Bell's theorem: Local realism v. counterfactual determinism

In summary, the conversation discusses the implications of Bell's Theorem and its potential violation of local realism or counterfactual determinism. The question is raised whether there exists a theory that is consistent with Bell's inequality and the predictions of quantum mechanics while still being completely local, and if not, whether this proves nonlocality. It is suggested that views such as Copenhagen and QBism retain locality by giving up certain assumptions, and the concept of "measurement independence" is mentioned as a possible explanation for the masking of non-local signalling.
  • #1
zvi
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This is kind of an offshoot from:
https://www.physicsforums.com/showthread.php?t=369328

Assume for a second that the controversial experiments are valid and Bell's theorem is true of the universe.
I have often seen the philosophical analysis that if Bell's Theorem is true then either local realism OR counterfactual determinism must be violated.
E.g. http://en.wikipedia.org/wiki/Counterfactual_definiteness

But all the interpretations of quantum mechanics I know do involve some breech of locality - namely there is entanglement.

So (1) can we say that Bell's Theorem proves that the world is not local (i.e. that there is entanglement) and if not what is the counter example of a theory which is totally local and yet is consistent with Bell's inequality and with the predictions of QM?

(2) if locality is indeed violated does anyone know of a really clear lay explanation for how locality can be broken but in such a way that it does not allow non-local signalling.

Hope my question is clear!

ThanksPS I just saw an older related thread
https://www.physicsforums.com/showthread.php?t=361173

However I don't see a conclusion there. Does there/can there exist a theory compatible with the predictions of QM which is abandons realism but is completely local (no entanglement). I have not seen such a theory. And if not can we not say that Bell=>nonlocality? Thanks.
 
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  • #2
zvi said:
So (1) can we say that Bell's Theorem proves that the world is not local (i.e. that there is entanglement) and if not what is the counter example of a theory which is totally local and yet is consistent with Bell's inequality and with the predictions of QM?
Copenhagen and other non-representational views like QBism. They retain locality by giving up the idea that values of Quantum Observables are outcomes on a single sample space.

zvi said:
(2) if locality is indeed violated does anyone know of a really clear lay explanation for how locality can be broken but in such a way that it does not allow non-local signalling.
Typically our ignorance of the precise conditions of the particle masks any possible signalling. I have heard that the non-local nature of Bohmian Mechanics actually produces the limit of experimental precision needed for this masking. @Demystifier can undoubtedly say more.
 
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  • #3
DarMM said:
Copenhagen and other non-representational views like QBism. They retain locality by giving up the idea that values of Quantum Observables are outcomes on a single sample space.
This is equivalent to abandoning one of the Bell inequality hypothesis, i.e., "Measurement independence"
$$p(\lambda|a,b)=p(\lambda)$$
Copenhagen does not necessarily mean that we have to abandon this "no conspiracy" assumption.
 

What is Bell's theorem?

Bell's theorem is a foundational principle in quantum mechanics that demonstrates the fundamental incompatibility between local realism and counterfactual determinism. It states that certain experiments involving entangled particles cannot be explained by both local realism and counterfactual determinism at the same time.

What is local realism?

Local realism is the idea that the properties of physical systems are predetermined and exist independently of observation. It also assumes that there is a finite speed at which information can be transmitted, meaning that events happening in one location cannot influence events in another location instantaneously.

What is counterfactual determinism?

Counterfactual determinism is the belief that the properties of physical systems are predetermined and can be known without actually measuring or observing them. This implies that the outcome of an experiment can be determined without actually performing the experiment.

What is the significance of Bell's theorem?

Bell's theorem has profound implications for our understanding of the physical world. It challenges the classical view of the universe as a deterministic and local reality, and instead suggests that there may be non-local and indeterminate aspects at play in the quantum realm.

What are some real-world applications of Bell's theorem?

Bell's theorem has led to the development of technologies such as quantum cryptography and quantum computing, which rely on the principles of quantum mechanics. It also continues to be a subject of study and debate in the fields of physics and philosophy, as it has implications for our understanding of the nature of reality and the limits of human knowledge.

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