Implications of violation of the Leggett–Garg inequality

[alphajoza]

Please consider the following premises and correct me if I'm wrong in anyone:

1. Based on the results of the experimental investigation of Bell's theorem and violation of the Bell's inequality, locality in tandem with reality is not applicable to quantum systems (no theory of local realistic hidden variables is possible);
2. Based on the results of the experimental investigation of violation of Leggett inequality (here and here), realism is not applicable to quantum systems (quantum realism: notion that physical systems possesses complete sets of definite values for various parameters prior to, and independent of, measurement);
3. Based on the results of the experimental investigation of violation of Leggett–Garg inequality (here), macroscopic realism is no longer acceptable.
   

My questions are:

1. I think that the violation of Bell's theorem is generally accepted in scientific community, what about the consensus on violation of Leggett inequality and Leggett–Garg inequality?
2. The result of violation of Leggett inequality (no quantum realism) is the endorsement of Copenhagen Interpretation (for this specific case)?
   

 

[alphajoza]

Emary, Lambert and Nori in an article called Leggett-Garg Inequalities stated that "Violation of a genuine Leggett-Garg test implies either the absence of a realistic description of the system or the impossibility of measuring the system without disturbing it".

I think that the implication of first premise (the absence of a realistic description of the system) is the violation of what we call Realism (macroscopic one) and what Einstein pointed out as: "I like to think the moon is there even if I am not looking at it". Is my understanding right?

What are the implications of the second premise (the impossibility of measuring the system without disturbing it)? (I know that it means like the observer-effect in QM, our act of measurement will have influences on the classical systems, but how much can this cause worry for foundation of science?)

 

[zonde]

I think that the implication of first premise (the absence of a realistic description of the system) is the violation of what we call Realism (macroscopic one) and what Einstein pointed out as: "I like to think the moon is there even if I am not looking at it". Is my understanding right?

I would say that from violation of Leggett inequality you can conclude that the question: "Does the moon have a certain spin along z-axis?" sometimes won't have certain answer. But from that statement to the statement that "the moon is not there when nobody is looking at it" is quite a leap and I see no justification for that leap.

What are the implications of the second premise (the impossibility of measuring the system without disturbing it)? (I know that it means like the observer-effect in QM, our act of measurement will have influences on the classical systems, but how much can this cause worry for foundation of science?)

You don't need Leggett inequality to conclude that. Just take unpolarized photon beam and three polarizers. Look at the beam through two polarizers that have their polarization axis orthogonal to each other. Observe that there is no light. Then insert third polarizer between the first two with polarization angle midway between the first two. Observe that there is some light passing through (1/8 of the initial intensity). Make conclusions.

 

[PeterDonis]

Moderator's note: I have moved a post here that was in a separate thread. @alphajoza please do not start two separate threads on the same topic.

 

[DrChinese]

Please consider the following premises and correct me if I'm wrong in anyone:

1. Based on the results of the experimental investigation of Bell's theorem and violation of the Bell's inequality, locality in tandem with reality is not applicable to quantum systems (no theory of local realistic hidden variables is possible);
2. Based on the results of the experimental investigation of violation of Leggett inequality (here and here), realism is not applicable to quantum systems (quantum realism: notion that physical systems possesses complete sets of definite values for various parameters prior to, and independent of, measurement);
3. Based on the results of the experimental investigation of violation of Leggett–Garg inequality (here), macroscopic realism is no longer acceptable.

My questions are:

1. I think that the violation of Bell's theorem is generally accepted in scientific community, what about the consensus on violation of Leggett inequality and Leggett–Garg inequality?
2. The result of violation of Leggett inequality (no quantum realism) is the endorsement of Copenhagen Interpretation (for this specific case)?

There are a number of "No-go" theorems that are pretty well accepted. Bell being the most well known and one of the easiest to follow. It is also one which lends itself to empirical testing. Others include Kochen–Specker, Leggett, Gleason, GHZ, PBR. All of these, in one way or another, point to rejection of the following:

Quantum Realism: the notion that [entangled] physical systems possesses complete sets of definite values for various parameters prior to, and independent of, measurement.

I would say that the above is almost universally rejected by physicists, regardless of their interpretation at this point. Even in Bohmian/DBB interpretations, it is difficult to assert that that entangled observables are independent of the nature of a measurement on the system. At the same time, I would say that most physicists also reject the following statement:

Quantum Locality: the notion that entangled physical systems of two particles consists of two independent particles.

So I personally agree with your main conclusion, that realism is not tenable. But the generally accepted view is that it is viable despite my comments above. So perhaps a bit of contradictory views, I don't think there is much practical impact on this.

 

[StevieTNZ]

The Leggett-Garg inequality is one I would like to see tested properly in my lifetime.

Some years back, an article appeared in Nature -- https://www.nature.com/articles/nphys1698
However, the experiment referred to did not take into account the clumsiness loophole.

 

[alphajoza]

There are a number of "No-go" theorems that are pretty well accepted. Bell being the most well known and one of the easiest to follow. It is also one which lends itself to empirical testing. Others include Kochen–Specker, Leggett, Gleason, GHZ, PBR. All of these, in one way or another, point to rejection of the following:

Quantum Realism: the notion that [entangled] physical systems possesses complete sets of definite values for various parameters prior to, and independent of, measurement.

I would say that the above is almost universally rejected by physicists, regardless of their interpretation at this point. Even in Bohmian/DBB interpretations, it is difficult to assert that that entangled observables are independent of the nature of a measurement on the system. At the same time, I would say that most physicists also reject the following statement:

Quantum Locality: the notion that entangled physical systems of two particles consists of two independent particles.

So I personally agree with your main conclusion, that realism is not tenable. But the generally accepted view is that it is viable despite my comments above. So perhaps a bit of contradictory views, I don't think there is much practical impact on this.

Thank a lot for your clarification; Just one remained question; Is the value definiteness of of Bohmian mechanics (postulation of an actual configuration that exists even when unobserved) in contrast with the result of Leggett inequality (NO Quantum Realism: the notion that [entangled] physical systems possesses complete sets of definite values for various parameters prior to, and independent of, measurement.)?

 

[DrChinese]

Thank a lot for your clarification; Just one remained question; Is the value definiteness of of Bohmian mechanics (postulation of an actual configuration that exists even when unobserved) in contrast with the result of Leggett inequality (NO Quantum Realism: the notion that [entangled] physical systems possesses complete sets of definite values for various parameters prior to, and independent of, measurement.)?

Our member Demystifier is better able to answer it that I. He is extremely well versed in the Bohmian position (excuse the intended pun). As best I understand from him (and I may have misunderstood him), the Bohmian view is in an observer dependent reality and is not consistent with classical realism - even though it is a hidden variable theory. After many discussions around definitions on this forum: I know that one person's words never quite stretch to meet another's. That gap leads to a lot back and forth. And don't be surprised if one physicist's use of the language is seemingly quite different than another's. They may actually agree (or vice versa).