A non-mathematical introduction to the idea of Nonlocality in quantum theory

IMPORTANT NOTE: Quantum theory should NOT be use to justify any new agey spiritualist movement. Quantum theory has been developed using the scientific method. Spiritualist types do not believe in the scientific method.

You don't need much (if any) mathematics to follow this introduction, but that doesn't mean that it is easy to follow!

Nonlocality is a difficult idea to explain in around 10 minutes. So you may find that you need to spend a lot longer than 10 minutes thinking about it. If that is the case then please don't worry - you are in good company, because to this day there are people discovering new things about nonlocality that weren't previously appreciated. Feel free to pause or forward the clip whenever convenient.

I didn't get time in the video to emphasize that all of this nonlocality stuff is about measurements that are done essentially simultaneously by Alice and Bob (for those that know relativity - `spacelike' separated). Quantum theory (if it is a correct description of nature) requires INSTANT non-locality in this setting - i.e. the response of Alice's system has dependence on Bob's choice of button the instant that he presses the button - and if Alice and Bob are far apart, this happens at much faster than the speed of light. However, this instant non-locality does not permit instant signalling - indeed no currently accepted physical theory, including quantum physics, allows signals to be sent at faster than the speed of light.

To emphasize why this is weird: if quantum theory is an accurate description of nature, then that would mean that nature somehow "knows" what is going on now at very distant locations (it is instantly non-local), but still does not allow us to "know" what is happening at distant locations now (it does not permit instant signalling). This type of instant non-locality is sometimes refererred to as "spooky action at a distance". No classical theory shares this property.

The clip also does not explain the manner in which Quantum systems can display non-locality in real laboratories. Well here is one way (there are many): two quantum particles (say photons - `particles' of light) are prepared in a particular way (a so-called `entangled state'), and then distributed to Alice and Bob. They choose to do one of two measurements on their sides (in the case of photons, these could be polarisation measurements, if you know what polarisation is) which can have one of two outcomes each. It turns out that if the photons were prepared in the right way, then you can get statistics that are non-local. To see that the statistics obtained from such quantum systems are non-local is a little more difficult than in the example of the Popescu-Rohrlich box discussed in the clip, as it requires a little bit more mathematics (although not that much more). That is why I used the PR boxes as an example of a non-local theory rather than directly explaining the statistics that quantum theory gives. Although actually the explanation I gave of the PR box is not totally complete - I didn't really define rigorously what it means for the statistics are non-local - but I hope that you can agree with the intuitive discussion presented in the clip. If you want to know more, look up "local hidden variables" at a reputable university website.

To be totally honest - there are many many many subtleties underlying the discussion of nonlocality - implicitly we have assumed certain things about the statistical nature of experimentation. I am well aware that I do not appreciate all of these subtleties myself. However, if you do want to understand those subtleties a little more you'll have to look elsewhere than a 10 minute youtube clip! If you have some formal training in quantum theory, you can find out more by looking up terms like "local hidden variables", "Bell's inequalities", "CHSH", or "the GHZ paradox".

A postscript: a lot of people make really speculative unjustified claims about connections between entanglement, quantum theory, and new age style spiritualism/ theories of consciousness, out of body experiences etc. Don't believe the hype - they might claim to be physicists but ultimately they are just using fancy words to peddle their unjustifiable beliefs. There are plenty of decent physicists out there working on the foundations of physics. If you want to learn more then go to the websites of researchers at government universities or reputable private ones.

Category:

Science & Technology

Tags:

• Quantum
• Nonlocality
• Physics
• Entanglement
• EPR
• Bell
• inequalities
• Nonlocal
• Non local

[martifingers]

A brave attempt to simplify what must be a very complex situation. Is it possible to say something about the experimental verification of this theory and at what stage in the development of quantum thoery did this occur?

[PhysicsMathCharlatan]

@martifingers

Thanks. I don't really know the history fully. But two major papers on the topic were the Einstein-Podolsky-Rosen paper (maybe 1935?) and the paper by John Bell on Bell inequalities (1960s).The PR box example discussed in the video cannot be achieved by any known physical theory, but a more restricted form of nonlocality is now regularly investigated in modern experiments, up to various controversies about statistical accuracy and possible loopholes in the experiments.

[qbrute]

You've got to stop keep saying "OK?" ... it's irritating!

[PhysicsMathCharlatan]

@qbrute

you're right it is annoying. ok sorry about that. i didn't realise that i was doing it at the time ok. though ok by the end i had stopped saying it as much. hope all else is ok with the explanation however. ok won't do it again. sorry.

[TheUnhiddenBible]

Very nice video ... thanks for posting this.

[heintron]

Why is this related to boyd rice?!

[Karlemids]

randomize.

[TaylerKnox]

You are not applying the scientific method - you are an atheist fascist without the courage to step up.

[ExcessivelyArgumen]

Or, maybe if Alice only presses A, a signal change by Bob within a ten second increment can mean one thing and its lack can mean something else. Alice doesn't have to know what signal Bob sent, just that Bob changed the signal and that change means something while its lack means something else.

[ExcessivelyArgumen]

Why not have Alice only press B and have Bob avoid sending any signals for ten seconds to say he's sad and send at least one signal within a ten second increment to say he's happy. Next ten seconds they switch. They agree on the time of day to start by postcard.

[TaterGumfries]

OK.