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Comment by tedsanders | Hacker News Reader

Suppose you and a distant friend both agree to wave a flag at 1200 GST.

To an alien observing Earth, it looks as if the "human flag-raising interaction" is an instant effect that "travels" faster than the speed of light, since the flag raising happens at the same time in two places.

But really, nothing is travelling. You and your friend just pre-agreed to do something at the same time at different places.

If energy or information is not transmitted, then what is? I'd argue nothing.

(-A physicist)

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Almost, but spooky action is quite a bit spookier. Suppose you and your friend both agree to MAYBE wave a flag every day at 1200 GST, then communicate no further about it. The alien would always observe that either both flags are being waved, or both are not.

It's even worse than that.

You each have four flags. The first flag is black on one side, red on the other. The second is black on one side, green on the other. The third is white on one side, red on the other, and the fourth is white on one side, green on the other. (In other words, one side is black or white, the other side is red or green.)

The flags have the peculiar property that as soon as they are exposed to enough light to see what color they are, they burst into flames. So you can look at one side of a flag, or the other, but you can never see both sides of the same flag.

You and your (entangled) friend each choose a flag. The alien observer observes the following:

1. Your choice of flags, considered independently of your friend's choice, is indistinguishable from random. Likewise, your friend's choice of flag, considered independently of your choice, is indistinguishable from random.

2. Nonetheless, if the alien looks at the black-white side of both flags, they are always the same. Likewise, if he looks at the red-green side, they are always the same.

(My physics education is limited to some undergrad level classes, so sorry if my interpretation is erroneous)

Could it be that spooky action has underlying causes that we can't tell yet? For example, reusing the flag metaphor- you and your friend decide to MAYBE wave a flag every day at 1200 GST, and then in practice it happens that every sunny day you both wave it (because hey it's sunny, everybody likes to go outside) and on rainy days no one does (because it's warm and dry inside in your house).

An alien with no knowledge of meteorology and our reaction to it observing this would always observe that either both flags are waved, or both are not, in a seeming random matter, where in fact it is not random at all.

What you are describing are hidden variables [1]. Instead of depending on the weather condition both parties could just agree on a secret flag waving schedule you will never be able to observe. Experimental tests have shown that the Bell Inequality [2] is violated and this rules out the class of local hidden variable theories. As far as I know non-local hidden variable theories are still possible.

[1] http://en.wikipedia.org/wiki/Hidden_variable_theory

[2] http://en.wikipedia.org/wiki/Bells_theorem

I believe that would be labeled a hidden-variable theory, which have apparently been mathematically eliminated from consideration.

Only local hidden variables has been eliminated (as opposed to nonlocal), and even local hidden variables can logically exist with some tortured reasoning.

It's far less spooky when you consider both you and your friend are basically the same person, and the criteria you use for considering whether or not to raise the flag are the same.

That's my understanding of entanglement, but I get the feeling it's a wrong view.

What you're suggesting sounds like a hidden variable theory, which have been thoroughly disproven (Bell's Theorem).

Quantum mechanics cannot work through merely pre-arranging actions.

To nitpick: Only LOCAL hidden variable theories are disproven by Bell's theorem.

However, I agree with your point that my example is not compatible with quantum mechanics.

I only meant to give the sense that nothing is travelling, even though you can semantically say something is.

I'm inclined to agree with your premise, but your analogy implies that it's as a result of our pre-synchronisation that there's no FTL (faster than light) communication taking place. Entanglement is weirder than that though as Bell inequalities attest - it's as if I raised my flag at 11:59 and you did too despite the prior agreement.

I'm not sure if current physics has an explanation, but the last time I read around this, it seemed that the relativist position was that no FTL communication was occurring between particles because a transfer of information would require a slower than light communication between us so that we both knew what to measure. Or to pose it a different way - even if the electrons seem to communicate faster than light, technically we can't tell that it happened until later.

It's still worthwhile to check that it really does work like that.

But somehow, no one ever titles these articles "Physics continues to work exactly as expected." :)

I wholly agree. :)

But it takes energy to arrange, synchronize, and release that energy (ie. that synchronization didn't come without an energy cost).

See my comment on reddit here for why it's not quite that simple:

http://www.reddit.com/r/askscience/comments/19w1ag/why_is_qu...

(Yes, the probabilities are fudged)

Except in the case of the top article,as far as I understand, information could in theory be transmitted thanks to time. A spin change can represent a 1 bit, and the lack of a spin change at a time can represent a 0 bit.

No, entanglement cannot transmit information. It does not (necessarily) violate speed-of-light causality.

Like eoinmurray92 said, entanglement can be useful for cryptography.

It can also be useful for quantum computing, where you need to find clever algorithms where the wrong answers "cancel out."

It can be used to share a cipher (one-time-pad) because you and I would know the arrangement of bits that the other person has, and no-one else would/could know. However the measurements on the qubit's are probabilistic, so we cannot control the arrangement of bit that we have. We will only know if the are correlated/anti-correlated.

I still live in hope we'll find a way to use it for communication. A system where you had to use a slower-then-light synchronization signal to maintain the entangled state, but didn't actually transmit information over non-entangled bands, would fundamentally revolutionize communication none-the-less.

Being able to send data across the Pacific at essentially the speed of light, which was uninterceptable, would still be a massive breakthrough. Work on "weak" measurements of superpostioned systems interests me to that end (http://en.wikipedia.org/wiki/Weak_measurement)

I am quite confident that quantum dots will be able to do this sort of thing soon (10yrs). They are shown to be reliable and resistant to decoherence.

I don't know much about that weak measurement stuff, only in really theoretical situations thus far. But it seems a nice way to probe entanglement.

Toshiba @ Cambridge have recently shown QKD over broadband networks over 90km using single photon sources.

Before that the distance record was held by Tyndall in Ireland.

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