Is quantum theory a microscopic theory?

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Demystifier
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If that is the case then QT does tell us something about the microscopic world and the philosophical doubts are proved meaningless.
Perhaps QT does tell us that, but MQT (M is for minimal) doesn't.
 
Demystifier
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what hope is there?
To go beyond the minimal. More precisely, to adopt some ontic interpretation of QM.
 
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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.
No, the issue of determinism is secondary in most quantum philosophy quibbles. The primary issue is the ontology. The things which are there even if nobody observes it. Minimal QM says almost nothing about ontology, especially about microscopic ontology, and that's what many philosophers (and a substantial number of scientists) find disturbing.
 
Demystifier
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But when one is counting single photons ... these seems like fundamentally microscopic events
The fact is that you are counting detector clicks. Whether those clicks correspond to single photons, well, that's an miscroscopic interpretation of your macroscopic events. And I'm not saying that such an interpretation is wrong, I am saying that such an interpretation requires going beyond the minimal instrumental view of QM. In effect, you are dealing with a quantum interpretation even if you don't want to. A physicist cannot really avoid dealing with quantum interpretations.
 
vanhees71
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Of course the things are there if nobody observes them because there are fundamental conservation laws. Read Kant, who clearly defined "substance" as something "persistent", and nothing has changed on that with quantum theory.

Of course, whether or not a photon is still there is just a question whether or not it was absorbed by something from the last observation, but where is the problem? Also the classical electromagnetic field excitations get absorbed all the time. The only things that are persistent are energy, momentum, and angular momentum, which are transferred to the matter the em. field is absorbed from.

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.
 
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So then do you agree with @atyy ’s definition of micro/macro and the notion that a classical microscopic theory is impossible?
I agree that classical microscopic theory is impossible. Bohmian mechanics, for instance, is not classical.
 
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Well, the strength of science tells us first to be open to learn how nature behaves, ....
you do have a nag for writing nice summaries, maybe a history book.:smile:
However, I am sure you know about all the controversies in physics whether in SR, GR, QM, QFT, cosmology ...etc
As I have said many times I think a lot of progress have been achieved, but obviously no coherent picture is there.
 
Demystifier
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Of course the things are there if nobody observes them because there are fundamental conservation laws.
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.
 
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vanhees71
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Hm, well, I'm sure if there were no conservation laws forbidding it, this would already have happened ;-)).
 
Dr. Courtney
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The fact is that you are counting detector clicks. Whether those clicks correspond to single photons, well, that's an miscroscopic interpretation of your macroscopic events. And I'm not saying that such an interpretation is wrong, I am saying that such an interpretation requires going beyond the minimal instrumental view of QM. In effect, you are dealing with a quantum interpretation even if you don't want to. A physicist cannot really avoid dealing with quantum interpretations.
Fair enough. But by this definition of microscopic, nothing is microscopic at the experimental level. Everything humans sense is a microscopic interpretation of macroscopic events. So no theory in science is microscopic by this definition, because humans do not directly observe microscopic events. We only have microscopic interpretations of macroscopic events.
 
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Fair enough. But by this definition of microscopic, nothing is microscopic at the experimental level. Everything humans sense is a microscopic interpretation of macroscopic events. So no theory in science is microscopic by this definition, because humans do not directly observe microscopic events. We only have microscopic interpretations of macroscopic events.
I agree. But the minimal instrumental version of QM tries to deny it.
 
DarMM
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Fair enough. But by this definition of microscopic, nothing is microscopic at the experimental level. Everything humans sense is a microscopic interpretation of macroscopic events. So no theory in science is microscopic by this definition, because humans do not directly observe microscopic events. We only have microscopic interpretations of macroscopic events.
It's a little more difficult than that.

To simplify in the standard quantum formalism a beam of light say could be measured in the photon basis or the field basis. The photon basis would correspond to a machine that clicks like you mentioned, however results for devices measuring in the field basis seem to contradict those from the photon basis. It seems difficult to piece them together as being the result of some underlying picture. You can only consider the beam of light to consist of photons if you measure it in the photon basis.

Thus in the standard reading the macroscopic device can't be detached from your description. There are photons because that is what you are measuring, not because there are photons around when your device is absent.
 
atyy
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vanhees71
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The problem here is that you formulate the things in too abstract a way. You cannot simply say, "I measure a beam of light in the photon basis or the field basis". I've no clue what you mean. So I have to guess: The "photon basis" may be the Fock basis, i.e., states of the em. field with a defined total number of photons. I'm a bit at lost how to realize such a measurement. Do you know of any real-world device that measures only photons if they are a prepared in a photon-number eigenstate? I've no clue, how to construct such a device with real-world materials. Also what do you mean by "field basis"? Are these coherent states?

There's a well-developed subfield of relativstic QFT called quantum optics, which clearly defines, what's observed in experiments. They have all kinds of measurement devices. Most of them are based on the photo effect, i.e., an electromagnetic wave (no matter in which state it is prepared, be it a Fock state (not so easy to do, but standard today with parametric down conversion, or a coherent/squeezed state (lasers are your friend), is interacting with electrons which undergo a transition from a bound to a continuum state, and this signal is amplified to make these photoelectrons countable. In this way you can measure correlation functions of the electromagnetic (in the usually sufficient lowest-order dipole approximation the electric) field.
 
Dr. Courtney
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I agree. But the minimal instrumental version of QM tries to deny it.
I suspect the "denial that every theory is macroscopic" is not unique to the instrumental version of QM.

How would biologists view assertions that cell theory and the germ theory of disease are not really microscopic theories but "microscopic interpretations of macroscopic events"? How would chemists view assertions that atomic theory (from Dalton) and kinetic theory are not really microscopic theories but "microscopic interpretations of macroscopic events"? I expect they can follow the logic, but this is not really how "microscopic" is used in other fields of science.

One could construct a similar logic regarding scientific reconstructions of past events since these are not observed directly but are inferred from modern observations. "The Big Bang theory is not properly a theory of origins, but it is a historical interpretation of modern events." In the same way that one cannot separate microscopic interpretations from the macroscopic things humans actually observe, one cannot separate historical interpretations from the modern events that humans actually observe.
 
atyy
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I suspect the "denial that every theory is macroscopic" is not unique to the instrumental version of QM.

How would biologists view assertions that cell theory and the germ theory of disease are not really microscopic theories but "microscopic interpretations of macroscopic events"? How would chemists view assertions that atomic theory (from Dalton) and kinetic theory are not really microscopic theories but "microscopic interpretations of macroscopic events"? I expect they can follow the logic, but this is not really how "microscopic" is used in other fields of science.

One could construct a similar logic regarding scientific reconstructions of past events since these are not observed directly but are inferred from modern observations. "The Big Bang theory is not properly a theory of origins, but it is a historical interpretation of modern events." In the same way that one cannot separate microscopic interpretations from the macroscopic things humans actually observe, one cannot separate historical interpretations from the modern events that humans actually observe.
The problem is we would like a theory of reality, but quantum mechanics (as we understand it) is simply not such a theory. Of course it is absurd to say that germs don't really exist or that the measuring apparatus is not made of electrons - but the formalism does not grant us the ability to say these things - it is an open problem of quantum mechanics.
 
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DarMM
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The problem here is that you formulate the things in too abstract a way
It's the old problem of complimentary between results in different basis. I could make things more precise, but it doesn't really change the point. Why would the details matter? You'll still get complimentary where the statistics in different bases can't be considered as marginals of one set of properties.
It was just an attempt to say why complimentarity makes viewing things in a way that's detached from your device is difficult, it's not intended as a completely accurate rendering of quantum optics.
 
DarMM
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I suspect the "denial that every theory is macroscopic" is not unique to the instrumental version of QM.

How would biologists view assertions that cell theory and the germ theory of disease are not really microscopic theories but "microscopic interpretations of macroscopic events"? How would chemists view assertions that atomic theory (from Dalton) and kinetic theory are not really microscopic theories but "microscopic interpretations of macroscopic events"? I expect they can follow the logic, but this is not really how "microscopic" is used in other fields of science.

One could construct a similar logic regarding scientific reconstructions of past events since these are not observed directly but are inferred from modern observations. "The Big Bang theory is not properly a theory of origins, but it is a historical interpretation of modern events." In the same way that one cannot separate microscopic interpretations from the macroscopic things humans actually observe, one cannot separate historical interpretations from the modern events that humans actually observe.
In principle one can reason like that in any science, but in reality such reasoning can only be found in quantum foundations. Why is quantum theory different? I think the main reason is that some physicists like to think quantum theory is very close to the final fundamental theory of everything, so they don't like to think that there is some hidden reality on which quantum theory has nothing to say. Instead, they prefer to think that there is no any hidden reality at all. All Copenhagen-like interpretations of QM are nothing but attempts to justify the ideology that QM must somehow be complete.
 
vanhees71
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It's the old problem of complimentary between results in different basis. I could make things more precise, but it doesn't really change the point. Why would the details matter? You'll still get complimentary where the statistics in different bases can't be considered as marginals of one set of properties.
It was just an attempt to say why complimentarity makes viewing things in a way that's detached from your device is difficult, it's not intended as a completely accurate rendering of quantum optics.
I'm not saying you should make things more precise, but you should make a statement about what you mean by "measuring" in the one or the other basis. Particularly to understand complementarity (one of Bohr's enigmatic terms) right, you need to think about many concrete experiments, i.e., descriptions of real-world measurement devices applied to concrete real-world preparations of measured objects. I don't know of a single example, where this has not resolved apparent quibbles with overly abstract formulations like "measuring in a basis".

Quantum optics is just the example, where the real-world descriptions of measurement and preparation processes become most transparent, because they are usually not that complicated after all. You need just a good grasp of classical optics. To understand all kinds of devices like lenses, polarizers, beam splitters even linear electrodynamics as taught in classical electrodynamics is sufficient. For parametric down conversion and preparation of proper Fock states also some nonlinear optics is useful. The quantization part is also not too hard. Usually you just need a good understanding of the quantization of the free electromagnetic field and some perturbation theory for the interactions with matter. The latter almost always can be described with non-relativistical QM.
 
DarMM
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I'm not saying you should make things more precise, but you should make a statement about what you mean by "measuring" in the one or the other basis. Particularly to understand complementarity (one of Bohr's enigmatic terms) right, you need to think about many concrete experiments, i.e., descriptions of real-world measurement devices applied to concrete real-world preparations of measured objects. I don't know of a single example, where this has not resolved apparent quibbles with overly abstract formulations like "measuring in a basis".
A good understanding of how concrete experiments work resolves the conceptual difficulties with complementarity? Even if I specified the exact devices and what they measure it wouldn't do this. I can't imagine how it would. It would still reduce to the fact that the statistics for two different measurements don't seem to be marginals of a third more complete measurement as is always the case in classical mechanics.

For example an ##x## measurement and a ##p## measurement on a single particle cannot in general be considered as a coarse graining of some ##(x,p)## measurement, preventing you from considering them as measurements of properties already present.
 
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