Bounding the speed of ‘spooky action at a distance’
arxiv.org
arxiv.org
So how fast is the interaction, really? Well, it turns out that if something moves even 0.0001% faster than light, we can always find a reference frame in which it moves a million times faster than light, or infinitely fast, or even in which it appears to move at some huge negative velocity: back in time. See "tachyonic antitelephone". In other words, once something is faster than light, it is, essentially, instantaneous. This is in accordance with standard predictions of quantum mechanics.
Source: I'm a physics grad student.
http://blog.rongarret.info/2011/09/faster-than-light-probabl...
http://en.wikipedia.org/wiki/Relational_quantum_mechanics
It's sort of like many worlds with a whole lot of unnecessary baggage stripped away.
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)
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.
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.
That's my understanding of entanglement, but I get the feeling it's a wrong view.
Quantum mechanics cannot work through merely pre-arranging actions.
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 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.
But somehow, no one ever titles these articles "Physics continues to work exactly as expected." :)
http://www.reddit.com/r/askscience/comments/19w1ag/why_is_qu...
(Yes, the probabilities are fudged)
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."
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 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.
This means that one cannot send information faster than light.
What is useful however is the fact that when you send entangled qubits very far away, and measure them is the fact that the results of the measurements are always correlated/anti-correlated.
As an example.
Alice has a pair of entangled qubits, they are in a superposition of 1 and 0 (lets say they are an correlated pair, such that when measured both quibits will always agree).
She sends one to Bob.
Bob and Alice now hold one qubit each out of the entangled pair. They both measure their qubits. The measurement is probabilistic, you have a 50/50 chance of getting a 1 or a 0, but Bobs quibits and Alices quibit will agree everytime.
Since you can't control the outcome of the experiment you cannot decide which bit to send and hence cannot send information.
The useful result is that Alice and Bob have a string of bits which only the two of them know the arrangement of. They can use the string of bits as a one-time pad for encryption.
I have simplified this immensely but I think the point gets across.
Source : I currently undertake research into generating qubits for quantum computation and communication purposes.
In a classical bit stream, ie loads of photons in a fiber or a radio wave, Eve can intercept 10% of the signal without Alice or Bob knowing, and she can recreate and send that 10% perfectly.
Alice and Bob can publicly announce measurement bases and a few of the bits to see if the line was touched.
You each take a card from the deck, and then travel a million miles away.
Then, at the crack of noon (synchronized to GST, of course), you both look at your cards. Your card is the 3 of Clubs.
You travel back to meet your friend, and discover that his card (which he measured at noon, just like you) was the 7 of Diamonds.
You repeat this experiment a gazillion times and are amazed!! No matter how many times you run it, you and your friend never get the same card!!!
It's as if the cards are sending spooky signals to each other!
And after you double check that you and your friend are both looking only at 1200 GST, you realize that this spooky signal travels faster than the speed of light (since the it takes no time to "travel" a million miles).
It's like magic, eh?
Not really. The fact there is one of each card means it's impossible for you and your friend to have the same card. No magical spooky signals are being sent.
(The real question is whether the quantum mechanical case is analogous to this contrived example. I think it is. The important difference in quantum mechanics it that you really don't know what card you have until you look at it. However, that doesn't mean that your card is sending a faster-than-light spooky signal to tell the other card what to be. And you certainly can't use it for signalling or anything useful.)
The point of my post is to convey the sense that this article headline has when it says that "spooky action" has a speed.
Then, in the last paragraph, I ask whether the example I gave is actually analogous to quantum mechanics. I think it is, particularly if you see things through the many-worlds interpretation.
In the many-worlds interpretation, you and your card exist in a superposition of 52 states, 1 for each possible card. When you look at your card, your wavefunction separates into 52 different experiences and that effect propagates out into the universe at a speed less than the speed of light. Nonetheless, when you compare your results with your friend, you find that you never got the same card, even though you looked at the same time.
Nitpick: Bell's Theorem doesn't rule out hidden variable theories. Only local hidden variable theories.
http://arxiv.org/abs/1205.2024
Since it's common practice to first post to the ArXiv before submitting to a journal (note, for instance, that the first version of their Nature paper was posted to the ArXiv 4 month before being published), I'm confident this paper is or soon will be submitted for peer review. It's always possible they made a mistake or are overstating their findings, but I don't think these guys are a bunch of hacks.
If its observable then how could information not be transmitted?
I'm the furthest thing from a physicist but if quantum entanglement causes an atoms polarity to change because millions of miles away another atom it is entangled with did the same, that would seem like an excellent way to encode information into bits, supposing you can change the atoms polarity at a whim.
That does put a crimp in my universal router plans, but that's really interesting by itself that just looking at the atom affects its polarity.
Think about it this way:
You have two cards(A and B) - you and your friend pick one randomly. Until you look at yours, it's both A and B at the same time. It's the same with your friend's card. So you both own cards that are in a "mixed" A/B state. And then, when you and your friend look at them, the state is "decided".
Therefore, you could send your friend and his card on a rocket million years away from Earth, and then look at the card. Then,you can tell with 100% guarantee which card your friend has. Is that information useful? No,not really. You were able to instantaneously find out the state of an object millions of light years away,but you can't use it to communicate.
You each take a card from the deck, and then travel a million miles away.
Then, at the crack of noon (synchronized to GST, of course), you both look at your cards.
Your card is the 3 of Clubs.
You travel back to meet your friend, and discover that his card (which he measured at noon, just like you) was the 7 of Diamonds.
You repeat this experiment a gazillion times and are amazed!! No matter how many times you run it, you and your friend never get the same card!!!
It's as if the cards are sending spooky signals to each other.
And after you double check that you and your friend are both looking only at 1200 GST, you realize that this spooky signal travels faster than the speed of light (since the it takes no time to "travel" a million miles).
It's like magic, eh?
Not really. The fact there is one of each card means it's impossible for you and your friend to have the same card. No magical spooky signals are being sent.
(I think the quantum mechanical case is analogous to this contrived example. The difference in real quantum mechanics it that you really don't know what card you have until you look at it. However, that doesn't mean that your card is sending a faster-than-light spooky signal to tell the other card what to be. And you certainly can't use it for signalling or anything useful.)
((Reposted to main thread with some slight edits, in case people find this comment useful or interesting.))
Thusfar that isn't the case - so it does seem to be spooky action at a distance. It's not just an inability to perform a suitable measurement beforehand.
But! Cat_A being alive is paired with Cat_B being dead. We know this. When we discover Cat_A being alive and thus Cat_B being dead we're not really finding out anything about Cat_B; we're just finding out something about the Cat_A / Cat_B pairing, and Cat_A is local to us.
(http://everything2.com/title/Quantum+entanglement+and+faster...)
Also, you can't change the polarity at a whim.
Nothing.
It's just as if you and a distant friend pre-agree to lift a flag at 1200 GST. The "flag-raising interaction" is instant, "travelling" faster than the speed of light, but really nothing is travelling faster than the speed of light. You and your friend just pre-agreed to do something at the same time in different locations.
(http://www.newscientist.com/article/dn12786-quantum-cryptogr...)
Entanglement doesn't send information.
It just protects it by giving you a private one-time pad to use for cryptography.