Undergrad The thermal interpretation of quantum physics

[stevendaryl]

I agree with 1, but not with 2. If this were so, then you'd simple have a bad measurement device. A measurement device gives a unique result, when measuring an observable (within its accuracy of course).

Well, that contradicts the linearity of quantum evolution, it seems to me.

 

[stevendaryl]

The equation for the measurement device's macroscopic pointer readings alone is not according to linear quantum-time evolution, as is the case for any open system. A measurement device necessarily has some dissipation to lead to an irreversible storage of the measured result.

There can't be something magical happening in open systems. The point of an open system is that you have the system of interest interacting with another system whose details we either do not know completely, or choose to abstract away from.

What you're doing by saying "Oh, we don't have to obey linear evolution because it's an open system" is that your pushing the contradiction out into the part of the composite system that you're ignoring (the environment). That's bogus.

 

[DarMM]

I agree with 1, but not with 2. If this were so, then you'd simple have a bad measurement device. A measurement device gives a unique result, when measuring an observable (within its accuracy of course).

Well if you model the atomic object, the device and the lab environment, then the total system as modeled by an external observer outside the lab will be a superposition. That's a fact.

So either you're saying that QM cuts off at some scale, or you're just saying the device subsystem enters a specific (but unknown for the outside observer) state. You can't say the whole lab state evolves into a single eigenstate.

 

[kith]

The only slighly mysterious thing is why Alice can predict Bob's measurement. Here I don't have a full explanation, but only arguments that show that nothing goes wrong with relativistic causality.

Like others, I don't think that this is only "slightly mysterious" but the central mystery. But I agree with you that some tangible calculations are missing.

My recollection of studying the dynamics of open quantum systems a while back is the following: The Lindblad equation, which encompasses transitions from pure system states to mixed system states, can be derived from unitary dynamics of the system together with the environment in general as well as in specific settings, if one makes certain assumptions (mainly the Born-Markov approximation). What I didn't find in the literature back then, was a model of an entangled system consisting of two subsystems with only one of them interacting with an environment. Depending on the level of detail, seeing decoherence in such an entangled system arise dynamically from a local interaction between a single subsystem and an environment might shed some light on the machinery of non-local correlations.

 

[N88]

The only slighly mysterious thing is why Alice can predict Bob's measurement. Here I don't have a full explanation, but only arguments that show that nothing goes wrong with relativistic causality.

Note also that the prediction comes out correct only when the entangled state is undisturbed and the detector is not switched off at the time the photon hits - things that Alice cannot check until having compared notes with Bob. Thus her prediction is not a certainty but a conditional prediction only.

Like others, I don't think that this is only "slightly mysterious" but the central mystery. But I agree with you that some tangible calculations are missing.

A question: Why is there anything mysterious when Alice can predict Bob's measurement with certainty?

Under idealised EPRB and relativistic-causality there are no tricks, so the mechanics appear to be straight-forward: Let Alice observe the result A = +1, and let her predict that Bob will observe B = -1. Then, via Bell (1964; eqns (1) & (13)), we have the following mechanics:

$$A = +1 = A(a,\lambda). \;\;B= B(a,\lambda)= -A(a,\lambda) = -1.$$
QED? For a theory that disagrees or that sees mystery here would, it seems to me, be suspicious.

 

[kith]

Then, via Bell (1964; eqns (1) & (13)), we have the following mechanics:

$$A = +1 = A(a,\lambda). \;\;B= B(a,\lambda)= -A(a,\lambda) = -1.$$

That's not the mechanics, that's the thing which needs to be explained. It's like balancing energies: you know that you have the correct result (actually, I haven't checked, if what you write is correct) because your initial and your final value are in agreement with experiment but you don't know how the system gets from the initial to the final state.

In order to understand, I want to know how the fundamental entities of the theory evolve in time. In classical mechanics, Newton's laws accomplish this and in QM, the Schrödinger equation should do it. But alas, a) we have the measurement problem and b) I can't picture how dynamical decoherence works in entangled systems, so I'm not satisfied.

Of course, there's the possibility, that these problems can't be solved and that this is the lesson, QM is trying to teach us. But then, the answer is still not "we know the mechanics" but "the mechanics can't be known".

 

[stevendaryl]

A question: Why is there anything mysterious when Alice can predict Bob's measurement with certainty?

Well, let's assume that Bob and Alice agreed ahead of time to measure spin along the z-axis. For definiteness, let's assume that in Alice's rest frame, she measures her particle's spin before Bob measures his. Then let's consider the statement "Bob will measure spin-down".

1. Immediately before Alice's measurement, she doesn't know what value Bob will get. So she doesn't know whether the statement is true or false.
2. Immediately afterward, when she gets spin-up, she knows that Bob will get spin-down. So she knows the statement is true.

Between 1 & 2, what happened? Did the truth value of the statement change because of her measurement? Or did she just learn its truth value? In other words, did the statement become true when Alice performed her measurement, or was it true beforehand, and Alice just didn't know it?

The first possibility, that it only became true when Alice performed her measurement, would seem to suggest that Alice's actions affected Bob's situation. That is, it suggests action-at-a-distance. The second possibility, that it was true before her measurement, and her measurement just informed her of this fact, seems to suggest a hidden-variable (the value of a variable before it's measured). But Bell's theorem shows that there can't be a hidden-variables explanation of the EPR experiment without nonlocality (or superdeterminism, there is that loophole in the argument).

 

[DarMM]

Of course, there's the possibility, that these problems can't be solved and that this is the lesson, QM is trying to teach us. But then, the answer is still not "we know the mechanics" but "the mechanics can't be known".

That's essentially the view of all Copenhagen flavors (Bohr, Heisenberg, Haag, Bub, Healey, Peres, Brukner, Zeilinger, Wheeler), Consistent Histories (Gell-Mann, Griffiths, Omnès) and QBism (Fuchs, Schack)

QM is a break from the previous representational theories of physics, it doesn't fundamentally tell you what is going on.

 

[stevendaryl]

In order to understand, I want to know how the fundamental entities of the theory evolve in time.

Right. Some people, though, would say that physics isn't about how the world evolves with time, but how our knowledge of the world evolves. I don't agree with that. "Knowledge" doesn't mean anything (to me) if there is isn't a truth that you can know. And making observations and measurements into the basic entities of physics seems perverse to me. Observations and measurements are physical acts performed by physical systems (if very complicated ones). It doesn't make sense to me to make them fundamental.

 

[DarMM]

Right. Some people, though, would say that physics isn't about how the world evolves with time, but how our knowledge of the world evolves. I don't agree with that. "Knowledge" doesn't mean anything (to me) if there is isn't a truth that you can know. And making observations and measurements into the basic entities of physics seems perverse to me. Observations and measurements are physical acts performed by physical systems (if very complicated ones). It doesn't make sense to me to make them fundamental.

The common explanation of this in the views I described above is that although Observations and Measurements aren't actually fundamental physically (i.e. out in the world) that QM represents a point where you can't do any better than a generalized probability theory and by being a probability theory it just has observations as a primitive notion in the theory (though not assumed to be so in reality).

In the QBism papers and the works of Gell-Mann, Griffiths and Omnès you can read that they think that "under" QM nature becomes non-mathematical or not fully susceptible to mathematization.

 

[atyy]

That's essentially the view of all Copenhagen flavors (Bohr, Heisenberg, Haag, Bub, Healey, Peres, Brukner, Zeilinger, Wheeler), Consistent Histories (Gell-Mann, Griffiths, Omnès) and QBism (Fuchs, Schack)

There are other Copenhagen flavours like Dirac's, which is consistent with Demystifier's - ie. we like Copenhagen, but we also believe that trying to solve the measurement problem in the spirit of Bohmian mechanics, GRW etc is or will become a viable research area.

Dirac: Of course there will not be a return to the determinism of classical physical theory. Evolution does not go backward. It will have to go forward. There will have to be some new development that is quite unexpected, that we cannot make a guess about, which will take us still further from Classical ideas but which will alter completely the discussion of uncertainty relations. And when this new development occurs, people will find it all rather futile to have had so much of a discussion on the role of observation in the theory, because they will have then a much better point of view from which to look at things. https://blogs.scientificamerican.com/guest-blog/the-evolution-of-the-physicists-picture-of-nature/

Demystifier: https://www.physicsforums.com/insights/stopped-worrying-learned-love-orthodox-quantum-mechanics/

 

[DarMM]

There are other Copenhagen flavours like Dirac

I agree with you that those were Dirac's views, but I think they deviate so strongly from Copenhagen that they deserve another name. I don't think it's a flavor of Copenhagen. He tends to be classified separately in historical studies. I've seen it called the "Dirac-VonNeumann" interpretation. It has elements Copenhagen doesn't have like the eigenvalue-eigenstate link.

See page 2 here for how he is often classified in Historical studies of QM and Quantum Foundations:
https://www.fetzer-franklin-fund.org/wp-content/uploads/2015/12/Ringbauer-EmQM15-part1.pdf

 

[julcab12]

Ill stick to "time goes at different speed for entangled particle".Such unusual effect is relevant classically, under strong gravitational well like Einstein Cross. In QM, mainly relational interpretation-- time dilation effect. Shown here in slide to 24:00min According to Rovelli with two pairs of entangled particle.

 

[Demystifier]

That's essentially the view of all Copenhagen flavors (Bohr, Heisenberg, Haag, Bub, Healey, Peres, Brukner, Zeilinger, Wheeler), Consistent Histories (Gell-Mann, Griffiths, Omnès) and QBism (Fuchs, Schack)

QM is a break from the previous representational theories of physics, it doesn't fundamentally tell you what is going on.

My problem with Copenhagen flavors of that kind is not that it doesn't tell you what is going on. My problem is that they try to make one step more by saying that nothing is going on at all. Not that we currently don't know what is going on, not even that it is in principle impossible to know what is going on, but that we do know that nothing is going on. Such a Copenhagen flavor is for me totally unjustified.

But where does such a flavor come from? I think it comes from a human desire to understand things completely, to believe that the best theories we currently have are in fact the final theories of everything. In essence, since QM doesn't say what is going on, the assumption that QM is the final theory implies that nothing is going on. So it's quite easy to understand where does such a Copenhagen flavor come from in a psychological sense.

Nevertheless, such a view is irrational and unscientific. If some questions don't have answers within our best theories, it doesn't mean that those questions don't have answers at all. It means that our best theories are not so perfect as we would like them to be. Perhaps it's not easy to accept, but this is what we should accept. QM is incomplete.

 

[DarMM]

My problem with Copenhagen flavors of that kind is not that it doesn't tell you what is going on. My problem is that they try to make one step more by saying that nothing is going on at all. Not that we currently don't know what is going on, not even that it is in principle impossible to know what is going on, but that we do know that nothing is going on. Such a Copenhagen flavor is for me totally unjustified.

I thought they said the second, i.e. "it is in principle impossible to know what is going on". That's what they say in their texts and papers. I can give you quotes to this effect if you wish.

I've never seen them say "nothing" is going on.

This has come up a few times, I'd be happy to discuss it on another thread.

 

[Demystifier]

I thought they said the second, i.e. "it is in principle impossible to know what is going on". That's what they say in their texts and papers. I can give you quotes to this effect if you wish.

I've never seen them say "nothing" is going on.

This has come up a few times, I'd be happy to discuss it on another thread.

I will open a separate thread for this.

EDIT: Here it is https://www.physicsforums.com/threa...-knowledge-or-restriction-on-ontology.968982/

 

[Lord Jestocost]

Right. Some people, though, would say that physics isn't about how the world evolves with time, but how our knowledge of the world evolves. I don't agree with that. "Knowledge" doesn't mean anything (to me) if there is isn't a truth that you can know. And making observations and measurements into the basic entities of physics seems perverse to me. Observations and measurements are physical acts performed by physical systems (if very complicated ones). It doesn't make sense to me to make them fundamental.

Sir Arthur Eddington in “The Philosophy of Physical Science: Tarner Lectures (1938)”:

“For the truth of the conclusions of physical science, observation is the supreme Court of Appeal. It does not follow that every item which we confidently accept as physical knowledge has actually been certified by the Court; our confidence is that it would be certified by the Court if it were submitted. But it does follow that every item of physical knowledge is of a form which might be submitted to the Court. It must be such that we can specify (although it may be impracticable to carry out) an observational procedure which would decide whether it is true or not. Clearly a statement cannot be tested by observation unless it is an assertion about the results of observation. Every item of physical knowledge must therefore be an assertion of what has been or would be the result of carrying out a specified observational procedure.” [italics in original]

 

[atyy]

I agree with you that those were Dirac's views, but I think they deviate so strongly from Copenhagen that they deserve another name. I don't think it's a flavor of Copenhagen. He tends to be classified separately in historical studies. I've seen it called the "Dirac-VonNeumann" interpretation. It has elements Copenhagen doesn't have like the eigenvalue-eigenstate link.

See page 2 here for how he is often classified in Historical studies of QM and Quantum Foundations:
https://www.fetzer-franklin-fund.org/wp-content/uploads/2015/12/Ringbauer-EmQM15-part1.pdf

Well, I don't mean in the historical sense. I mean in the common practice of physicists, where I'm using Copenhagen in a very broad sense, to be synonymous with what some might call the "orthodox" interpretation. Thus I'm using Copenhagen in the sense that major textbooks identify themselves with Copenhagen - Landau & Lifshitz, Messiah, Weinberg. The eigenvalue-eigenstate link has long gone out of use with POVMs etc. Also, I'm not sure if strict historical classification is useful. Historically, von Neumann did not support hidden variables because of flawed reasoning, yet the same flaw indicates that he was searching for the possibility of hidden variables.

Messiah in particular calls his interpretation Copenhagen, but does not rule out hidden variables.

 

[DarMM]

I understand what you mean, but what did Dirac actually agree with Bohr on that they could come under one term "Copenhagen". When you use it in the "common practice" sense what common elements does it refer to?

 

[atyy]

I understand what you mean, but what did Dirac actually agree with Bohr on that they could come under one term "Copenhagen". When you use it in the "common practice" sense what common elements does it refer to?

I think common practice is
(i) the informal realization that the postulates give measurement a special status, leading to something like the quantum-classical cut and the instrumental or FAPP view of QM
(ii) the formal or mathematical postulates in eg. Dirac, von Neumann, Weinberg, and stated in updated form by eg. Nielsen and Chuang- including the postulate for state update after a measurement outcome

So in my classification of Copenhagen, there are only 2 types depending on their reaction to the special status of measurement.
(ia) I'm only FAPP so I don't care (Bohr, Heisenberg).
(ib) I'm FAPP for QM, but I'm open to the measurement problem suggesting that there is a theory to be had beyond QM (Dirac, spirit of von Neumann, Messiah, Bell, Demystifier).

 

[DarMM]

I see. Basically in quantum foundations Copenhagen is used to only mean (ia) [though perhaps not phrased that way], so Bub, Healey and others in my list are in that camp.

 

[atyy]

I see. Basically in quantum foundations Copenhagen is used to only mean (ia) [though perhaps not phrased that way], so Bub, Healey and others in my list are in that camp.

Yes, in the communities or papers that need it, terminology is used differently from the broader textbook terminology. So there textbook Copenhagen is more often called "orthodox".

 

[A. Neumaier]

The Lindblad equation, which encompasses transitions from pure system states to mixed system states, can be derived from unitary dynamics of the system together with the environment in general as well as in specific settings, if one makes certain assumptions (mainly the Born-Markov approximation).

Yes, but this already produces nonlocal interaction terms!

Note also that the Born-Markov approximation turns all open systems into systems no longer described by a wave function but by a density operator of rank $>1$. In the Born-Markov approximation, this density operator is an improper mixture in the standard terminology, hence cannot be thought of as being ''in reality'' a classical mixture of pure states!

 

[A. Neumaier]

"Knowledge" doesn't mean anything (to me) if there is isn't a truth that you can know.

Yes, ''knowledge'' of anything else than truth is pure imagination or pretense.

The eigenvalue-eigenstate link has long gone out of use with POVMs etc.

But it still figures in Section 8.1 of the influential book

• M. Schlosshauer, Decoherence and the quantum-to-classical transition, Springer, New York 2007.

as part of the ''standard interpretation''; see top of p.331. It is also in Wikipedia, here and here.

von Neumann did not support hidden variables because of flawed reasoning, yet the same flaw indicates that he was searching for the possibility of hidden variables.

No. His reasoning was not flawed; his notion of ''hidden variables'' was well-defined and properly stated. Rather than searching for it he was ruling out the version of hidden variables that he considered physically meaningful.

It is a methodical mistake to assume that an old paper uses terminology in a sense shaped by the much later usage of the same term. von Neumann's notion was more restricted than the modern usage arising much later with Bohm and Bell.

Messiah in particular calls his interpretation Copenhagen, but does not rule out hidden variables.

Many authors term 'Copenhagen' their own version of the Copenhagen interpretation that Bohr and Heisenberg presented in 1927.

 

[kith]

So in my classification of Copenhagen, there are only 2 types depending on their reaction to the special status of measurement.
(ia) I'm only FAPP so I don't care (Bohr, Heisenberg).
(ib) I'm FAPP for QM, but I'm open to the measurement problem suggesting that there is a theory to be had beyond QM (Dirac, spirit of von Neumann, Messiah, Bell, Demystifier).

You have the positions "I don't care" and "I'm open to the measurement problem being solved by a more fundamental theory". What about the position "I don't think that the measurement problem can be solved by a more fundamental theory."? I think that one characterizes Bohr's position better because he emphasized the indispensability of classical concepts. Your Ia sounds more like shut-up-and-calculate.

 

[ftr]

@A. Neumaier With regards to your interpretation, suppose you have an ideal particle (say an electron) in a box (1D ..etc), what can you measure and what is the relation between repeated measurements? Thanks.

Edit: you can assume hypothetical( in case practical is hard) and practical measurements if you like.

 

[stevendaryl]

Bell tried to explain what he thought was nonlocal about the EPR experiment in an essay called "The Theory of Local Beables". My paraphrase of his argument is this:

This picture represents a joint measurement conducted by distant observers Alice and Bob. The horizontal axis represents space and the vertical axis represents time. Alice's measurement is conducted in a localized region of spacetime, region 1. Bob's measurement is conducted in region 2. Shown in the picture are the backwards lightcones for Alice's and Bob's experiments.

Bell's intuition about locality is that the result of Bob's measurement can only depend on facts about his backward lightcone. The result of Alice's measurement can only depend on facts about her backward lightcone. Therefore, the only way that Alice's measurement can give information about the results of Bob's measurement is if reveals facts about region 5, which is the intersection of their two lightcones.

So for example, suppose that there is a pair of shoes, one left shoe and one right shoe. In region 5, each shoe is placed into a different box, and one box is sent to Alice, and the other is sent to Bob. When in region 1 Alice opens her box and sees a left shoe, she immediately knows that Bob will see a right shoe when he opens his box in region 2. Even though this is nonlocal information, the information breaks into two local correlations: (1) Alice's result tells her something about what happened in Region 5. (2) What happened in Region 5 tells her about what will happen in region 2.

The EPR experiment seems to violate this notion of locality. Alice measuring the z-component of spin of her particle in region 1 tells her what Bob will get if he measures the z-component of spin of his particle in region 2. But Bell's inequality implies that there are no facts about what happened in Region 5 that would allow allow the prediction of Bob's result.

 

[vanhees71]

Ah, so you change the fundamental law of quantum mechanics and say that it applies never. For the only truly closed system we have access to is the whole universe, and you mentioned repeatedly that to apply quantum mechanics to it is nonsense.

So where does the dissipative description of the measurement device that you invoke come from, from a fundamental perspective?

I don't change any fundamental law. The fundamental laws are, if interpreted such that a physicist can make sense of it, what you've given in your papers we are discussing.

One last time: The macroscopic observables are coarse-grained, i.e., averages over many microscopic degrees of freedom. Another name is "collective modes" or the like. You can systematically derive semiclassical transport, classical transport, viscous and ideal hydrodynamics (including dissipation), Fokker-Planck/Langevin equations (including dissipation and fluctuation and their relation) etc. etc. from these principles. All this many-body physics applies to measurement devices as to any other macroscopic system, and there's no new fundamental rule, as claimed in some of the old flavors of the Copenhagen interpretation. It's one of the unnecessary philosophical additions (usually called quantum-classical cut) that has no scientific foundation whatsoever!

 

[vanhees71]

As a macrocopic system a measurement device cannot be described in all microscopic details that indeed follows unitary time evoution but it is described by statistical quantum theory mostly in terms of macroscopic, i.e., over many microscopic degrees of freedom averaged observables, leading to classical behavior of these macroscopic observables. E.g., there's no use to describe a galvanometer measuring a macroscopic electric current in all microscopic details using QED. However, you can use many-body quantum statistics to derive its macroscopic behavior.

 

[atyy]

You have the positions "I don't care" and "I'm open to the measurement problem being solved by a more fundamental theory". What about the position "I don't think that the measurement problem can be solved by a more fundamental theory."? I think that one characterizes Bohr's position better because he emphasized the indispensability of classical concepts. Your Ia sounds more like shut-up-and-calculate.

Good point. I'm not sure historically whether that is more what Bohr thought. Personally, I put it more as a necessary clause of of the "I am open" class, since it may be how nature and mathematics really work. It doesn't seem rational to believe in it without further evidence, since
(i) it may be true also of quantum gravity
(ii) Bohmian mechanics provides a conceptual counterexample for some domain of QM.