What sort of an experiment can refute QM or QFTs?

[MathematicalPhysicist]

When pondering about this, I think the falsification paradigm is a bit blunt and uselss. As long as one considers QFT as effective theories, I don't see an obvious way how to falsify it as you can probably always tweak the parameters, enlarge the state spaces etc. But if one tries to ask, how the effective theories FORM, one may be required to step outside the QFT paradigms. In this sense, I doubt there will be a single experiment that refutes QFT, I think it will be if an alternative paradigm demonstrates it's superioirty in solving some open questions. Superior maybe in the sense or computation complexity, compact representation etc. Ie. I think it's not sufficient for a theory to be consistent, it must also be solvable or computable by it's host agent - otherwise it is useless. It's in these sense I think the current paradigms may need rethinking. Ie. it is not enough that something is solvable in principle, if it is not solvable by the resources at hand, then what is it's value, and rational from a naturalness perspective?

/Fredrik

Well, I think we need both notions.
The ultimate notion of solvability, even if it's not feasible. and the more mundane doable testable theories.

 

[Fra]

Well, I think we need both notions.
The ultimate notion of solvability, even if it's not feasible. and the more mundane doable testable theories.

What I was thinking of was to put a stronger focus of fitness of a theory. I think the modern view on QM as a kind of information theory; where the expectations reflects the information we have about the system, is merely a first step towards a bigger revolution where both the encoded information and also the information processing is consider in it's physical processes, and this is then something that has to be modeled on the boundary, or the "observer side" of things. Here I think the often reductionist minded views on information theory may fail, and with it the fundamental QFT paradigms. I think we still have mote steps to take here, and I can perhaps imagine that the "trouble with QFT" as it stands, is that it will lead to problems such as unreaonable fine tuning, and beeing meaningfull only in a context of unlimited information capacity. I consider this a serious sign as suboptimal and not very fit.

/Fredrik

 

[Son Goku]

Run a loophole free CHSH test and see a violation of the Tsirelson bound. All quantum theories obey that bound.
Very hard to imagine Tsirelson exceeding frameworks being true given the rest of physics, but that's one example.

 

[zdcyclops]

Fair point but since I have not seen a consensus or a peer-reviewed resolution to these Schrodinger's cat questions about what exists between observations/interactions, Phinds and MathematicalPhysicist should probably not be asking them in this particular forum.

Objects exist even when they are not " looked at. " In quantum systems the only way to get information about quantum particles is to make a measurement or to look at them.

 

[vanhees71]

That's even true within classical physics. I never understood, why "the moon" shouldn't be there if nobody looks at it within quantum mechanics. There are fundamental conservation laws that tell us that the moon must be there when nobody looks, given the fact that it was there, when somebody has looked before.

 

[martinbn]

That's even true within classical physics. I never understood, why "the moon" shouldn't be there if nobody looks at it within quantum mechanics. There are fundamental conservation laws that tell us that the moon must be there when nobody looks, given the fact that it was there, when somebody has looked before.

If you prepare a spin one half system in the state |up>+|down> along a given axis, without measuring it what is the spin of the system along that axis?

 

[vanhees71]

It's indetermined. All the preparation implies are the probabilities to find the possible values of any measured observable.

 

[martinbn]

It's indetermined. All the preparation implies are the probabilities to find the possible values of any measured observable.

Then why do you have a problem with the same statement about the moon?! If the moon is not in an eigenstate of the position observables, then it is not there, it is not here, nor over there. You cannot say that it is in any specific location.

 

[CoolMint]

Objects exist even when they are not " looked at. " In quantum systems the only way to get information about quantum particles is to make a measurement or to look at them.

A quick summary of the key points:

Quantum theory is well established and grounded in experimental facts
It is a theory of measurements.
Who/what/when does the measurements is far less established and far less clear. Lots of assumptions and theories attempt to explain this weird situation with varying successes.

 

[vanhees71]

Then why do you have a problem with the same statement about the moon?! If the moon is not in an eigenstate of the position observables, then it is not there, it is not here, nor over there. You cannot say that it is in any specific location.

Then nothing would be ever anywhere, because the position eigenstates are not square integrable. It's as with all observables: Given a state (a "preparation") there are only probabilities for the outcome of measurements of any observable, which holds also for position. Position is never determined although the probability distribution for position can be arbitrarily sharply peaked around some value. Whether or not you can resolve the corresponding fluctuations depends on the resolution of your detector.

 

[martinbn]

Then nothing would be ever anywhere, because the position eigenstates are not square integrable. It's as with all observables: Given a state (a "preparation") there are only probabilities for the outcome of measurements of any observable, which holds also for position. Position is never determined although the probability distribution for position can be arbitrarily sharply peaked around some value. Whether or not you can resolve the corresponding fluctuations depends on the resolution of your detector.

Yes, in contrast to classical physics, where every single thing is there at any given time. Quantum mechanics is different, the moon is not there if you don't measure that observable. Einstein did have a problem with that, but you do you?

 

[Paul Colby]

Yes, in contrast to classical physics, where every single thing is there at any given time. Quantum mechanics is different, the moon is not there if you don't measure that observable. Einstein did have a problem with that, but you do you?

I didn't know the moon not there was an allowed transition?

 

[vanhees71]

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.

 

[martinbn]

As I said, I don't understand the conclusion that something is "not there", only because its position vector has no determined value.

No, it is not a conclusion, it is the meaning of "not being there". Shirley when Einstein says "God doesn't play dice." you are not thinking of a deity and actual dice.

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.

It is an example of bad use of expressions that can be (actually always are) misleading to anyone who reads only popular science. The one above is not so bad and was just part of the way Einstein phrased his thoughts.

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.

I suppose Einstein never accepted that.

 

[vanhees71]

Again, why shouldn't the moon not be there only because its position is more or less well known to me?

Of course Einstein was far from being as naive as these bon mots being quoted all the time without also quoting them in the context with his very clear and detailed criticism against the QT and the implications concerning "irreducible randomness". This also leads to the wrong picture, also sometimes errorneously claimed in the pop-sci literature, that Einstein hadn't understood QT.

 

[martinbn]

Again, why shouldn't the moon not be there only because its position is more or less well known to me?

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?

Of course Einstein was far from being as naive as these bon mots being quoted all the time without also quoting them in the context with his very clear and detailed criticism against the QT and the implications concerning "irreducible randomness". This also leads to the wrong picture, also sometimes errorneously claimed in the pop-sci literature, that Einstein hadn't understood QT.

 

[Demystifier]

That's even true within classical physics. I never understood, why "the moon" shouldn't be there if nobody looks at it within quantum mechanics. There are fundamental conservation laws that tell us that the moon must be there when nobody looks, given the fact that it was there, when somebody has looked before.

Because the Moon has only a few conserved quantities: energy, momentum, angular momentum and charge. Many other objects, which are not Moon, can have the same values of these conserved quantities. Why is Moon not transformed into one of those other objects?

 

[martinbn]

Again, why shouldn't the moon not be there only because its position is more or less well known to me?

Let me try this way. Suppose we have two disjoint regions of space, suppose $\psi$ and $\psi'$ are the wave functions that describe a particle being in each region (ignore mathematical subtleties). Suppose now that the particle is in a superposition of those two states. Where is the particle? Can you say there and point to a point in space, or just one of the two regions? If not, which I assume is your answer, then why is it hard for you to say that the particle isn't there?

 

[PeterDonis]

If the moon is not in an eigenstate of the position observables, then it is not there

I have always understood the claim that the moon is "not there" when nobody looks to mean, not that the moon has no specific position, but that it doesn't "exist" when nobody looks. That is the claim I take @vanhees71 and others to be arguing against.

 

[martinbn]

I have always understood the claim that the moon is "not there" when nobody looks to mean, not that the moon has no specific position, but that it doesn't "exist" when nobody looks. That is the claim I take @vanhees71 and others to be arguing against.

It is possible that it means exactlky that. Is there any way to know the context and what exactly Einstein meant? More importantly is there any reference that supports that QM states that?

 

[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.