Bell's Theorem, non-locality

CAF123

Hi.
I have read about Bell's Theorem in a couple of popular science books, namely 'Quantum Reality' by Nick Herbert and the recent book by John Gribbin.
However, I am still struggling to understand the overall idea.

Could anyone explain the concept? Is it taught at university at all?

Many thanks

harrylin

A good introduction was given by Bell in a presentation at CERN:
cdsweb.cern.ch/record/142461/files/198009299.pdf

CAF123

Ok, thanks.
What I understand now is that after the experiments conducted by Clauser and Aspect, it seems that there is experimental verification of the violation of Bells inequality, and that prior to what Bell thought, nature is non local which implies faster thn light communication.

However, in recent times statistical analysis on the subject showed that there are flaws in the experiments conducted and that the question of whether nature is local/non local is still an open question.

Is the above all correct?
If so, I have one question: from special relativity, nothing can exceed the speed of light, so how does this fit in with what was thought of as a non local reality? I.e interactions under non local reality imply that interactions can act instantaneously (faster than c). Does this not contradict relativity?

phinds

The bold part is incorrect.

In the case of entanglement, FTL interactions do NOT imply FTL communication since no information is transmitted.

CAF123

Ah ok, thanks.
What about my question at the bottom of my previous post?

bohm2

Yes, quantum entaglement appears to be instantaneous but the connection is as some have described it like an "invisible wire" (since it's private) between entangled particles. Thus, the entangled particles appear to pass "hidden signals" between them. And as long as these "signals" are hidden/private, no contradiction with relativity occurs.

CAF123

So does that mean it is possible for 'something' to travel faster than light?

phinds

No. It's as though you flipped a coin but didn't look at the result, then sliced it through between the faces and put half in one box at home and half in another box that you took with you then drove 100 miles and open the box and DAMN ... it's HEADS !!! Oh my gosh ... I'm 100 miles away from the other half but I KNOW instantaneously that it's tails. Well, so what. There was no information transferred.

CAF123

Thanks, that analogy helped a lot.

harrylin

It implies for example (there are even weirder interpretations!) faster than light influence.
Relativity is (with narrow escape!) not contradicted because it is just like QM a theory about observations; the inferred faster than light influences cannot be observed. That something may be going faster than light is only interpretation of what might be going on "out of our sight", it does not imply the detection of anything going faster than light. If faster than light signalling were possible, then that would break the PoR.

And indeed the discussions and debates are not finished. You can also also find ongoing discussions on this forums. For example:
http://physicsforums.com/showthread.php?t=369286
http://physicsforums.com/showthread.php?t=589134
http://physicsforums.com/showthread.php?t=590249
http://physicsforums.com/showthread.php?t=597171

phinds

Actually, for the analogy to be a bit more correct, you have to think of it as your having traveled a couple of light years. THEN it becomes apparent that when you call home and tell them that the coin in the box is tails, THAT is communication and does travel at or less than c and conversely if they want to open their box and tell you that yours is heads, they can do that and you'll find out a couple of years after they send the communication.

So again, the point is that interactions apparently can happen FTL via entanglement, but communications cannot.

harrylin

According to Bell's theorem that analogy is wrong (nothing "spooky"). Still, it's perhaps a good illustration! So I'll try to modify it to better reflect Bell's theorem:

It's as though you sliced an Aztek coin in two halves (each with half a head on one side), and you put one half in one box and the other half in another box. Next you thoroughly shake both boxes and open the one box .. it's HEADS. Oh my gosh ... the two halves are physically separated but according to Aztek Mechanics you know with 75% certainty that when you open the other box it will be tails!

Bell's theorem, in non-technical but perhaps clearer terms, is that only somewhat "spooky" models can match the predictions of QM.

CAF123

Why is it only a 75 percent certainty that the other box contains tails?

DrChinese

No, it is consensus that there are no flaws and there is no doubt about the experiments themselves. However, there are some theoretical "loopholes" in the entire process. One is the assumption that the sample detected represents the universe as a whole. Another is that Alice's setting does not affect Bob's result, and vice versa. These assumptions have been individually tested and found to be solid.

But in a somewhat unusual* twist, it has been decided that both of these should be tested simultaneously in a single experiment. That has not been done to date. I believe this is what you are referring to in your post. It is believed that such an experiment will be feasible in the future, and no unusual result is expected. It would definitely require radically new physics if the unexpected occurred.

*Unusual because no other scientific theory has been held to this standard that I am aware of.

harrylin

That's just an illustration, corresponding to the fact that the weird aspect of Bell's theorem is about surprising, rather magical looking statistics - if I chose my illustration right, then similarly no "normal" theory can explain that percentage. However, somehow (and regretfully) you must have misunderstood my illustration: normal theory has that the chance on tails after thorough shaking is 50%, independent of your observation of the half coin in the other box.

The clearest discussion of the statistics according to QM is perhaps "Herbert's proof", which is discussed in the second link of my post #10.

harrylin

Apparently you mean with "consensus" the simple majority (according to the dictionary, you may) which probably thinks as you claim.

For doubts that are nevertheless discussed in the scientific literature, see the recent and ongoing discussions on this forum as well as the physics FAQ:
http://math.ucr.edu/home/baez/physic...nequality.html

DrChinese

Yes. I don't count the Bell deniers, which are a small minority. There is little in which there is unanimous agreement.