A Is quantum theory a microscopic theory?

[Lord Jestocost]

Perhaps, but the minimal instrumental view of QM says nothing about that.

I don’t see any way out of the instrumentalist minimal interpretation. Our perceptions of events occurring on a macroscopic space-time scene (the “empirical reality”) cannot be traced back to the “behavior” of fundamental microscopic space-time realities (the mysterious “things” behind the space-time scene which we denote as a matter of convenience “electrons”, “atoms” etc. allow no space-time description).

J. Robert Oppenheimer in “Atom and Void: Essays on Science and Community”

“If we ask, for instance, whether the position of the electron remains the same, we must say "no"; if we ask whether the electron's position changes with time, we must say "no"; if we ask whether the electron is at rest, we must say "no"; if we ask whether it is in motion, we must say "no." The Buddha has given such answers when interrogated as to the conditions of a man's self after his death; but they are not familiar answers for the tradition of seventeenth- and eighteenth-century science.”

 

[Demystifier]

I don’t see any way out of the instrumentalist minimal interpretation. Our perceptions ...

A way out is to reject that science must only be based in perceptions.
https://www.physicsforums.com/threads/is-quantum-theory-a-microscopic-theory.974961/#post-6208601

 

[vanhees71]

What's a conceptual difficulty with complementarity? Quantum theory resolves all these difficulties, as far as I know, very well. Where are, in your opinion, issues with the position and momentum observables left?

 

[Lord Jestocost]

A way out is to reject that science must only be based in perceptions.

A way out might be that we at first accept that this what we call 'reality' is just a state of mind.
https://www.theguardian.com/science/blog/2009/mar/17/templeton-quantum-entanglement

 

[DarMM]

What's a conceptual difficulty with complementarity? Quantum theory resolves all these difficulties, as far as I know, very well. Where are, in your opinion, issues with the position and momentum observables left?

There's no issues with these observables and of course QM deals with how these things work very well. It's just odd, I don't think complementarity makes more intuitive sense once one considers the realistic details of the device. Let me try again.

Say we observe two quantities $N$ and $M$ with two separate devices and they can have outcomes $n$ and $m$. In classical mechanics their statistics are basically always modeled as some distribution over the space of pairs $(n,m)$. So even if you don't measure $M$ it can still be reasoned about.
Even if the world was fundamentally random but random in the sense of a classical stochastic theory this would be the case.

In QM however if $N$ and $M$ don't commute then this isn't true. The statistics of $N$ observations are not marginals for statistics of $(n,m)$ pairs, but simply a distribution over $n$ outcomes. This means when you measure $N$ you can't reason about some outcome $M$ had that you don't know. So basically $M$ events can't even be discussed. Only the quantity you measured has an outcome.

Now it seems to be what is true and QM models it perfectly but it's pretty weird and I don't think that weirdness goes away because of an accurate comprehension of devices. It seems that the device is embedded in one's description of the system in a way that isn't true in classical mechanics. Even in a fundamentally random classical theory.

 

[Mentz114]

Conservation laws are not enough. For example, from conservation laws alone you cannot deduce that the Moon is there when nobody observes it. As far as conservation laws are concerned, the Moon could spontaneously turn into a gigantic pink elephant of the same energy-momentum as that of the Moon, whenever it is no longer observed.

We only have conservation laws to constrain physical models. There is no need to 'deduce' the persistent existence of the moon unless you believe it is not there when your eyes are closed.

Earlier you deny that perceptions are relevant to physics and here you contradict this.

 

[vanhees71]

There's no issues with these observables and of course QM deals with how these things work very well. It's just odd, I don't think complementarity makes more intuitive sense once one considers the realistic details of the device. Let me try again.

Say we observe two quantities $N$ and $M$ with two separate devices and they can have outcomes $n$ and $m$. In classical mechanics their statistics are basically always modeled as some distribution over the space of pairs $(n,m)$. So even if you don't measure $M$ it can still be reasoned about.
Even if the world was fundamentally random but random in the sense of a classical stochastic theory this would be the case.

In QM however if $N$ and $M$ don't commute then this isn't true. The statistics of $N$ observations are not marginals for statistics of $(n,m)$ pairs, but simply a distribution over $n$ outcomes. This means when you measure $N$ you can't reason about some outcome $M$ had that you don't know. So basically $M$ events can't even be discussed. Only the quantity you measured has an outcome.

Now it seems to be what is true and QM models it perfectly but it's pretty weird and I don't think that weirdness goes away because of an accurate comprehension of devices. It seems that the device is embedded in one's description of the system in a way that isn't true in classical mechanics. Even in a fundamentally random classical theory.

It's only weird if you insist on a notion of "state" that is not in accordance with observations. As far as we know the notion of "state" is how QT describes it and not as how classical physics describes it. The weirdness goes away as soon as you accept that nature behaves as she does and doesn't care about what humans my consider weird.

 

[DarMM]

Well certainly, but I think most people will find it odd that you can't consider a microscopic system independently of the device like you can in other physical theories. It seems as if it should be possible to talk about things in and of themselves.

According to the standard reading of QM you can't. Your approach, i.e. just get used to it, is sensible but goes against the intuitions of many who feel science should give you a picture of the world. Not necessarily an intuitive picture. People are fine with GR despite the fact that it is unintuitive since it discusses things as they are when no measuring devices are present.

 

[Mentz114]

Conservation laws are not enough. For example, from conservation laws alone you cannot deduce that the Moon is there when nobody observes it. As far as conservation laws are concerned, the Moon could spontaneously turn into a gigantic pink elephant of the same energy-momentum as that of the Moon, whenever it is no longer observed.

Only the Moon has the same Hamiltonian and the same number of dof as the Moon. Your transformation would require dumping all internal energy dof. It is not true, except in a gross approximation.

 

[DarMM]

Only the Moon has the same Hamiltonian and the same number of dof as the Moon. Your transformation would require dumping all internal energy dof. It is not true, except in a gross approximation.

Note he did say the elephant is pink. A blue elephant would be forbidden by internal energy considerations.

 

[Jimster41]

Just trying to use this dialogue to get a better lay persons understanding: I am familiar with the complimentarity of position and momentum, but also there are spin states that are not compatible correct? There are also others?

Are the number of degrees of freedom that display such contextual inter-relation infinite or finite? I thought they were very finite ie. may be a dumb question.

 

[DarMM]

Just trying to use this dialogue to get a better lay persons understanding: I am familiar with the complimentarity of position and momentum, but also there are spin states that are not compatible correct? There are also others?

Are the number of degrees of freedom that display such contextual inter-relation infinite or finite? I thought they were very finite ie. may be a dumb question.

It depends on how you count it. For example position $x$ is incompatible with momentum $p$. However both are also incompatible with $xp$.
Technically there are an infinite number of incompatible observables, although you might want to only consider basic ones like $x$ and $p$.

In QFT however there is an infinite amount of even the basic ones.

 

[Jimster41]

In QFT however there is an infinite amount of even the basic ones.

Is that because in QFT the standard model particles are considered to be excited states of the field, and so the fundamental object is the field which could be excited a potentially infinite number of ways?

 

[DarMM]

Is that because in QFT the standard model particles are considered to be excited states of the field, and so the fundamental object is the field which could be excited a potentially infinite number of ways?

Without going into much detail, basically yes. There is still the issue mentioned above that in a Copenhagen reading "field" is a type of reaction in a device treated classically.

 

[microsansfil]

Perhaps, but the minimal instrumental view of QM says nothing about that.

Moreover, the concept of existence is a concept from the field of metaphysics, but not from the field of physics.

/Patrick

 

[Auto-Didact]

It's only weird if you insist on a notion of "state" that is not in accordance with observations. As far as we know the notion of "state" is how QT describes it and not as how classical physics describes it. The weirdness goes away as soon as you accept that nature behaves as she does and doesn't care about what humans my consider weird.

This often repeated meme that humans have a cognitive bias against QM due to natural selection isn't actually an answer but a copout; even worse, it is an incoherent philosophical ideology parading as science. The very existence of Bohmian mechanics even reduces this meme into absurdity.

The uncomfortableness isn't a matter of interpretative human psychology but a matter of mathematical self-consistency; the fact that in the minimal interpretation of QM things cannot be defined without making references to macroscopic devices simply means that this theoretical construction is de facto fundamentally logically inconsistent.

Putnam et al. have argued on this basis that QM actually falsifies standard logic and a new form of logic is needed, e.g. quantum logic; I myself have argued this point for years. The problem is that the 'necessity of such non-standard logics'-argument just seems to be flat out wrong.

 

[Jimster41]

...The problem is that the 'necessity of such non-standard logics'-argument just seems to be flat out wrong.

I didn’t quite follow that. You mean the argument for these non-standard logics is the wrong argument for the right logics?

 

[Auto-Didact]

I didn’t quite follow that. You mean the argument for these non-standard logics is the wrong argument for the right logics?

The Putnam argument - that QM falsifies the universal validity of standard logic and that there is therefore a necessity for a non-standard logic such as quantum logic - is wrong.

Contrary to Putnam et al., QM in fact does not falsify the validity of standard logic, because Bohmian mechanics can be completely described and understood using standard logic.

If anything the logical - and therefore mathematical - self-inconsistency of QM is exposed as being an inadequacy of the idealized mathematical framework underlying QM, which completely disappears once the extended mathematical framework of Bohmian mechanics is adopted.

This extended mathematical framework is essentially a proper complex analytic formulation of Hamilton-Jacobi theory; Bohmian mechanics is based on this formulation, while textbook QM instead makes do with the more limited Hamiltonian mechanics and then just pretends - through purely philosophical rhetoric - that the existence or construction of any such more extended mathematical frameworks is just impossible.

 

[Jimster41]

Is there any easy way to summarize the key difference between Bohmian Mechanics and QFT. My cartoon of QFT is that the fields are space-time non-local, which is also a big part of my cartoon of Bohm’s pilot wave. I could really use a cartoon of their disagreement.

 

[Auto-Didact]

Bohmian mechanics, just like QM and Newtonian mechanics, is a theory which respects Galilean relativity. The non-locality of BM (and QM) is due to the wavefunction existing and evolving in configuration space.

Quantum field theory on the other hand is a field theoretic extension of QM, which moreover respects special relativity. In this sense, QFT is a completely local relativistic field theory, where the quantum fields exist in flat spacetime.

 

[atyy]

A way out might be that we at first accept that this what we call 'reality' is just a state of mind.
https://www.theguardian.com/science/blog/2009/mar/17/templeton-quantum-entanglement

Reality is just a tool to predict the results of observations. Unfortunately, our minds seem to think that that our state of mind is ordered - then it asks - are there laws that govern the state of mind? - then it ends up again with the measurement problem.

 

[PeterDonis]

Reality is just a tool to predict the results of observations.

This doesn't seem right. I would say reality is whatever-it-is that is producing the actual results of our observations, and models are the tools we use to predict the results of our observations; we then compare the predicted results with the actual results to see how accurate our models are, and to improve them.

 

[atyy]

It's only weird if you insist on a notion of "state" that is not in accordance with observations. As far as we know the notion of "state" is how QT describes it and not as how classical physics describes it. The weirdness goes away as soon as you accept that nature behaves as she does and doesn't care about what humans my consider weird.

That is precisely why QM is weird - it doesn't allow one to describe how nature behaves as she does without caring about the observer. QM does not describe nature. As Bohr said, "There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature."

 

[Auto-Didact]

This doesn't seem right. I would say reality is whatever-it-is that is producing the actual results of our observations, and models are the tools we use to predict the results of our observations; we then compare the predicted results with the actual results to see how accurate our models are, and to improve them.

With respect to physics, reality is just another word for ontology, while our observations and models thereof are phenomenology; that the two need not immediately coincide conceptually is true for any epistemic question i.e. for any scientific question.

That is precisely why QM is weird - it doesn't allow one to describe how nature behaves as she does without caring about the observer. QM does not describe nature. As Bohr said, "There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature."

In other words, Bohr believed that QM was purely an epistemic theory without any ontology, just like statistics is an epistemic theory without any ontology. Physics has always been about ontology; any suggestion that this is not so is purely postmodern philosophical rhetoric which is astutely unaware of the history and philosophy of physics and mathematics and their relationship.

 

[atyy]

This doesn't seem right. I would say reality is whatever-it-is that is producing the actual results of our observations, and models are the tools we use to predict the results of our observations; we then compare the predicted results with the actual results to see how accurate our models are, and to improve them.

Of course. What I mean is that whether one takes the common sense view of reality or an operational view of reality as a tool, one ends up with the measurement problem. One can say I don't care about the problem, but one cannot say it doesn't exist (unless one believes it is already solved by MWI etc).

 

[julcab12]

The more I listen to these philosophical debates about apparent problems of QT and it's "ontology", the less I understand them. I come more and more to the conclusion that those people who have such problems just cannot accept that nature behaves in another way than thought based on our everyday experience with "classical phenomena", which is however an apparent phenomenon due to a much coarse grained observation of the relevant macroscopic degrees of freedom.

Its not a matter of acceptance. Even macroworld, Before the realization of gravitational lensing effect. People though of twin/identical galaxies and stars when in fact it is a distortion of single image. This is an example of incomplete knowledge.

http://www.einstein-online.info/spotlights/grav_lensing_history.1.html
Fall-out of the article in "Science"

Right after Einstein's brief text had been published, it was followed by a number of articles by well-known scientists, who picked up where Einstein had left off.
Fritz Zwicky (1898-1974), an astronomer at the California Institute of Technology, discussed the possibility of observing the lensing effect in the case of the recently discovered extragalactic nebula, in other words: other galaxies. The typical masses, sizes and mutual distances of galaxies are such that double images of a distant galaxy should be significantly more frequent than double images of stars: The necessary near-alignment of a closer object, a more distant object and an observer here on Earth is much more probable for galaxies than for stars.
Henry Norris Russell, the astronomer from Princeton, published an article in which he speculated about the inhabitants of a hypothetical planet orbiting the White Dwarf companion of the star Sirius. What would they see during a total eclipse - on the occasion when, from the point of view of these inhabitants, the White Dwarf star would move in front of the more distant Sirius? As White Dwarfs are very compact objects, light from Sirius passing close to the companion would be markedly deflected. Of course, astronomers on Earth would not be able to see this relativistic gala performance, and in fact Russell cites this scenario as a perfect test of relativity theory which, regrettably, is impossible to put into practice.
Indisputably, Einstein's little publication had lent credibility to the idea of gravitational lensing, and the concept became part of the general knowledge of theoretical astronomers.

 

[A. Neumaier]

Reality is just a tool to predict the results of observations.

More properly, observations are just a tool for predicting reality.

 

[PGaccount]

The uncomfortableness isn't a matter of interpretative human psychology but a matter of mathematical self-consistency; the fact that in the minimal interpretation of QM things cannot be defined without making references to macroscopic devices simply means that this theoretical construction is de facto fundamentally logically inconsistent.

The reason we don't have to talk about measurement in classical physics is the fact that we can always control and account for the influence of the measuring bodies on the objects under investigation. For example we can make the effect of the measuring bodies as small as we want, or if it is finite, we can control and take that finite effect into account in our description. This means that we can talk about the state of a system, for example the position of a particle, as something that exists independently of observation. This is not possible in quantum physics because the effect of the measuring bodies is uncontrollable. If a body is to serve as a clock, then there will be an uncontrollable exchange of energy with the clock, which cannot be separately taken into account in order to specify the state of the objects. Any attempt to do so would interfere with the capability of the body to serve its original purpose of functioning as a clock. Our inability to eliminate disturbances does not by itself imply a need to alter the classical concept of observation. The reason why we cannot talk about the behavior of the objects independently of their interaction with the apparatus is that these disturbances are uncontrollable.

 

[vanhees71]

This often repeated meme that humans have a cognitive bias against QM due to natural selection isn't actually an answer but a copout; even worse, it is an incoherent philosophical ideology parading as science. The very existence of Bohmian mechanics even reduces this meme into absurdity.

Bohmian mechanics only "exists" for the non-relativistic theory. For me that's the reason, why I don't think it's a solution to any of the (pseudo-)problems discussed in philosophical circles about the "meaning" of QT.

 

[vanhees71]

That is precisely why QM is weird - it doesn't allow one to describe how nature behaves as she does without caring about the observer. QM does not describe nature. As Bohr said, "There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature."

There is no classical world, only an abstract classical description ;-)).

The classical description, however, is inaccurate. As far as we know today, the classical notion of state (point in phase space and thus determinism for all possible observables) is not accordance with observations, but the quantum notion of state is, and according to this only such observables are determined, for which the state is prepared into be so.