Question on the Finality of Bells Theorem.

[JohnSSM]

Hi Folks...its nice to be back...

Ive got some confusion and am looking for links or perspectives to help clear it all up.

If Bell's theorem is proving that there is no such thing as unknown local variables, does this imply or prove that quantum entanglement (non local variables) is at work in some systems? Or does it prove that truly random actions are coming from "nowhere"...an effect without a cause?

Ive read a lot about Bell's theorem, and I think I understand it, but the bottom line is never made clear in my eyes.

Does it require that some communication is breaking the speed of light? Or that some pairs of particles simply do not need local position to influence each other? In my confusion, it seems to imply that some paired particles no longer need the field of spacetime to interact...or at least one of their states, does not...

The point is, I don't get it...

IS there a bottom line of what Bell's theorem really states? Thanks, J

[imatfaal]

Bell's theorem states that no physical model with local hidden variables can reproduce all of the predictions of quantum mechanics.

 

The theorem, inequality and experimental observations do not prove one or other interpretation of Quantum mechanics - but rather shows that experiments can be made which give results that CANNOT be explained by a local hidden variable model; ie rather than "rule in" certain ideas and concepts it "rules out" the possiblity of purely local hidden variables.

 

To truly get a handle on the importance of Bell's Inequality and Theorem - and the experiments that showed he was (mostly) correct - you must first read up about Einstein Podolsky Rosen "Hidden Variables". It is also worth familiarising your self with the simple statistical inequality (search on DrChinese Bell's Inequality)

 

To be honest there are still tiny loopholes that experimenters and theorists have not yet managed to plug - but the elegance of the inequality (which is purely statistical and can be understood without any real knowledge of QM) means that even though the case is not entirely proven the majority is persuaded.

[Strange]

There was a very good video explaining this posted a while ago: http://www.scienceforums.net/topic/87347-why-hidden-variables-dont-work/

[swansont]

Or does it prove that truly random actions are coming from "nowhere"...an effect without a cause?

 

Not sure where this would be coming from. Entanglement is quite the opposite of random and acausal.

[Enthalpy]

In some sense... The entanglement is a relation hence a loss of randomness, but the (pair of) state observed is still random, and some experiments do tell that the (pair of) state is not defined before observation. It's not a hidden variable that is defined but escapes us.

[swansont]

In some sense... The entanglement is a relation hence a loss of randomness, but the (pair of) state observed is still random, and some experiments do tell that the (pair of) state is not defined before observation. It's not a hidden variable that is defined but escapes us.

 

The relationship of the second particle after the first is measured is decidedly un-random.

[JohnSSM]

There was a very good video explaining this posted a while ago: http://www.scienceforums.net/topic/87347-why-hidden-variables-dont-work/

Ya...wow...that is exactly what I needed...

 

Question: Without the idea of entanglement (faster than light communication) or the EPR (local hidden variables), what else is there? The answer in the video was not clear and it seemed to imply "why worry about this spin issue?"...If measuring one can automatically effect the other, what are the options besides entanglement (nonlocal hidden variables)?

 

Its EPR (disproved) or entanglement, or what other option?

 

 

Something else that caught my attention...He said that spin wasnt really spin, but angular momentum...Wouldnt angular momentum require movement, if not spin?

[Strange]

Something else that caught my attention...He said that spin wasnt really spin, but angular momentum...Wouldnt angular momentum require movement, if not spin?

 

It is an intrinsic property of particles, like mass or charge; it is not due to rotation as it is in large objects.

 

 

Without the idea of entanglement (faster than light communication)

 

Faster than light communication is one way of describing what happens, but not very satisfactory. I prefer to think of it as just another example of non-locality: the (detected) behaviour of particles is affected by things which are physically remote as well as in the past or future (see also things like the delayed choice quantum eraser experiment).

[JohnSSM]

 

It is an intrinsic property of particles, like mass or charge; it is not due to rotation as it is in large objects.

 

 

Faster than light communication is one way of describing what happens, but not very satisfactory. I prefer to think of it as just another example of non-locality: the (detected) behaviour of particles is affected by things which are physically remote as well as in the past or future (see also things like the delayed choice quantum eraser experiment).

What are other examples of non-locality? ghosts? ha...but seriously...

 

I suppose I enjoy harping on things without answers...but if non-locality is real, doesnt some or any form of communication become implied? The ole "how" question...

[Strange]

What are other examples of non-locality? ghosts? ha...but seriously...

 

I would recommend Feyman's lectures on QED (videos or book). He explains a lot of this very clearly. One simple example is that the probability of a photon being reflected from the front surface of a sheet of glass depends on the thickness of the glass. This tells us that the photon somehow "knows" how thick the glass is.

 

http://www.vega.org.uk/video/subseries/8

 

 

I suppose I enjoy harping on things without answers...but if non-locality is real, doesnt some or any form of communication become implied?

 

The trouble is that there are good reasons for thinking that transferring information faster than light is impossible (and, indeed, entanglement cannot be used for communication). And in the photon reflection example above, what is doing the communication. That doesn't really work as an explanation. The "how" is simply non-locality.

[imatfaal]

None of the Bell's Theorem Experiments can show a difference between any of the Interpretations of Quantum Mechanics - and I believe that they fit with Bohmian Mechanics / Pilot Wave Theory as well.

 

The implication of how QM works is a completely separate issue to the questions. These questions that we have answered is that QM is spectacularly accurate - it is tested to a level of astounding accuracy; and that QM cannot be explained by a purely local hidden variables (ie classical) theory.

 

I think it is best that this thread sticks to Bell's Inequality and Theorem - a general "so what's all this about" for QM is too wide a topic.

[JohnSSM]

 

I would recommend Feyman's lectures on QED (videos or book). He explains a lot of this very clearly. One simple example is that the probability of a photon being reflected from the front surface of a sheet of glass depends on the thickness of the glass. This tells us that the photon somehow "knows" how thick the glass is.

 

http://www.vega.org.uk/video/subseries/8

 

 

The trouble is that there are good reasons for thinking that transferring information faster than light is impossible (and, indeed, entanglement cannot be used for communication). And in the photon reflection example above, what is doing the communication. That doesn't really work as an explanation. The "how" is simply non-locality.

I get it...good answer...

 

Ya, I understood why it cant be used for communication from that video...before, i thought it could be...

 

Bell's theorem came up in a discussion i was having and I didnt really know much about it...this guy used Bell's theorem to prove that the universe is in chaos and not fully dictated by rules...im still not sure if it does that or not...

None of the Bell's Theorem Experiments can show a difference between any of the Interpretations of Quantum Mechanics - and I believe that they fit with Bohmian Mechanics / Pilot Wave Theory as well.

 

The implication of how QM works is a completely separate issue to the questions. These questions that we have answered is that QM is spectacularly accurate - it is tested to a level of astounding accuracy; and that QM cannot be explained by a purely local hidden variables (ie classical) theory.

 

I think it is best that this thread sticks to Bell's Inequality and Theorem - a general "so what's all this about" for QM is too wide a topic.

I think ive narrowed down the question to be...Does Bell's theorem either prove or disprove quantum entanglement? Does is either prove or disprove randomness?

[Strange]

.this guy used Bell's theorem to prove that the universe is in chaos and not fully dictated by rules...im still not sure if it does that or not...

 

Absolutely not. The fact that we can use mathematics to derive testable (and tested) results such as Bell's theorem pretty much proves that the universe is dictated by rules. Sounds like "this guy" doesn't know what he is talking about.

[imatfaal]

Bell's theorem came up in a discussion i was having and I didnt really know much about it...this guy used Bell's theorem to prove that the universe is in chaos and not fully dictated by rules...im still not sure if it does that or not...

 

He was wrong or you misinterpreted him. Bell's theorem is an attempt to refute EPR local hidden variables theory.

 

I think ive narrowed down the question to be...Does Bell's theorem either prove or disprove quantum entanglement? Does is either prove or disprove randomness?

 

See above posts - which bit of my posts and Strange's and others are not clear. Bell's work is a theoretical framework to show that EPR were incorrect - subsequent experiments seem to bear this out.

 

Theorems CAN NEVER EVER prove anything! Experimental evidence can be brought to the community that shows that the predictions of the mathematical model are closely aligned to the results of the lab - the more the better. QM is as well tested as anything. Entanglement is about as settled as it is going to get - but what entanglement actually IS we don't know. The model makes astonishingly good predictions.

 

Randomness is a harder question. QM is inherently probablistic and we also have the Uncertainty principle - this is not classical physics. But I would say it is not random either.

!

Moderator Note

 

I have split the offtopic question to its own thread. It is a good question and deserves space...

 

[swansont]

!

Moderator Note

Question about photon reflection has also been split.

http://www.scienceforums.net/topic/92584-from-where-does-the-photon-reflect-split-from-finality-of-bells-theorem/