5 ms·
Bounding the speed of ‘spooky action at a distance’
- loceng 14y agoThis would be helpful for intergalactic domination. Kidding.
- deleted 14y ago[deleted]
- digz 14y agoI sent this to my friend who has a phd in physics and is a post-doc at Cambridge and he said it's bogus.
- ChrisClark 14y agoThen he didn't understand what you were asking. It can easily be faster than the speed of light because no information can be transmitted through this phenomenon.
- dasil003 14y agoWhoah you're jumping to a strange conclusion here. How do you know he didn't actually look at the paper and respond based on the contents rather than first thing that happened to pop into your mind?
- n3rdy 14y ago> Then he didn't understand what you were asking. It can easily be faster than the speed of light because no information can be transmitted through this phenomenon. 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.
- gambiting 14y agoBecause just observing the polarity changes it, so you can never be sure what position it was in originally. So you can affect an object that is light years away from you instantly, but it's useless for transmitting information, because while you know which state it is in, the party on the other end does not,and has no way of finding out without affecting the information and making it useless.
- n3rdy 14y ago> Because just observing the polarity changes it, so you can never be sure what position it was in originally. 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.
- gambiting 14y agoWell it does not _really_ change the 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.
- tedsanders 14y agoIt's like you and a friend have a deck of cards. 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.))
- tedsanders 14y agoIf no information or energy is transmitted, then what exactly is traveling faster than the speed of light? 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.
- Cushman 14y agoThings can travel faster than light. Say you sweep a spotlight across the moon. You could easily observe a spot of light traveling from one side of the moon to the other much faster than the speed of light. The spot of light is a "thing" the way we describe it; look at it, it's moving. But it can't be used to send information from one side of the moon to the other, so it's not a "thing" the way the universe describes it.
- mratzloff 14y agoMy understanding is there are scientists working on quantum entanglement communication. If so, I'm not sure why you assert no information transfer. (Not a physicist.)
- DanBC 14y agoIsn't that just tamper-proofing, rather than FTL communication? (http://www.newscientist.com/article/dn12786-quantum-cryptography-to-protect-swiss-election.html http://www.newscientist.com/article/dn12786-quantum-cryptogr...)
- tedsanders 14y agoRight. Entanglement doesn't send information. It just protects it by giving you a private one-time pad to use for cryptography.
- tedsanders 14y agoSuppose 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)
- _red 14y agoBut it takes energy to arrange, synchronize, and release that energy (ie. that synchronization didn't come without an energy cost).
- shardling 14y agoIt'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." :)
- tedsanders 14y agoI wholly agree. :)
- bpicolo 14y agoExcept 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.
- tedsanders 14y agoNo, 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."
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- scythe 14y agoSo what they actually measured is something that appears to go faster than light in the reference frame of measurement. This does not mean they measured the actual speed of the interaction. Instead, the point of this experiment is to lose a loophole in Bell test experiments: simply put, Bell experiments had previously proven that this seemingly-"non local" (bear with me) state exists and preserves dntanglement over long distances. The loophole was that it previously wasn't possible to time the ecperiment well enough to make sure that the whole thing didn't occur in some unexpected, subluminal way. Now we know it couldn't have occurred by some sort of classical interaction between the entangled qubits. 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.
- lisper 14y agoYou might enjoy these then: http://blog.rongarret.info/2011/09/faster-than-light-probably-not-but-you.html http://blog.rongarret.info/2011/09/faster-than-light-probabl... http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf
- scythe 14y agoMy favorite interpretation so far: http://en.wikipedia.org/wiki/Relational_quantum_mechanics http://en.wikipedia.org/wiki/Relational_quantum_mechanics It's sort of like many worlds with a whole lot of unnecessary baggage stripped away.
- _of 14y agoWatch out for arxiv.org, these papers may or may not have passed peer review. A lot of junk is deposited there.
- jessriedel 14y agoThis same group research group, including the same first author, published in Nature last year. http://arxiv.org/abs/1205.2024 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.
- mircopop 14y ago"There is no such thing as consensus science. If it's consensus, it isn't science. If it's science, it isn't consensus. Period.” Michael Crichton
- ryanatkn 14y agoThis is ignorant speculation, but what if the interaction doesn't depend on distance, and instead always takes the same fixed amount of time? What if that delta t is equal to 1 plank time, one tick in the cosmic 3D simulation? Of course that's an advanced experiment to run, but the implications would be fun.
- tedsanders 14y agoIt's like you and a friend have a deck of cards. 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.)
- Filligree 14y agoWhat you're describing is a hidden-variable theory, which has been ruled out by Bell's Theorem. Sorry.
- tedsanders 14y agoNo need to be sorry. :) 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.
- tedsanders 14y agoPhysics works as we already thought it did. Move along, folks. Nothing to see here. :)
- eoinmurray92 14y agoEveryone here is getting confused by the fact that you cannot control the polarity/polarization of the qubits before/as you send them. 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.
- icambron 14y agoCan you explain the one-time pad application a bit more? It seems to me that if Alice and Bob can share a bunch of entangled qbits, they could just as easily share a classical one-time pad. What am I missing?
- eoinmurray92 14y agoThe key is in the no cloning theorem. If Evil Eve intercepts some of the signal that Alice is sending Bob she will resend incorrect information 50% of the time. 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.
- eoinmurray92 14y agoIm not explaining it very well and its difficult to condense, perhaps I'll write a blog post that goes into more detail for a layman reader.