Talk:1591: Bell's Theorem

Explain xkcd: It's 'cause you're dumb.
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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: 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. 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)


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. 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: 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 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.