What Is the Frauchiger-Renner Theorem?

[Auto-Didact]

Do you have an example of such confusion?

QM actually has paradoxes and problems; this is the original cause of all the interpretations of QM and it is also why the foundations of QM, despite the success of QM itself, is such an active field today. It is clear that, despite the classification of the issues as done by historians of physics and workers in QM foundations, the confusion in the field has been exacerbated to some extent by how the field operates (e.g. see @martinbn and my posts above) and how it interacts with other fields in the practice of physics, compared to the functioning and interoperating of practically all other fields in physics and mathematics.

In order not to veer far off-topic too far in here, I offer some possible takes on the extra confusion in QM foundations, which has expanded since the inception of the field, in this thread.

 

[RUTA]

I'm not sure what to make of this Frauchiger paper since I'm not having enough time to translate their pretty complicated wording into clear formulas, but I've the impression that they make (hidden) assumptions about independence of states from observations which contradict what QT is actually saying (within the minimal interpretation), and this leads to apparent paradoxes. As I said, I'm not sure of that and I'm not able to point my finger at precisely the place where their argument goes wrong, but I've my suspicions with papers which do not provide a clear calculation in terms of bras and kets and rather write many words ;-)).

Let me summarize the issue as succinctly as possible without any formalism. If you want to read more details, see https://arxiv.org/abs/1710.07212 (BW's paper), or https://www.physicsforums.com/insights/wigners-friend/ (which I have updated accordingly) or http://users.etown.edu/s/stuckeym/WignersFriendConsciousness.pdf (which we just submitted for a book on QM and consciousness). The source of inconsistency in FR (and the typical rendering of Wigner's friend) stems from contradictory assumptions, just as you suspected vanhees71. Classical systems obey Boolean algebra while quantum systems obey non-Boolean algebra. If you assume you have a quantum system obeying non-Boolean algebra that communicates with its classical environment (the universe) that obeys Boolean algebra, then you can end up with self-inconsistent classical information being shared among observers in the form of contradictory measurement outcomes. Simply put, this is precisely what FR did. Here is a bit more detail, if you're interested.

If one assumes that physics is used to model all shared, self-consistent classical information (we would call that "objective reality"), then any prediction of shared, self-inconsistent classical information would constitute a "scientific contradiction" in the language of BW and would invalidate the scientific theory making said prediction. There are two different QM formalisms -- the "standard" formalism with its measurement-update and Born rules (BW call this "objective collapse"), and the "relative-state" formalism with its universal unitary evolution (no measurement-update, which BW call "no collapse"). These formalisms produce the same predictions except in Wigner-friend-type experiments (observers measuring each other). In those experiments, the two formalism do make different predictions, so deciding which is correct is an empirical matter (good luck screening off macroscopic measuring devices to test it though). Neither of these formalisms produces a scientific contradiction for Wigner's friend and neither was used by FR (and neither is used in a typical rendering of Wigner's friend).

Instead, FR used what BW call "subjective collapse" (some would say that's an oxymoron of course and that's their point). In other words, FR assume a screened-off quantum system shares classical information with the universe and then shows how this leads to shared, self-inconsistent classical information between observers. Really?? Wow, how can that be?? [Sarcasm] Because you can't use non-Boolean algebra to model a system that shares classical information per Boolean algebra without generating contradictions. Go figure.

This is not intended to dismiss the importance of FR's paper, which has spawned many valuable discussions. BW unveiled inconsistencies in FR that are introduced in the typical rendering of Wigner's friend and are extremely important for understanding the foundations of QM. At the end of the day, if you want to avoid scientific contradiction, you have choices such as no collapse with its subjective reality (QM predictions are relative to the observer) or objective collapse with its objective reality (QM predictions are agreed upon by everyone). Or, you can do a hybrid where people's memories and written records are changed by quantum measurement (Bohmian relative-state approach per https://dustinlazarovici.com/wp-content/uploads/comment_renner_new.pdf).

What FR do not show is that you must have Many Worlds, as Renner used to believe ("I have to admit that if you had asked me two years ago, I’d have said [our experiment] just shows that many-worlds is actually a good interpretation and you should give up" the requirement that measurements have only a single outcome, Renner said. https://www.quantamagazine.org/frau...where-our-views-of-reality-go-wrong-20181203/). Or, that there is a limit to the applicability of QM (Renner, however, has changed his mind. He thinks the assumption most likely to be invalid is the idea that quantum mechanics is universally applicable. Same Quanta Magazine article).

 

[vanhees71]

Do you have an example of such confusion?

Plus none of this gets away from the fact that if you hold a view with Masanes's four assumptions you have a contradiction, regardless of how well that view works otherwise.

Which of the following do you disagree with:

1. Quantum Mechanics applies objectively to all systems/is universal
2. Single World
3. Superobservers should use superposed states to describe observers, prior to their measurements of them
4. It's possible in principal to reverse measurements

I agree with 1; (2) is pointless since you can always assume that there are many unobservable "parallel universes" like in the many-worlds interpretation, but that's not physics, because what's principally not observable is simply not what's considered in the sciences. (3) seems to me the weak point of the Frauchiger-Renner argument (although, as I stressed above, I've not the time at the moment to really analyze their paper carefully): There are no "superobservers" but just observers. To gain information about a system you have to interact with it, i.e., to measure something the measurement apparatus must interact with it to at least partially entangle the pointer states of the measurement device with the measured observable of the system. If the system is a microscopic system, it's impossible not to disturbe the system in a significant way. So the assumption that there are superobservers who can measure something without disturbing the system significantly is simply a assumption contradicting basic quantum theory. (4) seems also problematic since a measurement involves macroscopic measurement devices and a "good measurement" involves some irreversibility to fix the result. Irreversibility here is of course meant in the thermodynamical sense since the time evolution of the complete states are of course reversible, because they are unitary operators, but in practice it's impossible to reverse a measurement process since it involves too many microscopic degrees of freedom that cannot be controlled precisely.

 

[vanhees71]

A slight correction regarding Bohr's attitude. Jan Faye writes in https://plato.stanford.edu/entries/qm-copenhagen/ :

"Second, many physicists and philosophers see the reduction of the wave function as an important part of the Copenhagen interpretation. But Bohr never talked about the collapse of the wave packet. Nor did it make sense for him to do so because this would mean that one must understand the wave function as referring to something physically real. Bohr spoke of the mathematical formalism of quantum mechanics, including the state vector or the wave function, as a symbolic representation. Bohr associated the use of a pictorial representation with what can be visualized in space and time. Quantum systems are not vizualizable because their states cannot be tracked down in space and time as can classical systems. The reason is, according to Bohr, that a quantum system has no definite kinematical or dynamical state prior to any measurement. Also the fact that the mathematical formulation of quantum states consists of imaginary numbers tells us that the state vector is not susceptible to a pictorial interpretation (CC, p. 144). Thus, the state vector is symbolic. Here “symbolic” means that the state vector's representational function should not be taken literally but be considered a tool for the calculation of probabilities of observables." [bold emphasis added by LJ]

Well, yes. Bohr was, if interpreted with some benevolence, the most physical of the Copenhagen interpreters. What I dislike with Bohr is his tendency to write many words which are overly complicated and unsharp, i.e., more like philosophy than theoretical physics. As Pauli has put it: Don't make so many words! To put it in my own words: The only way to precisely speak about theoretical physics is in terms of mathematics. Of course, I agree with Bohr (as interpreted in the quote above) that the mathematical "objects" of the quantum formalism, i.e., self-adjoint operators on Hilbert space, representing states and observables, are epistemic, i.e., they provide the description of what we know given some information about the system under consideration, and what we can know about the system according to QT is probabilistic information about the outcome of measurements given the preparation of the system in some (pure or mixed) state.

 

[vanhees71]

Let me summarize the issue as succinctly as possible without any formalism. If you want to read more details, see https://arxiv.org/abs/1710.07212 (BW's paper), or https://www.physicsforums.com/insights/wigners-friend/ (which I have updated accordingly) or http://users.etown.edu/s/stuckeym/WignersFriendConsciousness.pdf (which we just submitted for a book on QM and consciousness). The source of inconsistency in FR (and the typical rendering of Wigner's friend) stems from contradictory assumptions, just as you suspected vanhees71. Classical systems obey Boolean algebra while quantum systems obey non-Boolean algebra. If you assume you have a quantum system obeying non-Boolean algebra that communicates with its classical environment (the universe) that obeys Boolean algebra, then you can end up with self-inconsistent classical information being shared among observers in the form of contradictory measurement outcomes. Simply put, this is precisely what FR did. Here is a bit more detail, if you're interested.

If one assumes that physics is used to model all shared, self-consistent classical information (we would call that "objective reality"), then any prediction of shared, self-inconsistent classical information would constitute a "scientific contradiction" in the language of BW and would invalidate the scientific theory making said prediction. There are two different QM formalisms -- the "standard" formalism with its measurement-update and Born rules (BW call this "objective collapse"), and the "relative-state" formalism with its universal unitary evolution (no measurement-update, which BW call "no collapse"). These formalisms produce the same predictions except in Wigner-friend-type experiments (observers measuring each other). In those experiments, the two formalism do make different predictions, so deciding which is correct is an empirical matter (good luck screening off macroscopic measuring devices to test it though). Neither of these formalisms produces a scientific contradiction for Wigner's friend and neither was used by FR (and neither is used in a typical rendering of Wigner's friend).

Instead, FR used what BW call "subjective collapse" (some would say that's an oxymoron of course and that's their point). In other words, FR assume a screened-off quantum system shares classical information with the universe and then shows how this leads to shared, self-inconsistent classical information between observers. Really?? Wow, how can that be?? [Sarcasm] Because you can't use non-Boolean algebra to model a system that shares classical information per Boolean algebra without generating contradictions. Go figure.

This is not intended to dismiss the importance of FR's paper, which has spawned many valuable discussions. BW unveiled inconsistencies in FR that are introduced in the typical rendering of Wigner's friend and are extremely important for understanding the foundations of QM. At the end of the day, if you want to avoid scientific contradiction, you have choices such as no collapse with its subjective reality (QM predictions are relative to the observer) or objective collapse with its objective reality (QM predictions are agreed upon by everyone). Or, you can do a hybrid where people's memories and written records are changed by quantum measurement (Bohmian relative-state approach per https://dustinlazarovici.com/wp-content/uploads/comment_renner_new.pdf).

What FR do not show is that you must have Many Worlds, as Renner used to believe ("I have to admit that if you had asked me two years ago, I’d have said [our experiment] just shows that many-worlds is actually a good interpretation and you should give up" the requirement that measurements have only a single outcome, Renner said. https://www.quantamagazine.org/frau...where-our-views-of-reality-go-wrong-20181203/). Or, that there is a limit to the applicability of QM (Renner, however, has changed his mind. He thinks the assumption most likely to be invalid is the idea that quantum mechanics is universally applicable. Same Quanta Magazine article).

I think that's a very important point. If there are testable predictions between two interpretations (I'd rather say, it's indeed even different theories), then it's just an engineering challenge to invent an appropriate experiment to decide the issue.

BTW: For me "standard quantum mechanics" is the formalism with Born's rule, i.e., the standard probabilistic interpretation of states, but the assumption of a collapse is superfluous and unphysical since in contradiction to standard relativistic QFT it implies non-local interactions, while standard QFT only implies non-local correlations (formalized as "entangled states"). In other words, for me standard QT is the formalism, including Born's rule, with the minimal statistical interpretation for the epistemology. Whether there's an ontic interpretation or not is not important for physics.

 

[atyy]

Well, yes. Bohr was, if interpreted with some benevolence, the most physical of the Copenhagen interpreters. What I dislike with Bohr is his tendency to write many words which are overly complicated and unsharp, i.e., more like philosophy than theoretical physics. As Pauli has put it: Don't make so many words! To put it in my own words: The only way to precisely speak about theoretical physics is in terms of mathematics. Of course, I agree with Bohr (as interpreted in the quote above) that the mathematical "objects" of the quantum formalism, i.e., self-adjoint operators on Hilbert space, representing states and observables, are epistemic, i.e., they provide the description of what we know given some information about the system under consideration, and what we can know about the system according to QT is probabilistic information about the outcome of measurements given the preparation of the system in some (pure or mixed) state.

I think that's a very important point. If there are testable predictions between two interpretations (I'd rather say, it's indeed even different theories), then it's just an engineering challenge to invent an appropriate experiment to decide the issue.

BTW: For me "standard quantum mechanics" is the formalism with Born's rule, i.e., the standard probabilistic interpretation of states, but the assumption of a collapse is superfluous and unphysical since in contradiction to standard relativistic QFT it implies non-local interactions, while standard QFT only implies non-local correlations (formalized as "entangled states"). In other words, for me standard QT is the formalism, including Born's rule, with the minimal statistical interpretation for the epistemology. Whether there's an ontic interpretation or not is not important for physics.

If the quantum state is epistemic, then why would there be any problem with its collapse, since collapse would also be epistemic?

 

[DarMM]

(3) seems to me the weak point of the Frauchiger-Renner argument (although, as I stressed above, I've not the time at the moment to really analyze their paper carefully): There are no "superobservers" but just observers. To gain information about a system you have to interact with it, i.e., to measure something the measurement apparatus must interact with it to at least partially entangle the pointer states of the measurement device with the measured observable of the system. If the system is a microscopic system, it's impossible not to disturbe the system in a significant way. So the assumption that there are superobservers who can measure something without disturbing the system significantly is simply a assumption contradicting basic quantum theory.

Superobservers aren't assumed to measure a system without disturbing it.

 

[stevendaryl]

I think that's a very important point. If there are testable predictions between two interpretations (I'd rather say, it's indeed even different theories), then it's just an engineering challenge to invent an appropriate experiment to decide the issue.

But I think that some such engineering challenges are forever beyond solving. Take for example the interpretation that an observation by a conscious observer causes an instantaneous collapse of the wave function. In principle, that theory is distinguishable from one (such as the Bohmian interpretation) that has no wave function collapse, because the collapse destroys interference terms in the predictions of probabilities for future measurements. However, if two states of a system are macroscopically distinguishable, then observing interference between them is in practice impossible. And I think it always will be.

So even though I agree that there is a sense in which the different "interpretations" of QM are actually slightly different theories, which make slightly different predictions, I think it's unlikely that we will ever experimentally distinguish them.

 

[A. Neumaier]

The form of Copenhagen I am fairly sure is safe is Bohr's. I'm not yet so sure about Heisenberg's.

Just a comment that is often overlooked: Until at least the end of 1927 (Solvay conference), the quantum physicists in Göttingen and Copenhagen had a realistic view of quantum mechanics in which particles were always in stationary states (characterized by energy and momentum) and performed quantum jumps guided by the wave function. Thus in their writing, state = stationary state and not = wave function up to a phase! And only the wave function was sort of epistemic...

 

[Auto-Didact]

Just a comment that is often overlooked: Until at least the end of 1927 (Solvay conference), the quantum physicists in Göttingen and Copenhagen had a realistic view of quantum mechanics in which particles were always in stationary states (characterized by energy and momentum) and performed quantum jumps guided by the wave function. Thus in their writing, state = stationary state and not = wave function up to a phase! Only the latter was sort of epistemic...

Any good sources for this?

Incidentally, historian of mathematics Unguru wrote exactly about this topic in (Unguru 1975). To quote some recent work by a young researcher (Stenlund 2014):

Unguru claimed that lack of historical sense is a common feature of most mathematicians’ readings of ancient mathematical texts in that they tend to read the ancient texts only from the point of view of modern mathematics. He writes: “to read ancient mathematical texts with modern mathematics in mind is the safest method for misunderstanding the character of ancient mathematics …”

The historical approach, according to Unguru, is an approach which involves interpretation. It cannot “divorce itself from the attempt to unravel the original intentions of the text’s author,” which means that the interpreter has to be sensitive to the historico-cultural context.

 

[A. Neumaier]

Any good sources for this?

Born discusses the issue in the introduction to

M. Born, Das Adiabatenprinzip in der Quantenmechanik, Z. Phys. 40 (1927), 167-192.

where he argues against Schroedinger's continuum view. Bohr's paper at the Solvay conference still says the same.

I haven't seen my claim explicitly researched on from a comparative historical point of view. But I am doing a historical study myself, and have plenty of detailed evidence, that will be the content of a paper to be finished later this year. Once one realizes what I wrote, many otherwise difficult to understand things get a straightforward sense.

(Unguru 1975).

Do you mean the following?

Unguru, S. (1975). On the need to rewrite the history of Greek mathematics. Archive for history of exact sciences, 15(1), 67-114.

 

[Auto-Didact]

Born discusses the issue in the introduction to

M. Born, Das Adiabatenprinzip in der Quantenmechanik, Z. Phys. 40 (1927), 167-192.

where he argues against Schroedinger's continuum view. Bohr's paper at the Solvay conference still says the same.

Thanks.

I haven't seen my claim explicitly researched on from a comparative historical point of view. But I am doing a historical study myself, and have plenty of detailed evidence, that will be the content of a paper to be finished later this year. Once one realizes what I wrote, many otherwise difficult to understand things get a straightforward sense.

Nice, can't wait to read it!

Do you mean the following?

Unguru, S. (1975). On the need to rewrite the history of Greek mathematics. Archive for history of exact sciences, 15(1), 67-114.

Yes. Stenlund's work is also pretty illuminating. I just noticed that I severly misjudged his age

 

[A. Neumaier]

Do you mean the following?
Unguru, S. (1975). On the need to rewrite the history of Greek mathematics. Archive for history of exact sciences, 15(1), 67-114.

Yes.

This is a critque on van der Waerden's (and others) way of writing about Greek mathematics. At the end is an editorial comment saying:
''A defense of his views will be published by Professor van der Waerden in a succeeding issue.''
Do you have the precise reference?

 

[Auto-Didact]

I think its:
B.L. Van der Waerden, “Defense of a ‘Shocking’ point of View”, A.H.E.S., 15, 1975, p.205.

There seems to be some controversy between mathematicians on this subject going back all the way to Hertz. Stenlund writes about this viewpoint and its counter viewpoint at length in his piece I linked above.

Prof. Van der Waerden's view is probably the more popular view among most mathematicians, practitioners of mathematics and teachers; it is difficult to imagine we could all be mistaken.

However, recalling the words of Feynman, there is sufficient reason to be careful to distinguish ones expertise in a subject from ones expertise on the history of that subject:

What I have just outlined is what I call a ‘physicist’s history of physics’, which is never correct… a sort of conventionalized myth-story that the physicist tell to their students, and those students tell to their students, and it is not necessarily related to actual historical development, which I do not really know!

Stenlund rejoins in this view, stating:

The normal interest in history of mathematics (among mathematicians who write history of mathematics) is interest in our mathematical heritage. This interest therefore tends to be conditioned by the contemporary situation and is not always an interest in what actually happened in mathematics of the past regardless of the contemporary situation. Only history in the latter sense deserves to be called history.$^1$ But history and heritage are often confused and one consequence of this kind of confusion is that the transformation of mathematics at the beginning of modern times is concealed. Features of modern mathematics are projected upon mathematics of the past, and the deep contrasts between ancient and modern mathematics are concealed. As a consequence, the nature of modern mathematics as symbolic mathematics is not understood as the new beginning of mathematics that it was.

1. Grattan-Guiness, I., 2004, The mathematics of the past: distinguishing its history from our heritage. Historia Mathematica, vol. 31, pp. 163-185.

 

[A. Neumaier]

B.L. Van der Waerden, “Defense of a ‘Shocking’ point of View”, A.H.E.S., 15, 1975, p.205.

I found instead:

Van der Waerden, Bartel L. "Defence of a “shocking” point of view." Archive for History of Exact Sciences 15.3 (1976): 199-210.

Prof. Van der Waerden's view is probably the more popular view among most mathematicians, practitioners of mathematics and teachers; it is difficult to imagine we could all be mistaken.

However, recalling the words of Feynman, there is sufficient reason to be careful to distinguish ones expertise in a subject from ones expertise on the history of that subject:

I am wholly behind van der Waerden. In addition to being an influential mathematician, he had strong historical expertise (many books, articles, editions of original sources), and his arguments given are cogent. The controversy is about the meaning of a piece of mathematics, and that cannot be decided by a historian without a good knowledge of mathematical culture. Whatever is said today about mathematics must be said in today's language to be properly understood, and this language is mathematical and formal, symbolical. Insisting on not doing this is like insisting that any proper history of the Babylonians must be stated in the Babylonian language.

Feynman's words are of course also true. there is simplified history just amounting to attribution from the heritage point of view. This is done in textbooks. Van der Waerden's history is far from such a superficial textbook history.

 

[A. Neumaier]

Nice, can't wait to read it!

If you send me an email, I'll send you a draft version (which should be ready end of February, I hope) in advance.

 

[Auto-Didact]

I appreciate the offer but as things currently stand I already have a long back log of work and reading to do; seeing February is just around the corner I should probably just wait patiently and read the finished manuscript.

I am wholly behind van der Waerden. In addition to being an influential mathematician, he had strong historical expertise (many books, articles, editions of original sources), and his arguments given are cogent. The controversy is about the meaning of a piece of mathematics, and that cannot be decided by a historian without a good knowledge of mathematical culture. Whatever is said today about mathematics must be said in today's language to be properly understood, and this language is mathematical and formal, symbolical. Insisting on not doing this is like insisting that any proper history of the Babylonians must be stated in the Babylonian language.

Feynman's words are of course also true. there is simplified history just amounting to attribution from the heritage point of view. This is done in textbooks. Van der Waerden's history is far from such a superficial textbook history.

I see your point and I don't question van der Waerden's expertise at all (I'm a huge fan of his two-spinor formalism). There was a time when I was as ardent about such matters as you seem to be, but I don't have such strong views anymore. In fact, now I am quite impartial to the matter in its full generality.

I think we are all the better when historians and philosophers intercede and try to contribute to the history/philosophy of science; it keeps us from segregating too far into separate domains and also keeps both parties sharp. I try to keep an open point of view and have learned at the least to take pleasure in reading such literature from any of the sides, especially when the debate gets fierce; sometimes, I hope that such old texts will be able to reveal to me somethings which have become lost over time.

By the way, Stephen Wolfram has a long list of articles on related topics over at his site; this one in particular, on Mathematical Notation: Past and Future is quite interesting.

 

[A. Neumaier]

seeing February is just around the corner I should probably just wait patiently and read the finished manuscript.

I had meant: The draft is likely to be finished end of February; the final version more like end of the year...

 

[entropy2information]

Frauchinger and Renner make this assumption:

If you want to hold on to the assumption that quantum theory is universally applicable, and that measurements have only a single outcome, then you’ve got to let go of the remaining assumption, that of consistency: The predictions made by different agents using quantum theory will not be contradictory.

https://www.quantamagazine.org/frau...where-our-views-of-reality-go-wrong-20181203/

Why do they make this assumption?

 

[RUTA]

Frauchinger and Renner make this assumption:
https://www.quantamagazine.org/frau...where-our-views-of-reality-go-wrong-20181203/

Why do they make this assumption?

They are following the conventional approach to Wigner's friend. That conventional approach makes inconsistent assumptions which I point out in post #142.

 

[bhobba]

A slight correction regarding Bohr's attitude.

The problem with Bohr was that he was quite philosophical and a notorious mumbler. I am pretty sure he was understandable if you thought hard enough, and I think his good friend Einstein understood what he was getting at, although he mostly did not agree, but I find him quite obscure. That Bohr understood QM is not in doubt - in fact better than Heisenberg who he corrected in his mistaken views about the uncertainty principle.

Personally, for what its worth, IMHO the only early pioneers whose views stand up to modern scrutiny is Dirac and maybe Einstein. I tend to shy away from what those of that era thought and look at more modern views like those Murray Gell-Mann who I find quite lucid. Just me.

Added Later
As an example people that know my views know I think far too much is made of EPR. I find Murray's take on it much more lucid than the usual, IMHO overly sensationalist, takes on it like Henry Stapp:


Trouble is its the views of Stapp etc that the pop-sci press seem to concentrate on.

Thanks
Bill

 

[DarMM]

I think Bub's papers are sort of a lucid modern version of Bohr. For instance replacing the "Classical" side of the cut with the "Boolean algebra" side of the cut, since that is the crucial point. One doesn't require actual Classical Physics on the observer's side of the cut, just an event space to which true/false can be assigned consistently since that is what is required to even define an outcome.

He does similarly for other concepts from Bohr.

 

[bhobba]

As far as the paper being discussed goes I find it exasperating when statements like the following are made - Quantum theory cannot consistently describe the use of itself. It may be true, but people are working on the issue of the classical/quantum cut when of course everything is quantum. Progress has been made and I think it could eventually be completely resolvable. Certainly statements like that confuse beginners and even some at the intermediate level no end and should, again IMHO, be more judiciously expressed - it really is quite sensationalist the way its worded.

I personally have no issue with QM can't satisfy the three claimed assumptions simultaneously and do not see what the fuss is about. Just me.

Thanks
Bill

 

[Auto-Didact]

Certainly statements like that confuse beginners and even some at the intermediate level no end and should, again IMHO, be more judiciously expressed

This argument tends to get made, often by professors, but just out of curiosity: how many actual beginners or intermediaries are chased away by things like this? I would presume very few if any.

 

[bhobba]

This argument tends to get made, often by professors, but just out of curiosity: how many actual beginners or intermediaries are chased away by things like this? I would presume very few if any.

I do not think any get chased away but rather myths about QM are reinforced:
https://arxiv.org/abs/quant-ph/0609163

It isn't until you read an advanced book like Ballentine that they are 'corrected', by which time making that shift is harder than it should be. It even happened with me. I shudder when I think about some of my early posts of many years ago where I actually believed particles are literally everywhere at once etc - and I had read Ballentine by then. Getting things straight in QM seems particularly hard - it took me quite a while to develop my current views.

Thanks
Bill

 

[A. Neumaier]

replacing the "Classical" side of the cut with the "Boolean algebra" side of the cut, since that is the crucial point.

Let me just observe that 99.99% of all quantum mechanics done in practice and theory uses classical logic. Everything in quantum mechanics is Boolean in spite of the non-Boolean talk in some of the foundational work

 

[RUTA]

I think Bub's papers are sort of a lucid modern version of Bohr. For instance replacing the "Classical" side of the cut with the "Boolean algebra" side of the cut, since that is the crucial point. One doesn't require actual Classical Physics on the observer's side of the cut, just an event space to which true/false can be assigned consistently since that is what is required to even define an outcome.

He does similarly for other concepts from Bohr.

My language about Boolean vs non-Boolean is from Bub — I must have referenced his forthcoming paper Two Dogmas Redux somewhere in this thread. Hardy’s axiomatizations also emphasize the difference between classical probability theory and quantum probability theory. Garg and Mermin’s paper (ref 10 in my Insight Why the Quantum) make a similar point.

 

[Auto-Didact]

I do not think any get chased away but rather myths about QM are reinforced:
https://arxiv.org/abs/quant-ph/0609163

If no one is getting chased away, then this isn't a real issue. People - laymen, specialists outside QT and non-orthodox believers alike - are frustrated with QM and they should be; no other canonical physical or mathematical theory, contemporary or historical, suffers from the incoherent picture presented.

The fact that there are alternative ontological formulations like BM and the TSVF are clear indications that QM need not be the final picture. A monograph that makes an excellent case for this is Dürr & Teufel, Bohmian Mechanics.

It isn't until you read an advanced book like Ballentine that they are 'corrected', by which time making that shift is harder than it should be. It even happened with me.

Apart from Ballentine's introduction of BM halfway in the book, his remarks on the superiority of the many body SE over the Hartree-Fock scheme and his review of Bell's theorem, I honestly don't see what is so great about this book.

As an introductory textbook it's definitely not the best and as a monograph on QM written for a postgraduate audience it's nice but nothing to write home about. When others first recommended it to me, I was expecting a 'MTW for QM'; one should be able to imagine the disappointment I had when I finished it.

 

[Auto-Didact]

Let me just observe that 99.99% of all quantum mechanics done in practice and theory uses classical logic. Everything in quantum mechanics is Boolean in spite of the non-Boolean talk in some of the foundational work

As an undergrad I flirted awhile with Birkhoff-von Neumann (BvN) quantum logic, espousing essentially the same points as Hilary Putnam made of its fundamental nature.

This was until I came to realize that BvN quantum logic wasn't actually a proper non-standard logic and that a 'real' quantum logic doesn't differ as much from standard logic as to make a big deal out of it.

 

[atyy]

The fact that there are alternative ontological formulations like BM and the TSVF are clear indications that QM need not be the final picture. A monograph that makes an excellent case for this is Dürr & Teufel, Bohmian Mechanics.

TVSF is basically a reformulation of Copenhagen, and does not attempt to solve the measurement problem.

 

[kurt101]

As an example people that know my views know I think far too much is made of EPR. I find Murray's take on it much more lucid than the usual, IMHO overly sensationalist, takes on it like Henry Stapp:

I am not sure I understood your comment about Gell-Mann being lucid, but in the video Gell-Mann sounds to me like an evangelist for his particular interpretation. It sounds like he is certain his is the right one.

In the video Murray Gell-Mann says the following about the EPR experiment:

People say loosely, crudely, wrongly that when you measure one of the photons, it does something to the other one. It doesn't. All it happens is that you measure the property of one and you learn the corresponding property of the other one. Now what these people who try to confuse us will say is yes but you choose which property and there by you choose what state the other one will be in. The point is that the different measurements say of linear polarization of one revealing the linear polarization of the other or circular polarity of one revealing the circular polarization of the other, those measurements are made on different branches of history, decoherent with each other, only one of which occurs. So its simply not true. And Einstein's point of view which was if some variable could ever be measured with certainty it should have some sort of physical reality and a definite value. That’s just wrong, that’s just in contradiction to Quantum Mechanics. When two variables at the same time don't commute any measurement of both of them would have to be carried out with one measurement on one branch of history and the other measurement on another branch of history and that's all there is to it. I presented that in my book, and of course Jim and I have argued for that and some other people, but it doesn't seem to get across. People are still mesmerized by this confusing language of non-locality.

In Gell-Mann's EPR interpretation, how are the branches of history selected? When are the two branches of history selected?

Whatever selects the two branches of history will have to know what angles the polarizers will be at when the entangled photons interact with them. This would have to be communicated/known instantly and acted on at the time of measurement or predicted and acted on by something that had complete knowledge of the future (i.e. knows the entire state of the universe and knows how the future will unfold). So I don't see how Gell-Mann is avoiding action at a distance. Although Gell-Mann does not use the phrasing of "selecting a history", I don't see how he avoids it and selecting a history seems like "action".

 

[atyy]

I am not sure I understood your comment about Gell-Mann being lucid, but in the video Gell-Mann sounds to me like an evangelist for his particular interpretation. It sounds like he is certain his is the right one.

That shows complete unity and lucidty on interpretations across the physics community - everyone agrees "quantum theory needs no interpretation - except mine" :)

My apologies to Fuchs and Peres for misappropriating their idea :) http://www.phy.pku.edu.cn/~qhcao/resources/class/QM/PTO000070.pdf

 

[bhobba]

In Gell-Mann's EPR interpretation, how are the branches of history selected? When are the two branches of history selected?

Probabilistically. Probability theory is silent on questions like that. You may not like it, but it is a legit position to take. In order for you to say whatever selects it you must show something does - which of course does not have to be the case - nature may simply be like that.

Thanks
Bill

 

[bhobba]

I honestly don't see what is so great about this book.

Well many have a different view. He develops QM from just 2 axioms - the dynamics comes from symmetry. I know of no other book that does that.

Thanks
Bill

 

[DarMM]

Let me just observe that 99.99% of all quantum mechanics done in practice and theory uses classical logic. Everything in quantum mechanics is Boolean in spite of the non-Boolean talk in some of the foundational work

True, what's the relation to Bub's point or are you just pointing that out? Or are you saying that demonstrates what Bohr and Bub are talking about?

To me Bub is speaking of the event algebra being Boolean at the macrolevel to permit a truth function having a clean definition and that's the real property of the non-Quantum side of the cut, i.e. you can assign true/false to macroevents even if they result from interaction with the microscopic.

I would have thought this is a separate issue as to whether you can use classical logic to reason about the mathematics of QM, unless I'm missing the point.