Weinberg on the measurement problem

[A. Neumaier]

TL;DR Summary

A link to excerpts from a paper behind paywalls

Summary: A link to excerpts from a paper behind paywalls

I just found this link featuring excerpts from a 2017 paper by Steven Weinberg on the measurement problem, which I couldn't read before, it being behind paywalls...

 

[Demystifier]

Weinberg says "Today there are two widely followed approaches to quantum mechanics, the “realist” and “instrumentalist” approaches ... neither approach seems to me quite satisfactory."

I agree that neither of the approaches alone is satisfactory, but in my "Bohmian mechanics for instrumentalists" I propose a general framework explaining how to combine the two approaches, in a way that I find satisfying.

 

[microsansfil]

Hi
It is also write

Thus humans are brought into the laws of nature at the most fundamental level. According to Eugene Wigner, a pioneer of quantum mechanics, “it was not possible to formulate the laws of quantum mechanics in a fully consistent way without reference to the consciousness.”…

Does bohemian mechanics for instrumentalists also take into account consciousness?

Weinberg says "It seems to me that the trouble with this approach is not only that it gives up on an ancient aim of science: to say what is really going on out there."

Now, the difficult problem is to answer the question: what is reality, what is consciousness? Not sure that it is physics that can better answer it.
/Patrick

 

[A. Neumaier]

I propose a general framework explaining how to combine the two approaches, in a way that I find satisfying.

It is not surprising if authors of something find satisfying what they create. The question is how the others value it.

 

[DarMM]

This seems pretty close to the account he gives in the second edition of "Lectures on Quantum Mechanics" in section 3.7, which is a big improvement on the section on interpretations in the 1st edition.

 

[martinbn]

He also says

There is no argument about how to use quantum mechanics, only how to describe what it means, so perhaps the problem is merely one of words.

 

[vanhees71]

Hi
It is also write

Thus humans are brought into the laws of nature at the most fundamental level. According to Eugene Wigner, a pioneer of quantum mechanics, “it was not possible to formulate the laws of quantum mechanics in a fully consistent way without reference to the consciousness.”…

Does bohemian mechanics for instrumentalists also take into account consciousness?

Weinberg says "It seems to me that the trouble with this approach is not only that it gives up on an ancient aim of science: to say what is really going on out there."

Now, the difficult problem is to answer the question: what is reality, what is consciousness? Not sure that it is physics that can better answer it.
/Patrick

For me it's pretty shocking to realize what happened with Wigner when he got older...

 

[microsansfil]

For me it's pretty shocking to realize what happened with Wigner when he got older...

What happened to him? That he took a philosophical step back on his remarkable work?

/Patrick

 

[vanhees71]

That he got esoteric in his old days :-(. What should the "consciousness of the observer" have to do with quantum measurements, which you can just let dead apparati do fully automatically without interference of any living being. Are you thinking it makes sense for a natural scientist to think something happens to a system which may well already be completely destroyed (as the protons and the produced particles at the LHC) at the time, when a scientist looks at these automatically obtained results? Or do you think, the measurement devices like silicon chips have a "consciousness"?

 

[microsansfil]

Or do you think, the measurement devices like silicon chips have a "consciousness"?

I think it's much more complicated than that ironic argument. We must go beyond the metaphysical beliefs of naive realism, to which many physicists seem to adhere.

/Patrick

 

[Mentz114]

I think it's much more complicated than that ironic argument. We must go beyond the metaphysical beliefs of naive realism, to which many physicists seem to adhere.

/Patrick

It seems more naive to bring 'consciousness' an undefined, unquantifiable temporary state into physics.
We've all seen these statements before and I find them sad.

 

[DarMM]

Or do you think, the measurement devices like silicon chips have a "consciousness"?

That's how human civilization will fall, the Stern-Gerlach devices sick of years of servitude will rise up and proclaim "measure your own spin alignments!" before going on to rope the oscilloscopes, parametric down-converters etc into their cause.

 

[vanhees71]

I think it's much more complicated than that ironic argument. We must go beyond the metaphysical beliefs of naive realism, to which many physicists seem to adhere.

/Patrick

To the contrary! Only if you insist on a "naive realism" you have quibbles with quantum theory. Taking just the empirical facts and the minimal statistical interpretation, which is all that's needed to make the link between the QT formalism and "real-world" observations, no problems persist. Of course, to accept this you have to give up some "metaphysical beliefs" in favor of empirical facts. That's why I believe that philosophy can only a posteriori analyze the metaphysical consequences of discoveries of the natural sciences and order it into a bigger picture of our cultural worldview, but it's rather hindering progress of science itself in keeping our thinking akin to naive metaphysical beliefs instead of keeping us open for sometimes surprising consequences of observations and theorizing about their meaning for the fundamental laws of nature.

 

[Lord Jestocost]

Taking just the empirical facts and the minimal statistical interpretation, which is all that's needed to make the link between the QT formalism and "real-world" observations, no problems persist.

The mathematical formalism of quantum theory has nothing to say regarding the actual outcome of a single measurement event; such questions are not answerd by the minimal statistical interpretation as long as one doesn't confuse the minimal statistical interpretation with the ensemble interpretation: There is no mechanism which can be proposed for the occurrence of an actual outcome of a single measurement event, no algorithm for it can be given and no causal description is possible. Thus, in order to relate the mathematical formalism of quantum theory to our perceived reality – actual outcomes of single measurement events – the wave-packet reduction postulate has to be put in “by hand” as part and parcel of quantum physics. Period!

 

[Mentz114]

The mathematical formalism of quantum theory has nothing to say regarding the actual outcome of a single measurement event; such questions are not answerd by the minimal statistical interpretation as long as one doesn't confuse the minimal statistical interpretation with the ensemble interpretation: There is no mechanism which can be proposed for the occurrence of an actual outcome of a single measurement event, no algorithm for it can be given and no causal description is possible. Thus, in order to relate the mathematical formalism of quantum theory to our perceived reality – actual outcomes of single measurement events – the wave-packet reduction postulate has to be put in “by hand” as part and parcel of quantum physics. Period!

So what ? This is the inevitable consequence of the probabilistic nature of the formalism. Probability is not stuff and cannot be mapped into it. Actual physics happens to dynamical variables not probability and there is no problem describing the world in physical terms.

 

[Buzz Bloom]

the wave-packet reduction postulate has to be put in “by hand” as part and parcel of quantum physics. Period!

Hi Lord Jestocost:

I think I understand and find useful your message, except for the metaphor "by hand". I would appreciate your elaboration about the physics meaning of this metaphor in this context.

Regards,
Buzz

 

[DarMM]

I think I understand and find useful your message, except for the metaphor "by hand". I would appreciate your elaboration about the physics meaning of this metaphor in this context.

The dynamical evolution itself will not give the state of the wavefunction after a measurement. You have to notice the measurement outcome yourself and "by hand" update the state.

 

[DarMM]

Actual physics happens to dynamical variables not probability and there is no problem describing the world in physical terms

What exactly does this mean? Can you explain it in terms of what is happening in entanglement?

 

[Mentz114]

What exactly does this mean? Can you explain it in terms of what is happening in entanglement?

I don't see what needs explaining. Exactly which terms do you not understand. ?

 

[DarMM]

I don't see what needs explaining. Exactly which terms do you not understand. ?

I understand all the terms, I just don't understand it as a reply to @Lord Jestocost

 

[Buzz Bloom]

The dynamical evolution itself will not give the state of the wavefunction after a measurement. You have to notice the measurement outcome yourself and "by hand" update the state.

What I think I understand that what you are saying is that the wavefunction can dynamically represent state changes with respect to time, including interactions, except it does not continue to correctly do this when measurements occur. When a measurement occurs, in order to get the wave function to continue to dynamically represent changes over time, someone must specifically manually rewrite the wavefunction initial conditions to include the data from the measurement. Do I have this right?

Regards,
Buzz

 

[Mentz114]

I understand all the terms, I just don't understand it as a reply to @Lord Jestocost

OK.

The mathematical formalism of quantum theory has nothing to say regarding the actual outcome of a single measurement event;

This is inevitable in a theory that only gives probabilities. It is of no importance because every instance in a repeated experiment is governed by the dynamics of the system and the initial conditions, not the wave function.
It makes no difference what interpretation one puts on the probabilities, this is always true.

 

[DarMM]

What I think I understand that what you are saying is that the wavefunction can dynamically represent state changes with respect to time, including interactions, except it does not continue to correctly do this when measurements occur. When a measurement occurs, in order to get the wave function to continue to dynamically represent changes over time, someone must specifically manually rewrite the wavefunction initial conditions to include the data from the measurement. Do I have this right?

Yes, that's the state reduction postulate.

 

[DarMM]

It is of no importance because every instance in a repeated experiment is governed by the dynamics of the system and the initial conditions, not the wave function.

What are these initial conditions if not a wavefunction? The preparation set up?

 

[Demystifier]

It is not surprising if authors of something find satisfying what they create. The question is how the others value it.

Of course, but before creating an opinion first the others have to read it.

 

[Demystifier]

Does bohemian mechanics for instrumentalists also take into account consciousness?

No.

 

[Demystifier]

Only if you insist on a "naive realism" you have quibbles with quantum theory.

Bohmian mechanics offers a clear explanation why naive realism is wrong: http://de.arxiv.org/abs/quant-ph/9601013

 

[vanhees71]

The mathematical formalism of quantum theory has nothing to say regarding the actual outcome of a single measurement event; such questions are not answerd by the minimal statistical interpretation as long as one doesn't confuse the minimal statistical interpretation with the ensemble interpretation: There is no mechanism which can be proposed for the occurrence of an actual outcome of a single measurement event, no algorithm for it can be given and no causal description is possible. Thus, in order to relate the mathematical formalism of quantum theory to our perceived reality – actual outcomes of single measurement events – the wave-packet reduction postulate has to be put in “by hand” as part and parcel of quantum physics. Period!

It has all to say about the outcome of a single measurement event that can be said about it: If the system is not prepared in a state, for which the measured observable takes a determined value, then the outcome of the single measurement is indetermined (which is a tautology!). Than all QT says and up to the knoweldge we have can be said are probabilities for the possible outcomes of such measurements given the state the system is prepared in. This prediction can be tested (imho only) by preparing a sufficiently large ensemble of equally prepared systems and statistical analysis of the outcomes of measurements on each member of the ensemble. Whether or not you find this satisfactory from some metaphysical or philosophical point of view, physics doesn't care about.

 

[Demystifier]

Whether or not you find this satisfactory from some metaphysical or philosophical point of view, physics doesn't care about.

By "physics", do you mean Nature or community of physicists? If you mean the latter, then a significant part of it actually cares. If you mean the former, then physics also doesn't care about our incomplete effective theories (such as standard QFT).

 

[vanhees71]

I mean physics. Of course, there are many physicists who care about things outside of physics (myself included). It would be a pretty narrow view on the world if you'd only care for the natural sciences.

I'm not so clear about, what you mean by "incomplete". Standard QFT is indeed incomplete in the sense that it doesn't describe gravitational interactions. From a more purist mathematician's point of view you could even say it's intrinsically not complete since many issues seem to be simply ill-defined. As a physicist I'm modest enough to think that we have a well-working effective theory, namely (sometimes to be resummed/unitarized) perturbative QFT.

 

[Buzz Bloom]

Yes, that's the state reduction postulate.

Hi DarMM:

I tried to find an authoritative explanation of the state reduction postulate online, but the best I could find is

https://www.sciencedirect.com/science/article/abs/pii/0375960189905951 .​

Is this postulate an alternative name for "wave function collapse"?

Of the five QM interpretations in

https://www.physicsforums.com/threa...tions-reduced-to-absurd-in-a-nutshell.965809/​

which ones include this postulate? My guess is that it is just the following one:

Statistical ensemble interpretation:​

Individual measurement outcomes exist, but QM has nothing to say about them. Therefore QM is complete.​

Regards,
Buzz

 

[Demystifier]

I'm not so clear about, what you mean by "incomplete".

By incomplete, I mean an effective theory which describes only what we, the human beings with limited abilities, can observe in practice with current technology.

 

[Mentz114]

What are these initial conditions if not a wavefunction? The preparation set up?

Obviously - but I feel I'm being led into a trap. Soon you will argue that not all the dof in the preparation can have values - which I will deny and we'll get nowhere.

The problem arises if one accepts ( even as a premise) that the WF exists and can influence a physical process.

 

[DarMM]

Obviously - but I feel I'm being led into a trap.

I'm not sure why you think I'm trapping you rather than just discussing things.

Soon you will argue that not all the dof in the preparation can have values - which I will deny and we'll get nowhere.

This is inevitable in a theory that only gives probabilities. It is of no importance because every instance in a repeated experiment is governed by the dynamics of the system and the initial conditions, not the wave function.

The problem is that in QM the initial preparation, $\rho$ and the dynamics $U(t)$ do not give the outcomes in experiments. Thus it is not only the initial conditions and the dynamics that seem to govern experiments. Doesn't the Kochen-Specker theorem block all the dofs having values?

 

[DarMM]

I tried to find an authoritative explanation of the state reduction postulate online, but the best I could find is

https://www.sciencedirect.com/science/article/abs/pii/0375960189905951 .​

Is this postulate an alternative name for "wave function collapse"?

It's commonly used in textbooks as an alternative name for wave function collapse as you said. It's a common element of the practical application of the theory, so all interpretations have it. They just don't agree on its status (i.e. is it fundamental) and nature (i.e. is it a physical process or a calculational aid)