A Is quantum theory a microscopic theory?

[Demystifier]

Your answer seems to imply that only detections are macroscopic (macroscopic = detection).

No, I imply that all detections are macroscopic. But the converse is not true, some macro objects may not be detections.

 

[Demystifier]

There is no classical/quantum cut to be well defined within quantum theory.

True, but there is a measurement/no-measurement cut within the minimal instrumental view of QM.

 

[Demystifier]

There's only a measurement problem, if you insist on an ontic interpretation. The very success of QT in describing all known observables disproves the existence of any "measurement problem". QT precisely describes all results of measurement in the real world, and thus there's no measurement problem in any scientific sense.

1. As long as there is no measurement problem in QT, there is a problem of whether QT is about the microscopic world.
2. As long as there is no problem of whether QT is about the microscopic world, there is a measurement problem in QT.
3. As long as there is neither measurement problem in QT nor a problem of whether QT is about the microscopic world, there is a problem with logical consistency of QT.

 

[Demystifier]

Why should a theory not use classical objects in its formulation?

A theory can use classical objects in its formulation, but not if that theory (like QM) claims that it can derive the classical objects. Using classical objects in formulation of a theory more fundamental than classical physics would be like using mathematical analysis in ZFC axioms of set theory.

 

[Lord Jestocost]

I say that the minimal instrumental form of QM does not say anything about the micro world.

That's indeed the point. Cord Friebe, Holger Lyre, Manfred Stöckler, Meinard Kuhlmann, Oliver Passon and Paul M. Näger in “The Philosophy of Quantum Physics”:

“If one tries to proceed systematically, then it is expedient to begin with an interpretation upon which everyone can agree, that is with an instrumentalist minimal interpretation. In such an interpretation, Hermitian operators represent macroscopic measurement apparatus, and their eigenvalues indicate the measurement outcomes which can be observed, while inner products give the probabilities of obtaining particular measured values. With such a formulation, quantum mechanics remains stuck in the macroscopic world and avoids any sort of ontological statement about the (microscopic) quantum-physical system itself.”

 

[Demystifier]

That's indeed the point. Cord Friebe, Holger Lyre, Manfred Stöckler, Meinard Kuhlmann, Oliver Passon and Paul M. Näger in “The Philosophy of Quantum Physics”:

“If one tries to proceed systematically, then it is expedient to begin with an interpretation upon which everyone can agree, that is with an instrumentalist minimal interpretation. In such an interpretation, Hermitian operators represent macroscopic measurement apparatus, and their eigenvalues indicate the measurement outcomes which can be observed, while inner products give the probabilities of obtaining particular measured values. With such a formulation, quantum mechanics remains stuck in the macroscopic world and avoids any sort of ontological statement about the (microscopic) quantum-physical system itself.”

Yes, that's exactly what I say, nice to see that I am not alone.

 

[vanhees71]

So what?

 

[Mentz114]

I do not question the success of quantum theory. I question that quantum theory is about the microscopic world. Or more precisely, I question that one particular view of QM is about the microscopic world.

The core formalism of QT allows us to make predictions about microscopic things by observing macroscopic outcomes of experiments. For instance the behaviour of atoms interacting with the EM field in cavities.
I think I miss the point of your question. It is true that we can only predict probabilities, and that imposes a limit what we can infer.

 

[Lord Jestocost]

to make predictions about microscopic things by observing macroscopic outcomes of experiments.

The “microscopic things” are merely mental concepts which one uses to “describe” the behavior of measuring instruments in a given experimental context.

 

[Mentz114]

The “microscopic things” are merely mental concepts which one uses to “describe” the behavior of measuring instruments in a given experimental context.

No. Atoms really do exist ! The only mental concept involved is probability which does not share the same kind of existence.

 

[Schwann]

But we reveal the "existence" of microscopic atoms via macroscopic devices...

No. Atoms really do exist !

 

[Mentz114]

But we reveal the "existence" of atoms via macroscopic devices...

If that is the case then QT does tell us something about the microscopic world and the philosophical doubts are proved meaningless.

 

[Lord Jestocost]

No. Atoms really do exist !

The question is: Does an atom “exist” on its own in the full, common-sense notion of the word so that it can be given a description in its own right or is it only a phenomenon in case it is an observed/registered phenomenon?

 

[Jimster41]

So what?

Maybe there is some theory that is.
Though the question this thread poses... has me scratching my head... is there some Godel-like proof that there can never be? I mean any theory we could point to is going to be an invention starting in a classical experience (ours) of the frustratingly in-naccessible (in the classical sense) quantum realm so... what hope is there?

Is that the point of the OP?

 

[Mentz114]

The question is: Does an atom “exist” on its own in the full, common-sense notion of the word so that it can be given a description in its own right or is it only a phenomenon in case it is an observed/registered phenomenon?

I would apply that definition of existence to anything that is claimed to exist. A Rydberg atom in a microwave resonant cavity is a vey tiny thing in a relatively huge volume.. We cannot hope to affirm its existence as we might do for a baseball. But experiment shows that there is something there which is interacting with the EM field - as predicted. So that atom existed. That atom made headlines in the 1960's !

 

[Fred Wright]

This discussion reminds me of the day, many years ago, when my physics teacher (a Nobel prize nominee) handed back an exam paper on the Dirac equation and sternly said, "Fred, the neutron, she's not a little potato!"

 

[DarMM]

Well QM in the typical Copenhagen reading is a theory of the statistics of impressions microscopic systems leave in macroscopic objects. So it does concern the microscopic, but it's not a representational theory telling you what they are like in and of themselves.

 

[martinbn]

A theory can use classical objects in its formulation, but not if that theory (like QM) claims that it can derive the classical objects. Using classical objects in formulation of a theory more fundamental than classical physics would be like using mathematical analysis in ZFC axioms of set theory.

Well, can QM derive that? May be people think too highly of QM. If you admit that it isn't the most fundamental, the last word, then there wouldn't be "problems" that need interpretations to "solve" them.

 

[DarMM]

Well, can QM derive that? May be people think too highly of QM. If you admit that it isn't the most fundamental, the last word, then there wouldn't be "problems" that need interpretations to "solve" them.

The problem to many is that no-go theorems imply QM is the last word unless you're willing to have multiple worlds, retrocausality or nonlocality. A theory which is the last word and leaves in place classical objects outside it is unsatisfying to many.

 

[lucas_]

No, since we would like to say the classical measurement apparatus is made of electrons, which are quantum. However, quantum mechanics does not seem to allow us to say that.

Can you push it to the nucleus too. About your "Is the measurement apparatus (classical macroscopic) made of electrons (quantum microscopic)?". Can you also ask ""Is the measurement apparatus (classical macroscopic) made of atomic nuclei (quantum microscopic)?"? (and the answer not necessarily being yes)

 

[microsansfil]

That's indeed the point. Cord Friebe, Holger Lyre, Manfred Stöckler, Meinard Kuhlmann, Oliver Passon and Paul M. Näger in “The Philosophy of Quantum Physics”:

“If one tries to proceed systematically, then it is expedient to begin with an interpretation upon which everyone can agree, that is with an instrumentalist minimal interpretation. In such an interpretation, Hermitian operators represent macroscopic measurement apparatus, and their eigenvalues indicate the measurement outcomes which can be observed, while inner products give the probabilities of obtaining particular measured values. With such a formulation, quantum mechanics remains stuck in the macroscopic world and avoids any sort of ontological statement about the (microscopic) quantum-physical system itself.”

Our regards of the world only could be in terms of human experiences.

/Patrick

 

[vanhees71]

No. Atoms really do exist ! The only mental concept involved is probability which does not share the same kind of existence.

Why? As far as we know, nature, as far as we can observe the phenomena with present means, behaves statistically on a fundamental level. So, as far as we know today, that's the way nature is. Why should this feature not "share the same kind of existence" as atoms? What else are atoms than what we can observe about them?

 

[A. Neumaier]

What else are atoms than what we can observe about them?

They existed already before there were observers.

 

[vanhees71]

That doesn't matter. When there were no observers, there were also no more or less stupid theories about phenomena. There haven't even been phenomena at all, but that's now really too philosophical for a science forum ;-))).

 

[martinbn]

The problem to many is that no-go theorems imply QM is the last word unless you're willing to have multiple worlds, retrocausality or nonlocality. A theory which is the last word and leaves in place clasical objects outside it is unsatisfying to many.

Which theorems?

 

[A. Neumaier]

There haven't even been phenomena at all

So the history of the solar system is a myth about its nonexistent past, invented by modern astrophysicists to explain present observations?

 

[vanhees71]

Since when is any theory in the natural sciences "the last word"? If there were such a "last word", we could give up physics and just apply the ToE to engineering tasks...

 

[DarMM]

Which theorems?

PBR theorem, Bell's theorem, the Kochen-Specker theorem.

 

[DarMM]

Since when is any theory in the natural sciences "the last word"?

I'm talking about how no-go theorems constrain the form of future developments, not saying QM is definitively the last word.

 

[vanhees71]

So the history of the solar system is a myth about its nonexistent past, invented by modern astrophysicists to explain present observations?

If there are no observers with the ability to store information, there's no past.

Strictly speaking, indeed, to claim that the solar system exists for some billion years, how it maybe have formed from gases and dust, or even the entire 14 billion years of the history of the entire universe, is just an extrapolation based on our current knowledge under the assumption of the Copernicanian, cosmological principle. Though at the moment it seems as if this assumption is quite well established, particularly that the natural constants are really constants, you can never be sure about such extrapolations, which can only indirectly checked by observations, which then are interpreted using this principle again.

 

[A. Neumaier]

Strictly speaking, [...] is just an extrapolation based on our current knowledge

Like everything else we pretend to know. We observe finitely many instances of something and then extrapolate to a general law. The latter is called knowledge and understanding. But strictly speaking, there is no knowledge since whatever we know is based on a long sequence of such extrapolations, combined with logic.

Thus nothing of interest remains once we go to this level of strictly speaking.

 

[vanhees71]

Well, yes, physics teaches humility...

 

[DarMM]

Well all I have of vanhees are impressions on my monitor screen. I've never taken the vanhees-realist view of many on the forum.

 

[martinbn]

PBR theorem, Bell's theorem, the Kochen-Specker theorem.

These say that you cannot add certain hidden variable to the theory. And suggests that probably there aren't any. But they don't suggest that theory cannot be superseded by a better one.

 

[DarMM]

These say that you cannot add certain hidden variable to the theory. And suggests that probably there aren't any. But they don't suggest that theory cannot be superseded by a better one.

They also directly show a superseding theory would have the same issues. Since they are proved in a general framework not quantum theory. The next theory would have a similar issue with requiring classical devices unless you take the "outs" mentioned above.

 

[DarMM]

These say that you cannot add certain hidden variable to the theory. And suggests that probably there aren't any. But they don't suggest that theory cannot be superseded by a better one.

Let me be clearer it's not that QM can't be improved on, it's that unless you take one of the "outs" the newer theory is going to require classical devices as well and on that issue say no more than QM. Thus QM's approach to measurements would be the final word on that topic, even if the newer theory explains gravity better etc

 

[PGaccount]

Bohr clarified this in his so-called "phonomenon" terminology.

"However far the phenomena transcend the scope of classical physical explanation, the account of all evidence must be expressed in classical terms. This crucial point implies the impossibility of any sharp separation between the behavior of atomic objects and the interactions with the measuring instruments which serve to define the conditions under which the phenomena appear. The word "phenomenon" should be applied exclusively to refer to observations obtained under specified circumstances. In such terminology, the observational problem is free of any special intricacy since, in actual experiments, all observations are expressed by unambiguous statements referring, for instance, to the registration of the point at which an electron arrives at a photographic plate."

 

[Mentz114]

Why? As far as we know, nature, as far as we can observe the phenomena with present means, behaves statistically on a fundamental level. So, as far as we know today, that's the way nature is. Why should this feature not "share the same kind of existence" as atoms? What else are atoms than what we can observe about them?

I don't understand what you mean by 'this feature'.
I was saying that probability has no physical counterpart comparable to the existence of the atom.

 

[vanhees71]

I mean the "feature" of probabilistic events. Why should nature not behave probabilistically on a fundamental level? I think the main quibbles of philosophers and still even some scientists with QT is the fact that it's indeterministic, i.e., that there is probabilistic/statistical behavior on the fundamental level, i.e., not due to some incomplete knowledge as within the realm of classical theory.

E.g., within classical mechanics, if we'd know precisely the complete initial conditions of a die, we could always predict at which side it will fall, i.e., there were no probabilistic element in the description. The fact that it appears probabilistic is due to our incomplete knowledge of the initial conditions.

In QT, we have, instead, intrinsic or "irreducible" probabilities. Take a spin state of a single electron and suppose we have prepared it to be precisely in a pure $\sigma_z=1/2$ state. According to QT, there's no more precise way to know the system's spin state. But now the spin component in any other direction is indetermined. We only know that, when measuring it we'll also get a precise value of either +1/2 or -1/2, but which we'll get in some individual measurement we don't know, but only the probability $P(\vec{n},\sigma)=|\langle \vec{n} \cdot \vec{\sigma}=\sigma|\sigma_z=+1/2 \rangle|^2$ with which each of the possible values $\sigma=\pm 1/2$ may occur. These probabilities are there despite our complete knowledge about the electron's spin state, and it cannot be completed somehow by knowing some whatever hidden variables there might be. At least today, as far as I know, there's no such deterministic hidden-variable theory known that reproduces all the successful descriptions of nature within QT. The main problem seems to be that according to the outcome of accurate Bell tests such a theory would have to be non-local, and it's obviously very difficult to find non-local determinstic theories compatible with relativistic causality. The only known theory that is both relativistically causal and describes all known phenomena is relativistic microcausal/local QFT, but that's of course also indeterminstic as any kind of QT.

Now, in view of this lack of any working deterministic theory, my simple question is, why it is considered a problem that nature seems to be "irreducibly probabilistic/random" in the precise sense defined by QT? Shouldn't it be most "realistic" to just accept this irreducible randomness?

Of course, it may well be that there's some deterministic theory one day, which is more comprehensive (or at least as comprehensive) as QT to describe the natural world, but so far there's no idea how such a theory should look like, and all we can observe is that nature seems to behave as described by QT.

 

[Mentz114]

[]

Now, in view of this lack of any working deterministic theory, my simple question is, why it is considered a problem that nature seems to be "irreducibly probabilistic/random" in the precise sense defined by QT? Shouldn't it be most "realistic" to just accept this irreducible randomness?

Of course, it may well be that there's some deterministic theory one day, which is more comprehensive (or at least as comprehensive) as QT to describe the natural world, but so far there's no idea how such a theory should look like, and all we can observe is that nature seems to behave as described by QT.

I emphatically agree with this. Accepting that all we can predict is a probability is hard for some people, who also think that probability is 'stuff'. Accepting also gets rid of the 'problem', as you say.

This is a philiosophical topic so maybe some imprecise language should be expected.

 

[julcab12]

Now, in view of this lack of any working deterministic theory, my simple question is, why it is considered a problem that nature seems to be "irreducibly probabilistic/random" in the precise sense defined by QT? Shouldn't it be most "realistic" to just accept this irreducible randomness?

Probability exists only subjectively. Randomness is just the absence of knowledge on what it will happen when we will do something we don't have the complete control on it. Outcomes are relative configurations of experimentation. In UV spectroscopy(Definite V,hv, Unknown size and position), Quantum Optic Experimenter(No definite Frequency, somewhat defined position and size-Localized), High Energy experimenter(Has "All"). In any setup, outcomes are incomplete. Randomness is only a feature and less complete picture than a temporal one.

 

[akvadrako]

Now, in view of this lack of any working deterministic theory, my simple question is, why it is considered a problem that nature seems to be "irreducibly probabilistic/random" in the precise sense defined by QT?

First, I would like to ask, if there is no collapse, where is the irreducible randomness in QT?

There is one reason I can think of why irreducible randomness might be problematic, but it's a concern that's far ahead of our current theorizing. The issue is computers can't create randomness – they always need an outside source. We can't write a RAND() function based off other primitive operations. If we try to figure out how the universe exists, probably how it created itself, there is no outside source to draw upon. So it's easier to imagine something evolved from nothing if no decision was ever made.

 

[Lord Jestocost]

No. Atoms really do exist !

Maybe, the following comment by @atyy puts it in a nutshell:

"I think most scientists don't care whether atoms exist or not. One just makes a model, uses the model to make predictions, and if the predictions match experimental results closely enough, then the model is accepted as a good approximation of reality. Atoms are just the name for something in some model."

https://www.physicsforums.com/threads/how-do-we-know-atoms-exist.282832/post-2092560

 

[vanhees71]

Probability exists only subjectively. Randomness is just the absence of knowledge on what it will happen when we will do something we don't have the complete control on it. Outcomes are relative configurations of experimentation. In UV spectroscopy(Definite V,hv, Unknown size and position), Quantum Optic Experimenter(No definite Frequency, somewhat defined position and size-Localized), High Energy experimenter(Has "All"). In any setup, outcomes are incomplete. Randomness is only a feature and less complete picture than a temporal one.

That's precisely what I asked! Why do you think that randomness is "just the absence of knowledge"? Why shouldn't nature behave randomly in a way as described by QT?

 

[ftr]

That's precisely what I asked! Why do you think that randomness is "just the absence of knowledge"? Why shouldn't nature behave randomly in a way as described by QT?

I think I have commented on such question before. Because we believe in science and science tells us that there is a reason for everything. randomness with no reason seems utterly illogical. Now, if it is indeed that way, people want to know why, that's all.

 

[DarMM]

First, I would like to ask, if there is no collapse, where is the irreducible randomness in QT?

There is one reason I can think of why irreducible randomness might be problematic, but it's a concern that's far ahead of our current theorizing. The issue is computers can't create randomness – they always need an outside source. We can't write a RAND() function based off other primitive operations. If we try to figure out how the universe exists, probably how it created itself, there is no outside source to draw upon. So it's easier to imagine something evolved from nothing if no decision was ever made.

I think this ties in with an issue we see in understanding quantum theory, is the universe an algorithm or describable by an algorithm on the fundamental level.

 

[Mentz114]

Maybe, the following comment by @atyy puts it in a nutshell:

"I think most scientists don't care whether atoms exist or not. One just makes a model, uses the model to make predictions, and if the predictions match experimental results closely enough, then the model is accepted as a good approximation of reality. Atoms are just the name for something in some model."

https://www.physicsforums.com/threads/how-do-we-know-atoms-exist.282832/post-2092560

No. Atoms in materials exist. We can image them. Rutherford 'saw' gold atoms deflecting alpha-particles !
The link above is just one persons opinion. But anyone may believe whatever suits their particular view of the universe ...

 

[Jimster41]

What I don't get is why there is structured, non-local randomness at the horizon (at the classical/quantum boundary).

And why is that horizon, though seemingly un-avoidable, dependent on some observer's frame of reference... why is it always waiting for some observer to be... picked but supposedly not involved in picking. I mean the problem with the Pilot wave, to my mind, is it suggests the future is mapped by a "wave". A wave isn't so offensive but how much of my future is determined by that wave? And why then isn't the present and past (mine for example) more like a wave? What I'm saying is there is a huge distance between that microscopic description and the classical reality we see. But all that reality is a function of that... thing (microscopic waves). So why the big difference in the description? I mean that's part of what we are struggling with here... the difference between what's going on down there and what we experience is profound.

Maybe a theory that had a better way of dealing with complicated (like more realistically sized) causality networks could better describe the rubbery space-time horizon - so it's not just a choice between an idealized microscopic (toy) Wave or a set of classical objects.

I mean the cool thing about multi-fractals that I can't get out of my head is they give you some math to create really rich mixtures of pure (or nearly pure) periodicity, pure (or nearly pure) randomness, and sets of things that are more classical seeming (unique-but-self-similar) objects. Are there any multi-fractal models of molecules?

 

[julcab12]

That's precisely what I asked! Why do you think that randomness is "just the absence of knowledge"? Why shouldn't nature behave randomly in a way as described by QT?

Randonmess in its natural or mathematical form is a placeholder or almost meaningless. Absence of knowledge is a natural direction. Randonmess is always associated by incompleteness in a dynamical sense. Some considered it as placeholder like flat space in geometry. Flat doesn't hold in nature like randonmess. If we narrow it down. The only thing that's meaningful is interactions.

 

[julcab12]

I think I have commented on such question before. Because we believe in science and science tells us that there is a reason for everything. randomness with no reason seems utterly illogical. Now, if it is indeed that way, people want to know why, that's all.

It has meaning in its form. No more than a 'flat space' in geometry does.