Bell's Theorem and Counterfactual Definiteness

  • Thread starter Jolb
  • Start date
418
28
Bell's Theorem and "Counterfactual Definiteness"

Bell's Theorem and Aspect's experiments pretty much show that quantum theory is nonlocal--the violation of Bell's inequalities show that somehow the wavefunction is acting on the entangled particles faster than c.

However, I've heard that Bell's inequalities violate either locality or "counterfactual definiteness."

Now I'm very confused about what that means. Is that complete garbage or is there some possibility that quantum theory is local and we just need to abandon "counterfactual definiteness" instead? I thought quantum theory was already counterfactually non-definite, being as though you can't simultaneously measure complementary properties (like position and momentum) due to the Heisenburg uncertainty principle.

What does "counterfactual definiteness" mean in that context?
 
346
7
Re: Bell's Theorem and "Counterfactual Definiteness"

Bell's Theorem and Aspect's experiments pretty much show that quantum theory is nonlocal--the violation of Bell's inequalities show that somehow the wavefunction is acting on the entangled particles faster than c.
Actually, I thought Bell's theorem at best showed that a hidden variable theory which reproduced the results of QM would be non-local.

I was under the impression that QM, as it stands, is compatible with locality - though there are some difficult issues about whether all interpretations of QM (in particular those with wave function collapse) are indeed non-local
 

f95toli

Science Advisor
Gold Member
2,932
425
Re: Bell's Theorem and "Counterfactual Definiteness"

Bell's Theorem and Aspect's experiments pretty much show that quantum theory is nonlocal
No, it shows that theories that are both local and realistic can not be correct.

I am pretty sure that "counterfactual definiteness" refers to what is commonly knowns as "realism", realistic theories/interpretations basically assume that objects exist and have properties even when they are not measured.

Hence, a violation of Bell's inequality is still compatible with local theories, as long as you give up realism. Of course it is also compatible with non-local, non-realistic theories (which some claim are supported by experiments)
 
865
0
Re: Bell's Theorem and "Counterfactual Definiteness"

I am pretty sure that "counterfactual definiteness" refers to what is commonly knowns as "realism", realistic theories/interpretations basically assume that objects exist and have properties even when they are not measured.
Accepting the Wikipedia version of "counterfactual definiteness," this sounds correct. http://en.wikipedia.org/wiki/Counterfactual_definiteness:
In some interpretations of quantum mechanics, counterfactual definiteness (CFD) is the ability to speak meaningfully about the definiteness of the results of measurements, even if they were not performed.[1]
Compare this to the EPR criterion for reality, which is most likely where we get the physics/QM version of the term "realism" from:
If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of reality corresponding to that quantity.
The two are equivalent because taking an actual measurement would disturb the system. To meet the EPR criterion, you must be able to speak about what the result of a measurement definitely would be if it were taken - you need counterfactual definiteness. Of course, actual reality may not necessarily conform to Einstein's view of what realism is, so be careful not to read too much into the terminology. Everyone thinks that the job of physics is to describe reality, or at least to model it as best as possible, even those who deny "realism."
 
Last edited:

DrChinese

Science Advisor
Gold Member
7,190
1,008
Re: Bell's Theorem and "Counterfactual Definiteness"

Bell's Theorem and Aspect's experiments pretty much show that quantum theory is nonlocal--the violation of Bell's inequalities show that somehow the wavefunction is acting on the entangled particles faster than c.

However, I've heard that Bell's inequalities violate either locality or "counterfactual definiteness."

Now I'm very confused about what that means. Is that complete garbage or is there some possibility that quantum theory is local and we just need to abandon "counterfactual definiteness" instead? I thought quantum theory was already counterfactually non-definite, being as though you can't simultaneously measure complementary properties (like position and momentum) due to the Heisenburg uncertainty principle.

What does "counterfactual definiteness" mean in that context?
Alll of the above replies are correct, as is your viewpoint. The issue is the language more than anything. Some people don't like the term "realism" and prefer "counterfactual definiteness" instead. You could use the term "Is the moon there when no one is looking" and get the same effect (Einstein used this analogy). Any way you describe it, it has 3 characteristics: a) the specific observable is NOT measured; and b) it should have a specific value; and c) the probability of it having that value should be between 0 and 1.

A lot of people assume QM must be non-local because of Bell; however, as you point out, simply accepting reality as matching the HUP avoids having to discard locality. Most folks think these positions are a matter or personal choice.
 
453
0
Re: Bell's Theorem and "Counterfactual Definiteness"

Bell's Theorem and Aspect's experiments pretty much show that quantum theory is nonlocal--the violation of Bell's inequalities show that somehow the wavefunction is acting on the entangled particles faster than c.
No, they don't show this.

The locality condition (manifested as factorability of joint, entangled state representation) in Bell-type lhv formulations contradicts the statistical dependence between the separated data accumulations. However, this statistical dependence (which is a necessary byproduct of the pairing process) is entirely due to local transmissions/interactions vis, eg., coincidence circuitry.

The Bell locality condition is also incompatible with standard qm formulation (not factorable, nonseparable) of joint, entangled state.

But this doesn't make standard qm a nonlocal theory any more than violations of Bell-type inequalities indicate that Nature is nonlocal.
 

Related Threads for: Bell's Theorem and Counterfactual Definiteness

Replies
10
Views
2K
Replies
1
Views
3K
Replies
157
Views
10K
Replies
23
Views
2K
Replies
1
Views
526
Replies
42
Views
3K
Replies
2
Views
720
Replies
16
Views
1K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top