# Is quantum theory a microscopic theory?

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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
Gold Member
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
Gold Member
Which theorems?
PBR theorem, Bell's theorem, the Kochen-Specker theorem.

DarMM
Gold Member
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
Gold Member
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
Gold Member
Well, yes, physics teaches humility...

DarMM
Gold Member
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
Gold Member
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
Gold Member
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

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 seperation 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
Gold Member
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
Gold Member
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
Gold Member
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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.

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.

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
Gold Member
2018 Award
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."

vanhees71
Gold Member
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.

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DarMM
Gold Member
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
Gold Member
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."

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
Gold Member
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?

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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 doesnt hold in nature like randonmess. If we narrow it down. The only thing thats meaningful is interactions.

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.