1591: Bell's Theorem

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Bell's Theorem
The no-communication theorem states that no communication about the no-communication theorem can clear up the misunderstanding quickly enough to allow faster-than-light signaling.
Title text: The no-communication theorem states that no communication about the no-communication theorem can clear up the misunderstanding quickly enough to allow faster-than-light signaling.

Explanation

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If you can address this issue, please edit the page! Thanks.

In quantum mechanics, entanglement is a situation where measurements between two or more particles that are not yet determined are nonetheless correlated. For example, two electrons can be in a state where, considered individually, each can be in a superimposed spin-up/spin-down state, but nevertheless correlated so that if one electron is found to be in the spin-up state, the other must be in the spin-down state and vice-versa. Since the entangled particles may be at significant distance from each other, such correlations make it seem like the particles have some method of communicating faster than light to ensure these correlations.

Bell's Theorem states "No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics." and is the basis for arguing faster than light mechanisms in Quantum Mechanics, if entanglement is not due to a hidden variable, then somehow a super-light speed mechanism ensures the entanglement. Entanglement can't be used for faster than lightspeed communication though, so Bell's theorem does not prove faster than lightspeed communication is possible.


Ponytail begins reading Bell's theorem to Cueball, who is standing 5 meters away. Cueball responds with a misunderstanding of Bell's Theorem in 1 nanosecond. The speed of light in a vacuum is 299,792,458 meters per second. In one nanosecond, the light from Ponytail would only have traveled 0.299 meters, thus Cueball misunderstands Bell's Theorem faster than the light from Ponytail reading the Theorem can reach him, which implies that faster-than-light communication occurred to set up the misunderstanding.

The title text, with 4 negatives, tries to be as confusing as possible.

Transcript

[First frame captioned: t = 0 nanoseconds]
Ponytail, holding a piece of paper and facing to the right: This is called Bell's Theorem. It was first–

[A double-headed arrow links the characters in the two frames. The arrow is labelled "5 meters".]

[Second frame captioned: t = 1 nanosecond]
Cueball, facing to the left towards Ponytail: Wow, faster-than-light communication is possible!

Caption: Bell's Second Theorem: Misunderstandings of Bell's Theorem happen so fast that they violate locality.


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Discussion

I'm sure some people here have this memorised, but light travels just under 30 centimetres in a nanosecond. For our Metric-ally challenged friends, that's about one foot – so 5 metres takes around 16.67 nanoseconds. I leave the comic explanation to smarter people than me. Paddles (talk) 13:02, 16 October 2015 (UTC)

I have seen Admiral Grace Hopper demonstrate this with approximately foot-long lengths of wire representing "light-nanoseconds". It's accurate to one part in 50 (although not as accurate as the one-part-in-1000 "30 centimeters" measurement). PsyMar (talk) 20:33, 16 October 2015 (UTC)
The problem with that nifty rule-of-thumb is that it is technically correct, but practically useless. The 30cm/ns is for light in a vacuum. For an electrical signal in a wire (or light in a fibre, for that matter) the effective speed is roughly 20cm/ns. -- Popup (talk) (please sign your comments with ~~~~)

The comic only shows that the two characters are 5m apart at chest level. What if there was a miniature wormhole or distortion in time in a separate area, making this seemingly "FTL" communication scientifically possible? Forrest (talk)14:19, 16 October 2015 (UTC)

For an explanation of Bell's theorem in the words of the man himself, and targeted at an educated lay audience, this is essential reading: https://cds.cern.ch/record/142461/files/198009299.pdf 162.158.35.36 16:22, 16 October 2015 (UTC) : Tim B posting as Anon

Wow, the explanation needs some explaining. Can the first part about quantum mechanics be simplified, moved, or have something clearer put in front of it? I don't feel up to the task, but the section is not very helpful. -DanB (talk) 17:32, 16 October 2015 (UTC)

Yeah, the explanation isn't actually an explanation at all. Can someone who understands Bell's Theorem write an explanation for the joke in the comic? The current explanation appears to be a non sequitorial digression. I'm really curious as to what the actual joke is about. 108.162.249.155 04:20, 9 March 2016 (UTC)

In the widely separated electrons section, isn't it necessary that the two electrons measured be from the same source? If so, the explanation could use that small edit, but I'm not sure I'm remembering right. Miamiclay (talk) 05:35, 17 October 2015 (UTC)

Yes.

I think this whole explanation is suffering from "Bell's second theorem".

Can anyone cite an experiment or proof that *altering* the state of one half of an entangled electron pair *after* they have been separated to a significant distance has any effect upon the other half? So far as I have learned, the two electrons in question are driven to opposite states by close proximity: When separated, they maintain cyclical synchrony until the state of one electron is measured. Environmentally induced state changes have not been shown to propagate between entangled particles after they are separated; They simply retain oppositional synchrony until disentangled by observation (or other interference). Any information derived was imparted at the point of entanglement, or during transit, or by measurement. Introducing new information (state change) to one half of an entangled pair after separation interrupts the synchronous effect, disrupting the entanglement. This is not useful from a communications standpoint.

Correct, there is nothing that changes about the second particle when the first particle is measured

Nothing in quantum mechanics actually violates classical mechanics; Rather, quantum mechanics acknowledges that our ability to measure a near-infinite (but still finite) set of variables is limited by the effect of our own observation & by our inability to quantify all relevant variables prior to comparison. Thus "quantum uncertainty" & wave function collapse are merely an admission that any data set is necessarily incomplete, while reserving the possibility of predicting deterministic outcomes by reasoned observation of the limited data available.

At least, that's what the cat told me. 108.162.221.47 06:54, 17 October 2015 (UTC)

That is exactly what Bell's theorem states and what the experiments behind it showed. It is a bit technical, but the best layman description I have seen was on Ars Technica: http://arstechnica.com/science/2010/01/a-tale-of-two-qubits-how-quantum-computers-work/

162.158.92.91 09:41, 17 October 2015 (UTC)

The first rule of the No Communication Theory is that you don't talk about the No Communication Theory. -Pennpenn 108.162.250.162 22:44, 18 October 2015 (UTC)

"This means that any complete description..." - isn't this exactly the misunderstanding the comic is making fun of? I don't think one can make such an inference without first unscientifically presupposing some interpretations of QM to be correct.