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#40828 - 10/09/11 09:50 PM Uncertainty
Bill S. Offline
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Registered: 08/20/10
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Loc: Essex, UK

The cover story in New Scientist 30.04.11, seemed to suggest that uncertainty was on the way out. However, it did a U-turn before the end, in an uncertain sort of way.

The thing that puzzles me is this: Uncertainty says that complementary quantities, such as position and momentum cannot both be measured precisely. More of one; less of the other….

In the “Bob and Alice” experiment they cite, it seems that Bob can obtain a measurement of one quantity that may be more accurate than that obtained by Alice; however, he cannot obtain a measurement of the other quantity.

If my understanding of this is correct, I fail to see how this undermines uncertainty. Surely, uncertainty does not say you cannot get an accurate measurement of one quantity, only that if you do you won’t find out much about the other.
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#40838 - 10/10/11 04:04 AM Re: Uncertainty [Re: Bill S.]
Orac Offline
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Registered: 05/20/11
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The article is trying to bring together Uncertainty principle and Bell's inequality which are two corner stones of QM.

Bell's inequality in QM we usually use alice and bob representing position A & B in space.

Here is the technical background (http://en.wikipedia.org/wiki/Bell's_theorem)

However Sascha or David Schneider has a laymanized version which may be simpler
(http://www.science20.com/alpha_meme/disproving_local_realism-79216)
(http://www.drchinese.com/David/Bell_Theorem_Negative_Probabilities.htm)
(http://www.drchinese.com/David/Bell_Theorem_Easy_Math.htm)

The experiment is very simple to do with just a polarizer and two observers who have random orientation. They will never agree on the outcome.

Mathematically you can proove the weird result is true.

If you think it isn't you are welcome to try the Quantum Randi Challenge (http://www.science20.com/alpha_meme/official_quantum_randi_challenge-80168)


Initially Bell via QM was dealing with light but as you know we now have spins in atoms and those spins are subject to the same weird law (and we have tested it) so what it is telling you is no two observers will agree what is happening with an atom????

So welcome to the problem of reality posed by QM which comes down to this

Quote:

Classically measuring A and B, C existed even if we didn't measure it. QM via Bell's inequality says C never existed if you don't measure or observe it.


The moon featured in an ironic question about the result, Einstein once asked of physicist Abraham Pais: "Do you really believe that the moon only exists when you are looking at it?"

Does anything exist if noone is observing it? See we can prove there is no local reality.

So now lets bring in uncertainty suppose Alice measures a spin or QM characteristic she is "certain" about the result the question is what does this mean to Bob?

The article is trying to combine the two concepts and keep a local reality between Alice and Bob via uncertainty .... we so thrive for our common local reality even though you can proove its not real :-)

In case you are tempted to say QM spins are not real you may want to have a look at
http://www.physorg.com/news/2011-10-sideways.html

Thats transporting electrical energy without any electrons or charge movement ... the world really is weird :-)


Edited by Orac (10/10/11 04:24 AM)
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#40852 - 10/10/11 11:23 AM Re: Uncertainty [Re: Bill S.]
Bill S. Offline
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Registered: 08/20/10
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Loc: Essex, UK
Orac, thanks for that. It will be a while before I can follow up the links, but when I do I anticipate I will have more questions.
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#40896 - 10/11/11 06:36 PM Re: Uncertainty [Re: Bill S.]
Bill S. Offline
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Registered: 08/20/10
Posts: 3570
Loc: Essex, UK
Quote:
In case you are tempted to say QM spins are not real....


Certainly it is real, but is it "spin". Kallog would probably point out that you could call it "ugbugoo", and it would still be real. smile
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