Living Opponents of the Copenhagen Interpretation

[vanhees71]

Opponents of Copenhagen are either crackpots or Ballentine (and Ballentine is wrong).

I still don't get, why the minimal statistical interpretation is wrong, because it's a subset of Copenhagen. One should say which flavor of Copenhagen anyway; e.g., I can well agree with all flavors without a collapse. An example is Bohr, who stressed that QT is a description about our objective (possible) knowledge about a quantum system and that the preparation procedures and measurement apparati select what we observe. There is no clear explicit statement concerning the collapse question (anyway, Bohr is usually not very explicit and clear, but that's another story). What I disagree with is the statement that there is a separate quantum dynamics and classical dynamics and there is a "cut" between these to realms. So far quantum theory has been seen as the most comprehense model, and the classical behavior of macroscopic systems is rather well understood from quantum many-body theory, where one derives transport equations, hydrodynamics, etc. from the full quantum Kadanoff-Baym equations via some coarse-graining formalism like the gradient expansion.

What I also don't buy is the collapse hypothesis, which only makes problems rather than explaining anything. So what's left as a physical theory is the usual postulates about quantum kinematics and dynamics + the Born postulate, and that's the minimal statistical interpretation. A quantum state refers to a single system being operationally defined as a (equivalence class) of preparation procedures but leads only to probabilistic knowledge about the outcome of further measurements which can empirically validated only using a large enough ensemble and statistical analysis. That's all what's need to use QT as the most successful physical theory, i.e., to map the formalism to observable objective facts about Nature, and that's all what physics is about.

Ontological or other philosophical questions beyond this is the problem of philosophers not of physicists! I don't see in which respect Ballentine's ensemble point of view is wrong (perhaps there are details he got wrong in his book, but I still think it's the best book on interpretations I've seen yet; also the book by Peres and Weinberg's new QM book are very good too).

 

[atyy]

I still don't get, why the minimal statistical interpretation is wrong, because it's a subset of Copenhagen. One should say which flavor of Copenhagen anyway; e.g., I can well agree with all flavors without a collapse. An example is Bohr, who stressed that QT is a description about our objective (possible) knowledge about a quantum system and that the preparation procedures and measurement apparati select what we observe. There is no clear explicit statement concerning the collapse question (anyway, Bohr is usually not very explicit and clear, but that's another story).

Yes, the minimal statistical interpretation is at best a flavour of Copenhagen. What I don't like about Ballentine's work is that he explicitly opposes Copenhagen in his review article, but at best the Copenhagen he opposes is such a caricature and misunderstanding of Copenhagen. In his book he doesn't identify his opponent as Copenhagen, but the interpretation he opposes is the one he identifies as Copenhagen in his review.

What I disagree with is the statement that there is a separate quantum dynamics and classical dynamics and there is a "cut" between these to realms. So far quantum theory has been seen as the most comprehense model, and the classical behavior of macroscopic systems is rather well understood from quantum many-body theory, where one derives transport equations, hydrodynamics, etc. from the full quantum Kadanoff-Baym equations via some coarse-graining formalism like the gradient expansion.

Copenhagen does not assign specific dynamics to the classical realm. The term "classical" refers to the the fact that we get a particular or definite experimental outcome on any single run of an experiment. This terminology goes back at least to the English translation of Landau and Lifshitz, and is standard in the literature. For example, http://www.quantiki.org/wiki/Channel_(CP_map): "Any device taking classical or quantum systems of a certain type as input and (possibly different) classical or quantum systems as output is a channel.", "Measurements are channels with classical range", "Preparations are channels with classical domain".

What I also don't buy is the collapse hypothesis, which only makes problems rather than explaining anything. So what's left as a physical theory is the usual postulates about quantum kinematics and dynamics + the Born postulate, and that's the minimal statistical interpretation. A quantum state refers to a single system being operationally defined as a (equivalence class) of preparation procedures but leads only to probabilistic knowledge about the outcome of further measurements which can empirically validated only using a large enough ensemble and statistical analysis. That's all what's need to use QT as the most successful physical theory, i.e., to map the formalism to observable objective facts about Nature, and that's all what physics is about.

In quantum mechanics one can use measurement as a method of state preparation. Collapse in quantum mechanics is a tool to describe the relationship between preparation and the preceding measurement used as a preparation procedure.

Ontological or other philosophical questions beyond this is the problem of philosophers not of physicists! I don't see in which respect Ballentine's ensemble point of view is wrong (perhaps there are details he got wrong in his book, but I still think it's the best book on interpretations I've seen yet; also the book by Peres and Weinberg's new QM book are very good too).

Ballentine is wrong in the following:
1. Opposing Copenhagen or characterizing Copenhagen with a caricature.
2. Claiming that pure states are not in some sense the "complete" information about an individual system. Within Copenhagen, after a classical/quantum cut is taken, pure states are the most "complete" information in the sense that they are extremal points of the space of density operators. One can identify pure states with individual systems or ensembles, and no mistake is made as long as one adds that the theory only predicts probabilities.
3. Claiming that Copenhagen cannot predict the results of Stern-Gerlach experiments correctly. In Copenhagen, if a measurement is made, there is a classical apparatus producing a definite result, or at least a quantum ancilla on which a measurement is later made. Ballentine is mssing the ancilla in his caricature of Copenhagen's version of Stern-Gerlach. So Copenhagen gets it right, Ballentine's caricature of Copenhagen gets it wrong. A correct version of the Stern-Gerlach with ancilla is shown in Zurek's http://arxiv.org/abs/quant-ph/0306072.
4. Claiming that one can do away with state reduction, because effective state reduction can be derived from decoherence without any additional assumptions. Here he has made the error of assuming that proper and improper mixtures are equivalent which is equivalent to assuming state reduction as Haroche and Raimond explain in https://www.amazon.com/dp/0198509146/?tag=pfamazon01-20.
5. If he is presenting some version of Copenhagen, then the wave function is already just a tool, and there is no problem with state reduction. His objection to state reduction makes more sense if he is considering the wave function as real, as in Many-Worlds.

 

[vanhees71]

Only because something is in Landau Lifshitz (whose volume 3 is, however, excellent) it needs not to be true, indeed. An there is no physically valid definition or any empirical hint for a cut, and as far as I know, it was Bohr, who emphasized the necessity for classical dynamics of a measurement apparatus. So that's part of the Copenhagen interpretation. Anyway, that's just semantics.

Further the collapse postulate is almost never fulfilled in practice. Usually a quantum system gets destroyed when it's measured but not prepared in a new state. Let's discuss a concrete example, where both aspects of measurements, gaining information about a system and destroying it and using the measurement for the preparation of a system for further measurements.

Let's consider a photon from an entangled state gets absorbed when it is registered by a photo plate or CCD or however you measure it (call the experimentalist Alice). If you have let Alice's photon go through an ideal polarizer, then the 2nd photon (measured by Bob before or after Alice's measurement) is in a definite polarization state (if Alice's photon was measured to be horizontally polarized in the direction determined by the polarizer, then Bob's is usually vertically polarized when the original entangled biphoton was in the singlet state). You can take this as a preparation procedure for a definite polarization state of Bob's single photon through measurement of the other photon in an entangled pair, if Bob does his polarization measurement after Alice did hers, but already at this point you come into trouble with causality if Bob does his measurement before Alice. This is, what I call a preparation procedure (however, a very tricky one). No classical dynamics is needed. Going throuh a polarization filter and detction of the one photon is perhaps difficult to describe in quantum-theoretical detail, but there's no hint that you need any classical dynamics to describe the meausrement/preparation procedure. So you need no cut.

What is, in my opinion, really unsolved with regard to ontology within quantum dynamics, is how Bob's photon gets into the pure polarization state. Here comes the place, where I think one has to invoke something like a collapse, when insisting on an ontological interpretation of quantum states, and this is highly problematic, because then Bob's photon must instantly go somehow into the definite pure polarization state, at Bob's place, when I measure the one photon due to the entanglement the other photon's state which may be registered at the same or a previous time (or at any space or timelike distance in spacetime). Note however, that the photon is not well localized. So the collapse, if it's a real process in an ontological interpretation of quantum states, place over long distances instantaneously, which violates relativistic causality.

In my opinion, this issue can only be solved by giving up an ontological interpretation and thus to give up the collapse postulate. Then I'm right at the ensemble interpretation, which does not associate an ontological meaning to states but taking them as the precise description of our knowledge about the quantum under investigation, which is probabilistic, i.e., takes an epistemical interpretation of quantum states. Then the association of the pure state after the measurement of the first photon is just due to the gain of our knowledge about the polarization of the measured photon and the knowledge that both photons were previously prepared in that entangled Bell state. This knowledge, however I can only gain when I take notice about the polarization state of the first photon. For this I need to communicate the measured polarization state of Alice's photon to Bob to take it as a preparation process for Bob's photon, before he has measured its polarization. This communication of information can take place at most with the speed of light, and thus Bob's measurement to validate the polarization state can only occur after (i.e., at a time like distance) to Alice's measurement. No problems with causality occur. That the correlation of polarizations is 100% for the photons in the entangled state is due to the very first preparation process, when the pair was created. Thus the epistemological interpretation of the ensemble interpretation saves the causality structure of quantum theory, which of course is at the heart of constructing all the most successful relativistic quantum theories in terms of microcausal local quantum field theories.

However, many people insist on an ontological interpretation of the notion of states in physics, and then you are left with this unsolved problem of preparations of states. For physics, which is just about the description of nature not about finding out ontological meaning of our knowledge, this is quite irrelevant, and thus it's a metaphysical problem of philophers :-).

 

[atyy]

Only because something is in Landau Lifshitz (whose volume 3 is, however, excellent) it needs not to be true, indeed. An there is no physically valid definition or any empirical hint for a cut, and as far as I know, it was Bohr, who emphasized the necessity for classical dynamics of a measurement apparatus. So that's part of the Copenhagen interpretation. Anyway, that's just semantics.

Just to be clear, my criticism of Ballentine is that he is wrong when he says (1) Copenhagen is wrong (2) textbook quantum mechanics with state reduction is wrong. Yes, Copenhagen-type/instrumental/operational interpretations have a cut, Landau and Lifshitz have a cut, and Weinberg's description of Copenhagen has a cut. Copenhagen-type interpretations do have a problem, but Ballentine has neither identified it not solved it.

Further the collapse postulate is almost never fulfilled in practice. Usually a quantum system gets destroyed when it's measured but not prepared in a new state. Let's discuss a concrete example, where both aspects of measurements, gaining information about a system and destroying it and using the measurement for the preparation of a system for further measurements.

Let's consider a photon from an entangled state gets absorbed when it is registered by a photo plate or CCD or however you measure it (call the experimentalist Alice). If you have let Alice's photon go through an ideal polarizer, then the 2nd photon (measured by Bob before or after Alice's measurement) is in a definite polarization state (if Alice's photon was measured to be horizontally polarized in the direction determined by the polarizer, then Bob's is usually vertically polarized when the original entangled biphoton was in the singlet state). You can take this as a preparation procedure for a definite polarization state of Bob's single photon through measurement of the other photon in an entangled pair, if Bob does his polarization measurement after Alice did hers, but already at this point you come into trouble with causality if Bob does his measurement before Alice. This is, what I call a preparation procedure (however, a very tricky one). No classical dynamics is needed. Going throuh a polarization filter and detction of the one photon is perhaps difficult to describe in quantum-theoretical detail, but there's no hint that you need any classical dynamics to describe the meausrement/preparation procedure. So you need no cut.

What you are saying is we can take the measurement apparatus to be quantum, and use only unitary evolution, and no additional assumptions, and recover definite outcomes. If you can show this, you would have shown that decoherence solves the measurement problem. Decoherence does not solve the measurement problem, unless additional assumptions are introduced.

What is, in my opinion, really unsolved with regard to ontology within quantum dynamics, is how Bob's photon gets into the pure polarization state. Here comes the place, where I think one has to invoke something like a collapse, when insisting on an ontological interpretation of quantum states, and this is highly problematic, because then Bob's photon must instantly go somehow into the definite pure polarization state, at Bob's place, when I measure the one photon due to the entanglement the other photon's state which may be registered at the same or a previous time (or at any space or timelike distance in spacetime). Note however, that the photon is not well localized. So the collapse, if it's a real process in an ontological interpretation of quantum states, place over long distances instantaneously, which violates relativistic causality.

In my opinion, this issue can only be solved by giving up an ontological interpretation and thus to give up the collapse postulate. Then I'm right at the ensemble interpretation, which does not associate an ontological meaning to states but taking them as the precise description of our knowledge about the quantum under investigation, which is probabilistic, i.e., takes an epistemical interpretation of quantum states. Then the association of the pure state after the measurement of the first photon is just due to the gain of our knowledge about the polarization of the measured photon and the knowledge that both photons were previously prepared in that entangled Bell state. This knowledge, however I can only gain when I take notice about the polarization state of the first photon. For this I need to communicate the measured polarization state of Alice's photon to Bob to take it as a preparation process for Bob's photon, before he has measured its polarization. This communication of information can take place at most with the speed of light, and thus Bob's measurement to validate the polarization state can only occur after (i.e., at a time like distance) to Alice's measurement. No problems with causality occur. That the correlation of polarizations is 100% for the photons in the entangled state is due to the very first preparation process, when the pair was created. Thus the epistemological interpretation of the ensemble interpretation saves the causality structure of quantum theory, which of course is at the heart of constructing all the most successful relativistic quantum theories in terms of microcausal local quantum field theories.

However, many people insist on an ontological interpretation of the notion of states in physics, and then you are left with this unsolved problem of preparations of states. For physics, which is just about the description of nature not about finding out ontological meaning of our knowledge, this is quite irrelevant, and thus it's a metaphysical problem of philophers :).

It's actually the other way round. If states are not ontological or "physically real and existing", in comparison to measurement outcomes, then we can have collapse. Major approaches that treat the wave function as real such as Bohmian Mechanics or Many-Worlds do not have collapse. Also, if you now treat the wave function as not ontological, you have introduced some form of Heisenberg cut, because the wave function is not "out there" but "in your head" so to speak.

 

[vanhees71]

Just to be clear, my criticism of Ballentine is that he is wrong when he says (1) Copenhagen is wrong (2) textbook quantum mechanics with state reduction is wrong. Yes, Copenhagen-type/instrumental/operational interpretations have a cut, Landau and Lifshitz have a cut, and Weinberg's description of Copenhagen has a cut. Copenhagen-type interpretations do have a problem, but Ballentine has neither identified it not solved it.

I've to read Ballentine's book again, but I don't think he says that Copenhagen is wrong. Anyway, I think if you take the collapse as a real thing happening in the physical world it must contradict Einstein causality, and there is no hint of this being true whatsoever. To the contrary, the successful relativistic QFTs are based on the very assumption that this is not the case: It's local and microcausal. So there is no instantaneous interaction at a distance by construction. Of course, the caveat is that so far nobody could prove the mathematical consistency of this scheme for physically relevant theories.

What you are saying is we can take the measurement apparatus to be quantum, and use only unitary evolution, and no additional assumptions, and recover definite outcomes. If you can show this, you would have shown that decoherence solves the measurement problem. Decoherence does not solve the measurement problem, unless additional assumptions are introduced.

I can't show this, but I don't see, where one finds it necessary to introduce a non-quantum dynamics, because the classical behavior of macroscopic objects, including measurement apparati are well-understood as approximations of quantum dynamics.

It's actually the other way round. If states are not ontological or "physically real and existing", in comparison to measurement outcomes, then we can have collapse. Major approaches that treat the wave function as real such as Bohmian Mechanics or Many-Worlds do not have collapse. Also, if you now treat the wave function as not ontological, you have introduced the classical/quantum cut, because the wave function is not "out there" but "in your head" so to speak.

I don't know, whether Bohm is consistent with relativity yet. Within non-relativistic theory there is no problem with a collapse, because instantaneous interactions are no contradiction with basic principles. So there is no need for non-local dynamics on top of quantum theory, as long as it doesn't provide additional observational consequences, and this is not the case as far as I know. So this additional Bohmian orbits are simply superfluous and can be cut away with Occam's razor. The same is true for unobservable parallel universes in the many-world interpretation. It simply doesn't help to solve the problems with quantum theory.

The minimal interpretation just states the facts and makes the necessary connections between observations in the real world and the formalism provided by quantum theory. I don't claim that it solves the measurement problem but it's a consistent scheme to use quantum theory to describe the outcome of real experiments.

 

[Fredrik]

Yes, the minimal statistical interpretation is at best a flavour of Copenhagen. What I don't like about Ballentine's work is that he explicitly opposes Copenhagen in his review article, but at best the Copenhagen he opposes is such a caricature and misunderstanding of Copenhagen. In his book he doesn't identify his opponent as Copenhagen, but the interpretation he opposes is the one he identifies as Copenhagen in his review.

I'm puzzled by this comment. You're implying that Ballentine is opposing a bad idea that's based on misunderstandings. So what's the problem? Is it really that he used the term "Copenhagen" to describe this bad idea 40 years earlier? Surely that's not a problem with his book?

It's interesting that you're labeling Ballentine's views as Copenhagen, and the views he's opposing as "not Copenhagen". I would do that too. The view he's opposing is essentially the MWI with the many worlds removed by magic. The view he's advocating is essentially just the idea that the state can be used to assign probabilities to possible results of experiments. I don't even consider that an interpretation. It's just QM. But if I had to call it something, I'd call it "Copenhagen". The term "the minimal statistical interpretation" has gained some popularity at PF, but I haven't heard it elsewhere. I don't want to use a term that includes "statistical interpretation", because that term should refer to the 1970 version, in which particles are assumed to have well-defined positions even when their wavefunctions are spread out. Is it the removal of that assumption that makes it "minimal"? Who defined it that way?

Oh, and I don't really want to call it "Copenhagen" either, because that term has completely lost its meaning.

Ballentine is wrong in the following:
1. Opposing Copenhagen or characterizing Copenhagen with a caricature.

So he's using the wrong terminology...in an article that came out more than 40 years ago. And he's not using that terminology in the book. I don't see the problem. Also, there's no right way to use the term "Copenhagen" anymore, because its meaning has been changed by people who misunderstood Bohr in different ways.

2. Claiming that pure states are not in some sense the "complete" information about an individual system. Within Copenhagen, after a classical/quantum cut is taken, pure states are the most "complete" information in the sense that they are extremal points of the space of density operators. One can identify pure states with individual systems or ensembles, and no mistake is made as long as one adds that the theory only predicts probabilities.

This argument doesn't make sense. What Ballentine is opposing is the view that a state represents is all the "actual properties" of the system. You can't argue that he's wrong by saying that pure states are extreme points in a convex set. That isn't just a weak argument. The argument isn't even related to the thing you're applying it to.

3. Claiming that Copenhagen cannot predict the results of Stern-Gerlach experiments correctly.

I haven't read this recently, so I can't comment at this time.

4. Claiming that one can do away with state reduction, because effective state reduction can be derived from decoherence without any additional assumptions. Here he has made the error of assuming that proper and improper mixtures are equivalent which is equivalent to assuming state reduction as Haroche and Raimond explain in https://www.amazon.com/dp/0198509146/?tag=pfamazon01-20.

The table of contents offers no clues about where in the book they have done this. That's OK though. Not sure I would want to take the time to read it. I expect that if they prove a claim like that, then their version of "reduction" or "collapse" isn't something that I would describe in those terms.

5. If he is presenting some version of Copenhagen, then the wave function is already just a tool, and there is no problem with state reduction. His objection to state reduction makes more sense if he is considering the wave function as real, as in Many-Worlds.

I don't remember what he said about this. Aren't the negative comments about state reduction part of his criticism of the view that the wave function is "real"?

 

[Nugatory]

There's no reason to not to let this thread run as long as the current participants are enjoying it... but I do think that we may have left OP and his question somewhere behind us?

 

[atyy]

I've to read Ballentine's book again, but I don't think he says that Copenhagen is wrong. Anyway, I think if you take the collapse as a real thing happening in the physical world it must contradict Einstein causality, and there is no hint of this being true whatsoever. To the contrary, the successful relativistic QFTs are based on the very assumption that this is not the case: It's local and microcausal. So there is no instantaneous interaction at a distance by construction. Of course, the caveat is that so far nobody could prove the mathematical consistency of this scheme for physically relevant theories.

In Ballentine's book he doesn't explicitly say that Copenhagen is wrong. However, in his 1970 review he does oppose Copenhagen, and the caricature of Copenhagen opposed in his review is the same view he opposes in his book. I have no problems with an Ensemble/minimal statistical interpretation if one says it is a flavour of Copenhagen, and Copenhagen is basically right.

I can't show this, but I don't see, where one finds it necessary to introduce a non-quantum dynamics, because the classical behavior of macroscopic objects, including measurement apparati are well-understood as approximations of quantum dynamics.

There are two properties of the classical world: (1) definite measurement outcomes (2) specific classical dynamics. The classical limit of quantum mechanics can get (2), so that is not the problem. But quantum mechanics itself seems not to be able to get (1), which is why there is the widely acknowledged measurement problem. Within Copenhagen, the subjective cut is introduced to get (1), in Bohmian mechanics hidden variables are introduced to get (1), and in MWI all possible outcomes are introduced to get (1).

I don't know, whether Bohm is consistent with relativity yet. Within non-relativistic theory there is no problem with a collapse, because instantaneous interactions are no contradiction with basic principles. So there is no need for non-local dynamics on top of quantum theory, as long as it doesn't provide additional observational consequences, and this is not the case as far as I know. So this additional Bohmian orbits are simply superfluous and can be cut away with Occam's razor. The same is true for unobservable parallel universes in the many-world interpretation. It simply doesn't help to solve the problems with quantum theory.

The minimal interpretation just states the facts and makes the necessary connections between observations in the real world and the formalism provided by quantum theory. I don't claim that it solves the measurement problem but it's a consistent scheme to use quantum theory to describe the outcome of real experiments.

Yes, it is not really collapse that Bohmian mechanics and other interpretations are introduced to solve. It is the problem of definite measurement outcomes. I agree that it is still a matter for research whether there is a satisfactory Bohmian mechanics for relativistic quantum field theory. However, if the problem of lattice chiral fermions interacting with non-abelian gauge fields can be solved, then there will be a lattice standard model (let's take Hamiltonian lattice theory), which for any finite lattice spacing will not be relativistic, in which case there should probably be a Bohmian standard model also.

Anyway, as I have said, I have no problems with the minimal interpretation (with a cut, and with collapse) taken be a flavour of Copenhagen. Basically once the wave function is not necessarily ontological, there is a subjective cut, and there is no problem with collapse since that is (at least partly) updating our knowledge.

 

[carllooper]

If asked how a wave function becomes a particle detection one might very well say: "I don't know, but one way to describe it is that the wave function collapses".

I don't think Copenhagen introduces the concept as anything much more than this. It certainly doesn't point back to any physical mechanism that anyone had elaborated in QM at the time of Copenhagen

And Einstein will get this. He'll understand by this that QM is incomplete, rather than incorrect.

For Bohr it's obvious that if there is any reality behind the measurements it's not going to be one that fits any classical conception of reality. Einstein (and others) will pursue a somewhat different course. If Bohr introduces the cut it's because about the only thing that remains classically real are the particle detections. One can assign them an unambiguous location in space (and the same would apply if one used relativistic space-time for this as well).

Bohr cleverly anchors the whole thing in terms of that which is the least problematic. Remember it's at a time when relativity is still quite new, and QM even newer. The audience for QM is classically trained. In other words Bohr anchors QM in terms which everyone can, at the very least, agree on. And that's really smart.

But what other way can it be done anyway?

If one starts with the concept of a wave function and tries to derive a particle detection, it doesn't quite work. But does it need to work that way anyway? To the extent that any proposed mechanism might yield new physics that would be just fine. It would be good for physics. But until then it's only good for science fiction (ie. philosophy). Of course nothing wrong with that.

C

 

[atyy]

I don't remember what he said about this. Aren't the negative comments about state reduction part of his criticism of the view that the wave function is "real"?

I read the rest of your post as well, but basically the problem is we not only have multiple interpretations of QM, we have multiple interpretations of Ballentine! I have never read Ballentine as opposing the wave function as "physically real" until this thread where that interpretation of Ballentine is mentioned by bhobba, kith and you. I have always read Ballentine as opposing a Copenhagen-style interpretation which one can also call instrumental/operational/orthodox/textbook/shut-up-and-calculate, which is certainly the impression conveyed by his review. I understood the distinctive elements he is opposing to be the assignment within these Copenhagen-style interpretations of a pure state as the most complete possible description of a single system, and state reduction. Although the book does not call what he is opposing "Copenhagen", it is the same view he earlier called "Copenhagen", and in both the review and the book he makes it seem that the view he is opposing is a mainstream view. The only mainstream view with state reduction I know is the Copenhagen-style interpretation, while two major approaches that treat the wave function as "physically real" are Bohmian Mechanics and MWI, neither of which have state reduction as fundamental.

The comment that assuming proper and improper mixtures to be equivalent is as good as assuming state reduction is found in section 2.5.4 "Decoherence Models versus the Copenhagen Interpretation" in Haroche and Raimond's https://www.amazon.com/dp/0198509146/?tag=pfamazon01-20. As far as I can tell, Ballentine believes he can get effective state reduction from unitary evolution alone. Ballentine also opposed the explanation of the quantum Zeno effect based on state reduction, ie. he opposed Sudarshan and Misra, and Wineland and colleagues. I think it is fair to assume that when Sudarshan and Misra and Wineland and colleagues use state reduction, they are using a Copenhagen-style interpretation, not some non-mainstream "the wave function is "real" and "really" collapses" view of quantum mechanics. So I basically don't think Ballentine is teaching "standard quantum mechanics".

 

[bhobba]

IIf one starts with the concept of a wave function and tries to derive a particle detection, it doesn't quite work

Cant follow that.

A wave-function is a state expanded in terms of eigenfunctions of position so must include the idea of particle detection.

Thanks
Bill

 

[carllooper]

Cant follow that.

A wave-function is a state expanded in terms of eigenfunctions of position so must include the idea of particle detection.

Yes, that's why you can't derive a particle detection from it. The wave function already includes the idea of a particle detection. The particle detection is not an output, or an outcome, of the wave function. Rather it is an input. The wave function includes within it's conception that which was used to derive the wave function in the first place. The wave function describes the statistics of particle detections, but in a formal and recomposable way, ie. generalises the phenomena of particle detections, by decomposing such data into the concept of single particles, (wave functions) which can then be manipulated in a flexible way (though different compositions, according to shut-up-and-calculate robots) to predict the behaviour of ensembles (compositions) within a range of varying setups. Or indeed any setup (as far as we know).

But it doesn't include within it a solution to the so called measurement problem. But nor does it create the problem. Although the throwaway concepts such as "collapse" might be a contributor.

The problem emerges in that somewhat conflicted zone between classical philosophy and modern physics. I don't know how physics can resolve that. Or classical philosophy can resolve it. It seems to me to be a false problem, but who knows - perhaps it isn't.

cheers
C

 

[vanhees71]

Well, the main problem seems to be to know, what's the Copenhagen interpretation. In my opinion, if you take out the cut and the collapse as physical processes you are at the minimal interpretation. So you may well call the minimal (ensemble) interpretation a flavor of Copenhagen, and with this everybody agrees.

It doesn't solve the measurement problem from first principles, i.e., how it comes to definite outcomes via quantum dynamics between the measured object and the measurement apparatus. It takes the measurement in a naive way as it's used in the lab. You put a photo plate in the way of an electron (or any other massive particle), and when it hit's the plate you have detected the electron at a place with an accuracy given by the resolution of the photo plate. To check quantum theory, you need many equally stochstically independently prepared electrons described by a wave function (which you know through the preparation in the corresponding state) and then you can check within the accuracy of your position measurement whether this wave function describes the position probability distribution correctly. That's it. You can call it the "shut up and measure" interpretation. The question how the electron leaves the trace on the photo plate in terms of quantum theory is perhaps too complicated to be understood in all detail ever, but it's not necessary to have a working interpretation of the quantum theoretical formalism connecting it with the position measurement of an electron, and "for all practical purposes" this is all you need (although Bell didn't like this "FOPP" statements at all :-) ).

 

[kith]

I have never read Ballentine as opposing the wave function as "physically real" until this thread where that interpretation of Ballentine is mentioned by bhobba, kith and you.

I didn't say this and I think introducing "realism" in this discussion opens an unnecessary additional can of worms. I wrote that he is arguing against collapse as a physical process and he does so by expanding the boundary between what is included in the quantum system and what is not. This is something all major interpretations allow for. So he isn't criticizing any of them but is debunking old and wrong ideas which come from people who misunderstood Bohr or -as I can tell from personal experience- from standard textbooks being vague on this matter.

 

[atyy]

Well, the main problem seems to be to know, what's the Copenhagen interpretation. In my opinion, if you take out the cut and the collapse as physical processes you are at the minimal interpretation. So you may well call the minimal (ensemble) interpretation a flavor of Copenhagen, and with this everybody agrees.

It doesn't solve the measurement problem from first principles, i.e., how it comes to definite outcomes via quantum dynamics between the measured object and the measurement apparatus. It takes the measurement in a naive way as it's used in the lab. You put a photo plate in the way of an electron (or any other massive particle), and when it hit's the plate you have detected the electron at a place with an accuracy given by the resolution of the photo plate. To check quantum theory, you need many equally stochstically independently prepared electrons described by a wave function (which you know through the preparation in the corresponding state) and then you can check within the accuracy of your position measurement whether this wave function describes the position probability distribution correctly. That's it. You can call it the "shut up and measure" interpretation. The question how the electron leaves the trace on the photo plate in terms of quantum theory is perhaps too complicated to be understood in all detail ever, but it's not necessary to have a working interpretation of the quantum theoretical formalism connecting it with the position measurement of an electron, and "for all practical purposes" this is all you need (although Bell didn't like this "FOPP" statements at all :) ).

But in the FAPP spirit, there is no problem with treating the wave function as the physical state of an individual system, and collapse as being at least partly due to physical disturbance of the state during measurement, right?

kith: thanks for your clarification about your interpretation of Ballentine! Anyway, since you learned partly form Cohen-Tannoudji, perhaps you can also comment on this. I too read Cohen-Tannoudji, and IIRC, they hedge their bets about whether collapse is caused by a physical disturbance. I think they say something vague like after getting a measurement outcome, we have to update what we know about the system, leaving open the possibility that there was a physical disturbance. Technically, the reason for not ruling out the physical disturbance is that we don't seem to be able to map the state reduction postulate onto a pure Bayes's rule. Wiseman and Milburn's make this interesting comment: "The most general formulation of classical measurement was achieved simply by adding back-action to Bayes’ theorem. The most general formulation of quantum measurement should thus be regarded as the quantum generalization of Bayes’ theorem, in which back-action is an inseparable part of the measurement." http://books.google.com/books?id=ZNjvHaH8qA4C&vq=back&source=gbs_navlinks_s (bottom of p33)

 

[kith]

Technically, the reason for not ruling out the physical disturbance is that we don't seem to be able to map the state reduction postulate onto a pure Bayes's rule.

To allow for a physical disturbance implies that the Heisenberg cut divides the world in a quantum world where QM is valid and a classical world where QM is false and classical mechanics is valid instead. But wherever that barrier lies, once we enter the classical regime, QM's statistical predictions reduce to the ones of classical mechanics. So QM predicts the correct results of measurements in both regimes. If there's a disturbance or a barrier, it can't be verified by experiments even in principle, which is why I think calling it physical isn't justified.

I don't have access to Wiseman's and Milburn's book. What seems interesting is that they include classical physics in their discussion. Many comments on the foundations raise all kind of problems with QM but don't mention the tacit assumptions of classical mechanics or reflect on science in general.

 

[atyy]

To allow for a physical disturbance implies that the Heisenberg cut divides the world in a quantum world where QM is valid and a classical world where QM is false and classical mechanics is valid instead. But wherever that barrier lies, once we enter the classical regime, QM's statistical predictions reduce to the ones of classical mechanics. So QM predicts the correct results of measurements in both regimes. If there's a disturbance or a barrier, it can't be verified by experiments even in principle, which is why I think calling it physical isn't justified.

But can it be verified by experiments that there is no physical disturbance?

 

[kith]

So we have something about which we cannot get any insight by performing experiments. I wouldn't call such a thing "physical".

 

[atyy]

So we have something about which we cannot get any insight by performing experiments. I wouldn't call such a thing "physical".

So "physical" and "not physical" is itself not physical? I guess more directly: what do you mean by "To allow for a physical disturbance implies that the Heisenberg cut divides the world in a quantum world where QM is valid and a classical world where QM is false and classical mechanics is valid instead."

Actually, my question initially was that if the wave function is not necessarily real, and everything we do with it is FAPP anyway, then it would seem that there is no problem with treating the wave function as FAPP physical. So for example, do you really object to language in which the measurement is conceived as causing a disturbance? Here are some examples:

"The inevitable back action noise (uncontrollable disturbance)" http://web.stanford.edu/~rsasaki/AP226/text4.pdf

"The most general formulation of quantum measurement should thus be regarded as the quantum generalization of Bayes’ theorem, in which back-action is an inseparable part of the measurement." http://books.google.com/books?id=ZNjvHaH8qA4C&vq=back&source=gbs_navlinks_s

"Proof of Heisenberg's error-disturbance relation" http://arxiv.org/abs/1306.1565

"Since the earliest days of quantum mechanics, a common idea associated with the measurement process has been that it necessarily disturbs or interferes with the system being observed. For instance Bohr, in his reply to the Einstein-Podolsky-Rosen paper [1], writes that the quantum description “may be characterized as a rational utilization of all possibilities of unambiguous interpretation . . . compatible with the finite and uncontrollable interaction between the objects and the measuring instruments” [2,3]." http://arxiv.org/abs/quant-ph/0009101

 

[kith]

So for example, do you really object to language in which the measurement is conceived as causing a disturbance?

You were talking specifically about a physical state reduction process and not about the general interaction between the measurement apparatus and the system. The problem with this view is that if you shift the boundary, you won't find the state reduction in the quantum dynamics. So you have to postulate that the boundary cannot be shifted and that the quantum regime ends somewhere. This is very strange because the statistical predictions of QM remain valid beyond this barrier.

 

[Fredrik]

we have multiple interpretations of Ballentine! I have never read Ballentine as opposing the wave function as "physically real" until this thread where that interpretation of Ballentine is mentioned by bhobba, kith and you. I have always read Ballentine as opposing a Copenhagen-style interpretation which one can also call instrumental/operational/orthodox/textbook/shut-up-and-calculate,

Then my interpretation of Ballentine is exactly the opposite of yours. He's supporting a Copenhagen-style interpretation which one can also call instrumental/operational/orthodox/textbook/shut-up-and-calculate. (I'm not sure that "orthodox" is right. I'd have to look up how that's defined). He's rejecting the view that a pure state "provides a complete and exhaustive description of an individual system". That's an exact quote from the description of the class of interpretations that he rejects. It's at the start of section 9.3 if you want to check.

I would define an "interpretation of QM" as a speculative statement about what really going on at all times, including at times between state preparation and measurement. Ballentine offers no interpretation of QM in this sense. He just describes a way to think about pure states that's appropriate if you don't subscribe to any particular interpretation of QM. This gives us an interpretation of pure states that's guaranteed to be appropriate, regardless of what else a pure state may represent.

The comment that assuming proper and improper mixtures to be equivalent is as good as assuming state reduction is found in section 2.5.4 "Decoherence Models versus the Copenhagen Interpretation" in Haroche and Raimond's https://www.amazon.com/dp/0198509146/?tag=pfamazon01-20.

Thanks. I haven't decided if I will check it out yet. The suggestion that there could be a difference between "proper" and "improper" is utterly bizarre to me. Hm, maybe saying that there's a difference is the same as saying that a pure state "provides a complete and exhaustive description of an individual system". A person who says that may want to resort to using he magical kind of collapse to not have to deal with many worlds. I should probably at least skim it to see what it's about, but I have to go to bed now. Maybe tomorrow.

 

[atyy]

You were talking specifically about a physical state reduction process and not about the general interaction between the measurement apparatus and the system. The problem with this view is that if you shift the boundary, you won't find the state reduction in the quantum dynamics. So you have to postulate that the boundary cannot be shifted and that the quantum regime ends somewhere. This is very strange because the statistical predictions of QM remain valid beyond this barrier.

Actually, I was asking two different questions, one about the interpretation of quantum mechanics and the concept of "really physical", and the other about the interpretation of Ballentine and language like "FAPP physical". Anyway, yes, let's stick for the moment to "really physical" in the context of state reduction. The idea I'm interested in understanding is whether one can say that state reduction is only updating our knowledge and does not correspond to anything physical. I do like the approach that state reduction is only updating our knowledge, that it is like throwing a die, and having the probability distribution collapse when you get a definite outcome. However, can this be justified beyond an analogy? To my knowledge, no one has done it yet. Some attempts to draw a close analogy to Bayesian updating indicate that the analogy is not complete (eg. http://arxiv.org/abs/quant-ph/0106133, http://arxiv.org/abs/1107.5849, and the Wiseman and Milburn book). For this reason, I don't rule out that state reduction may also be partly physical, in contrast to only updating our knowledge). So I am interested if you have an argument that makes the Bayesian analogy closer.

In the argument you give, it seems that you are more willing to consider unitary evolution as more physical than state reduction, so by "quantum dynamics" so mean unitary evolution. However, within Copenhagen, I don't know if unitary evolution is privileged, and in principle both unitary evolution and state reduction are quantum dynamics. Both are equally not necessarily real, and just tools to calculate probabilities of measurement outcomes.

 

[atyy]

Then my interpretation of Ballentine is exactly the opposite of yours. He's supporting a Copenhagen-style interpretation which one can also call instrumental/operational/orthodox/textbook/shut-up-and-calculate. (I'm not sure that "orthodox" is right. I'd have to look up how that's defined). He's rejecting the view that a pure state "provides a complete and exhaustive description of an individual system". That's an exact quote from the description of the class of interpretations that he rejects. It's at the start of section 9.3 if you want to check.

Well, our interpretations of Ballentine are not exactly opposite. I do think the view Ballentine is proposing is a Copenhagen-style view. Our difference lies in what we think he is opposing. Possibly because I did also know the 1970 review, and treated the book as an updated version of the review, I understood the view he is opposing to be Copenhagen. Even if the book does not mention Copenhagen, it seemed to be the same view he opposed in the review, and he still makes it seem like the view is a mainstream view with state reduction. So I read the book entirely within Copenhagen, as a debate between two flavours of Copenhagen, and the claim the Ballentine's flavour (Copenhagen B) was better than the flavour he opposed (Copenhagen A). Within Copenhagen A, I understood "a pure state provides a complete and exhaustive description of an individual system" to mean that one can think of a pure state as the state of an individual system, and it is complete and exhaustive within the theory (a choice of measurement apparatus and quantum system, operators and commutation relations, and Hilbert space) because a pure state is an extreme point of the space of density operators. On my initial reading, my puzzlement was that his Copenhagen B alternative "A pure state describes the statistical properties of an ensemble of similarly prepared systems." was not at all different from Copenhagen A, since in Copenhagen A one adds that the theory only predicts expectation values. I had always used both Copenhagen A and Copenhagen B interchangeably before reading Ballentine. To make sense of his claim, since he does attack state reduction in what I understood to be Copenhagen A, I thought he was claiming that state reduction was not required in Copenhagen B, and that if one used Copenhagen B, state reduction can be derived from unitary evolution without the introduction of any additional postulate.

However, I don't think what I understood to be his claim is true. I do think that Copenhagen A in which a pure state is thought to label an individual system, and in which there is state reduction, is a valid version of quantum mechanics. I do not think that Copenhagen B manages to derive state reduction from unitary evolution without any additional postulate. So I believe his claim of the superiority of Copenhagen B over Copenhagen A to be untrue. I also think that since his derivation of state reduction from unitary evolution fails, that his Copenhagen B is in fact lacking a postulate of Copenhagen-style quantum mechanics, and is therefore incomplete, and therefore wrong. I do believe there are valid approaches to QM without state reduction (reviewed in Wiseman and Millburn's book), but these do not easily accommodate a common-sense reality (whereas Copenhagen does accommodate it on one side of the cut, which can be shifted). I do of course acknowledge that there can be theories that reproduce QM without state reduction such as Bohmian mechanics, and interesting approaches to QM interpretation such as MWI that don't have state reduction, but introduce many worlds. If he is rejecting common-sense reality, introducing hidden variables or many-worlds, I believe these are unusual enough that he has to explicitly state them in order for us to understand that that is what he is doing.

Of course, if the alternative he opposes is not Copenhagen A but some non-mainstream view like physical collapse of the wave function (whatever that means - I have never heard it within QM - to me that refers to GRW or CSL), then I could hardly object to it. But I think I have a good case for my reading! Maybe in the future, when hidden variables have been experimentally discovered, and the interpretation of QM settled, there will be vigourous debates over the various Schools of Ballentine Interpretation:)

 

[Fredrik]

Our difference lies in what we think he is opposing. Possibly because I did also know the 1970 review, and treated the book as an updated version of the review, I understood the view he is opposing to be Copenhagen. Even if the book does not mention Copenhagen, it seemed to be the same view he opposed in the review, and he still makes it seem like the view is a mainstream view with state reduction.

I've read the review too, but it was years ago, so I don't remember details like how he used the term "Copenhagen". I don't doubt that he would call the A view in the book "Copenhagen", when "collapse" is also included. This seems to be very common. I think it's how Copenhagen was explained to me in my first QM course. Ballentine doesn't say it explicitly, but the A view without collapse is a many-worlds interpretation. So I would say that he's rejecting a larger class of interpretations, not just a flavor of Copenhagen.

To me, the most remarkable detail in the review is that in the interpretation presented there, particles have well-defined positions at all times, even when their wavefunctions are spread out. At first I thought that this was completely insane and impossible. I eventually realized that one of the reasons I thought so was that I had not been able to completely let go of the idea that the wavefunction is the system.

Within Copenhagen A, I understood "a pure state provides a complete and exhaustive description of an individual system" to mean that one can think of a pure state as the state of an individual system, and it is complete and exhaustive within the theory (a choice of measurement apparatus and quantum system, operators and commutation relations, and Hilbert space) because a pure state is an extreme point of the space of density operators.

I think the intended meaning of "provides a complete and exhaustive description of an individual system" is completely impossible to define within the theory. The claim that a pure state "provides a complete and exhaustive description of an individual system" is supposed to tell us something new about pure states, something that the theory isn't telling us, but if we use the theory to define the statement so that it's implied by the theory, then the statement is not telling us anything. It's supposed to be an interpretation of QM, not something that QM says is true by definition.

My interpretation of the claim that a pure state "provides a complete and exhaustive description of an individual system" is that it says that the pure state (and time evolution) is telling us what is "actually happening" to the system at all times, including at times between state preparation and measurement. It's telling us that the pure state "describes" all the "properties" of the system.

In statements like these, terms like "actually happening", "describes" and "properties" are primitives, i.e. terms left undefined. I know that the suggestion that it's OK to leave something undefined sounds like complete nonsense to a lot of people, so I will elaborate a bit. Every definition of a term has the problem that it can only be understood by a person who understands the terms used in the definition. So every definition seems to just lead to another definition. To avoid an infinite chain of definitions, we have to leave some terms undefined. These undefined terms are called "primitives". The fact that we're leaving them undefined doesn't prevent us from explaining how to think about them. These explanations are called "elucidations".

his Copenhagen B alternative "A pure state describes the statistical properties of an ensemble of similarly prepared systems." was not at all different from Copenhagen A,
[...]
I had always used both Copenhagen A and Copenhagen B interchangeably before reading Ballentine.

This is understandable, since you seem to have interpreted "provides a complete and exhaustive description of an individual system" in a way that makes the statement (the defining assumption of the A class of interpretations) vacuously true. If we do this, then there's no difference between A and B. (The defining assumption of B is also true in A).

To make sense of his claim, since he does attack state reduction in what I understood to be Copenhagen A, I thought he was claiming that state reduction was not required in Copenhagen B, and that if one used Copenhagen B, state reduction can be derived from unitary evolution without the introduction of any additional postulate.

I think that state reduction has a very different meaning in A and B. In A, it's some kind of magical nonsense that eliminates the need to conclude that there are many worlds. In B, it's the idea that an interaction that leaves a macroscopic (and therefore FAPP classical) object in one of several easily distinguishable positions, will either destroy the system or leave it in a state that's FAPP indistinguishable from an eigenstate. (Or something like that...I haven't thought about this enough to find a favorite explanation of what state reduction is. Isham describes it as something even more mundane, as a selection of a sub-ensemble on which to make measurements).

I do think that Copenhagen A in which a pure state is thought to label an individual system, and in which there is state reduction, is a valid version of quantum mechanics.

I think it's probably not. If I find out that I'm wrong, I may feel a little silly about having used language like "magical nonsense" to describe aspects of this intepretation, but for now, I'll take my chances. :)

 

[kith]

In the argument you give, it seems that you are more willing to consider unitary evolution as more physical than state reduction, so by "quantum dynamics" so mean unitary evolution.

Yes, I think that the term quantum dynamics should be reserved for what the system does as long as we use a quantum description. State reduction refers to a situation where we let the system interact with something which is not included in the quantum description. So quantum dynamics and state reduction are not on equal footing.

If we shift the boundary, the quantum dynamics of the combined system gives predictions which can be checked experimentally. This quantum dynamics doesn't involve state reduction within the combined quantum system. So calling only the former "physical" seems pretty straightforward. Terms like "FAPP physical" and "really physical" make my head spin and I don't see how they help to clarify anything.

But this is starting to go in circles and be mostly about terminology. I already had a number of discussions about state reduction with you and although I learned quite a bit from them, I never had the impression that we got past terminology issues and to the core of the disagreement. I'm not sure if there is real disagreement (maybe this is an argument for Copenhagen ;)) but I don't see a way to simply jump past these terminology issues. And right now, I'm not interested in dissecting lots of terminology for a few grains of real issues.

 

[naima]

There are are physicists who defines QM physical processes as unitary processes
Read http://arxiv.org/pdf/1004.5073.pdf
"We know that any physical process can be thought of as a unitary process in an enlarged Hilbert space"

 

[atyy]

I think it's probably not. If I find out that I'm wrong, I may feel a little silly about having used language like "magical nonsense" to describe aspects of this intepretation, but for now, I'll take my chances. :)

I'm pretty sure that Copenhagen A as I understood it is valid quantum mechanics, because it is identical to Copenhagen B with state reduction. However, I do agree that state reduction in Copenhagen A is magical nonsense - but it doesn't matter, because Copenhagen A, being a variety of Copenhagen, does not believe the wave function is necessarily real, and it is just a tool to calculate the probabilities of measurement outcomes. So in Copenhagen A, the wave function, unitary evolution and state reduction are all magical nonsense. In short, Copenhagen A makes successful predictions that have not been falsified at present, but it does admit that it has a measurement problem.

 

[atyy]

But this is starting to go in circles and be mostly about terminology. I already had a number of discussions about state reduction with you and although I learned quite a bit from them, I never had the impression that we got past terminology issues and to the core of the disagreement. I'm not sure if there is real disagreement (maybe this is an argument for Copenhagen ;)) but I don't see a way to simply jump past these terminology issues.

Yes, the issue I'd like to discuss is tangential to most of what is in this thread (living opponents of Copenhagen!). I think there isn't a refined enough standard terminology within Copenhagen to discuss reality/physicality, so I will start another thread about it. Regarding our old discussions, I always thought they were about whether state reduction is needed or not. My answer is that it is needed within Copenhagen, and I always thought you were agreeing with Ballentine that it is not needed if one uses his Ensemble interpretation. But now that I realize your interpretation (#64) and my interpretation (#73) of Ballentine are quite different, I do wonder! My default approach to quantum mechanics is indeed to assume Copenhagen. ;)

 

[carllooper]

This is Bohr summarising his semi-classical position:

"The new progress in atomic physics was commented upon from various sides at the International Physical Congress held in September 1927, at Como in commemoration of Volta. In a lecture on that occasion, I advocated a point of view conveniently termed "complementarity," suited to embrace the characteristic features of individuality of quantum phenomena, and at the same time to clarify the peculiar aspects of the observational problem in this field of experience. For this purpose, it is decisive to recognise that, however far the phenomena transcend the scope of classical physical explanation, the account of all evidence must be expressed in classical terms. The argument is simply that by the word "experiment" we refer to a situation where we can tell others what we have done and what we have learned and that, therefore, the account of the experimental arrangement and of the results of the observations must be expressed in unambiguous language with suitable application of the terminology of classical physics."

I interpret this as Bohr looking for a way to ensure QM is, before anything else, understood as a valid contribution to the history of physics, as opposed to some other discipline. One can imagine a lot of classically trained physicists walking around the conference in a huff saying things like "what is all this rubbish". And Bohr is trying to win them over to QM. Not in any dishonest way of course - just in a way that can be understood (to the extent it can) within a given climate and a certain history.

I'd have used different words from "phenomena transcending the scope of classical physical explanation". I'd have put it the other way - that classical physical explanation transcends the phenomena. The phenomena inspires or requires the introduction of certain limitations on classical concepts, rather than suggesting anything that transcends such concepts.

The vexed question of "reality" is something quite difficult to elaborate in physics. Its a lot easier to address this in philosophy than it is physics. But as a result it can then become very difficult to map such back into physics. Only a very small subset of philosophy maps back into physics. And typically it will be (understandably) the most conservative aspects of philosophy that do so. I'd suggest physics ends up doing it's own philosophy. It knows what philosophy it needs to construct. It creates that which belongs to physics.

C

 

[bhobba]

One can imagine a lot of classically trained physicists walking around the conference in a huff saying things like "what is all this rubbish". And Bohr is trying to win them over to QM.

QM was a mishmash of ideas until Dirac came up with his transformation theory in December 1926 (although some mathematical issues with that damnable Dirac Delta function remained - later fixed by Von-Neumann - but fully resolved with the development of Rigged Hilbert Spaces):
http://www.lajpe.org/may08/09_Carlos_Madrid.pdf

And indeed Copenhagen was only just completed in 1927 with a number of separate theories each calling itself QM prior to that. It was only after Diracs great breakthrough Copenhagen could have been finalised. So it's highly doubtful Bohr would have been trying to win anyone over in 1927 since it had such a long gestation and most physicists would have been aware of the situation. They knew classical physics was wrong by that stage - no convincing was required. The convincing would have been for the final form of QM that was then just developed.

But once Dirac's final form of QM was published, and especially after his book, Principles Of Quantum Mechanics, was available in 1930, opposition to QM as a theory disappeared. Even Einstein always carried around a copy of Dirac, and he spoke admiringly of Dirac of whom he said 'in my opinion we owe the most logically perfect presentation of quantum mechanics.' As a theory it was then mainstream - with little or no opposition, even by Einstein. Einstein's view at that stage had morphed - the 1930 Solvay conference was his last attempt to find fault with it - he failed - and it was reported he tipped his hat to Bohr when Bohr used Einsteins own principle of equivalence to show Einstein's latest objection was flawed. His debates with Bohr and others now convinced him the theory was correct - but incomplete. It was that conviction that reached its full flower in EPR.

Also by that stage most physicists were not concerned with its foundations - indeed Dirac's book and Von-Neumann's equally influential text - Mathematical Foundations of Quantum Mechanics - published in 1932, were pretty much agnostic towards philosophical issues (caveat - there is the issue of the Von-Neumann regress and conciousness - but that is another story):
http://www.physicsandmore.net/resources/Shutupandcalculate.pdf
'Both the two books were completely indifferent toward philosophical issues raised by Bohr, Schrödinger, Einstein and others, and were brilliant trend-setters in a precise formulation of quantum theory in mathematical terms because the authors felt that there was no point in half measures where quantum theory was concerned, and the need for the day was a set of concrete rules for mathematical calculations. Between them, the two books set forth the basic principles that were so brilliantly being made use of by the likes of Pauli, Schrödinger and Dirac in waves after waves of remarkable papers that were setting the entire world of physics on fire. Ever since, quantum theory has time and again demonstrated the power of the decree: shut up and calculate. Of course, there were people who asked questions.For instance, Feynman asked questions, but mostly he asked those in silence and never rested till he himself provided the answers. John S. bell asked questions, and was not satisfied a bit with the received wisdom but he, too, calculated and gave the world the famous inequality bearing his name that, more than anything else, brought in the information theoretic revolution in quantum theory dating from the nineteen eighties.'

The prevailing mood, as alluded to by the quote above, was shut up and calculate - which is often attributed to Dirac, or maybe Feynman, but, as the article correctly points out, is by David Mermin.

That has been pretty much the case since then. Most physicists couldn't really care a hoot - a few do - but for most - blah.

Of course to philosophy types, and students starting out, it's often the most important thing. But students soon get used to it and what they thought were problems melt away as they apply it or think about it more carefully - that happened to me - but took a while. Still some physicists and mathematicians are fascinated with its foundations - but it's not really a big concern generally - amongst scientists that is.

Thanks
Bill

 

[carllooper]

So it's highly doubtful Bohr would have been trying to win anyone over since the situation was a bit of a mess.

To suggest it is doubtful that Bohr can't be doing this because QM does not yet include that which Dirac will later contribute makes no sense at all. QM is what it is in Sept 1927. A work in progress, with promising developments, and a reason for being.

And while Einstein might be tipping his hat to Bohr in 1930, that doesn't stop him thinking through the problem further and coming back in 1935 with a new line of attack in the form of EPR, and temporarily throwing Bohr off balance. Bohr recovers his wits when he realizes that since you can't communicate information over the FTL channel, such a channel isn't physical, ie. it's 'philosophical' (or science fiction as I like to call it).

C

 

[bhobba]

To suggest it is doubtful that Bohr can't be doing this because QM does not yet include that which Dirac will later contribute makes no sense at all. QM is what it is in Sept 1927. A work in progress, with promising developments, and a reason for being.

It was completed in December 1926 - your quote was from 1927 - it was completed then, but maybe it hadn't percolated through the physics community yet. My suspicion is that was more the concern than than Bohr's views on complementarity.

What I am suggesting is that once that was done, because everyone knew that classical physics was kaput, Bohr didn't need to convince anyone about QM. What he was on about was his philosophical view of it which by that stage people didn't care that much about since shut up and calculate was taking hold.

And while Einstein might be tipping his hat to Bohr in 1930, that doesn't stop him thinking through the problem further and coming back in 1935 with a new line of attack in the form of EPR, and temporarily throwing Bohr off balance. Bohr recovers his wits when he realizes that since you can't communicate information over the FTL channel, such a channel isn't physical, ie. it's 'philosophical' (or science fiction as I like to call it).

I thought that's what I said. He no longer thought it incorrect, which is a big turn around, he thought it incomplete, which was what EPR was about.

That wasn't how Bohr refuted Einstein:
http://philosophyfaculty.ucsd.edu/faculty/wuthrich/teaching/2013_146/146QLect04_BohrEinsteinEPR.pdf
“Indeed the finite interaction between object and measuring agencies conditioned by the very existence of the quantum of action entails—because of the impossibility of controlling the reaction of the object on the measuring instruments, if these are to serve their purpose—the necessity of a final
renunciation of the classical ideal of causality and a radical revision of our attitude towards the problem of physical reality. In fact, as we shall see, a criterion of reality like that proposed by the named authors contains—however cautious its formulation may appear—an essential ambiguity when it is applied to the actual problems with which we are here concerned.'

Its highly likely Einstein knew what Bohr would say. However, as he wryly noted, everyone's response seemed to be different.

That said, my post wasn't primarily about Einstein, it was about the shut up and calculate view that took hold after QM was completed by Dirac - most people weren't that worried about Bohr's philosophical musings. Although it must be said when pushed on the matter physicists would retreat to Bohr's orthodoxy.

Thanks
Bill

 

[jbmolineux]

Any philosophical principles that Einstein may have used may very well have played an important, if not key role in his contribution to physics. Indeed, personally, I'm sure of it. But how can we conclude that if we "get those principles wrong" science will go astray? Bohr was equally inspired by philosophical principles, but a somewhat different set of principles, yet was perfectly able to make a contribution to physics.

The way in which physics generally works is that ideas are put to a physical test. If there's agreement between the physical test and the ideas that conceive it, then the idea is considered provisionally correct. Or useful. In other words, the idea could be philosophically right or wrong, but in terms of the physics it points out, (to the extent that it does) it wouldn't actually matter. What matters is whether it's physically so, (ie. physically wrong or provisionally correct). Not whether it's philosophically so.

Of course, in practice, it can get a lot more complicated than this.

C

Carllooper, I believe it is self-evident that if we get the philosophical principles underlying science wrong, that science will go astray. Unfortunately my sense is that not only is this not believed these days, but that the opposite idea has practically become orthodoxy--namely, the idea that the only meaning of a scientific theory is in its ability to make accurate predictions.

You say "what matters is if a theory is 'physically right or wrong' not whether it is 'philosophically so'." By a theory being "physically right or wrong" here, you could mean two things: (1) you could mean "physically" in the sense of having to do with the real physical matter of the world, and thus be saying something like "a physically true theory correctly describes what is actually out there in the world," or (2) which is practically the opposite of that, you could mean "physically" in the sense of "as in physics"--which, these days, often explicitly repudiates the idea that theories are describing a "world out there," but rather insists on the above point about just being about accurate experimental predictions.

In my view, a theory will only be able to make accurate predictions consistently by truly describing the actual world.

I know you will say, "what does it matter?"--let physics talk about experiments and philosophers talk about the meaning of scientific claims. But I think it does matter, and I think that the infiltration of physics by wrong philosophical assumption has affected it significantly.

I will try to say why I think this, but let me first give the disclaimer that I know very well that I am not an expert, and I am confident that there are many detailed replies to this that I will not be able to follow. What I have to say is more of a sense than it is grounded in something I fully understand. I am aware of this, and that's why I am interested in studying physics, and why I started this thread!

To me it appears that what has happened is that physicists started with choice #2 above (that physics does not describe a "world out there" but just makes predictions that either can, or can't be experimentally verified) and were led by that conclusion to disbelief in the out-there reality of the very particles they are supposed to be studying. You can clearly trace the lineage in the latter "scientific conclusion" to the former philosophical error. You can see how this would affect physical research. If you don't believe that the thing you are studying really exists (at least in any way you can make sense of) that is OBVIOUSLY going to affect the paths that you choose to study it!

Currently physics is moving forward largely under that understanding, and it seems to me that it is greatly affecting the path. What I see is a whole bunch of theories which all have to do with the ways that complex equations can match experiments--with fairly little progress on achieving consensus on theoretical advancement. That would be consistent with a mis-step of the type that I describe above, where a philosophical error would retard scientific progress.

And I feel like that explanation makes sense with what we know. Whereas classical physicists had the considerable advantage of being able to use their mental intuitions to form theories, today's physicists are trained in the art of NOT doing that. You have to accept that your intuitions (including the very fundamental intuition that physical theories are about the "real world" or the intuition that "everything has an explanation for being as it is," which was so important for Einstein) are of no use, and they are actually a hindrance to advancement. Well, if those intuitions are as essential to scientific advancement as I think they are (and as Einstein found them to be), then transforming the entire physics community into an institution that trains everyone who goes through it to mutilate those intuitions will obviously have a very negative effect.

Personally I simply can't conceive of why, if you don't actually believe the theory is about an independent objective world, you would expect experiments to match theories, or why you would care. In my view, you simply can't, and everyone carries that assumption around with them no matter how well they've trained themselves to mutilate it. So progress in physics may go on in spite of the error, but the error (and its having become orthodox) could be a significant hindrance.

Of course, I know that there are ways that we really do have to overcome our intuitions. I know we don't have the capacity to intuit general relativity, and to think in terms of it you have to in a sense twist your mind into a shape it just won't go. And furthermore, I know there really are things going on "down there" that are pretty much mind-blowing.

But whereas scientific advancement has often occurred by trying so solve paradox, it seems to me that part of the philosophy behind QM is very much the opposite. Rather than trying to solve the paradox, there is a sense in which they are deliberately embracing it. Rather than saying, "there is something that is going on here that we don't understand and we need to study it further," they are saying, "we understand exactly what is going on here, and it is that the particles cease to exist"--at just the point where they reach the limit of their ability to understand them! Thus it is with the alleged "completeness" of quantum theory. It amounted to transforming their inability to understand into a "scientific conclusion" (that was really a philosophical error, in my view) that (a) is taught to all students of science nowadays, and (b) actually puts a stop to the process of scientific inquiry.

So...if the philosophical error lead to physicists believing that there was no further need to study something because they had already understood everything there was to know about it, you can see why philosophy affects physical research, and so matters a good deal.

 

[bhobba]

Carllooper, I believe it is self-evident that if we get the philosophical principles underlying science wrong, that science will go astray.

I STRONLY disagree with that:
https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCAQFjAA&url=http://www.phys.washington.edu/users/vladi/phys216/Weinberg_Against_philosophy.doc&ei=fVJoVIXUNeL2mQWH_oCwCA&usg=AFQjCNHg_elaIirwh-1Q7Al_kVaI8Fz8YA&sig2=h2vnb14frw18jhYKClXrLw

But this forum is not the place to discuss it. The philosophy forums is the place.

Unfortunately my sense is that not only is this not believed these days, but that the opposite idea has practically become orthodoxy--namely, the idea that the only meaning of a scientific theory is in its ability to make accurate predictions.

It's not. Wienbergs view, which is the same as mine, is pretty common.

My post concerned the statement - 'One can imagine a lot of classically trained physicists walking around the conference in a huff saying things like "what is all this rubbish". And Bohr is trying to win them over to QM.'

I think by that time no one wasn't won over to QM. What Bohr was on about is his particular Copenhagen view - in this case complementarity. But shut up and calculate would soon predominate, if it hadn't already.

In my view, a theory will only be able to make accurate predictions consistently by truly describing the actual world.

Does Euclidean geometry describe the actual world? And if it does precisely how does QM differ from it.

Thanks
Bill

 

[Fredrik]

"Fredrik said:"

I didn't. The quote in bhobba's post #80 is from carllooper's post #79.

 

[Fredrik]

I STRONLY disagree with that:

Weinberg is dismissing the idea that philosophers can tell physicists which theories are plausible and which ones are not. I think everyone in physics agrees with him. But the idea that you can't find the right theory just by thinking about it is a philosophical principle that it's important to get right. So it would be more accurate to say that Weinberg is supporting the quoted sentence than to say that he's disagreeing with it.

In my view, a theory will only be able to make accurate predictions consistently by truly describing the actual world.

Now this is something I strongly disagree with. bhobba's example of Euclidean geometry shows that a theory can make good predictions even if's only an approximate description of the world. Here's an example of a theory that makes good predictions without even approximately describing the world:

Define $\Omega=\{1,2,3,4,5,6\}$. Let $\Sigma$ be the set of all subsets of $\Omega$. For each $E\in\Sigma$, let $|E|$ denote the cardinality of $E$, i.e. the number of distinct elements of $E$. Define $P:\Sigma\to[0,1]$ by
$$P(E)=\frac{|E|}{|\Omega|}$$ for all $E\in\Sigma$. Now let's turn this piece of mathematics into a theory about the real world by specifying that for each $E\in\Sigma$, $P(E)$ is the fraction of times we'll get a result in the set $E$ if we repeatedly throw a standard six-sided die a large number of times.

This theory isn't even approximate description of the world. In particular, it says nothing about what's actually happening to the die between state preparation (the throw) and measurement (the moment when it has landed with some side up).

 

[carllooper]

What I am suggesting is that once that was done, because everyone knew that classical physics was kaput, Bohr didn't need to convince anyone about QM. What he was on about was his philosophical view of it which by that stage people didn't care that much about since shut up and calculate was taking hold.

Ah ok. I see what you are saying.

 

[bhobba]

Weinberg is dismissing the idea that philosophers can tell physicists which theories are plausible and which ones are not. I think everyone in physics agrees with him. But the idea that you can't find the right theory just by thinking about it is a philosophical principle that it's important to get right. So it would be more accurate to say that Weinberg is supporting the quoted sentence than to say that he's disagreeing with it.

Its the old issue of being 'anti' philosophy. That is itself a philosophical position. But I think most understand what's meant without being ultra analytical about it.

Still - you are correct.

Thanks
Bill

 

[carllooper]

That wasn't how Bohr refuted Einstein:

It was one of Bohr's insights that emerged during his analysis of EPR. There is an agreement to be found between relativity and QM rather than a disagreement. The concept of information becomes clearer.

 

[bhobba]

bhobba's example of Euclidean geometry shows that a theory can make good predictions even if's only an approximate description of the world. Here's an example of a theory that makes good predictions without even approximately describing the world:

That's true, but the idea I was trying to get across is that QM is a mathematical model - specifically its a model about this primitive, loosely defined, thing called an observation. The archetype of all mathematical models is Euclidean geometry. Its primitives are these things called points and lines - again loosely defined - a point has position but no size, a line length but no width. The same with probability theory - its primitives are called events - and again loosely defined - in this case VERY loosely defined, so much so you get the gist by seeing examples.

I believe mathematical models describe the world in a certain domain of applicability. But that's just my view. My point though is whatever view you have of Euclidean geometry - its exactly the same for QM.

Thanks
Bill

 

[bhobba]

It was one of Bohr's insights that emerged during his analysis of EPR. There is an agreement to be found between relativity and QM rather than a disagreement. The concept of information becomes clearer.

I would agree with that.

Thanks
Bill

 

[carllooper]

You say "what matters is if a theory is 'physically right or wrong' not whether it is 'philosophically so'." By a theory being "physically right or wrong" here, you could mean two things: (1) you could mean "physically" in the sense of having to do with the real physical matter of the world, and thus be saying something like "a physically true theory correctly describes what is actually out there in the world," or (2) which is practically the opposite of that, you could mean "physically" in the sense of "as in physics"--which, these days, often explicitly repudiates the idea that theories are describing a "world out there," but rather insists on the above point about just being about accurate experimental predictions.

I don't why they would be opposed.

Physics (2) is, by definition, the study of the physical world (1).

I don't know what kind of physics would repudiate the concept of a "world out there". Physics certainly poses some challenges to classical philosophy, but it's not within physics scope to solve that for philosophy. Philosophy has to solve that.

Experiment plays an important role in physics. Without an experiment, it can be difficult to decide if something has some physical meaning or remains science fiction. Physics is very particular about that - far more than science fiction.

Prediction is only required if one needs to test out any potential conflicts or otherwise between a theory and it's experimental side. If the theory didn't include any predictions then there's no way to map the theory to the experiment. But many theorisations don't need to involve prediction. They can be working within a context in which the physical or experimental aspect has already been tested, and are elaborating the theoretical side, ie. in a mathematically consistent way that wouldn't violate the theory already tested.

C

 

[atyy]

But whereas scientific advancement has often occurred by trying so solve paradox, it seems to me that part of the philosophy behind QM is very much the opposite. Rather than trying to solve the paradox, there is a sense in which they are deliberately embracing it. Rather than saying, "there is something that is going on here that we don't understand and we need to study it further," they are saying, "we understand exactly what is going on here, and it is that the particles cease to exist"--at just the point where they reach the limit of their ability to understand them! Thus it is with the alleged "completeness" of quantum theory. It amounted to transforming their inability to understand into a "scientific conclusion" (that was really a philosophical error, in my view) that (a) is taught to all students of science nowadays, and (b) actually puts a stop to the process of scientific inquiry.

But here you are simply wrong. Quantum mechanics is widely acknowledged to have a problem called the "measurement problem". It is the most important problem in the foundations of quantum mechanics. Von Neumann's proof of the alleged "completeness" (to use your term) of quantum mechanics is widely known to be wrong, particularly after Bell's 1966 explanation of the error.

 

[jbmolineux]

I STRONLY disagree with that:
https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCAQFjAA&url=http://www.phys.washington.edu/users/vladi/phys216/Weinberg_Against_philosophy.doc&ei=fVJoVIXUNeL2mQWH_oCwCA&usg=AFQjCNHg_elaIirwh-1Q7Al_kVaI8Fz8YA&sig2=h2vnb14frw18jhYKClXrLw

But this forum is not the place to discuss it. The philosophy forums is the place.
It's not. Wienbergs view, which is the same as mine, is pretty common.

My post concerned the statement - 'One can imagine a lot of classically trained physicists walking around the conference in a huff saying things like "what is all this rubbish". And Bohr is trying to win them over to QM.'

I think by that time no one wasn't won over to QM. What Bohr was on about is his particular Copenhagen view - in this case complementarity. But shut up and calculate would soon predominate, if it hadn't already.
Does Euclidean geometry describe the actual world? And if it does precisely how does QM differ from it.

Thanks
Bill

Carl, I couldn't agree more with Weinberg's article. He literally demonstrates exactly what I am arguing--how bad philosophy has repeatedly led physics astray.

But the remedy for bad philosophy is not no philosophy. As Weinberg himself says, everyone has underlying philosophical assumptions. It's getting them right that matters, not simply not having them. The philosophy that says "physics should have no philosophy" is just one more bad philosophy. Thus "shut up and calculate" has allowed "the particle that cannot be measured does not exist" to dominate. And since you stop searching for what you don't believe exists, it has literally put a stop to scientific inquiry. Or so it seems to me.

The remedy for bad philosophy is good philosophy, and Weinberg seems to demonstrate a good deal of it in rejecting much of the bad philosophy that has come to dominate the academy. Since the bad philosophy has (according to Weinberg's article) done so much damage to physics--isn't that all the more reason for the importance of the conversation not being won by those who are in error? Doesn't that show precisely that good philosophy is important? As I think CS Lewis says, "good philosophy needs to exist, if for no other reason than that bad philosophy must be answered."

As to your question about Euclidean geometry, I'm not sure I would say that it "describes the world." I believe mathematical theories are different than scientific theories in that they aren't about the physical world. That's why you don't test them with measurements or experiments. But theories in physics ARE about the world, which is why you do test them with measurements and experiments.

Where I believe QM goes wrong (and apparently where Wienberg also believes it goes wrong) is in the claim that what cannot be measured cannot be the content of a theory.

 

[carllooper]

To me it appears that what has happened is that physicists started with choice #2 above (that physics does not describe a "world out there" but just makes predictions that either can, or can't be experimentally verified) and were led by that conclusion to disbelief in the out-there reality of the very particles they are supposed to be studying. You can clearly trace the lineage in the latter "scientific conclusion" to the former philosophical error. You can see how this would affect physical research. If you don't believe that the thing you are studying really exists (at least in any way you can make sense of) that is OBVIOUSLY going to affect the paths that you choose to study it!

No this is not the case at all. Physics does describe the world out there, but it's just not in a way that classical philosophy might describe it. For example Kant assumes a world in which time and space are separate. And this might be something one might like to use in physics. But if you are trying to use it with Relativity Theory it won't work, because Relativity employs the concept of time and space as not separate. It's not for any philosophical reason. And it's not due to the infiltration of any wrong philosophy. It's that an aspect of the world out there makes sense in terms of Relativity Theory. Or to put it another way: the world out there doesn't make nonsense of it.

Regarding particles.

The reality or otherwise of particles depends on what you mean by reality. A particle itself is defined by the mathematics. In many philosophies mathematics and reality are the same thing. So in those philosophies you could say the particle is real. In other philosophies its the particle detection which is real and the particle which isn't. But who's to say which philosophy is correct? And does it matter? As long as one understands what is being meant, in a particular context, by terms such as "particle", or "reality", or "non-existent" or "actual" etc. that's what matters.

C

 

[jbmolineux]

But here you are simply wrong. Quantum mechanics is widely acknowledged to have a problem called the "measurement problem". It is the most important problem in the foundations of quantum mechanics. Von Neumann's proof of the alleged "completeness" (to use your term) of quantum mechanics is widely known to be wrong, particularly after Bell's 1966 explanation of the error.

Yes, I am aware that QM has a measurement problem. But historically the measurement problem combined with the positivist idea that "only the measurable is meaningful" to the actual belief that the unmeasurable is nonexistent, which a stop to inquiry, as the Weinberg article above spells out. I have heard that later the repudiation of "completeness" by Bell came to be accepted, but obviously decades of research-that-could've-been were lost by what were ultimately philosophical mistakes.

Honestly, I am unable, even, to sort out where the conversation went from there. Perhaps you guys can help me to understand. Here is how I understand what happened (and please tell me where I am wrong here).

1. Logical positivists, in their zeal to "get metaphysics out of philosophy" put forth the empirical criterion of meaning, which says that the only meaningful propositions were those that could be empirically verified.

1. Once this was accepted, as soon as one considers some fact which "exists" (in the ordinary sense) but cannot be perceived for any reason--cannot be spoken about meaningfully at all. This was problematic and counter-intuitive, and led to many absurdities. But it was the clear implication of the empirical criterion of meaning.

1. Heisenberg brought to bear the empiricist criterion of meaning onto the position / velocity of particles at the quantum level, where the mere act of observing them by shinning light on them would affect them (the measurement problem). He pointed out that under the empirical criterion, such particles actually do not have a velocity or a position.
2. This was embraced by some (Copenhagen), and flatly rejected by others, including Einstein, who believed that this result contradicted the fundamental scientific intuition.
3. Eventually, the positivists won over most of the physics community. I believe Von Neuman's later-to-be-repudiated "completeness" proof was part of it. On this basis, the idea became orthodox in physics that particles do not "exist in the classical sense" but exist in a in a state of quantum uncertainty, and literally do not have both a position and velocity.
4. In the next few decades logical positivism would die out in philosophy (as it came to be accepted that is was directly self-defeating), and apparently Von Neuman's completeness was eventually repudiated by Bell.
5. As far as I understand it, Bell's theorem was tested experimentally a number of times in a way that is supposed to have vindicated the positivist QM interpretation.

This last step, unfortunately, is where I get lost. I haven't really been able to understand the Bell Test experiments, which is why I want to learn physics. But it seems to me that as soon as positivism is repudiated in philosophy and "completeness" in physics, that there would be a metaphysical revolution in physics to get things back on track. As the Weinberg article pointed out, bad philosophy had been leading physics astray for decades. If you could get it back on track, it seems that the astray-leading should stop. But it seems to me that physics is still so-heavily influenced by bad philosophy that it still hasn't really got back on track. (And, in my view, "shut up and calculate" amounts to "there is no objective physical world to be measured," or certainly allows it to flourish….) Or maybe I just don't understand the Bell Test experiments, and how they somehow vindicate some philosophical ideas that I have been considering errors.

Answers to a few questions might help me here:

• Why is it impossible to know both the position and the velocity of a particle by the rebound of a photon hitting it?

• On Entanglement experiments - why can't the cause be explained by the same properties in each at the source? (I know I am out of my league with this question, and I believe the answer might be Bell's theorem itself...is that true?)
• Why couldn't there be an entanglement-type experiment where both particles were sent out "in the same way" so that you learn the information from one to know about the other? In other words, can you do something at the beginning to ensure that the position and velocity are the same, and then measure the position of one and the velocity of the other?

 

[bhobba]

I believe mathematical theories are different than scientific theories in that they aren't about the physical world. That's why you don't test them with measurements or experiments. But theories in physics ARE about the world, which is why you do test them with measurements and experiments.

That's a misconception.

Both Euclidean Geometry and QM are mathematical models and make predictions that can be tested. Pure mathematics is something different again .

Where I believe QM goes wrong (and apparently where Wienberg also believes it goes wrong) is in the claim that what cannot be measured cannot be the content of a theory.

QM doesn't say that. Its simply a theory whose primitive is observations. It silent on the issue of what's going on aside from that - but we have conjectures (called interpretations) that have their own take.

Thanks
Bill

 

[Fredrik]

Quantum mechanics is widely acknowledged to have a problem called the "measurement problem".

The only reason for this is that what most people think of as QM is a great theory of physics plus the unnecessary and unscientific assumption that a pure state "provides a complete and exhaustive description of an individual system". Quantum mechanics as I would define it, doesn't have a measurement problem. There are things that this theory is unable to tell us, but that's not a problem. It's either something that only a better theory can answer, or something that's forever beyond the reach of science. If it's the latter, that's certainly a problem, but it's a problem with the real world, not a problem with the theory.

This is how Wikipedia describes the measurement problem:

If observers and their measuring apparatus are themselves described by a deterministic wave function, why can we not predict precise results for measurements, but only probabilities?​

This sounds like a very good question, until you realize that the claim that matter is described by pure states is either a tautology (if you define "describes" in a way that makes this idea true) or an unnecessary and unscientific assumption added on top of a perfectly fine theory (if you leave "describes" undefined). It's the same assumption that Ballentine worded "a pure state provides a complete and exhaustive description of an individual system".

To a person who hasn't made this assumption, the question above looks very naive. Consider my theory of a six-sided die defined in post #86. No reasonable reasonable person would ask "If dice are described by this theory, why can we not predict precise results for measurements, but only probabilities?" The reason why people ask such silly questions about QM is that the standard presentation of the theory makes it very tempting to literally identify pure states with systems. The temptation is so strong that a lot of people are simply unable to see that when they do, they have left science behind and made an unnecessary assumption

 

[bhobba]

Physics does describe the world out there, but it's just not in a way that classical philosophy might describe it.

Modern physical theories are mathematical models, exactly the same as the archetypical mathematical model - Euclidean Geometry.

Since antiquity it has been recognised as THE pristine intellectual achievement:
http://poetry.about.com/od/poems/l/blmillayeuclid.htm
Euclid alone has looked on Beauty bare.
Let all who prate of Beauty hold their peace,
And lay them prone upon the Earth and cease
To ponder on themselves, the while they stare
At nothing, intricately drawn nowhere
In shapes of shifting lineage; let geese
Gabble and hiss, but heroes seek release
From dusty bondage into luminous air.
O blinding hour, O holy, terrible day,
When first the shaft into his vision shone
Of light anatomized! Euclid alone
Has looked on Beauty bare. Fortunate they
Who, though once only and then but far away,
Have heard her massive sandal set on stone.

All modern physics does is carry on that exemplary tradition.

Thanks
Bill

 

[jbmolineux]

No this is not the case at all. Physics does describe the world out there, but it's just not in a way that classical philosophy might describe it. For example Kant assumes a world in which time and space are separate. And this might be something one might like to use in physics. But if you are trying to use it with Relativity Theory it won't work, because Relativity employs the concept of time and space as not separate. It's not for any philosophical reason. And it's not due to the infiltration of any wrong philosophy. It's that an aspect of the world out there makes sense in terms of Relativity Theory. Or to put it another way: the world out there doesn't make nonsense of it.

Regarding particles.

The reality or otherwise of particles depends on what you mean by reality. A particle itself is defined by the mathematics. In many philosophies mathematics and reality are the same thing. So in those philosophies you could say the particle is real. In other philosophies its the particle detection which is real and the particle which isn't. But who's to say which philosophy is correct? And does it matter? As long as one understands what is being meant, in a particular context, by terms such as "particle", or "reality", or "non-existent" or "actual" etc. that's what matters.

C

Carl, I certainly don't believe that any philosophical error on Einstein's part caused any problems in physics! Kant's idea that time and space are mind-dependent seems to me to be a philosophical precursor to relativity.

Yes, different people have different ideas of what constitutes "reality," "non-existent," "particle," etc.--as you point out. But then you go on to say that "as long as one understands what is being meant"--that’s what matters. But since there is no agreement about what is meant by those terms, how can it be understood what is being meant?

Further, the question of whether something even exists if it can't be measured by the technology of a certain time is certainly relevant! If scientists are taught something doesn't exist, it puts a stop to the inquiry that drives scientific progress!