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

[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

 

[physika]

The problem to accept quantum theory for us is that we have to give up determinism.

But Bohmian Mechanics is deterministic.

 

[gentzen]

I also cannot explain myself better. The problem to accept quantum theory for us is that we have to give up determinism

But Bohmian Mechanics is deterministic.

The determinism of Bohmian Mechanics is mostly in the form of a mathematical model. It is a nice and simple mathematical model, allowing realism even for small closed models. However, those small closed models are still not a good approximation of the small open systems actually relevant in most cases. And if you want to have appropriate randomness over larger time scales, you need to go to bigger and bigger models (not as far as the wavefunction of the universe like MWI, but still not bounded).

On the other hand, for short time scales, Bohmian Mechanics shows that deterministic like behavior is not in contradiction to QM, and that actual discontinuous quantum jumps are not required. (Here I am talking in a mathematical "conservation of difficulty" kind of way.) Which makes sense, because it would be strange if randomness could be conjured up out of thin air exactly in the moment a measurement takes place. Even more so, because it is not even clear what constitutes a measurement, and in which exact moment it takes place.

 

[CoolMint]

The randomness hypothesis is way overused these days. Students probably get the idea that randomness can creep up into the classical realm.
If the 'randomness' of qm was real, it would likely have led to a non-classical world.
I think even the HUP suggests that we are dealing with the unpredictability of certain qualities, rather than 'randomness'.
A limit to what can be known at tiny scales.

 

[PeterDonis]

Thread closed for moderation.

 

[PeterDonis]

After Mentor review, the thread will remain closed.