B What sort of an experiment can refute QM or QFTs?

[PeterDonis]

Quantum mechanics is different, the moon is not there if you don't measure that observable.

With regard to position, the point @vanhees71 was making is that its eigenstates are not square integrable, so they are not physically realizable--nothing is ever in a position eigenstate. By your argument that would mean nothing is ever "there", including things like detector pointers that we supposedly use to read off the results of measurements. Which would seem to imply that we can never obtain a measurement result for anything.

 

[PeterDonis]

Is there any way to know the context and what exactly Einstein meant?

I haven't been able to find anything useful online. I think Pais' biography of Einstein includes a discussion of the back and forth with Bohr that, AFAIK, included Einstein making that remark about the moon, but I don't have a copy of that book so I can't check.

More importantly is there any reference that supports that QM states that?

That supports that QM states that the moon isn't there if nobody is looking at it? If by "QM" we mean just the basic math of QM, without adopting any particular interpretation, I don't see how, since the claim that the moon isn't there if nobody is looking at it, where "there" means something like "exists", is an interpretation.

Of course your statement about a quantum system that is not in an eigenstate of an observable not having a definite value for that observable is true, and true of basic QM independent of any interpretation. (Actually, strictly speaking that's not quite true because there are interpretations, such as the thermal interpretation of @A. Neumaier, that do not treat eigenstates the same way.) But its implications once we recognize that the eigenstates of the position observable are not physically realizable are also somewhat unsettling (see my post #91).

 

[physika]

Why is Moon not transformed

Yes, it retains its identity through nothing.
appears and disappears and is the same.
...lol...

 

[PeterDonis]

Why is Moon not transformed into one of those other objects?

Because the Moon's quantum degrees of freedom are different from the quantum degrees of freedom of those other objects.

 

[Demystifier]

Because the Moon's quantum degrees of freedom are different from the quantum degrees of freedom of those other objects.

What do you mean by different? They are all made of field degrees of freedom of the Standard Model.

 

[PeterDonis]

What do you mean by different?

In a different part of configuration space. Or, if you like, different dimensions in Hilbert space.

 

[Demystifier]

In a different part of configuration space. Or, if you like, different dimensions in Hilbert space.

OK, but there are no conservation laws that prevent transition from one part of the configuration space to another. Hence it doesn't help to answer the question as formulated by @vanhees71.

 

[vanhees71]

If it is there, where is that there? What are the coordinates? You agreed that if you haven't measured them those observables do not have values. Just like the spin one half system, if it is in the state |up>+|down> what is the value of the spin along that axis?

I've said this repeatedly: The "coordinates" are indetermined. You find the particle with some probability (distribution) at each position. It's not different for the spin component in the z-direction if you prepare it not in an eigenstate of the corresponding self-adjoint operator: It's value is indetermined and the probability to find the one or the other possible value is given by Born's rule.

 

[martinbn]

I've said this repeatedly: The "coordinates" are indetermined. You find the particle with some probability (distribution) at each position. It's not different for the spin component in the z-direction if you prepare it not in an eigenstate of the corresponding self-adjoint operator: It's value is indetermined and the probability to find the one or the other possible value is given by Born's rule.

Then you agree that there is not "there" that refers to the location of the particle. Therefore the particle isn't "there".

 

[martinbn]

With regard to position, the point @vanhees71 was making is that its eigenstates are not square integrable, so they are not physically realizable--nothing is ever in a position eigenstate. By your argument that would mean nothing is ever "there", including things like detector pointers that we supposedly use to read off the results of measurements. Which would seem to imply that we can never obtain a measurement result for anything.

Yes, I understand that, but it is not important for the discussion. It definitely wasn't not what Einstein had troubles with.

I haven't been able to find anything useful online. I think Pais' biography of Einstein includes a discussion of the back and forth with Bohr that, AFAIK, included Einstein making that remark about the moon, but I don't have a copy of that book so I can't check.

I have the book, I will try to check. But it is up to people that use the quote to explain what they mean by it in the context they are using it.

That supports that QM states that the moon isn't there if nobody is looking at it? If by "QM" we mean just the basic math of QM, without adopting any particular interpretation, I don't see how, since the claim that the moon isn't there if nobody is looking at it, where "there" means something like "exists", is an interpretation.

I suppose one has to use the interpretation that Einstein used, or specify which interpretation one uses.

Of course your statement about a quantum system that is not in an eigenstate of an observable not having a definite value for that observable is true, and true of basic QM independent of any interpretation. (Actually, strictly speaking that's not quite true because there are interpretations, such as the thermal interpretation of @A. Neumaier, that do not treat eigenstates the same way.) But its implications once we recognize that the eigenstates of the position observable are not physically realizable are also somewhat unsettling (see my post #91).

 

[CoolMint]

As I said, I don't understand the conclusion that something is "not there", only because its position vector has no determined value. I also don't understand, why it is sometimes claimed (usually in popular-science writing) that a particle can be "at two places at ones", only because it's somehow prepared in a state, where its wave function peaks around two (or more) different regions. As soon as you accept that there is "irreducible randomness" in nature and that QT describes right that, such obviously meaningless paradoxa vanish into nothing.

Then you agree that there is not "there" that refers to the location of the particle. Therefore the particle isn't "there".

The disagreement between the two of you seem to come down to whether quantum fields are real and thus the Moon has some existence between measurements based on the resolution of the argument whether the quantum fields are real or not.
This debate cannot be settled but if logical arguments need to be put forth, it's easier to defend the position that they are not real. And are not "there".

 

[martinbn]

The disagreement between the two of you seem to come down to whether quantum fields are real and thus the Moon has some existence between measurements based on the resolution of the argument whether the quantum fields are real or not.
This debate cannot be settled but if logical arguments need to be put forth, it's easier to defend the position that they are not real. And are not "there".

No, this is not the argument. The disagreement comes from the fact that @vanhees71 is not willing to accept that someone might use the phrase "is not there" to mean "is not in a position eigenstate".

What do you mean by "not real" when it comes to the fields?

 

[CoolMint]

No, this is not the argument. The disagreement comes from the fact that @vanhees71 is not willing to accept that someone might use the phrase "is not there" to mean "is not in a position eigenstate".

What do you mean by "not real" when it comes to the fields?

That the respective quantum field has no physical existence between measurements.

 

[martinbn]

That the respective quantum field has no physical existence between measurements.

Do you mean that the field disappears, it ceases to exist, then it appears again? I am quite confident that neither of us thinks this, nor is talking about it.

 

[vanhees71]

Then you agree that there is not "there" that refers to the location of the particle. Therefore the particle isn't "there".

But the particle is "there", because the total probability sums to 1. It's only indetermined, where this "there" is.

 

[vanhees71]

Do you mean that the field disappears, it ceases to exist, then it appears again? I am quite confident that neither of us thinks this, nor is talking about it.

Then, why do you insist that the particle weren't "there", only because its position is indetermined?

 

[physika]

That the respective quantum field has no physical existence between measurements.

Then, the field does not exists.
Begin to exists because "something" exists before it and that "something" measures it ?

 

[martinbn]

Then, why do you insist that the particle weren't "there", only because its position is indetermined?

Because that is what I mean by "being there" that the "position is determined". Perhaps we should stop. It is starting to resemble a discussion about what a reference frame is. If you cannot accept a different use of the phrase "the particle is there" other than yours then there is no point in repeating ourselves.

 

[martinbn]

But the particle is "there", because the total probability sums to 1. It's only indetermined, where this "there" is.

It "is there" if by that you mean that it exists in space-time. It "is not there" if by that you mean that it is not in a position eigenstate. I really don't understand why this is so hard for you the understand. In any case I cannot explain myself any better. May be it is my English, but I've tried everything I could.

 

[vanhees71]

There are no position eigenstates! That implies that nothing is anywhere, according to your definition of "being there". That doesn't make sense, does it? For "being there" it's obviously sufficient for us to be able to localize the particle within some volume. This can be done with any accuracy you are able to technically achieve, but there'll be always a finite resolution of the position.

I also cannot explain myself better. The problem to accept quantum theory for us is that we have to give up determinism, which has been a paradigm for more than 300 years before the advent of quantum theory.

 

[physika]

I also cannot explain myself better. The problem to accept quantum theory for us is that we have to give up DETERMINISM, which has been a paradigm for more than 300 years before the advent of quantum theory.

Give up Cause-Effect ?
Nothing have an antecedent ?

 

[PeterDonis]

The disagreement comes from the fact that @vanhees71 is not willing to accept that someone might use the phrase "is not there" to mean "is not in a position eigenstate".

Since nothing can ever be in a position eigenstate, because those states are not physically realizable, this meaning of "not there" does not seem to be useful, since everything is always "not there" by this criterion.

More to the point of the specific Einstein quote under discussion, everything would be "not there" even when somebody looks. So that can't be what Einstein meant.

 

[PeterDonis]

there are no conservation laws that prevent transition from one part of the configuration space to another.

Yes, there are. For example, if the Moon just vanished from its orbit around the Earth and reappeared in orbit around, say, Venus, that would violate local conservation of stress-energy.

 

[CoolMint]

Give up Cause-Effect ?
Nothing have an antecedent ?

Bell's theorem says that the world cannot be both local and realistic at the same time which seem to be required for the form of causality you require.

'A classical notion of a causal structure is therefore untenable in any framework compatible with the basic principles of quantum mechanics and classical general relativity.'

https://www.nature.com/articles/s41467-019-11579-x

 

[vanhees71]

Give up Cause-Effect ?
Nothing have an antecedent ?

No, it has nothing to do with causality. The state of the system is completely determined by the unitary time evolution given the initial condition ("the preparation").

The point is that the determination of the state doesn't imply that all observables take determined values. The preparation of the state only determines the probabilities for the outcome of measurements. Thus in QT causality is fulfilled (as it should if physics should make any sense at all, because if Nature wouldn't behave according to causal laws, we couldn't discover these laws of course), but it's probabilistic, i.e., indeterministic.

For a very thorough discussion of this, see the introductory chapter of

J. Schwinger, Quantum Mechanics, Symbolism of Atomic
Measurements, Springer, Berlin, Heidelberg, New York (2001).

 

[Demystifier]

Yes, there are. For example, if the Moon just vanished from its orbit around the Earth and reappeared in orbit around, say, Venus, that would violate local conservation of stress-energy.

I think that @vanhees71 only had global conservation laws in mind.

If one insists on local conservation laws, not only in average in large ensembles but also in individual systems, it seems to me that this makes random quantum changes impossible.

 

[PeterDonis]

If one insists on local conservation laws, not only in average in large ensembles but also in individual systems, it seems to me that this makes random quantum changes impossible.

How so?

 

[Lord Jestocost]

Is there any way to know the context and what exactly Einstein meant?

Abraham Pais in “Einstein and the quantum theory” (Reviews of Modern Physics, Vol. 51, No. 4, 1979):

“We often discussed his notions on objective reality. I recall that during one walk Einstein suddenly stopped, turned to me and asked whether I really believed that the moon exists only when I look at it. The rest of this walk was devoted to a discussion of what a physicist should mean by the term ‘to exist’." [Bold by LJ]

 

[PeterDonis]

Abraham Pais in “Einstein and the quantum theory” (Reviews of Modern Physics, Vol. 51, No. 4, 1979):

Who was Einstein talking to in that excerpt?

 

[Lord Jestocost]

Abraham Pais