Decoherence vs Observer Wave Function Collapse & MWI: Questions

[Bmarcus]

TL;DR

Questions about decoherence vs. observer wave function collapse and multi-worlds interpretation. Am I right that Zeh's decoherence theory does not involve an observer, and esp not a conscious observer? Also, am I right that his theory does not involve or align with the "multi-worlds" interpretation? Tx

I have a couple of questions about decoherence vs. observer wave function collapse and multi-worlds interpretation. Am I right that Zeh's decoherence theory does not involve an observer, and esp not a conscious observer? Also, am I right that his theory does not involve or align with the "multi-worlds" interpretation? PLEASE NOTE: I have a nearly zero math skills, so I would very much appreciate responses that don't involve complex formulas, calculus, trigonometry (whatever that is :) and whatnot! Thx much!

 

[Nugatory]

Am I right that Zeh's decoherence theory does not involve an observer, and esp not a conscious observer?

Pretty much, yes.

Also, am I right that his theory does not involve or align with the "multi-worlds" interpretation?

The many-worlds interpretation is consistent with decoherence. It pretty much has to be, as decoherence is implied by the mathematical formalism of quantum mechanics and all interpretations are attempts to explain how that formalism corresponds to reality. In fact, decoherence provides a natural explanation for how the “worlds” “separate” (the scare quotes are because we are hard up against the limits of math-free description).

 

[Lord Jestocost]

TL;DR Summary: Questions about decoherence vs. observer wave function collapse and multi-worlds interpretation. Am I right that Zeh's decoherence theory does not involve an observer, and esp not a conscious observer? Also, am I right that his theory does not involve or align with the "multi-worlds" interpretation? Tx

I have a couple of questions about decoherence vs. observer wave function collapse and multi-worlds interpretation. Am I right that Zeh's decoherence theory does not involve an observer, and esp not a conscious observer? Also, am I right that his theory does not involve or align with the "multi-worlds" interpretation? PLEASE NOTE: I have a nearly zero math skills, so I would very much appreciate responses that don't involve complex formulas, calculus, trigonometry (whatever that is :) and whatnot! Thx much!

In a recent review on decoherence, Erich Joos states (https://arxiv.org/abs/quant-ph/9908008):

“Does decoherence solve the measurement problem? Clearly not. What decoherence tells us is that certain objects appear classical when observed. But what is an observation? At some stage we still have to apply the usual probability rules of quantum theory.” [Bold by LJ]

 

[Demystifier]

In a recent review on decoherence, Erich Joos states (https://arxiv.org/abs/quant-ph/9908008):

Recent?

 

[vanhees71]

In a recent review on decoherence, Erich Joos states (https://arxiv.org/abs/quant-ph/9908008):

“Does decoherence solve the measurement problem? Clearly not. What decoherence tells us is that certain objects appear classical when observed. But what is an observation? At some stage we still have to apply the usual probability rules of quantum theory.” [Bold by LJ]

What measurement problem? Our experimental colleagues observe with ever better precision quantum phenomena by constructing ever better measurement instruments, and the theory has been found always to be correct. So again my question: Where the heck is "the measurement problem"?

Decoherence indeed explains, why macroscopic objects almost always, seem to obey the rules of classical physics or on the other hand, why it is so difficult to reveal "quantum phenomena" on everyday macroscopic objects. On the other hand, also for macroscopic objects, there has been demonstrated that they indeed behave according to quantum theory. For me one of the most astonishing example is the use of the LIGO detectors to also demonstrate the quantization of (oscillatory) motion of an effectively 10kg heavy oscillator:

https://doi.org/10.1126/science.abh2634

 

[LittleSchwinger]

I would suggest having a read of a full recent model of how macroscopic commutativity arises, such the Allahverdyan et al (2011). Environmental Decoherence is actually not the dominant reason for classicality. Equilibration processes, thermalisation and the contraction of the algebra of observables are stronger effects. See the following recent paper by Frohlich for a rigorous worked model of the latter.

 

[Demystifier]

What measurement problem? Our experimental colleagues observe with ever better precision quantum phenomena by constructing ever better measurement instruments, and the theory has been found always to be correct. So again my question: Where the heck is "the measurement problem"?

The measurement problem is not in the real world, i.e. the problem has nothing to do with actual measurements in the laboratory. Instead, the problem is an internal problem of the theory, existing only within the theory itself. Namely, the theory talks about measurements, but in its minimal form the theory does not explain precisely what a measurement is. This is not a problem from an engineering/instrumental point of view, but can be a problem from a mathematical point of view if you think of quantum theory as a mathematical theory in which every concept should be defined precisely in mathematical terms. If you think of quantum theory as a theory for engineers, you cannot see any measurement problem. So if you really want to understand where do some people see a problem, you must think of it from their perspective, which is not an engineering/instrumental perspective. If you don't want to change your perspective because the engineering/instrumental perspective is perfectly fine for you, that's OK, but then don't ask others to explain you what's the measurement problem. It's absolutely impossible to explain what's the problem from the engineering/instrumental point of view, because from this point of view there is no measurement problem at all.

It is possible to write the book
"The measurement problem for mathematicians"
or
"The measurement problem for philosophers",
but it is impossible to write the book
"The measurement problem for experimentalists".

 

[Lord Jestocost]

What measurement problem? Our experimental colleagues observe with ever better precision quantum phenomena by constructing ever better measurement instruments, and the theory has been found always to be correct. So again my question: Where the heck is "the measurement problem"?

Decoherence indeed explains, why macroscopic objects almost always, seem to obey the rules of classical physics or on the other hand, why it is so difficult to reveal "quantum phenomena" on everyday macroscopic objects. On the other hand, also for macroscopic objects, there has been demonstrated that they indeed behave according to quantum theory. For me one of the most astonishing example is the use of the LIGO detectors to also demonstrate the quantization of (oscillatory) motion of an effectively 10kg heavy oscillator:

https://doi.org/10.1126/science.abh2634

Of course, the term “measurement problem” is dispensable in Joos’ remark.

 

[WernerQH]

Instead, the problem is an internal problem of the theory, existing only within the theory itself.

Exactly. But @vanhees71 is not an engineer, so why isn't he able to see it? If quantum theory is a mature theory, and if measurement is an integral part of the theory, why is the community of theorists still divided about whether or not there is a measurement problem? I don't believe in the second if, but people who do have a "meta"-physical problem at their hands: why does it take a century to reach consensus on a mature theory?

 

[Demystifier]

But @vanhees71 is not an engineer, so why isn't he able to see it?

When being engineer helps a problem go away, then he puts on an engineer hat. Many practical physicists do that.

but people who do have a "meta"-physical problem at their hands: why does it take a century to reach consensus on a mature theory?

In the language of model theory in mathematical logic, that's because the axioms of quantum theory are not categorical, i.e. there are many inequivalent models that satisfy the axioms.
https://en.wikipedia.org/wiki/Categorical_theory
Physicists refer to these models as interpretations.

 

[vanhees71]

The measurement problem is not in the real world, i.e. the problem has nothing to do with actual measurements in the laboratory. Instead, the problem is an internal problem of the theory, existing only within the theory itself. Namely, the theory talks about measurements, but in its minimal form the theory does not explain precisely what a measurement is.

That's not a problem but a feature. Physical theories, at least on the fundamental level, provide abstract definitions for observables. How to measure them in the lab is not determined and it shouldn't be. It's open to technological development. Of course, if a theory has no relation to observations, i.e., if its abstract definitions for observables simply cannot be realized (approximately) in real-world technology, it's an empty mathematical game (like, e.g., string theory).

This is not a problem from an engineering/instrumental point of view, but can be a problem from a mathematical point of view if you think of quantum theory as a mathematical theory in which every concept should be defined precisely in mathematical terms. If you think of quantum theory as a theory for engineers, you cannot see any measurement problem.

There are also no mathematical problems with the theory, at least not for non-relativistic QM, which was mathematically strictly formulated very early on by von Neumann et al. It also lead to the development of functional analysis as a subfield within pure mathematics.

So if you really want to understand where do some people see a problem, you must think of it from their perspective, which is not an engineering/instrumental perspective. If you don't want to change your perspective because the engineering/instrumental perspective is perfectly fine for you, that's OK, but then don't ask others to explain you what's the measurement problem. It's absolutely impossible to explain what's the problem from the engineering/instrumental point of view, because from this point of view there is no measurement problem at all.

So there is no problem with QT as a physical theory. That's all I'm saying.

It is possible to write the book
"The measurement problem for mathematicians"
or
"The measurement problem for philosophers",
but it is impossible to write the book
"The measurement problem for experimentalists".

I don't think that mathematicians and philosophers have much to say about measurement problems, which indeed are an engineering problem. You don't need a book the "measurement problem for experimentalists", because that's what experimental-physics books are all about!

 

[vanhees71]

When being engineer helps a problem go away, then he puts on an engineer hat. Many practical physicists do that.In the language of model theory in mathematical logic, that's because the axioms of quantum theory are not categorical, i.e. there are many inequivalent models that satisfy the axioms.
https://en.wikipedia.org/wiki/Categorical_theory
Physicists refer to these models as interpretations.

Interpretations have nothing to do how you mathematically realize QT. The different mathematical formulations like "matrix mechanics", "wave mechanics", "Dirac's bra-ket formalism", "path integrals", are all just the same theory. Interpretational issues are purely philosophical problems which should be strictly separated from the mathematical foundations and physics applications.

 

[WernerQH]

Physical theories, at least on the fundamental level, provide abstract definitions for observables. How to measure them in the lab is not determined and it shouldn't be.

This is just rationalization that you have internalized. Pure abstractions have never led to new physical theories; they were added later for their "rigorous" formulations.

There are also no mathematical problems with the theory, at least not for non-relativistic QM, which was mathematically strictly formulated very early on by von Neumann et al.

You seem to think of Bohr and von Neumann as prophets, that a better (physical) formulation of QM cannot possibly be found, ever.

 

[Lord Jestocost]

Jan Faye in “Niels Bohr and the Philosophy of Physics: Twenty-First-Century Perspectives” (edited by Jan Faye and Henry J. Folse, published 2017):

“In Bohr's view there is no measurement problem. Many physicists and philosophers claim there is a problem. But if you, like Bohr, do not take the wave function to designate an objective property of the system but to be a manual for thinking about the system, then the problem becomes a figment of the imagination. The claim that there is a problem simply presupposes that scientific realism is correct, and that scientific theories tell us how the world really is.” [Bold by LJ]

 

[A. Neumaier]

[Bold by LJ]

Who is this?

 

[mattt]

Whoi is this?

The poster himself

 

[vanhees71]

This is just rationalization that you have internalized. Pure abstractions have never led to new physical theories; they were added later for their "rigorous" formulations.

Theory is exactly about abstractions. Of course, from pure thought never came out new physical theories. There's a mutual "interaction" between theory and experiment. What I meant is that also in a theoretical book on Newtonian mechanics, you'll not find specific explanations for how to measure position, time, velocity, acceleration, and forces.

You seem to think of Bohr and von Neumann as prophets, that a better (physical) formulation of QM cannot possibly be found, ever.

No, particularly Bohr is for me one of the main culprits having obscured QT, triggering all these debates about "interpretation"; von Neumann did a great job in making (non-relativistic) QT mathematically rigorous. His physics is not so convincing.

 

[Lord Jestocost]

No, particularly Bohr is for me one of the main culprits having obscured QT .....

Maybe, one reason for your statement might be that you didn't find in Bohr's writings what you expected to find.

 

[vanhees71]

That's for sure true. I never understood a paper written by Bohr ;-).

 

[Demystifier]

There are also no mathematical problems with the theory

Imagine that a mathematician reads a book entitled "Mathematics of quantum mechanics". At some place in the book the word "measurement" appears, but the book never gives a mathematical definition of measurement. Can you imagine that the mathematician would think of it as a problem? And what kind of problem would that be, from the point of view of the mathematician? A mathematical problem? A physical problem? A philosophical problem? A linguistic problem? A pseudo-problem?

 

[Demystifier]

I don't think that mathematicians and philosophers have much to say about measurement problems, which indeed are an engineering problem.

Are you familiar with the von Neumann theory of quantum measurement? What's your opinion on that theory?

 

[vanhees71]

I'm not familiar with the details of von Neumann, but his famous proof for the impossibility of hidden variables is flawed. That was discovered by Grete Hermann already in the 1930ies and then rediscovered by Bell.

 

[Demystifier]

I'm not familiar with the details of von Neumann, but his famous proof for the impossibility of hidden variables is flawed. That was discovered by Grete Hermann already in the 1930ies and then rediscovered by Bell.

First, von Neumann theory of quantum measurements is not directly related to the von Neumann proof of impossibility of hidden variables. Second, his proof is mathematically correct. Physically, however, the proof is pretty much irrelevant because it assumes that hidden variables should satisfy a certain very restricting property. In the modern language, his proof can be considered as ruling out a certain class of non-contextual hidden variables.

Anyway, any modern serious analysis of the measurement problem (which you don't see as a problem at all) is based on the von Neumann theory of quantum measurements. Even Ballentine uses it, without calling it so, in Sec. 9.2.

 

[Demystifier]

When being engineer helps a problem go away, then he puts on an engineer hat.

One of the best examples of this way of thinking among physicists is this: "Quantum entanglement cannot be used for superluminal signaling, therefore quantum nonlocality does not violate Lorentz invariance." This argument never made much sense to me. It's a category mistake, in conflates practical issues (the inability to do something in the laboratory) with a purely mathematical property (invariance under a group of mathematical transformations). Nevertheless, many physicists, and even some mathematicians, find this argument convincing.

 

[gentzen]

In the language of model theory in mathematical logic, that's because the axioms of quantum theory are not categorical, i.e. there are many inequivalent models that satisfy the axioms.
https://en.wikipedia.org/wiki/Categorical_theory
Physicists refer to these models as interpretations.

Interpretations have nothing to do how you mathematically realize QT. The different mathematical formulations like "matrix mechanics", "wave mechanics", "Dirac's bra-ket formalism", "path integrals", are all just the same theory. Interpretational issues are purely philosophical problems which should be strictly separated from the mathematical foundations and physics applications.

In "Tutorium Quantenmechanik" by Jan-Markus Schwindt in "Chapter 4 Interpretations" one of the subsections is

4.6 New Age Interpretation​

Fig. 4.3 Source https://www.smbc-comics.com/comic/2011-03-20

 

[Lord Jestocost]

I'm not familiar with the details of von Neumann, but his famous proof for the impossibility of hidden variables is flawed. That was discovered by Grete Hermann already in the 1930ies and then rediscovered by Bell.

I am not sure whether you are right there. For example, Dennis Dieks in “Von Neumann’s Impossibility Proof: Mathematics in the Service of Rhetorics”:

“We shall attempt to tell a story that is more historically accurate and less ideologically charged. Most importantly, von Neumann never claimed to have shown the impossibility of hidden variables tout court, but argued that hidden-variable theories must possess a structure that deviates fundamentally from that of quantum mechanics. Both Hermann and Bell appear to have missed this point; moreover, both raised
unjustified technical objections to the proof."