Why the Quantum | A Response to Wheeler's 1986 Paper - Comments

[RUTA]

Weinberg seems to favor the "ant's-eye view" per Wilczek. On p 147 in The Geometric Analogy of Gravitation and Cosmology he writes

At one time it was even hoped that the rest of physics could be brought into a geometry formulation, but this hope has met disappointment, and the geometric interpretation of the theory of gravitation has dwindled to a mere analogy ... it simply doesn't matter whether we ascribe these predictions to the physical effect of gravitational fields on the motion of planets and photons or to a curvature of space and time. (The reader should be warned that these views are heterodox and would meet with objections from many general relativists.)

His view, as he makes clear elsewhere, is the action of gravitational fields on matter not the 4D view of spacetime curvature. That dynamical view of physical reality then leads him to believe QM is not complete. Here is a Weinberg quote in Sabine's book (p. 126-7)

You can very well understand quantum mechanics in terms of an interaction of the system you're studying with an external environment which includes an observer, but this involves a quantum mechanical system interacting with a macroscopic system that produces the decoherence between different branches of the initial wave function. And where does that come from? That should be described also quantum mechanically. And, strictly speaking, within quantum mechanics itself there is no decoherence.

This is a nonstarter if you accept the 4D view (Wilczek's "God's-eye view") of QM as I explain in the Insight. Your model of physical reality will greatly influence how you do physics. That's why, as Becker argues, it's important for physicists to reflect seriously on their models. They don't need to make a career of studying different models, as in foundations, but they should all be aware of existing or possible alternative models within their own fields.

 

[vanhees71]

Well, this I can agree with. One should always be open-minded. What I disagree with is the claim that philsophy is of any help to solve physics problems. As you can well see from the quote of Sabine's book, Weinberg doesn't argue philosophically but physically. On the other hand, is this really a problem? There are effective descriptions of decoherence as approximations of QT. Usually this goes via influence-functional methods of quantum-kinetic theory, resulting in master equations for open quantum systems. I find this already a pretty satisfactory explanation for the "classicality" of behavior of macroscopic objects, including measurement devices in the sense of the decoherence program.

There's of course one point, which however is again pretty metaphysical: What's the meaning of the quantum state of the entire universe. Is the entire universe an open system as well? This seems to be a pretty disturbing idea since the universe is, by definition, just everything. So what makes the universe open, if it includes everything? On the other hand, according to standard cosmology (particularly with inflation) we can only observe a tiny bit of the entire universe. So can we interpret the observable part of the universe, which is the only thing that can be described by physics as we usually define it since unobservables are not subject of any serious physics, as an open system? But then there should be interactions of the observable part with the "rest", but that's impossible because by definition the rest is beyond some horizon, i.e., the parts of the observable universe cannot interact with the "rest". If you have such a comprehensive view, I can admit that there is a fundamental problem with the interpretation of quantum theory, but as my just given examples show, it seems as well not so easy to be solved within the scientific method, because it may concern principally unobservable entities, and thus are no longer subject to the scientific realm of human knowledge. Maybe this teaches us that our objective knowledge is in principle always incomplete. Well, the natural sciences teaches us humility. Starting from being the center of the universe (an idea of philosophers by the way ;-)), we've become a humble little accident in a totally unimportant little galaxy at a place that is in no way distinguished from any other place in the universe :-)).

 

[RUTA]

Decoherence requires a classical environment, so it cannot explain classicality as arising from quantum systems, it can only explain how the quantum and classical relate to each other. In the IJQF version of the paper we made an argument for quantum-classical contextuality along those lines, but that's the part of the paper we took out for submission to a physics journal :-)

Again, this is where your model of physical reality bears significantly on how you do physics. If your model of physical reality is quantum-classical, i.e., both are equally fundamental, then you don't spend any of your research time trying to better quantum mechanics. Both Hardy and Weinberg have spent years trying to do that without success. As Weinberg discovered, "It is very hard to do better than quantum mechanics'' (p 124 of Sabine's book). Hardy tried to find information-theoretic postulates that would uniquely specify QM over classical probability theory and superquantum correlations. He writes, "Either there do exist higher theories in this hierarchy or there do not. For many years I tried to find such theories, and I tried to prove that such theories do not exist. I also tried to find other reasonable axioms that rule out higher theories in this hierarchy" (p 3 of https://arxiv.org/pdf/1303.1538.pdf). What he ended up with are five postulates satisfied by both classical and quantum probability theories. If Hardy's model of physical reality were quantum-classical instead of "quantum rather than classical," he might stop with this last result.

 

[Boing3000]

I really liked this insight, but I am confused about dynamical vs adynamical.

Whether or not you consider this apparently simple 4-dimensional (4D) constraint (conservation of angular momentum on average)

The concept of average (and ensemble) it what is not "real". As far as I understand QM (or even classic statistical mechanics) it is the observer effect.
Is there a formal way to slice a 4D volume (non arbitrary, for all FoR) where this quantity is conserved ?
(keep in mind a am a layman in your response)

 

[RUTA]

I really liked this insight, but I am confused about dynamical vs adynamical.

The concept of average (and ensemble) it what is not "real". As far as I understand QM (or even classic statistical mechanics) it is the observer effect.
Is there a formal way to slice a 4D volume (non arbitrary, for all FoR) where this quantity is conserved ?
(keep in mind a am a layman in your response)

If you’re a layman and you understood my Insight, give yourself a pat on the back. I wrote that for my undergrad QM students and colleagues on PF.

Apparently, the concept of average is “more real” in QM than the facts for any given trial. That’s the point of the argument. Doing a Lorentz boost to some other FoR (Charlie’s) in motion wrt Alice and Bob would not obscure this result because Charlie would still see the spacelike correlations.

 

[vanhees71]

Decoherence requires a classical environment, so it cannot explain classicality as arising from quantum systems, it can only explain how the quantum and classical relate to each other. In the IJQF version of the paper we made an argument for quantum-classical contextuality along those lines, but that's the part of the paper we took out for submission to a physics journal :-)

But the "classical environment" can be described as a coarse-grained quantum-many-body system in the sense that you can derive the classical Boltzman transport equation by a gradient expansion or an $\hbar$ expansion of the full Kadanoff-Baym equations. There is no necessity for a quantum-classical cut, because the classical behavior of macroscopic systems (in usual everyday states) can be understood from QT via suitable approximations.

In the same sense Newtonian mechanics is valid as an approximation of relativistic mechanics in its range of applicability (slow motions and not too strong gravitational and em. fields).

 

[RUTA]

But the "classical environment" can be described as a coarse-grained quantum-many-body system in the sense that you can derive the classical Boltzman transport equation by a gradient expansion or an $\hbar$ expansion of the full Kadanoff-Baym equations. There is no necessity for a quantum-classical cut, because the classical behavior of macroscopic systems (in usual everyday states) can be understood from QT via suitable approximations.

In the same sense Newtonian mechanics is valid as an approximation of relativistic mechanics in its range of applicability (slow motions and not too strong gravitational and em. fields).

Yes but to get from many quantum systems to a classical system via ED requires a classical environment. ED is an add-on to QM and QM requires CM. We had a nice quote from Landau & Lifshitz saying QM is unique among theories of physics in that it requires its limiting theory (CM). What we have now is a quantum-classical self-consistency with ED and QM and CM. So if you can accept a quantum-classical model of physical reality as we proposed, you’re not going to look for some purely quantum theory underwriting QM. Both Weinberg and Hardy seem to disagree and consequently they spend much time looking for that more fundamental theory. Again your model of physical reality largely determines your approach to physics.

 

[vanhees71]

That measurement apparati are macroscopic seems to be evident since we need macroscopic bodies to be able to read off the measurement result. I cannot follow your other statements clearly since I've no clue what the acronym ED might mean. Anyway, QM does not need CM to be formulated.The fundamental postulates are independent of CM.

 

[RUTA]

ED = environmental decoherence. Here is the explicit quote (p 3 Landau & Lifshitz, 1977)

Thus quantum mechanics occupies a very unusual place among physical theories: it contains classical mechanics as a limiting case, yet at the same time it requires this limiting case for its own formulation.

In order to construct the QM propagator you use the classical action. QM is built around CM. That's why Weinberg is not happy with it (p 124 Sabine, 2018)

You would like to understand macroscopic things like experimental apparatuses and human beings in terms of the underlying theory. You don't want to see them brought in on the level of axioms of the theory. ... In my view we ought to take seriously the possibility of finding some more satisfactory other theory to which QM is only a good approximation. ... I have tried very hard to develop that more satisfactory other theory without success ... It is very hard to do better than QM.

So, why not simply work with a quantum-classical model of physical reality? There's nothing in Nature demanding "quantum rather than classical." And we still have a beautiful quantum decomposition of classical systems (as you point out), even if such decompositions require a classical context. We just can't apply that decomposition in toto (as you point out).

We'll put these points back into foundations of physics versions of the paper :-)

 

[vanhees71]

Of course, in fact everything known for the past 120 or so years demancs a quantum rather than classical description. Classicality is an approximate description valid for macroscopic observables for many-body systems, derivable from QT. This does, however, not imply that macroscopic systems always necessarily behave "classically".

In fact, there is no physical decomposition into a classical and a quantum world; at least there's not the slightest empirical evidence for something like this (known also as the "quantum-classical cut" in the infamous Copenhagen class of interpretations; it's the 2nd-most ugly and unnecessary assumption of the adepts of the Copenhagen quantum gibberish; only the idea of a collapse is uglier and more misleading!). It is just a matter of preparation techniques to reveal quantum behavior of larger and larger objects. Already buckyballs are pretty large objects consisting of 60 carbon atoms, and they can, appropriately cooled be prepared in a way to get quantum-interference effects in the double-slit experiment. It's also clear that it is very easy already for such "mesoscopic" systems to make them behave "classically" by just not cooling them enough. The thermal e.m. radiation of a few photons is already enough "coupling to the environment" to get enough decoherence to justify a classical description.

Other really macroscopic systems are known to show quantum behavior even before modern QT has been discovered. One historically important example is the specific heat of solids at low temperatures. Famously Einstein and in a refined way Debye early on explained (at least qualitatively) the observed behavior of the specific heat at low temperatures applying the "old quantum theory" to the collective modes of lattice vibrations.

 

[RUTA]

Our quantum-classical model invokes a quantum-classical cut as necessary to use QM (which must be done). Depending on the context, that cut can include screened-off elephants, there is no definitive "size" defining this cut, we don't deviate at all from the practice of QM. It's just a matter of whether or not one is happy with this form of "contextuality" rather than demanding "reductionism" as Weinberg seeks. Again, Nature doesn't demand reductionism and all indications are otherwise, as Weinberg notes. So, do you continue to spend your most precious commodity (your time) seeking "some more satisfactory other theory to which QM is only a good approximation"? Or, do you spend your time looking for new theories of physics, e.g., quantum gravity, via quantum-classical contextuality? Your research direction is determined by your choice for a model of physical reality, precisely as Becker points out.

 

[atyy]

Of course, in fact everything known for the past 120 or so years demancs a quantum rather than classical description. Classicality is an approximate description valid for macroscopic observables for many-body systems, derivable from QT. This does, however, not imply that macroscopic systems always necessarily behave "classically".

In fact, there is no physical decomposition into a classical and a quantum world; at least there's not the slightest empirical evidence for something like this (known also as the "quantum-classical cut" in the infamous Copenhagen class of interpretations; it's the 2nd-most ugly and unnecessary assumption of the adepts of the Copenhagen quantum gibberish; only the idea of a collapse is uglier and more misleading!). It is just a matter of preparation techniques to reveal quantum behavior of larger and larger objects. Already buckyballs are pretty large objects consisting of 60 carbon atoms, and they can, appropriately cooled be prepared in a way to get quantum-interference effects in the double-slit experiment. It's also clear that it is very easy already for such "mesoscopic" systems to make them behave "classically" by just not cooling them enough. The thermal e.m. radiation of a few photons is already enough "coupling to the environment" to get enough decoherence to justify a classical description.

Other really macroscopic systems are known to show quantum behavior even before modern QT has been discovered. One historically important example is the specific heat of solids at low temperatures. Famously Einstein and in a refined way Debye early on explained (at least qualitatively) the observed behavior of the specific heat at low temperatures applying the "old quantum theory" to the collective modes of lattice vibrations.

This is completely wrong. There is no quantum reality in Copenhagen.

 

[Lord Jestocost]

In fact, there is no physical decomposition into a classical and a quantum world; at least there's not the slightest empirical evidence for something like this (known also as the "quantum-classical cut" in the infamous Copenhagen class of interpretations; it's the 2nd-most ugly and unnecessary assumption of the adepts of the Copenhagen quantum gibberish; only the idea of a collapse is uglier and more misleading!).

There seems to be some misunderstanding! N. P. Landsman writes in "Between classical and quantum" (https://arxiv.org/abs/quant-ph/0506082):

"Describing quantum physics in terms of classical concepts sounds like an impossible and even selfcontradictory task (cf. Heisenberg, 1958). For one, it precludes a completely quantum-mechanical description of the world: ‘However far the phenomena transcend the scope of classical physical explanation, the account of all evidence must be expressed in classical terms.’ But at the same time it precludes a purely classical description of the world, for underneath classical physics one has quantum theory.⁶⁶ The fascination of Bohr’s philosophy of quantum mechanics lies precisely in his brilliant resolution of this apparently paradoxical situation.

The first step of this resolution that he and Heisenberg proposed is to divide the system whose description is sought into two parts: one, the object, is to be described quantum-mechanically, whereas the other, the apparatus, is treated as if it were classical. Despite innumerable claims to the contrary in the literature (i.e. to the effect that Bohr held that a separate realm of Nature was intrinsically classical), there is no doubt that both Bohr and Heisenberg believed in the fundamental and universal nature of quantum mechanics, and saw the classical description of the apparatus as a purely epistemological move without any counterpart in ontology, expressing the fact that a given quantum system is being used as a measuring device.⁶⁷ For example: ‘The construction and the functioning of all apparatus like diaphragms and shutters, serving to define geometry and timing of the experimental arrangements, or photographic plates used for recording the localization of atomic objects, will depend on properties of materials which are themselves essentially determined by the quantum of action’ (Bohr, 1948), as well as: ‘We are free to make the cut only within a region where the quantum mechanical description of the process concerned is effectively equivalent with the classical description’ (Bohr, 1935).⁶⁸"

 

[vanhees71]

This is completely wrong. There is no quantum reality in Copenhagen.

There's no quantum reality in Copenhagen, but in the minimal statistical interpretation there is. It's just excepting the fundamental result of quantum theory that Nature is intrinsically probabilistic and cannot be described with local deterministic models.

 

[vanhees71]

There seems to be some misunderstanding! N. P. Landsman writes in "Between classical and quantum" (https://arxiv.org/abs/quant-ph/0506082):

"Describing quantum physics in terms of classical concepts sounds like an impossible and even selfcontradictory task (cf. Heisenberg, 1958). For one, it precludes a completely quantum-mechanical description of the world: ‘However far the phenomena transcend the scope of classical physical explanation, the account of all evidence must be expressed in classical terms.’ But at the same time it precludes a purely classical description of the world, for underneath classical physics one has quantum theory.⁶⁶ The fascination of Bohr’s philosophy of quantum mechanics lies precisely in his brilliant resolution of this apparently paradoxical situation.

The first step of this resolution that he and Heisenberg proposed is to divide the system whose description is sought into two parts: one, the object, is to be described quantum-mechanically, whereas the other, the apparatus, is treated as if it were classical. Despite innumerable claims to the contrary in the literature (i.e. to the effect that Bohr held that a separate realm of Nature was intrinsically classical), there is no doubt that both Bohr and Heisenberg believed in the fundamental and universal nature of quantum mechanics, and saw the classical description of the apparatus as a purely epistemological move without any counterpart in ontology, expressing the fact that a given quantum system is being used as a measuring device.⁶⁷ For example: ‘The construction and the functioning of all apparatus like diaphragms and shutters, serving to define geometry and timing of the experimental arrangements, or photographic plates used for recording the localization of atomic objects, will depend on properties of materials which are themselves essentially determined by the quantum of action’ (Bohr, 1948), as well as: ‘We are free to make the cut only within a region where the quantum mechanical description of the process concerned is effectively equivalent with the classical description’ (Bohr, 1935).⁶⁸"

Between Bohr's (mis)understanding of quantum theory and today are 83 years with tremendous progress not only in the possibility to test quantum theory experimentally but also in the understanding of how the classical behavior of classical systems, including measurement devices, can be understood in terms of many-body quantum theory. The possibility of a local deterministic description of Nature is ruled out with the amazingly accurate measurements of all kinds of Bell tests. The emergence of a "classical world" is of course statistical as is all of many-body physics.

 

[vanhees71]

Our quantum-classical model invokes a quantum-classical cut as necessary to use QM (which must be done). Depending on the context, that cut can include screened-off elephants, there is no definitive "size" defining this cut, we don't deviate at all from the practice of QM. It's just a matter of whether or not one is happy with this form of "contextuality" rather than demanding "reductionism" as Weinberg seeks. Again, Nature doesn't demand reductionism and all indications are otherwise, as Weinberg notes. So, do you continue to spend your most precious commodity (your time) seeking "some more satisfactory other theory to which QM is only a good approximation"? Or, do you spend your time looking for new theories of physics, e.g., quantum gravity, via quantum-classical contextuality? Your research direction is determined by your choice for a model of physical reality, precisely as Becker points out.

If it were my expertise and if I had some good idea somehow I'd rather try to find a way to formulate a consistent quantum theory of gravitation than tackle some vague philosophical problems with no clear scientific content. I don't believe in the scholastic idea of finding any useful science without a firm confirmation on empirical grounds. That seems to be the reason why we still have no real breakthrough in understanding the most pressing issue in the foundation of physics, i.e., to find a consistent unification of QT (so far relativistic local and microcausal QFTs) and gravity (so far GR, which is a classical relativistic field theory). I think the trouble is that we have not the slightest clue about what effects a quantization of gravity we have to expect since there are no observations hinting at such effects.

 

[RUTA]

If it were my expertise and if I had some good idea somehow I'd rather try to find a way to formulate a consistent quantum theory of gravitation than tackle some vague philosophical problems with no clear scientific content. I don't believe in the scholastic idea of finding any useful science without a firm confirmation on empirical grounds. That seems to be the reason why we still have no real breakthrough in understanding the most pressing issue in the foundation of physics, i.e., to find a consistent unification of QT (so far relativistic local and microcausal QFTs) and gravity (so far GR, which is a classical relativistic field theory). I think the trouble is that we have not the slightest clue about what effects a quantization of gravity we have to expect since there are no observations hinting at such effects.

And if you tried to tackle QG, you’d need a starting point (“some good idea somehow”), which depends on some tacit or explicit model of physical reality you’re trying to map using empiricism and mathematics (= physics). You can’t escape the need for this model, as Becker so nicely showed in his book. Given that many brilliant physicists have worked decades without finding QG suggests to me that we should consider new models. That’s what Hardy and others in QIT argue is the value of their reconstruction project. The manner by which our model bears on QG is explained in chap 6 of our book, so I do have “some good idea” on how to proceed (and I am doing so!). This is physics, not “some vague philosophical problems with no clear scientific content.”

 

[stevendaryl]

I do not believe that the minimal interpretation is really any different from the Copenhagen interpretation when it comes to requiring a classical/quantum split. In the minimal interpretation, the meaning of quantum amplitudes is that they give statistics for measurement results. That seems to me to require a distinction between "measurements" and other interactions. That's basically the same as the classical/quantum split.

 

[RUTA]

And if you tried to tackle QG, you’d need a starting point (“some good idea somehow”), which depends on some tacit or explicit model of physical reality you’re trying to map using empiricism and mathematics (= physics). You can’t escape the need for this model, as Becker so nicely showed in his book. Given that many brilliant physicists have worked decades without finding QG suggests to me that we should consider new models. That’s what Hardy and others in QIT argue is the value of their reconstruction project. The manner by which our model bears on QG is explained in chap 6 of our book, so I do have “some good idea” on how to proceed (and I am doing so!). This is physics, not “some vague philosophical problems with no clear scientific content.”

For example, here are some papers inspired by our model:

Modified Regge Calculus as an Explanation of Dark Energy,” W.M. Stuckey, Timothy McDevitt and Michael Silberstein, Classical & Quantum Gravity 29 055015 (2012). http://arxiv.org/abs/1110.3973.

“Explaining the Supernova Data without Accelerating Expansion,” W.M. Stuckey, Timothy McDevitt and Michael Silberstein. Honorable Mention in the Gravity Research Foundation 2012 Awards for Essays on Gravitation, May 2012. International Journal of Modern Physics D 21, No. 11, 1242021 (2012) DOI: 10.1142/S0218271812420217 http://users.etown.edu/s/STUCKEYM/GRFessay2012.pdf.

“End of a Dark Age?” W.M. Stuckey, Timothy McDevitt, A.K. Sten, and Michael Silberstein. Honorable Mention in the Gravity Research Foundation 2016 Awards for Essays on Gravitation, May 2016. International Journal of Modern Physics D 25, No. 12, 1644004 (2016) DOI: 10.1142/S0218271816440041 http://arxiv.org/abs/1605.09229

This first is specifically the result of our approach to QG. The resolution of DM is via the contextuality already inherent in GR (multiple values of mass for same matter). Different models of physical reality will produce different physics.

 

[atyy]

If it were my expertise and if I had some good idea somehow I'd rather try to find a way to formulate a consistent quantum theory of gravitation than tackle some vague philosophical problems with no clear scientific content. I don't believe in the scholastic idea of finding any useful science without a firm confirmation on empirical grounds. That seems to be the reason why we still have no real breakthrough in understanding the most pressing issue in the foundation of physics, i.e., to find a consistent unification of QT (so far relativistic local and microcausal QFTs) and gravity (so far GR, which is a classical relativistic field theory). I think the trouble is that we have not the slightest clue about what effects a quantization of gravity we have to expect since there are no observations hinting at such effects.

We have had a real breakthrough in quantizing gravity - string theory and gauge/gravity duality.

 

[RUTA]

We have had a real breakthrough in quantizing gravity - string theory and gauge/gravity duality.

There's definitely no consensus for that approach and it's been around for decades. If that's your belief, keep at it though!

 

[PeterDonis]

the contextuality already inherent in GR (multiple values of mass for same matter)

Can you explain in more detail what this means?

 

[vanhees71]

We have had a real breakthrough in quantizing gravity - string theory and gauge/gravity duality.

Well, there's not yet a single observable predictio from string theory. AdS/CFT has some applications even in my field of relativistic heavy-ion collisions, but to call it a breakthrough is a bit too enthusiastic ;-)).

 

[RUTA]

Can you explain in more detail what this means?

See this paper (attached):
“Could GR Contextuality Resolve the Missing Mass Problem?” W.M. Stuckey, Timothy McDevitt, A.K. Sten, and Michael Silberstein. Honorable Mention in the Gravity Research Foundation 2018 Awards for Essays on Gravitation, May 2018.

and this one referenced therein (also attached with errata):
“The Observable Universe Inside a Black Hole,” W.M. Stuckey, American Journal of Physics 62, No. 9, 788 – 795 (1994).

The idea is simple, as I've written many times on PF. When you combine two different GR solutions (two spacetime regions with different geometries) into one new solution, the mass of the matter responsible for the combined solution can be different for observers in each of the two different spacetime regions. In the AJP paper, we have a sphere of FLRW dust surrounded by Schwarzschild vacuum. The mass of the dust as measured by co-moving observers in the FLRW dust sphere equals the mass M of the Schwarzschild metric for the flat-space FLRW model and is less/greater than that mass in the open/closed models. So per GR, mass is a geometric property of spacetime, not an intrinsic property of matter.

 

[romsofia]

This is completely wrong. There is no quantum reality in Copenhagen.

I'm not sure I can agree with this statement. In the Copenhagen interpretation, as I understand it, is we take a state vector, and from this state vector, we can decompose it into a bunch of elements. We then assign a probability distribution to this set, and give weights to each element. However, until the wavefunction "collapses", nothing is "real" for the classical world. The classical world is ignorant of the underlying probability distribution. We only "see" the outcome!

So can we not consider that a quantum reality? It could be that I'm too invested in the math of the interpretation, and not the interpretation itself.

EDIT: Feel free to PM me as well, I don't want to divert the discussion from the main thread as I haven't read every post. Hopefully this isn't off-topic!

 

[Fra]

Between Bohr's (mis)understanding of quantum theory and today are 83 years with tremendous progress

As I see it the probabilisitic foundation required for QM is anchored in the classical "certainty".

The fact that one can in principle describe classical systems as emergent from a complex many-body QM picture, does not mean we do not need the classical measurement device.

Such a fallacious conclusions sits in the same category as those that suggest solving the observer problem by removing the observer, and instead attaching things in a metaphysical or mathematical realm and claim its objective.

This is a deep necessary insight that Bohr appears to have had. You can not make certain statistical predictions, without a certain distributions, and certain symmetries. These are manifested only on the classical side of things in the infinite ensembles etc; or in the "observer" part of this, if we are to generalize beyond classical observers.

This is easy to see if you analyse this from the point of view of inference. It should also be intuitive for any experimental work as the accuracy and confidence in the statistical predictions, requires a solid control and knowhow of the classical measurement devices. But from the perspective of mathematical physics, the statistical predictions of QM is anchored in axioms, that sit in the mathematical realm and its very easy to be seduced and confused by this.
And that essense is what i read out of Bohrs original view as well is that he understood this, this is why a proper formulation of quantum theory itself REQUIRES the classical reference. I think this is a fundamental insight.

We certainly need to improve this to understand QG and unification, but can't see anyone so far has done better than Bohr. We obviously grossly improved and developed the SM for particle physics and QFT, but the foundations remain at Bohr level.

/Fredrik

 

[vanhees71]

To be more precise, we need something that behaves with good enough accuracy classically, and quantum many-body theory shows that many-body quantum systems are behaving to good accuracy classically. That's all you need to explain why quantum theory is successful in providing its probabilistic description of the outcome of measurements on quantum systems with macroscopic measurement apparati. There's nothing, however, hinting at a "quantum classical cut", i.e., there's nothing contradicting QT in favor of a classical description, but for many-body systems very often the classical description is a very accurate description for macroscopic "coarse-grained quantities", which are sufficiently accurate to describe the relevant behavior of many-body systems, including measurement apparati. Particularly there's no difference between measurement devices and any other kind of matter since indeed measurement devices are composed of the same elementary particles as anything around us.

 

[stevendaryl]

There's nothing, however, hinting at a "quantum classical cut", i.e., there's nothing contradicting QT in favor of a classical description

But the formalism doesn't actually make any predictions without such a cut. Without a distinction between measurements and other interactions, or between macroscopic and microscopic, there are no probabilities in QM, and the theories only predictions are probabilistic.

 

[vanhees71]

In QM of course everything is probabilistic from the very beginning, but there is no cut anywhere in the formalism. Where do you need that cut?

 

[stevendaryl]

In QM of course everything is probabilistic from the very beginning

The evolution of the wave function is deterministic. Probabilities come in when you make a division between a macroscopic system (the measuring device) and the system being measured. That division is necessary for there to be any probabilities at all.

 

[vanhees71]

The "wave function" is a probability amplitude by definition (within the standard minimal interpretation). Thus it's probabilistic from the very beginning, without any necessity to introduce classical concepts.

 

[stevendaryl]

The "wave function" is a probability amplitude by definition (within the standard minimal interpretation). Thus it's probabilistic from the very beginning, without any necessity to introduce classical concepts.

That's not true. The wave function gives probabilities for measurement results. Without distinguishing measurement results from other properties, there are no probabilities in QM.

To have probabilities you have to have events---the things that have associated probabilities. The events for QM are measurement results.

 

[lavinia]

That's not true. The wave function gives probabilities for measurement results. Without distinguishing measurement results from other properties, there are no probabilities in QM.

To have probabilities you have to have events---the things that have associated probabilities. The events for QM are measurement results.

Why aren't there situations where quantum states are naturally falling into eigenstates of some operator - without measurement - for instance on a star?
This could happen trillions of times and thus a probability distribution. Or is any time a quantum states projects onto an eigen state of an operator a measurement by definition?

 

[stevendaryl]

Why aren't there situations where quantum states are naturally falling into eigenstates of some operator - without measurement - for instance on a star?
This could happen trillions of times and thus a probability distribution. Or is any time a quantum states projects onto an eigen state of an operator a measurement by definition?

I wasn't giving my opinion about it---I was describing the orthodox interpretation of quantum mechanics, which is that the probabilities in quantum mechanics are probabilities of measurement results.

An alternative interpretation which I think is empirically equivalent is to forget about measurements, and instead think of QM as a stochastic theory for macroscopic configurations. What I think is nice about this approach is that it doesn't single out measurements, and it doesn't require the assumption that a measurement always gives an eigenvalue of the operator corresponding to the observable being measured. It doesn't require observers, so you can apply QM to situations like distant stars where there are no observations. On the other hand, it's got the same flaw as the orthodox interpretation, in that it requires a macroscopic/microscopic distinction.

Getting back to your specific comment, I'm not sure what you mean by "naturally falling into eigenstates". Could you elaborate?

 

[vanhees71]

That's not true. The wave function gives probabilities for measurement results. Without distinguishing measurement results from other properties, there are no probabilities in QM.

To have probabilities you have to have events---the things that have associated probabilities. The events for QM are measurement results.

What else than measurement results should any physical theory describe? Physics is about objectively observables facts of nature. It's not an empty mathematical game of thought, where you solve Schrödinger's equation just for fun without needing any "meaning" of the wave function, i.e., just because for some reason you like the puzzle to solve the equation.