Insights How I Stopped Worrying and Learned to Love Orthodox Quantum Mechanics - Comments

[stevendaryl]

I did several times in this thread. Then you use the words in different meanings. In this way one cannot discuss scientific issues. That's all I'm saying.

Well, I think you've misdiagnosed the problem.

 

[Demystifier]

That's what makes no sense to me. If detector clicks are natural phenomena that are ultimately described by the physics of particles and fields, then how can they be more real than what they're made out of? To me, that's a schizophrenic point of view.

The pre-quantum theories of physics were not schizophrenic in this way. Bohmian mechanics is not schizophrenic in this way.

Here the problematic word is "ultimately". What if description by the physics of particles and fields is not ultimate but merely provisional? Would it be schizophrenic even then? Different levels of descriptions require different effective paradigms (see Anderson's "More is Different"), and there is nothing schizophrenic about that. On the other hand, even Bohmian mechanics can make you schizophrenic if you apply it to make free will decisions about everyday life actions. (Should I marry Ana or Rebecca? Well, it's already determined by initial Bohmian positions, so there is nothing I can do about it. Except that I can. Which is impossible. But obviously true. Arrrghhh!)

 

[Demystifier]

Hi, firstly, ontic and epistemic are not stuff of philosophy.. even brilliant physicists like Sean Carrol believes in ontic psi as when he made clear in:

http://blogs.discovermagazine.com/c...hysicality-of-the-quantum-state/#.Wa87v7pFxOx

“According to instrumentalism, palaeontologists talk about dinosaurs so they can understand fossils, astrophysicists talk about stars so they can understand photoplates, virologists talk about viruses so they can understand NMR instruments, and particle physicists talk about the Higgs Boson so they can understand the LHC. In each case, it’s quite clear that instrumentalism is the wrong way around. Science is not “about” experiments; science is about the world, and experiments are part of its toolkit.”

Also remember PBR theorem revolves around ontic and epistemic psi, so these are serious physics stuff.

Well, many physicists have also a good understanding of philosophy, and Sean Carroll is one of the best examples. The authors of the PBR theorem went even further, they found a way to translate philosophical terms into scientific ones, which is why their work is so important. But still, most physicists (who are not interested in quantum foundations) are not familiar with concepts of ontology and epistemology.

That said. If psi is really ontic, and there is some kind of actual Hilbert Space in the vacuum or whatever the ontic nature may be based on.. is there possibility that we have new force of nature (or new field such as higgs field like thing) that only work in the dynamics within the actual Hilbert space (or other mechanisms) that produces the ontic psi, etc.? Do you know of references with regards to this? Thank you.

I'm afraid I don't understand your question. Are you implying that electromagnetic force, for instance, does not work within actual Hilbert space? What do you mean by that?

 

[stevendaryl]

Here the problematic word is "ultimately". What if description by the physics of particles and fields is not ultimate but merely provisional? Would it be schizophrenic even then?

If it turns out the QM is not fundamental, but is just a heuristic approximation to a more accurate theory, then I would no longer care whether it is schizophrenic, and would instead turn my scrutiny to that replacement theory.

 

[fanieh]

Well, many physicists have also a good understanding of philosophy, and Sean Carroll is one of the best examples. The authors of the PBR theorem went even further, they found a way to translate philosophical terms into scientific ones, which is why their work is so important. But still, most physicists (who are not interested in quantum foundations) are not familiar with concepts of ontology and epistemology.I'm afraid I don't understand your question. Are you implying that electromagnetic force, for instance, does not work within actual Hilbert space? What do you mean by that?

I meant supposed there was a real Hilbert space.. then you need a set of new forces of nature for the real Hilbert space to work, forces we can't detect because it only works within the machinery that produces all this quantum ontology (for example imagine a super computer inside each of the Planck space in the vacuum whose only job is to produce quantum probabilities and bind them to objects (or whatever)).

 

[Demystifier]

If it turns out the QM is not fundamental, but is just a heuristic approximation to a more accurate theory, then I would no longer care whether it is schizophrenic, and would instead turn my scrutiny to that replacement theory.

Fair enough. And what if, as I propose, non-relativistic QM with Bohmian interpretation is fundamental while relativistic QFT is emergent?

 

[stevendaryl]

Fair enough. And what if, as I propose, non-relativistic QM with Bohmian interpretation is fundamental while relativistic QFT is emergent?

Like I said, if that's the case, then I would no longer care about whether QM seems schizophrenic.

 

[Demystifier]

I meant supposed there was a real Hilbert space.. then you need a set of new forces of nature for the real Hilbert space to work, forces we can't detect because it only works within the machinery that produces all this quantum ontology (for example imagine a super computer inside each of the Planck space in the vacuum whose only job is to produce quantum probabilities and bind them to objects (or whatever)).

I guess it's something like MWI applied not to Standard Model but to the true theory of everything. Well, it's possible but I am not aware of any actual reference.

 

[Demystifier]

Like I said, if that's the case, then I would no longer care about whether QM seems schizophrenic.

Yes, that's why my article is entitled "How I Stopped Worrying and Learned to Love Orthodox QM".

 

[fanieh]

I guess it's something like MWI applied not to Standard Model but to the true theory of everything. Well, it's possible but I am not aware of any actual reference.

I'll give clearer example. In Einstein time, he didn't know of the strong and weak forces because we hadn't know about the quarks and beta decay then. Is it possible there would be a fifth and sixth fundamental forces of nature whose domain of applicability is related to the quantum ontology (or mechanism within such) only?

 

[Demystifier]

Is it possible there would be a fifth and sixth fundamental forces of nature whose domain of applicability is related to the quantum ontology (or mechanism within such) only?

It's possible. See e.g. https://arxiv.org/abs/1703.08341 , Sec. 4.1, last paragraph.

 

[martinbn]

The minimalist interpretation of quantum mechanics seems to do that. I'm sure you've heard it said by many physicists that

(John Wheeler)

In the minimalist interpretation, we are using quantum mechanics to compute transition probabilities between macroscopic states: We start with a preparation procedure and proceed to a measurement. Quantum mechanics gives probabilities for the various possible measurement results, given the preparation procedure. So in this formulation, it seems to be viewing some things as definite---we chose a definite preparation procedure, we got a definite measurement result. But the microscopic details are not assumed to have definite values. The microscopic details seem to be treated as mere calculational tools for predicting macroscopic outcomes, which are the real things.

Yes, but there is a big difference between particles and fields are not real and values of observables are not meaningful without a measurment.

 

[atyy]

Yes, that's why my article is entitled "How I Stopped Worrying and Learned to Love Orthodox QM".

It's funny that Weinberg hasn't come to this conclusion although he knows the lesson of Wilson perfectly well. I guess he is still yearning for the old days in which particle physicists thought they were working on fundamental physics.

 

[fanieh]

It's possible. See e.g. https://arxiv.org/abs/1703.08341 , Sec. 4.1, last paragraph.

I see. May I know if the Hilbert Space in Bohmian Mechanics is located in the quantum vacuum or outside the vacuum or outside spacetime? Please describe where it is located. Thank you.

 

[martinbn]

I see. May I know if the Hilbert Space in Bohmian Mechanics is located in the quantum vacuum or outside the vacuum or outside spacetime? Please describe where it is located. Thank you.

This as phrased doesn't make sense. What do you mean by "the Hilbert space is located" ?

 

[fanieh]

This as phrased doesn't make sense. What do you mean by "the Hilbert space is located" ?

according to atyy: https://www.physicsforums.com/threads/pbr-theorem.789046/page-3

"The configuration space (Hilbert space) is real in dBB. It's not much different from the extra-dimensions of string theory."

Reference https://www.physicsforums.com/threads/pbr-theorem.789046/page-3

 

[martinbn]

Then atyy needs to explain the meaning of that.

 

[stevendaryl]

Yes, but there is a big difference between particles and fields are not real and values of observables are not meaningful without a measurment.

Can you expand on what the big difference is?

 

[martinbn]

Can you expand on what the big difference is?

Well, the particle is real, it exists out there. It is an objective entity, not an abstract construct. On the other hand the coordinates are part of the mathematical description and are things that need not make sense at all times.

 

[Demystifier]

I see. May I know if the Hilbert Space in Bohmian Mechanics is located in the quantum vacuum or outside the vacuum or outside spacetime? Please describe where it is located. Thank you.

In our mind.

 

[fanieh]

In our mind.

I thought Bohmian Mechanics Required the Wave Function to be real.. and needed the PBR theorem to have the wave function real:

https://en.wikipedia.org/wiki/PBR_theorem

"The theorem was first published as an arXiv preprint with Pusey as the principal author,[1] a subsequent version published in Nature Physics,[2] that states the theorem that either the quantum state corresponds to a physically real object and is not merely a statistical tool, or else all quantum states, including non-entangled ones, can communicate by action at a distance."

Is there something wrong with the PBR Theorem?

If the wave function in BM was not real. How can it affect the local particle?

I think it was this line of reasoning or logic that made atyy made the following statements (in the thread mentioned earlier):

"1. “In the Ψ-ontic view, the wave function is a wave like an EM wave. However, the wave function is a wave in Hilbert space, and whereas an EM wave is a wave in spacetime.

2. In both MWI and dBB, the wave function is not a wave in spacetime , it is a wave in Hilbert space.

3. The wave function exists only in Hilbert space in all interpretations of QM, so yes, it is real only in Hilbert space in Ψ-ontic proposals such as MWI and dBB.

4. The configuration space (Hilbert space) is real in dBB. It's not much different from the extra-dimensions of string theory.”"

Reference: https://www.physicsforums.com/threads/pbr-theorem.789046/page-3

How come didn't you have the same reasoning as atyy which was based on the PBR theorem.. did you see any flaw with the PBR line of reasoning? What are they, if any? Thank you.

 

[Demystifier]

I thought Bohmian Mechanics Required the Wave Function to be real

If you want to seriously discuss QM, you must first be well familiar with classical mechanics. Are you? Let me assume that you are. Then wave function is "real" in Bohmian mechanics in the same sense in which principal function of classical Hamilton-Jacobi equation is "real" in classical mechanics. (And if you have no idea what I am talking about, then go and learn classical mechanics first.)

 

[Georgios Bosch]

A very interesting read, thank you. If you don't mind sharing, what was the deep conceptual error in arXiv:1309.0400?

 

[stevendaryl]

Here's what seems strange to me. You have system A, an electron, say. For simplicity, only consider the property of being spin-up in the z-direction. You have system B, some measuring device. Among other things, it has a pointer that can swing from pointing left, where there is a label "U" to pointing right, where there is a label "D". You somehow connect the two systems so that system B measures the spin of system A: If system A is spin-up in the z-direction, then system B will go into the state of pointing to "U", and if system A is spin-down in the z-direction, then system B will go into the state of pointing to "D".

So for people say that properties of system A are meaningless, or have no definite value, until they are measured by system B seems weird if they are both quantum systems. Does system B need a third system, C to make its pointer-value meaningful? That would lead to an infinite regress.

The way I feel about it is that unless one can formulate the Rules of Quantum Mechanics in a way that does not mention, at the fundamental level, any macroscopic quantities such as "measurement", "preparation procedure", "average over many, many systems", then we don't really understand quantum mechanics. That might be fine. There might be limits to what we can understand. But I object to people pretending otherwise.

 

[Mentz114]

Here's what seems strange to me. You have system A, an electron, say. For simplicity, only consider the property of being spin-up in the z-direction. You have system B, some measuring device. Among other things, it has a pointer that can swing from pointing left, where there is a label "U" to pointing right, where there is a label "D". You somehow connect the two systems so that system B measures the spin of system A: If system A is spin-up in the z-direction, then system B will go into the state of pointing to "U", and if system A is spin-down in the z-direction, then system B will go into the state of pointing to "D".

So for people say that properties of system A are meaningless, or have no definite value, until they are measured by system B seems weird if they are both quantum systems. Does system B need a third system, C to make its pointer-value meaningful? That would lead to an infinite regress.

The way I feel about it is that unless one can formulate the Rules of Quantum Mechanics in a way that does not mention, at the fundamental level, any macroscopic quantities such as "measurement", "preparation procedure", "average over many, many systems", then we don't really understand quantum mechanics. That might be fine. There might be limits to what we can understand. But I object to people pretending otherwise.

I never understood why "properties of system A are meaningless, ... until they are measured by system B" and I believe it is not always true.
I can't offer any words of sympathy for your problem but I did find this paper which describes a possible experimental realization of your systems A and B (which you may not have seen). It even has a dial and a pointer.

Continuous Stern-Gerlach effect: Principle and idealized apparatus
HANS DEHMELT
Proc. Nat'l.Acad.Sci.
USA
Vol.83,
April 1986
Physics

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC323282/pdf/pnas00312-0017.pdf
( "I enjoyed discussions with W.E.Lamb,Jr., E.M.Purcell, I.I.Rabi, J.S.Bell, and M.0.Scully" who they ?)

 

[Demystifier]

A very interesting read, thank you. If you don't mind sharing, what was the deep conceptual error in arXiv:1309.0400?

Eq. (182) is only valid after the measurement. On the other hand, Eq. (185) contains a tacit (but wrong) assumption that (182) is valid at all times.
http://www.thethingswesay.com/there...integrity-than-his-behavior-when-he-is-wrong/
http://www.quote-coyote.com/quotes/authors/l/bruce-lee/quote-3714.html

 

[vanhees71]

Here's what seems strange to me. You have system A, an electron, say. For simplicity, only consider the property of being spin-up in the z-direction. You have system B, some measuring device. Among other things, it has a pointer that can swing from pointing left, where there is a label "U" to pointing right, where there is a label "D". You somehow connect the two systems so that system B measures the spin of system A: If system A is spin-up in the z-direction, then system B will go into the state of pointing to "U", and if system A is spin-down in the z-direction, then system B will go into the state of pointing to "D".

So for people say that properties of system A are meaningless, or have no definite value, until they are measured by system B seems weird if they are both quantum systems. Does system B need a third system, C to make its pointer-value meaningful? That would lead to an infinite regress.

The way I feel about it is that unless one can formulate the Rules of Quantum Mechanics in a way that does not mention, at the fundamental level, any macroscopic quantities such as "measurement", "preparation procedure", "average over many, many systems", then we don't really understand quantum mechanics. That might be fine. There might be limits to what we can understand. But I object to people pretending otherwise.

I think the problematic thing is to call properties which are not prepared as "meaningless". If you have a system in a state, the observables that have no determined value are of course not meaning less but measurable, and in measuring them you usually have an influence on the state of the measured system. Which one this is, depends on the interaction between measurement apparatus and measured system. In the special case of von-Neumann-filter measurements (I'd rather call them a certain kind of preparation procedure) you have prepared a state, where the observable takes the corresponding determined value.

 

[martinbn]

Here's what seems strange to me. You have system A, an electron, say. For simplicity, only consider the property of being spin-up in the z-direction. You have system B, some measuring device. Among other things, it has a pointer that can swing from pointing left, where there is a label "U" to pointing right, where there is a label "D". You somehow connect the two systems so that system B measures the spin of system A: If system A is spin-up in the z-direction, then system B will go into the state of pointing to "U", and if system A is spin-down in the z-direction, then system B will go into the state of pointing to "D".

So for people say that properties of system A are meaningless, or have no definite value, until they are measured by system B seems weird if they are both quantum systems. Does system B need a third system, C to make its pointer-value meaningful? That would lead to an infinite regress.

The way I feel about it is that unless one can formulate the Rules of Quantum Mechanics in a way that does not mention, at the fundamental level, any macroscopic quantities such as "measurement", "preparation procedure", "average over many, many systems", then we don't really understand quantum mechanics. That might be fine. There might be limits to what we can understand. But I object to people pretending otherwise.

This is what I don't understand. Why do you insist on the theory being of certain type? Why is it not ok to mention these notions? It seems to me it is a matter of taste. Almost as saying as long as the theory uses differential equations it is not a good explanation. It is incomplete until a purely algebraic description is found.

 

[Demystifier]

This is what I don't understand. Why do you insist on the theory being of certain type? Why is it not ok to mention these notions? It seems to me it is a matter of taste. Almost as saying as long as the theory uses differential equations it is not a good explanation. It is incomplete until a purely algebraic description is found.

In physics, there is a widespread belief that fundamental laws must be fully microscopic. You can compare it with a widespread belief in pure math that all math must be based on set theory. Proposing that macro laws could be fundamental can be compared to a proposal that math should be based on category theory (rather than set theory). Yes, some people propose it, but the mainstream does not buy it.

 

[stevendaryl]

This is what I don't understand. Why do you insist on the theory being of certain type?

I'm not insisting on anything. I'm just explaining why I feel there is something not yet understood about quantum mechanics. My feeling is that macroscopic properties should be derivable from microscopic properties, so that in principle, any mention of macroscopic properties should be eliminable. That's part of the reductionist program, it seems to me.

 

[stevendaryl]

I think the problematic thing is to call properties which are not prepared as "meaningless". If you have a system in a state, the observables that have no determined value are of course not meaning less but measurable, and in measuring them you usually have an influence on the state of the measured system.

But to me, calling something "measurable" is the issue. A property is measurable if some procedure can make it's value correlated with a macroscopic property (such as a pointer position). But what makes pointer positions different than properties such as the z-component of spin? Why does the first not need to be measured to have a value? Of course, that would lead to an infinite regress, but how do you stop the regress? It seems to me by saying that there is something special about pointer positions.

Can one electron measure the spin of another electron?

 

[stevendaryl]

In physics, there is a widespread belief that fundamental laws must be fully microscopic. You can compare it with a widespread belief in pure math that all math must be based on set theory. Proposing that macro laws could be fundamental can be compared to a proposal that math should be based on category theory (rather than set theory). Yes, some people propose it, but the mainstream does not buy it.

I think it's different from that. I couldn't care less whether you base your math on set theory or category theory. But if you had a law of physics that mentions macroscopic systems---say, that cats always land on their feet--it seems to me that either the law should be derivable from particle dynamics (since a cat is made up of particles, after all) or else particle dynamics is actually violated when the particles are part of a cat.

I suppose a third possibility is some kind of superdeterminism. The cat's particles just obey ordinary particle dyanmics, but the initial conditions are such that the cat always lands on its feet.

 

[stevendaryl]

This is what I don't understand. Why do you insist on the theory being of certain type? Why is it not ok to mention these notions? It seems to me it is a matter of taste. Almost as saying as long as the theory uses differential equations it is not a good explanation. It is incomplete until a purely algebraic description is found.

The macroscopic description of a measurement situation might be something like this:

• if you prepare an electron in the spin state \alpha |U\rangle + \beta |D\rangle and perform a measurement of spin in the z-direction, then the device will make a transition to the "Measured spin-up" state with probability |\alpha|^2 and to "Measured spin-down" with probability |\beta|^2.

Since "Measured spin-up" and "Measured spin-down" are presumably states of the measuring device, and the measuring device is made up of ordinary particles, then it seems that in principle, the above rule should be re-expressible in the form:

• If you prepare a collection of particles in such-and-such a state, then later they will be in such-and-such a state with probability |\alpha|^2. (Not "measured to be" in that state, because the system already includes the measuring device, which presumably doesn't need to be measured by a third system. Or does it?)

It seems that in principle, it should be possible to eliminate the measurement aspect of the theory and re-express it as a theory of pure particles. If that can't be done, that seems pretty weird to me. On the other hand, we're pretty sure that it can't be done, because the dynamics of pure particles is deterministic (Schrodinger's equation) regardless of how many particles are involved. So there's something weird going on.

 

[vanhees71]

But to me, calling something "measurable" is the issue. A property is measurable if some procedure can make it's value correlated with a macroscopic property (such as a pointer position). But what makes pointer positions different than properties such as the z-component of spin? Why does the first not need to be measured to have a value? Of course, that would lead to an infinite regress, but how do you stop the regress? It seems to me by saying that there is something special about pointer positions.

Can one electron measure the spin of another electron?

The difference between "macroscopic" and "microscopic" observables is that the former are coarse grained, i.e., averages over many microscopic degrees of freedom, which have the tendency to behave classical.

The infinite regress you mention simply stops by construction of a macroscopic measurement apparatus and its verification by experiments to really measure what it's supposed to measure. Physicists must be to a certain amount practitioners and must indeed stop to worry about such purely philosophical problems. Although Bell didn't like the expression "for all practical purposes" ("FAPP"), at a certain point you must get practical not to get lost in infinite regress of purposeless philosophical pondering. It's the art of the physicist to disinguish between interesting and pointless questions!

Finally, as I said before, one should say that an observable of a quantum system takes a determined value if the system is prepared in a corresponding state. Just to measure it doesn't give it a certain value. Quite often the measured system (like a photon) is destroyed in the act of measurement, and then you can only say, you've measured some value on this individual system. To check the predictions of QT you need to prepare a sufficiently large ensemble to test the probabilities predicted to a given significance level.

 

[Demystifier]

I think it's different from that. I couldn't care less whether you base your math on set theory or category theory. But if you had a law of physics that mentions macroscopic systems---say, that cats always land on their feet--it seems to me that either the law should be derivable from particle dynamics (since a cat is made up of particles, after all) or else particle dynamics is actually violated when the particles are part of a cat.

I have another mathematical analogy that can be useful. Saying that cat is made of particles is like saying that a function f(x) is made of points - at each local point x you have to specify f(x). However, you can make a Fourier transform and say that the function is not made of local points but of global functions sin and cos. In other words, you can have locality in the k-space rather than the x-space. Which space is fundamental? We don't know a priori. If forces of nature are local in the x-space, then it seems reasonable that x-space is more fundamental than the k-space. But if forces are not local in x-space (as violation of Bell inequalities suggests), then perhaps "fundamental" does not mean "micro". Perhaps we need to make some big functional transform of all our known laws of physics and obtain a truly local laws in some completely different space. To connect this (farfetched?) speculation with something more familiar, perhaps this big transform is somehow related to AdS/CFT and EPR=ER conjectures.

 

[Physics Footnotes]

The macroscopic description of a measurement situation might be something like this:

• if you prepare an electron in the spin state \alpha |U\rangle + \beta |D\rangle and perform a measurement of spin in the z-direction, then the device will make a transition to the "Measured spin-up" state with probability |\alpha|^2 and to "Measured spin-down" with probability |\beta|^2.

Since "Measured spin-up" and "Measured spin-down" are presumably states of the measuring device, and the measuring device is made up of ordinary particles, then it seems that in principle, the above rule should be re-expressible in the form:

• If you prepare a collection of particles in such-and-such a state, then later they will be in such-and-such a state with probability |\alpha|^2. (Not "measured to be" in that state, because the system already includes the measuring device, which presumably doesn't need to be measured by a third system. Or does it?)

It seems that in principle, it should be possible to eliminate the measurement aspect of the theory and re-express it as a theory of pure particles. If that can't be done, that seems pretty weird to me. On the other hand, we're pretty sure that it can't be done, because the dynamics of pure particles is deterministic (Schrodinger's equation) regardless of how many particles are involved. So there's something weird going on.

This is a very elegant articulation of the so-called Measurement Problem, and makes very clear why it is called a 'problem', namely: the experiments used to justify quantum mechanics are, by that very theory, not dynamically possible!

Despite what you may read to the contrary (here or elsewhere), this problem has not been resolved, and so it should be no surprise that it is causing you so much head-scratching. Many physicists like to pretend it has been solved by hand-wavy arguments littered with terms like 'decoherence', 'coarse-graining', 'Ehrenfest's Theorem', and so forth, however no such techniques have succeeded in putting this problem in the solved tray. (If it was true that this problem had been solved, you would not have otherwise perfectly level-headed physicists desperately introducing unobservable multi-verses with abandon!)

One can certainly avoid the problem by resorting to interpretations that avoid one or more assumptions you have made, but they then have their own problems, resulting in no consensus whatsoever about the best way to respond to the problem.

 

[Lord Jestocost]

I'm not insisting on anything. I'm just explaining why I feel there is something not yet understood about quantum mechanics. My feeling is that macroscopic properties should be derivable from microscopic properties, so that in principle, any mention of macroscopic properties should be eliminable. That's part of the reductionist program, it seems to me.

There is – so to speak - indeed a “problem” with quantum theory (QT) and it seems to me to be an “insoluble” problem – despite opposite claims. The “problem” can be explained in a simple way: There is one equation and one quantity which define the theory – the Schroedinger equation and the associated wave function – and those don’t describe how we - as conscious observers - experience our world. That is the fundamental essence of Schroedinger’s cat fable.

If you accept QT as a fundamental physical theory, you have to apply the theory straightforward at all stages, there is no way out. However, QT allows at no level definite outcomes to be realized, whereas at the level of our human consciousness it seems a matter of direct experience that such outcomes occur. That means, suddenly, when you make a measurement (observation), there is somehow a “cut" or "collapse”, something seems to become “concrete” and “real” stuff. And QT says nothing about it: the conceptual transition from quantum to classical “knowing” had to be put in “by hand”. This is indeed something not yet understood about quantum theory when considering it as a fundamental physical theory about "reality".

 

[bhobba]

Despite what you may read to the contrary (here or elsewhere), this problem has not been resolved, and so it should be no surprise that it is causing you so much head-scratching

Nobody here says its been solved. Consistently we all say issues remain.

The question is - are they worth worrying about. A number of people here, including myself and Vanhees, believe in the Ensemble Interpretation of Einstein - updated for modern times of course. But unlike Einstein many of us accept its just the way the world is and don't get worked up over it. There is no way to tell the difference between an improper mixed state and a proper one so who cares? Yes they are different but so what?

Just our view, but we are happy with it. Einstein probably wouldn't be - but to each their own.

Bottom line - no it has not been fully solved, but its purely a matter of opinion if its worth getting upset about. Every theory, every single one has things it accepts, it simply a matter of taste if you get worked up over them or not. I for one am perfectly happy with the state affairs. Some get worked up about foundational issues in probability, but most don't care a hoot and simply use it. Its not shut-up and calculate - its more like - well yes they are their but exactly why is it a worry? Its not like any theory explains everything.

I like to study various interpretations, not because I want to get to the bottom of how it all really works or anything like that. I am perfectly happy with the Ensemble interpretation. It just helps me understand the formalism better. For example its easy to get the impression from the formalism it has collapse - it doesn't but its not really clear until you study non-collapse interpretations - or even exactly how nuanced the question of just what collapse means - it's by no means straight forward.

Thanks
Bill

 

[atyy]

One can certainly avoid the problem by resorting to interpretations that avoid one or more assumptions you have made, but they then have their own problems, resulting in no consensus whatsoever about the best way to respond to the problem.

There need not be a best way without experiment. We would like to know all ways of responding, then deciding the best way by experiment.

 

[atyy]

Its not shut-up and calculate - its more like - well yes they are their but exactly why is it a worry? Its not like any theory explains everything.

Indeed why worry about quantum gravity - the existing theory is perfectly fine.

 

[bhobba]

Indeed why worry about quantum gravity - the existing theory is perfectly fine.

Well actually it is - up to the plank scale:
https://arxiv.org/abs/1209.3511

We want to know beyond that.

QM does not have that issue - as far as we know it works everywhere. It explains all phenomena in its domain - in gravity there is a domain about which we really know nothing. That may require a revision in QM to resolve - but any theory - any theory at all is just provisional.

Thanks
Bill

 

[vanhees71]

This is a very elegant articulation of the so-called Measurement Problem, and makes very clear why it is called a 'problem', namely: the experiments used to justify quantum mechanics are, by that very theory, not dynamically possible!

It's only not dynamically possible, if you insist on a collapse assumption, but that's not even part of many flavors of the Copenhagen interpretation!

 

[bhobba]

and those don’t describe how we - as conscious observers - experience our world.

Why yes - that is the central mystery - why do we get outcomes at all - colloquially of course - not technically which requires considerable detail to explain properly.

But the question is why don't you just accept that's how nature is? Why get worked up about it? Even if you answer it , and its experimentally proven, there will be another unknown that replaces it. Its just a matter of taste if you like some assumptions and not others.

If the issue interests you - great - research away but I get this sneaky feeling those that harp on it have some sort of evangelist bent this is the Earth shattering thing about QM that needs immediate attention. Sorry - but I don't agree.

Thanks
Bill

 

[kith]

@Demystifier, I find it very interesting to read about your evolving views on QM. Thanks for writing it up!

I don't know if this is incidental but David Wallace has written an article with the same film reference in its name and about a similar topic (Decoherence and Ontology, or: How I Learned To Stop Worring And Love FAPP). He also talks about emergent structures and uses analogies about quasi-particles, albeit from a MWI perspective.

 

[kith]

I also have a question regarding arXiv:1703.08341. If all particles are actually quasi-particles emerging from the behaviour of hypothetical non-relativistic "atoms", how are these "atoms" different from the aether?

For photons, don't the same counterarguments apply to this idea as to the aether in classical electromagnetism?

 

[kith]

But the question is why don't you just accept that's how nature is? Why get worked up about it?

Personally, I'm still figuring out how much I want to get worked up about it. ;-)

I have a bit of sympathy for the view that we can't remove the observer from science, that QM is a broad hint in this direction and that there's a limit to our understanding of Nature. But this still leaves a number of things to ponder about. How much exactly can we say and where is the limit? Can this view be reconciled with everyday realism? What does QM tell us about the nature of probabilities? etc.

 

[Lord Jestocost]

The question is - are they worth worrying about. A number of people here, including myself and Vanhees, believe in the Ensemble Interpretation of Einstein - updated for modern times of course. But unlike Einstein many of us accept its just the way the world is and don't get worked up over it. There is no way to tell the difference between an improper mixed state and a proper one so who cares? Yes they are different but so what?

I care. There is a difference, because even if the "syntaxes" (when looking at the density matrices) seem to be the same, the "semantics" are fundamentally different. Physics should as a matter of principle avoid to present interpretations which might somehow be subject to "confirmation bias".

 

[fanieh]

The macroscopic description of a measurement situation might be something like this:

• if you prepare an electron in the spin state \alpha |U\rangle + \beta |D\rangle and perform a measurement of spin in the z-direction, then the device will make a transition to the "Measured spin-up" state with probability |\alpha|^2 and to "Measured spin-down" with probability |\beta|^2.

Since "Measured spin-up" and "Measured spin-down" are presumably states of the measuring device, and the measuring device is made up of ordinary particles, then it seems that in principle, the above rule should be re-expressible in the form:

• If you prepare a collection of particles in such-and-such a state, then later they will be in such-and-such a state with probability |\alpha|^2. (Not "measured to be" in that state, because the system already includes the measuring device, which presumably doesn't need to be measured by a third system. Or does it?)

It seems that in principle, it should be possible to eliminate the measurement aspect of the theory and re-express it as a theory of pure particles. If that can't be done, that seems pretty weird to me. On the other hand, we're pretty sure that it can't be done, because the dynamics of pure particles is deterministic (Schrodinger's equation) regardless of how many particles are involved. So there's something weird going on.

You are basically asking why the detector in the double slit experiment can only detect one electron and not have multiple hits for only one electron emitted. But isn't it that according to:

1. Bohmian Mechanics.. there is a trajectory for the one electron being emitted so it hits the detector at one point...
2. Many Worlds.. there are multiple hits in the screen.. but we only viewed one of them because we are entangled with only one of them...
3. Copenhagen.. the wave function may pass through both slits but it collapses into one hit when it reached the screen...

These are the explanations why there is only one detector hit in the screen and supposed to address your "So there's something weird going on" ... are you saying you don't believe in the explanations? I can't seem to get your point... it's as if the interpretations are not related to your concern? Kindly elaborate.

 

[bhobba]

Personally, I'm still figuring out how much I want to get worked up about it. ;-)

That's the issue isn't it.

Some get almost evangelistic about it, others just shrug and accept say what Ballentine says.

Still others like me, while agreeing mostly with Ballentine gain insight by studying various interpretations to understand the formalism better.

And either one of those can get really 'into it' as many threads here show.

Thanks
Bill

 

[fanieh]

That's the issue isn't it.

Some get almost evangelistic about it, others just shrug and accept say what Ballentine says.

Still others like me, while agreeing mostly with Ballentine gain insight by studying various interpretations to understand the formalism better.

And either one of those can get really 'into it' as many threads here show.

Thanks
Bill

Something I want to know. If quantum state and even the quantum fields are just smoke and mirrors or not really there.. but statistical.. why are there forces of nature such as the electroweak force.. if objects are just smoke and mirror.. why do they seem to exist as stable object. Are you saying that symmetry and gauge symmetry is what created our universe.. so it's a battle or difference between pure mathematical symmetry and quantum state having objective properties.. but can't they occur at same time.. that is.. our universe results from mathematical symmetry and quantum state can be real? Or only one thing is true and why is that?