises to predict". But since you don't want to look at individual silver atoms, there is neither symmetry breaking nor randomness. So you may be right that this specific experiment does not need any interpretative assumptions to match QT predictions to observations. But without a reasonable model for a screen, it is hard to determine the QT predictions.

 

[A. Neumaier]

Your description of a spin measurement contradicts the observations.

I didn't describe Stern-Gerlach but an experiment where a pointer is coupled to a qubit such that it does not move if up is measured but moves when down is measured.

You can take the qubit to be the presence or absence of a photon, and the pointer to be the tip of the needle of a meter measuring the photocurrent generated through magnificaion of the photodetection event.

Unitary dynamics predicts for this setting that the needle will be in a nonthermal state with a macroscopic uncertainty of the pointer.

 

[WernerQH]

You can take the qubit to be the presence or absence of a photon, and the pointer to be the tip of the needle of a meter measuring the photocurrent generated through magnificaion of the photodetection event.

This is an experiment that can only be performed in (some) theoreticians' minds. Experimentalists will laugh at this.

 

[A. Neumaier]

This is an experiment that can only be performed in (some) theoreticians' minds. Experimentalists will laugh at this.

All experiments discussed in this forum are thought experiments. Real experiments are always much more complex than these.

But you can also think of the pointer as a pixel on a screen that turns from white to black when a photon arrives, and position as the grey level rather than a physical position. The presence or absence of a photon is a qubit, The mathematical analysis remains the same.

 

[WernerQH]

The presence or absence of a photon is a qubit, The mathematical analysis remains the same.

A qbit is not the same as a bit. Indeed, that's the mystery: how a qbit gets transformed into a bit. For example, how a circularly polarized photon chooses the path to take in a Nicol prism. In my view neither the qbit nor the wave function are real, only the detection events are real.

 

[A. Neumaier]

A qbit is not the same as a bit.

I didn#t claim they were. Before measurement there is a 2D qubit space of wave functions in superpositions of absent and present, and upon measurement, one of the two materializes and a classical bit results. This is precisely the measurement situation under discussion.

Indeed, that's the mystery: how a qbit gets transformed into a bit.

The mystery - explained neither by the statistical interpretation nor by statistical mechanics added on top.

From the tensor product of a thermal macroscopic state and a qubit state, the von Neumann dynamics produces a complex nonthermal state instead of one of the two observed tensor products of a thermal macroscopic state and a reduced qubit state (unless someone observes the combined system, which leads to von Neumann's regress to consciousness).

In my view neither the qbit nor the wave function are real, only the detection events are real.

But when detection events are real, detectors are real. Now detectors are large quantum systems. How large must a quantum system be in order to count as real? Which of its aspects are then real? Are these determined by its quantum state?

Endless questions for the adherents to this view.

 

[EPR]

But when detection events are real, detectors are real. Now detectors are large quantum systems. How large must a quantum system be in order to count as real? Which of its aspects are then real? Are these determined by its quantum state?

Endless questions for the adherents to this view.

Information availability seems to be key. From my lengthy research on this topic, it's the information availability which acts as a measurement/detection on quantum systems.
And this rule appears to encompass the entire Universe with all quantum systems manifesting as macroscopic detectors, objects, etc with this 'measurement' which in reality is information availability about the quantum system.

Shield the object and it starts to manifest its quantumness.
It's not weird - we just appear to have assumed in the past how the world works on very sketchy and flimsy information.

 

[A. Neumaier]

Information availability seems to be key.

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

 

[WernerQH]

Before measurement there is a 2D qubit space of wave functions in superpositions of absent and present, and upon measurement, one of the two materializes and a classical bit results.

Endless questions for the adherents to this view.

Obviously you think of the wave function as something physical. And like many physicists you are lured into thinking like this because the wave function has an existence continuous in time, like the "quantum system" that it is supposed to describe. But this is an assumption. If the detector is there whenever we look at it, it is of course an obvious assumption that it is always there. But this need not extend to the smallest scales of space and time. We perceive a motion picture as continuous, although we know it is not. Charge conservation seems to imply that an electron is always there. But there is no experimental evidence that the world-line of an electron must be continuous at the zeptosecond scale. It may in fact be a dotted line; the electron actually being nowhere (rather than "smeared out") between its interaction events.

The notion of an "electron" is, of course, based on a classical concept. It may be more appropriate to see it as a name we attach to a trail of events in spacetime. The essence of an "electron" is the Dirac propagator, and QED is a statistical theory describing correlations between events in spacetime.

I realize that you dislike the statistical interpretation of quantum theory, and I agree that it is unsatisfactory when it is based on an ontology of "particles" with definite properties and existing for finite intervals of time. But you can apply statistics to quite different things!

 

[A. Neumaier]

But you can apply statistics to quite different things!

But wne can apply statistics only to things that exist.

Certain quantum systems consisting of N particles exist and for large N they have properties (such as the center of mass) even when nobody is looking at them - detectors are an example. Decreasing N by one doesn't change the property of existing. By induction, quantum systems exist and have properites (such as the center of mass) down to N=0 (the vacuum), at which point we cannot decrease N anymore.

Or at which N do you suggest that quantum systems stop existing or having a center of mass even when nobody is looking at them? What causes the change in ontology?

 

[WernerQH]

But we can apply statistics only to things that exist.

The click of a Geiger counter is not a "thing". But of course you can do statistics of events. (Queueing theory is just one example!)

If you think that, fundamentally, physics must necessarily be about "things", you are out of luck.

 

[Jarvis323]

The click of a Geiger counter is not a "thing".

What do you mean? Intuitively I wouldn't think that is any less of a "thing" than anything else.

 

[A. Neumaier]

The click of a Geiger counter is not a "thing".

The Geiger counter is the existing thing of which the click is a property. If the quantum system called Geiger counter does not exist, neither the click exists.

If you think that, fundamentally, physics must necessarily be about "things", you are out of luck.

No. I found something innovative this way whereas you only repeat an abstruse tradition. I consider the former to be more lucky than the latter.

 

[PeroK]

Certain quantum systems consisting of N particles exist and for large N they have properties (such as the center of mass) even when nobody is looking at them - detectors are an example. Decreasing N by one doesn't change the property of existing. By induction, quantum systems exist and have properites (such as the center of mass) down to N=0 (the vacuum), at which point we cannot decrease N anymore.

I would say that "centre of mass" is a fuzzy concept. With a large number of particles, the concept is hardly fuzzy at all, but as you remove particles, the property becomes fuzzier and fuzzier.

That's certainly true of other properties: what if we replace "centre of mass" with "age", and apply the same logic? A human being has a well-defined age that cannot be induced into its constituent particles. If you remove the particles that make up me, at what point does what's left stop being 58 years old?

 

[A. Neumaier]

I would say that "centre of mass" is a fuzzy concept. With a large number of particles, the concept is hardly fuzzy at all, but as you remove particles, the property becomes fuzzier and fuzzier.

An object that has a fuzzy property surely exists, no matter how fuzzy the property. And the amount of fuzziness of a property is another (fuzzy) property of the object.

That's certainly true of other properties: what if we replace "centre of mass" with "age", and apply the same logic?

Yes.

A human being has a well-defined age that cannot be induced into its constituent particles. If you remove the particles that make up me, at what point does what's left stop being 58 years old?

When you are dead. There are objective procedures to decide that.

 

[PeroK]

When you are dead. There are objective procedures to decide that.

That's a clever answer, but it doesn't get you round the fundamental problem.

 

[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.