I Einstein & Bohr Debate: Epistemological Paradox

[Vividly]

TL;DR Summary

Quantum physics

Im reading a book called “Quantum” the deabate between Einstien and Bohr. I’ve read reviews on it and this one particular review said the book doen’t delve into the “epistimological paradox” theat caused the two debaters to take their stand in their arguement. My question is, what was their “stand” in regards to this epistimological paradox?

 

[Andrew Mason]

Summary: Quantum physics

Im reading a book called “Quantum” the deabate between Einstein and Bohr. I’ve read reviews on it and this one particular review said the book doen’t delve into the “epistimological paradox” theat caused the two debaters to take their stand in their arguement. My question is, what was their “stand” in regards to this epistimological paradox?

Well, a lot has been written about it over the years and one of the things the writers who have written about it don't seem to agree on entirely is what their fundamental positions and disagreements were. Essentially it was a philosophical debate over whether physical behaviour was "knowable" or not. Bohr's Copenhagen Interpretation of quantum mechanics was that it provided as complete a picture of nature as could be found and that there were aspects that were unknowable: a particle can exist in many states and it is not possible to determine at any particular time what state it may be in. Einstein used arguments like "God does not play dice with the universe" and proposed the argument concerning Schrodinger's cat. He also criticised the fact that - at that time - it did not take into account relativity. Dirac, Yukawa, Feynman and others later addressed that.

The debate was not science at all, but philosophy (the theory of what it means to know something - or something like that, I think). However, it may have given an impetus to different ideas and ways of looking at things that may have impacted the development of quantum mechanics in some way.

AM

 

[andresB]

The debate was not science at all, but philosophy

I would not call a discussion about the universality of Heisenberg uncertainty principle with propposed experimental setups as "not science".

 

[bhobba]

I would not call a discussion about the universality of Heisenberg uncertainty principle with propposed experimental setups as "not science".

It is science. But we have seen papers on the issue here. It's invariably a misconception about so called weak measurements, so one could call discussions on it both misunderstandings and philosophy and is especially bad when both happen together. The issue with Bohr is he and Einstein were good friends (despite a few interesting episodes at the end of Einsteins life that were probably caused by Einstein being simply too jaded to debate it further) and so Einstein understood quite well what Bohr meant, however for the rest of us mortals Bohr was a well known mumbler and known to not express himself always clearly. This has led to a number of different versions of Copenhagen and nobody is 100% sure exactly what Bohr subscribed to. He certainly did subscribe to QM being a complete theory and Einstein did not - that seems the main issue. From our current vantage the most common view is due to not having a complete quantum theory of gravity it probably is incomplete - so Einstein may get the last laugh. Really though we do not know. There is also something of a difference in the QM founders approach to science as revealed by a debate between Heisenberg and Dirac:
http://philsci-archive.pitt.edu/1614/1/Open_or_Closed-preprint.pdf
Personally I side with Dirac and reject Kuhn like views

Weinberg's remarks are also of interest here:
https://physicstoday.scitation.org/doi/pdf/10.1063/1.2155755
Basically both Einstein and Bohr were wrong - but for me Einstein was probably on the right track - QM is likely incomplete.

Thanks
Bill

 

[Lord Jestocost]

Einstein used arguments like "God does not play dice with the universe"...

…and Bohr replied in a striking way: “But still, it cannot be for us to tell God, how he is to run the world.”

 

[Andrew Mason]

I would not call a discussion about the universality of Heisenberg uncertainty principle with propposed experimental setups as "not science".

I am not saying that science wasn't discussed. But the disagreement, which was the subject of the debate, was essentially philosophical. The debate was not focused on the accuracy of the experimental data or the validity of the experiments. The debate appears to have been, as far as I can tell, about the true state of the physical world that lay beyond the known experimental results.

AM

 

[DrChinese]

... My question is, what was their “stand” in regards to this epistimological paradox?

Einstein's views were on display in the so-called EPR paper (1935 - Einstein is the "E") entitled: "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?"

EPR:
https://journals.aps.org/pr/pdf/10.1103/PhysRev.47.777
Bohr:
https://www.informationphilosopher.com/solutions/scientists/bell/Bell_On_EPR.pdf

You will get many different takes on this paper, which Bohr replied to with an identical title. I personally rate the EPR paper as more important than the Bohr response. Here's my take:

EPR: If you make certain reasonable assumptions about reality and entanglement (which had no experimental results at that time): then there must exist predetermined values for non-commuting observables. Such values (often called "hidden variables") would violate the Heisenberg Uncertainty relations, a lynchpin of the then-new Quantum Mechanics.

Bohr/anti-EPR: One of those "reasonable" assumptions is invalid.

The exact logic EPR used for their argument was correct as far as it went, and is very clever. However, the assumptions themselves were ultimately shown to contain a subtle flaw. The EPR thought experiment itself was compatible with the predictions of QM and the Uncertainty Principle. But their conclusion required those results to be extended further than QM allows. The flaw was fully exposed 30 years later in Bell's blockbuster paper:

https://www.informationphilosopher.com/solutions/scientists/bell/Bell_On_EPR.pdf

Bell showed that EPR's extension was in direct contradiction to previously unknown predictions of QM. Read these papers and you will get the arguments direct. Note that the audience for these papers were contemporary scientists, so the language is somewhat arcane this many years later and with the benefit of decades of advances.

 

[vanhees71]

Well, Einstein didn't like the EPR paper too much. He felt that it buried is point under erudition. Indeed, he wrote a much clearer paper single-authored somewhat later. I guess, there's an English translation somewhere. The original paper is

A. Einstein, Quantenmechanik und Wirklichkeit, Dialectica 2, 320 (1948),
https://doi.org/10.1111/j.1746-8361.1948.tb00704.x

 

[DrChinese]

From EPR: "One could object to this conclusion on the grounds that our criterion of reality is not sufficiently restrictive. Indeed, one would not arrive at our conclusion if one insisted that two or more physical quantities can be regarded as simultaneous elements of reality only when they can be simultaneously measured or predicted. On this point of view, since either one or the other, but no both simultaneously, of the quantities P and Q can be predicted, they are not simultaneously real. This makes the reality of P and Q depends upon the process of measurement carried out on the first system, which does not disturb the second system in any way. No reasonable definition of reality could be expected to permit this".

Notice the bold sentence contains a reasonable assumption, identified by EPR themselves. This was the subtle flaw in EPR (if you want to call that a flaw).

P and Q being quantum momentum and position, which are non-commuting observables and therefore obey the Heisenberg relations.

 

[DrChinese]

Well, Einstein didn't like the EPR paper too much. He felt that it buried is point under erudition. Indeed, he wrote a much clearer paper single-authored somewhat later. I guess, there's an English translation somewhere...

You thoughtfully provided that previously in another thread:

https://www.informationphilosopher.com/solutions/scientists/einstein/dialectica.html

 

[vanhees71]

From EPR: "One could object to this conclusion on the grounds that our criterion of reality is not sufficiently restrictive. Indeed, one would not arrive at our conclusion if one insisted that two or more physical quantities can be regarded as simultaneous elements of reality only when they can be simultaneously measured or predicted. On this point of view, since either one or the other, but no both simultaneously, of the quantities P and Q can be predicted, they are not simultaneously real. This makes the reality of P and Q depends upon the process of measurement carried out on the first system, which does not disturb the second system in any way. No reasonable definition of reality could be expected to permit this".

Notice the bold sentence contains a reasonable assumption, identified by EPR themselves. This was the subtle flaw in EPR (if you want to call that a flaw).

P and Q being quantum momentum and position, which are non-commuting observables and therefore obey the Heisenberg relations.

In the original EPR paradox there is no paradox (as there is none in any other example as QT is consistent, if not interpreted using the collapse assumption but using the ensemble interpretation, which I think Einstein accepted but didn't considered as "complete", but that's interpretation and thus belongs in the corresponding sub-forum).

The gedanken experiment was the decay of a particle originaly at rest in two particles. Of course "at rest" means within the restrictions of the uncertainty principle, i.e., the initial state must be described by a true normalizable state ("wave packet"). In the final state the relative momentum and the center of mass position are compatible observables and both are pretty well determined. The momenta and positions of the single particles are of course entangled and thus you have the corresponding correlations. Within the ensemble interpretation the correlations are due to the "preparation" (i.e., the creation of the particles in the decay of a pretty well localized particle pretty well at rest). There's no "spooky action at a distance" to explain the correlations through faster-than-light signals due to the local measurements of the single-particle properties.

 

[EPR]

Well, their debate revolves around the century old question - does classical reality exist apart from the brain? And do we see reality with the eyes or with the brain?
The answers to these questions will surprise you.
The scientific consensus is that Einstein lost the EPR debate about how the world works.

 

[PeterDonis]

The scientific consensus is that Einstein lost the EPR debate about how the world works.

Please give a reference for this claim.

 

[EPR]

Please give a reference for this claim.

Physics - Closing the Door on Einstein and Bohr’s Quantum Debate (aps.org)

 

[PeterDonis]

Physics - Closing the Door on Einstein and Bohr’s Quantum Debate (aps.org)

This reference in no way supports your claim of a "scientific consensus" that Einstein lost the debate. It is one particular scientist's opinion, and as references provided by others in this thread show, that opinion is by no means a "consensus".

 

[PeterDonis]

does classical reality exist apart from the brain? And do we see reality with the eyes or with the brain?
The answers to these questions will surprise you.

Please give a reference for this claim.

 

[PeterDonis]

Moderator's note: Thread moved to QM interpretations and foundations forum.

 

[EPR]

This reference in no way supports your claim of a "scientific consensus" that Einstein lost the debate. It is one particular scientist's opinion, and as references provided by others in this thread show, that opinion is by no means a "consensus".

It's not an opinion but experimentalists performing increasingly sophisticated tests of Bell’s inequalities that prove Einstein wrong.
Teaching standard quantum mechanics in all universities across the globe supports Bohr's stance, not Einstein's intuition-guided objections on introducing indeterminancy and probabilities in physics(espoused in his quote that 'God doesn't play dice').

 

[PeterDonis]

It's not an opinion but experimentalists performing increasingly sophisticated tests of Bell’s inequalities that prove Einstein wrong.

Einstein never claimed that the experimental predictions of quantum mechanics were incorrect. So showing that those predictions are correct in experiments cannot possibly prove Einstein wrong, since he agreed that that's what would happen.

Teaching standard quantum mechanics in all universities across the globe supports Bohr's stance

You cannot possibly know this to be true. Please stop making claims that you cannot support.

 

[EPR]

Please give a reference for this claim.

Cognition happens in the brain, not in the eyes.
' Cognition can be defined as the mental processes involved in acquiring and processing information that are necessary for everyday living;"
Postoperative Cognitive Dysfunction - ScienceDirect As for the question whether classical reality exists as such outside of the brain - i am doubtful. Experiments like entanglement challenge that notion. Our most comprehensive physical models and theories make no claims about classical reality existing in time and space.

 

[EPR]

Einstein never claimed that the experimental predictions of quantum mechanics were incorrect. So showing that those predictions are correct in experiments cannot possibly prove Einstein wrong, since he agreed that that's what would happen.

I never claimed he challenged QM's predictions!

Show me where i did.

 

[PeterDonis]

Thread closed for moderation.

 

[berkeman]

After a Mentor discussion, @EPR has been thread banned, and the thread is reopened.

 

[bhobba]

In discussing the Bohr Einstein debates I think it is important to understand the interpretation of QM Einstein adhered to:
https://www.informationphilosopher.com/solutions/scientists/ballentine/AJP72.pdf

To be clear he was not the only one that adhered to it, but some think he did not believe in QM - he did - and even had an interpretation used by many even today. It was reviewed/revived in a classic paper by Ballentine:
https://www.informationphilosopher.com/solutions/scientists/ballentine/PR70.pdf

It is interesting contrasting it to Copenhagen which has different variations. I generally do not like giving links to Lubos because his style IMHO can often be counterproductive, but he does detail a common version:
https://motls.blogspot.com/2011/05/copenhagen-interpretation-of-quantum.html

I personally am with Einstein. However I will only participate here and there in this discussion as IMHO it is best to come to your own conclusion. Also thinking both are wrong is a valid position.

Thanks
Bill

 

[vanhees71]

Well, it was for quite a long time that indeed many writings on the Bohr Einstein debate judged that Bohr "won the debate". Only for some years people are more careful about it and appreciate Einstein's profound insights on the interpretation of quantum mechanics more, though you have a hard time as a follower of the statistical minimal interpretation (which I think indeed comes the closest to Einstein's point of view), as one sees in this forum.

The reason is a bit puzzling for me since I never understood the hype about Bohr's writings. To be honest, I'm not able to understand most of Bohr's writings very well, because they are pretty unclear, because they don't contain enough math to unambigously express what Bohr really wants to say. The same holds for Heisenberg.

For a long time the "Copenhagen doctrine" (though it never was a clearly defined concept, because there are many flavors of the Copenhagen interpretation with different meanings of only vaguely defined notions, particularly the "collapse of the state").

It think the change of mind came slowly with Bell's theoretical work on the question how to decide between Einstein's claim that quantum theory is incomplete and Bohr's claim that it is complete.

I think for Einstein QT is incomplete in the sense that there should be local hidden-variable theories to resolve the problem that on the one hand QT only describes the statistical behavior of measurements on ensembles of equally prepared quantum systems but that on the other hand the preparation in the quantum state has to refer to each single system within such an ensemble. There's no problem if you consider the quantum state (statistical operator) just as a way to mathematically describe the preparation of a single system, implying only the usual probabilistic meaning via Born's rule (in its general form for general as well mixed as pure states). Some people consider such an epistemic view as incomplete and want an ontological description, i.e., they want to say "an electron IS one-to-one mapped to some mathematical entity in the theory", as is the electromagnetic field in classical electrodynamics. I think that also explains, why Einstein tried to find a unified classical field theory for the about last 30 years of his life.

The breakthrough was that Bell, with deriving his famous inequalities valid for all local determinstic hidden-variable theories, found a physically testable prediction, i.e., he brought a rather vague philosophical question, which was not well defined to decide by experiments, to a sharply defined physical question, which can be (and today in fact is) answered by experiment.

At the time it was a risk to work on the foundations of QT and questioning the Copenhagen doctrine. A somewhat sad illustrative story related to this is the history of Everett's thesis (now known as the many-worlds interpretation). Fortunately courageous experimentalists like Aspect took up the challenge to experimentally realize the first Bell-test experiments, and the breakthrough was that indeed Bohr was right in the sense that there are at least no local deterministic hidden-variable theories that can explain the outcome of the Bell-test experiments, namely that Bell's inequality is violated and with an amazing significance precisely as predicted by quantum theory.

I'm aware that I should give references for this. I have only two books in mind, but there's for sure tons of papers on this historical subject out there, but I'm not a librarian. So here are my two references (though more in the popular-science end of the spectrum):

https://www.amazon.com/dp/1491531045/?tag=pfamazon01-20
https://www.hippiessavedphysics.com/

 

[Lord Jestocost]

I personally am with Einstein. However I will only participate here and there in this discussion as IMHO it is best to come to your own conclusion.

To my mind, there is no need to say much more about the ‘Bohr-Einstein’- debate than done by Werner Heisenberg in his book “Physics and Philosophy”:

“The third group, finally, expresses rather its general dissatisfaction with the results of the Copenhagen interpretation and especially with its philosophical conclusions, without making definite counterproposals. Papers by Einstein, von Laue and Schrödinger belong to this third group which has historically been the first of the three groups.
However, all the opponents of the Copenhagen interpretation do agree on one point. It would, in their view, be desirable to return to the reality concept of classical physics or, to use a more general philosophic term, to the ontology of materialism. They would prefer to come back to the idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them.”

 

[vanhees71]

To my mind, there is no need to say much more about the ‘Bohr-Einstein’- debate than done by Werner Heisenberg in his book “Physics and Philosophy”:

“The third group, finally, expresses rather its general dissatisfaction with the results of the Copenhagen interpretation and especially with its philosophical conclusions, without making definite counterproposals. Papers by Einstein, von Laue and Schrödinger belong to this third group which has historically been the first of the three groups.
However, all the opponents of the Copenhagen interpretation do agree on one point. It would, in their view, be desirable to return to the reality concept of classical physics or, to use a more general philosophic term, to the ontology of materialism. They would prefer to come back to the idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them.”

That's why I don't like Heisenberg's writings. Why the heck should nature behave differently than she does only because Heisenberg would like to "come back to the idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist"?

What a hybris!

Ironically he doesn't come up with such an idea himself, blaming Einstein, von Laue, and Schrödinger to not make "definite counterproposals". At least Einstein did! Schrödinger's writings on the subject (both within the Copenhagen interpretation and in his criticism against it) is imho far more profound than Heisenberg's philosophical writings. I'm not aware that von Laue bothered much for QT at all. At least I'm not aware about any scientific paper on QT by von Laue.

 

[Lord Jestocost]

Maybe, you misunderstand something. It's not Heisenberg who wants to "come back to the idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them."

 

[martinbn]

That's why I don't like Heisenberg's writings. Why the heck should nature behave differently than she does only because Heisenberg would like to "come back to the idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist"?

What a hybris!

Wait! What's wrong with that!? They do exist objectively.

 

[Lord Jestocost]

They do exist objectively.

Come on, where do you know? A bold declaration has nothing to do with science.

As Bertrand Russell remarks in "An Outline of Philosophy": „We cannot find out what the world looks like from a place where there is nobody, because if we go to look there will be somebody there.”

 

[martinbn]

Come on, where do you know? A bold declaration has nothing to do with science.

As Bertrand Russell remarks in "An Outline of Philosophy": „We cannot find out what the world looks like from a place where there is nobody, because if we go to look there will be somebody there.”

I meant that they exist as much as the stones and trees do.

 

[PeroK]

I meant that they exist as much as the stones and trees do.

Although, of course, this debate may be sidetacked by philosophical ideas about the nature of existence, there is a key difference between a stone and an electron:

The stone is essentially defined without recourse to a particular mathematical model. But, the existence of the electron depends on the standard model of particle physics. If you change that model radically you may remove the whole concept of an electron. What is regarded as a stone does not change in the same way. Before the atomic model was adopted, physicists like Max Planck believed that matter may be a continuous substance. If we are using a stone in a experiment, then the existence of the stone is not dependent on the model of the fundamental particles that we have: continuous matter, atomic model, electron-proton-neutron model, standard model.

A better example, might be the existence of quarks: that they could, perhaps, be removed from the theory and replaced by something else entirely different is more likely than that we lose the electron.

If the electron definitely exists, then does also the quark?

Another example, perhaps, is the existence of EM fields. Using classical EM theory it appears, of course, that EM fields are real: you can measure them and if they are not real, then what makes the electron move a certain way? After all, in classical EM theory, the field is all you have!

But, then, you introduce the theory of QED and the EM fields are no longer fundamental: Coulomb's law breaks down at high energies, for example, and you lose the apparently obvious existence of these EM fields.

 

[PeterDonis]

they exist as much as the stones and trees do

But that is precisely the point at issue, so you can't just assume it.

 

[Lynch101]

Well, it was for quite a long time that indeed many writings on the Bohr Einstein debate judged that Bohr "won the debate". Only for some years people are more careful about it and appreciate Einstein's profound insights on the interpretation of quantum mechanics more, though you have a hard time as a follower of the statistical minimal interpretation (which I think indeed comes the closest to Einstein's point of view), as one sees in this forum.

The reason is a bit puzzling for me since I never understood the hype about Bohr's writings. To be honest, I'm not able to understand most of Bohr's writings very well, because they are pretty unclear, because they don't contain enough math to unambigously express what Bohr really wants to say. The same holds for Heisenberg.

In What is Real? The Unfinished Quest for the Meaning of Quantum Physics by Adam Becker, he suggests that Bohr's writing didn't make sense to most people, so much so that when his response to Einstein was published, there were a number of pages out of order and no one really noticed. But such was his standing in the quantum physics community, people assumed that he had addressed Einstein's misgivings simply by the fact that he had published a response.

 

[AlexCaledin]

"Bohr's answer to the EPR ... can be summarized in a short sentence: quantum mechanics is not separable."

R. Omnès "Understanding Quantum Mechanics" p. 274, 275

(- but, if so, the Collapse must always be universal, reducing the whole universe state)

 

[martinbn]

But that is precisely the point at issue, so you can't just assume it.

But if the smallest parts do not exist objectively, what does it mean for the other ones, made from them, to exist obectively.

 

[PeterDonis]

if the smallest parts do not exist objectively, what does it mean for the other ones, made from them, to exist obectively

You'd have to ask the people who claim that the smallest parts don't exist objectively unless they are measured. Unfortunately Heisenberg is no longer around to ask. But such interpretations do exist, so you can't just rule them out by fiat.

 

[bhobba]

Wait! What's wrong with that!? They do exist objectively.

Here in the macro world - yes. But, without specifying any particular interpretation, the idea of existing objectively has been modified somewhat. In QM what is going on between observations the theory is silent on. That puts a 'cloud' over the whole idea of objective reality existing at all times. It still may - but the theory does not affirm it.

Thanks
Bill

 

[bhobba]

But if the smallest parts do not exist objectively, what does it mean for the other ones, made from them, to exist obectively.

Well Freeman Dyson sorted that one out when he figured out why objects here in the macro world are solid. In the microworld things are a lot more fuzzy. There is of course an area of 'overlap' and there all sorts of 'strange' things happen eg
https://www.nist.gov/news-events/ne...ks-demonstrate-direct-coupling-vibrating-ions

As technology advances expect even weirder effects.

Thanks
Bill

 

[martinbn]

You'd have to ask the people who claim that the smallest parts don't exist objectively unless they are measured. Unfortunately Heisenberg is no longer around to ask. But such interpretations do exist, so you can't just rule them out by fiat.

Which interpretations are those? Just to be clear. There are interretations, in fact that is the standard theory, in which observables do not have values until measured. But i am not aware if iterpretations that say that the quantum systems do not exist, and at the same time the classical systems do exist.

Of course all this may be just semantics. If by exists people(Heisenberg) mean all/some observables have definite values, then i agree, but that is just strange use of language.

 

[martinbn]

Here in the macro world - yes. But, without specifying any particular interpretation, the idea of existing objectively has been modified somewhat. In QM what is going on between observations the theory is silent on. That puts a 'cloud' over the whole idea of objective reality existing at all times. It still may - but the theory does not affirm it.

Thanks
Bill

That is not quite true. The theory says that between measurements the system's state evolves according to the Schrodinger equation.

 

[Lynch101]

I meant that they exist as much as the stones and trees do.

This might be an esoteric bridge too far, but if we were to drill down into the idea that "stones" and "trees" exist, we can see that "stones" and "trees" are just concepts without any intrinsic existence. Each is made up of their constituent parts with the term "tree" and "stone" being the conceptual label for what is, in truth, a more complex process.

EDIT: to say that "trees" and "stones" exist is akin to a category mistake (or perhaps an inverse category mistake).

 

[atyy]

But if the smallest parts do not exist objectively, what does it mean for the other ones, made from them, to exist obectively.

They are only "made" from them in the quantum state. So the "made from" does not have the same level of reality as the measurement outcomes. Basically, it is not saying that the true constituent parts (which we have not identified and may have no straightforward relation to "electrons") from which real things are made don't exist. It is saying that we are agnostic about the reality of "electrons", "photons" etc that are concepts we use to construct the quantum state.

 

[vanhees71]

You'd have to ask the people who claim that the smallest parts don't exist objectively unless they are measured. Unfortunately Heisenberg is no longer around to ask. But such interpretations do exist, so you can't just rule them out by fiat.

This statement is one of the examples why I'm very critical against Heisenberg's mostly philosophical writings. QT doesn't say that things only exist when measured but that no matter in which state an object is prepared in there are always observables whose values are indetermined, and these values are objectively indetermined, i.e., there's no unknown "hidden variable" which we just don't know but that these values are just indetermined. This does not imply that the object doesn't exist at all. I don't know, how, from the quantum-theoretical foundations, one can come to such an absurd conclusion.

 

[Lord Jestocost]

But if the smallest parts do not exist objectively, what does it mean for the other ones, made from them, to exist obectively.

As a physicist I would say: At the end, it’s up to you (and thus up to your psychological predispositions) when you think of appearances as experiences of “existing objects” ("object" means that it can be given a description in its own right).

Here I am in line with Wheeler^* who states: "In today’s words Bohr’s point – and the central point of quantum theory – can be put into a single, simple sentence. 'No elementary phenomenon is a phenomenon until it is a registered (observed) phenomenon.'”

^* J. A. Wheeler in “Law without law” in "Quantum Theory and Measurement“ (edited by John Archibald Wheeler and Wojciech Hubert Zurek), Princeton, New Jersey 1983, pages 182-213

 

[PeterDonis]

i am not aware if iterpretations that say that the quantum systems do not exist

I actually overstated what I take to be Heisenberg's position. He did not, as I understand it, claim positively that quantum systems do not exist if they are not being measured. He only claimed that you could not say positively that they do exist if they are not being measured.

 

[PeterDonis]

The theory says that between measurements the system's state evolves according to the Schrodinger equation.

The theory says that the mathematical object, the state vector (or wave function), evolves according to the Schrodinger equation between measurements.

But the claim that the state vector describes an actually existing, individual quantum system is interpretation-dependent; not all interpretations make that claim. Some interpretations say it only describes our knowledge about the probabilities of possible measurement results; some interpretations say it describes statistical properties of an ensemble of systems that are prepared by the same preparation procedure. In either of these interpretations, no claim is necessarily made that individual quantum systems exist when they are not being measured. One can add such a claim to the interpretation, but one is not forced to.

 

[vanhees71]

Take an electron. If you know that there is an electron at one time and you also know the Hamiltonian you can precisely calculate the probability whether an electron does exist or not. If more than 1 (relevant) electron is involved in an experiment, of course you can't say whether an individual electron exists but only the number of electrons at any time. Also note that the state of one electron doesn't need to be a pure state, i.e., it doesn't need to be described by a state vector but by a statistical operator as part of a larger system. This is all interpretation independent!

 

[Morbert]

But if the smallest parts do not exist objectively, what does it mean for the other ones, made from them, to exist obectively.

There's an important distinction between assertions about properties of a system, and assertions about primitive objects/parts that make up a system. The latter category implies a primitive ontology (PO) approach, which is at odds with a neo-Copenhagen approach. I'm not aware of any PO approach that asserts primitive objects as subjective and the systems they make up as objective.

 

[martinbn]

There's an important distinction between assertions about properties of a system, and assertions about primitive objects/parts that make up a system. The latter category implies a primitive ontology (PO) approach, which is at odds with a neo-Copenhagen approach. I'm not aware of any PO approach that asserts primitive objects as subjective and the systems they make up as objective.

That is what i was trying to say too.