A Is quantum theory a microscopic theory?

[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.