# Is superluminal communication via quantum entanglement impossible? **URL:** \ **Category:** Factual Questions\ **Created:** [December 7, 2011, 12:56pm UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366 "2011-12-07T12:56:45Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Qwertol](https://avatars.discourse-cdn.com/v4/letter/q/e274bd/32.png) [@Qwertol](https://boards.straightdope.com/u/Qwertol)\ **Post date:** [December 7, 2011, 12:56pm UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/1 "2011-12-07T12:56:45Z")
Or we simply don’t know it yet? I read some articles on Wikipedia, like no-cloning theorem or no-communication theorem but they seem unsettling. So what is the truth? ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [December 7, 2011, 12:58pm UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/2 "2011-12-07T12:58:37Z")
What do you know. It worked. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [December 7, 2011, 1:36pm UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/3 "2011-12-07T13:36:32Z")
As I understand it, it would be possible if you could deliberately collapse a quantum superposition into a desired state - because that would collapse its counterpart into the opposite state. This could be used to modulate a signal and transmit information. But you can’t. Observing a particle collapses its state (and its entangled twin the opposite), but you just don’t get to control how. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [December 7, 2011, 1:42pm UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/4 "2011-12-07T13:42:02Z")
Both our current quantum dynamics understanding and our current general relativity understanding say “No”. At some point, we may find out our understanding is wrong, of course, but right now the answer is pretty clear. ---
**Author:** ![Darth\_Panda](https://avatars.discourse-cdn.com/v4/letter/d/ee7513/32.png) [@Darth\_Panda](https://boards.straightdope.com/u/Darth_Panda)\ **Post date:** [December 7, 2011, 1:45pm UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/5 "2011-12-07T13:45:10Z")
Oh, what an entangled web we weave, when the speed of light we must deceive. ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [December 7, 2011, 1:54pm UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/6 "2011-12-07T13:54:52Z")
Basically, no. Entanglement makes it possible to set up experiments to make it seem like particles have communicated faster than light, but it doesn’t open that door for you to put a message in that channel. It’s like flipping a coin here and having a simultaneous coin flip a light year away come out the opposite way with 100% certainty. You can go over to the other coin, see that it came out tails if yours was heads, or heads if yours was tails, and figure the coins must have somehow coordinated to make that happen, but you can’t actually communicate using that effect. A naive person might think, “the light-year away coin always gives the opposite answer as mine, so instead of flipping my coin, I’ll place it heads up on the table if I want to send a message of ‘tails’ one light-year away,” but it turns out that any attempt to cheat like that breaks the entanglement and the correlation doesn’t happen. ---
**Author:** ![Candyman74](https://avatars.discourse-cdn.com/v4/letter/c/ad7895/32.png) [@Candyman74](https://boards.straightdope.com/u/Candyman74)\ **Post date:** [December 7, 2011, 2:09pm UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/7 "2011-12-07T14:09:40Z")
Basically you don’t get any more information than you already had. You already know - before looking at it - that your particle will be the opposite of the distant particle. You look at it, you see it’s one thing, and therefore know that the distant particle is the opposite - but you already knew that. What you didn’t know beforehand is whether your particle was heads or tails. You just know that whichever it is, the other one will be the opposite. Same for the guy at the other end. You don’t “see” it flip or anything. You can’t put a row of particles there and send a message by checking which ones change. There’s no change to see in the normal sense - you simply find out what way up the particle is when you check. From your POV it may as well have always been that way up, because you had no way of knowing till you checked, and once you check it’s set. ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [December 7, 2011, 2:43pm UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/8 "2011-12-07T14:43:43Z")
It might violate some causality laws and create a temporal anomaly. Let me try something - my aim may be a little off (it is difficult to calibrate for such small jumps) but I’ll try let you know about two hours ago. ---
**Author:** ![coremelt](https://avatars.discourse-cdn.com/v4/letter/c/4491bb/32.png) [@coremelt](https://boards.straightdope.com/u/coremelt)\ **Post date:** [December 8, 2011, 12:49am UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/9 "2011-12-08T00:49:12Z")
> [@DrFidelius](#): > > It might violate some causality laws and create a temporal anomaly. Let me try something - my aim may be a little off (it is difficult to calibrate for such small jumps) but I’ll try let you know about two hours ago. I was waiting for this when you did the first post … I know it’s not possible for whatever reason, but the idea that pops into my head is to somehow to use collapsed/non-collapsed as a way to send the message. However as I understand the person on the other end can’t in anyway check if it’s collapsed without collapsing it… doh! ---
**Author:** ![rockystone](https://avatars.discourse-cdn.com/v4/letter/r/e47c2d/32.png) [@rockystone](https://boards.straightdope.com/u/rockystone)\ **Post date:** [December 8, 2011, 4:39am UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/10 "2011-12-08T04:39:10Z")
OK. Here’s a thought I had. I get that you cant control the head/tail up/down yin/yang properties of the particle-pair that you generate. I don’t see why this is an insurmountable barrier to communication. Just keep sending until the message you want to send accidentally generates itself, then stop. Thats not as stupid as I just made it sound. Imagine this- You have a system that generates, say, one thousand particle-pairs per second. On the sending end, we decide that “up” = one, “down” = zero, therefore, on the recieving end, “down” = one, “up” = zero. We want to send the message “Hello”. That begins with “H”, which is ASCII character 72 (decimal). I don’t actually know what that looks like in Binary, so I’ll pretend it’s 01101001. Our first task is to send a zero, so we just keep generating particle-pairs until we generate a zero, then we stop generating for, say, ten milliseconds, then start generating again until we send a 1, pause again, then restart to send the third Bit, and so on. That ten-millisecond pause is the signal to the recieving end that “the last bit we sent is the one we wanted to send, so write it down”. Refinements. If we send two zeros in a row, it’s obvious that we are trying to send a one, so there is no need to keep going until we do (send a one), we don’t even need to pause, just continue sending for the next Bit. So, to send “011”, it could look like any of these- 0\_1\_1\_ 111\_1\_ 11001\_ 0\_001\_ 1100000\_ The Underline denotes the ten-millisecond pause. The last example included the forth bit (a zero) So thats the basic idea, the random nature of the output does not preclude communication, as long as we provide ourselves with a method of saying “Yes, that one, that’s the one I meant.” Go ahead, shoot me down. ---
**Author:** ![coremelt](https://avatars.discourse-cdn.com/v4/letter/c/4491bb/32.png) [@coremelt](https://boards.straightdope.com/u/coremelt)\ **Post date:** [December 8, 2011, 4:48am UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/11 "2011-12-08T04:48:24Z")
You’ve missed something, you can’t generate and send particle pairs a light instantly, you need to somehow keep particle pairs entangled while 1 part of the pair is transferred a distance away. Then you collapse the particle pair and the one on the other end collapses the opposite way. Let see you pre-entangle 1 billion particle pairs, send them 1 light year away. You start collapsing them and looking for random ones that send you message, that part is fine. There’s no way to communicate to the person at the other end at what point you stopped, without having another FTL communication channel already existing. ---
**Author:** ![The\_Second\_Stone](https://avatars.discourse-cdn.com/v4/letter/t/bbe5ce/32.png) [@The\_Second\_Stone](https://boards.straightdope.com/u/The_Second_Stone)\ **Post date:** [December 8, 2011, 4:49am UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/12 "2011-12-08T04:49:41Z")
> [@Candyman74](#): > > Basically you don’t get any more information than you already had. > > You already know - before looking at it - that your particle will be the opposite of the distant particle. You look at it, you see it’s one thing, and therefore know that the distant particle is the opposite - but you already knew that. > > What you didn’t know beforehand is whether your particle was heads or tails. You just know that whichever it is, the other one will be the opposite. Same for the guy at the other end. > > You don’t “see” it flip or anything. You can’t put a row of particles there and send a message by checking which ones change. There’s no change to see in the normal sense - you simply find out what way up the particle is when you check. From your POV it may as well have always been that way up, because you had no way of knowing till you checked, and once you check it’s set. The thing is, if I know you are going to send either a “yes” or a “no” at a certain time and I don’t check until that time, how is it that I cannot send even that bit of information, given we have a few qubits set up to try it? ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [December 8, 2011, 5:07am UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/13 "2011-12-08T05:07:57Z")
> [@Darth\_Panda](#): > > Oh, what an entangled web we weave, when the speed of light we must deceive. 🙂 Two thumbs up. This too works in context here, I believe: “I know that you believe you understand what you think I said, but I’m not sure you realize that what you heard is not what I meant. –Robert McCloskey ---
**Author:** ![rockystone](https://avatars.discourse-cdn.com/v4/letter/r/e47c2d/32.png) [@rockystone](https://boards.straightdope.com/u/rockystone)\ **Post date:** [December 8, 2011, 5:53am UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/14 "2011-12-08T05:53:31Z")
> [@coremelt](#): > > You’ve missed something, you can’t generate and send particle pairs a light instantly, you need to somehow keep particle pairs entangled while 1 part of the pair is transferred a distance away. Then you collapse the particle pair and the one on the other end collapses the opposite way. > > Let see you pre-entangle 1 billion particle pairs, send them 1 light year away. You start collapsing them and looking for random ones that send you message, that part is fine. There’s no way to communicate to the person at the other end at what point you stopped, without having another FTL communication channel already existing. Oh. I’d gotten the impression that sending the particles was not a problem (at least theoretically) and every one was gnashing their teeth because they couldnt use it to talk, but now it seems that the problem lies in transporting the message/particles. I can’t help you there, sorry ---
**Author:** ![coremelt](https://avatars.discourse-cdn.com/v4/letter/c/4491bb/32.png) [@coremelt](https://boards.straightdope.com/u/coremelt)\ **Post date:** [December 8, 2011, 5:59am UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/15 "2011-12-08T05:59:09Z")
> [@rockystone](#): > > Oh. I’d gotten the impression that sending the particles was not a problem (at least theoretically) and every one was gnashing their teeth because they couldnt use it to talk, but now it seems that the problem lies in transporting the message/particles. I can’t help you there, sorry The problem is both, first you have to somehow transport particles vast distances while keeping them entangled in the first place. Second even if you could do that, you still can’t communicate using them, your method wouldn’t work. ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [December 8, 2011, 12:35pm UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/16 "2011-12-08T12:35:56Z")
> [@The\_Second\_Stone](#): > > The thing is, if I know you are going to send either a “yes” or a “no” at a certain time and I don’t check until that time, how is it that I cannot send even that bit of information, given we have a few qubits set up to try it? You don’t “send” anything. You measure the state of a particle and it randomly comes up 0 or 1. You can’t even tell if it’s still entangled or not. It’s only when you compare notes with the person measuring the other particle that you discover that your measurement mirrored theirs. And “comparing notes” can only happen at light speed. ---
**Author:** ![davidm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/davidm/32/225_2.png) [@davidm](https://boards.straightdope.com/u/davidm)\ **Post date:** [December 8, 2011, 12:48pm UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/17 "2011-12-08T12:48:00Z")
It can’t be used for communication. It can, however, be used for unbreakable encryption since it can be used to generate an unpredictable random key on both ends that no one else can intercept. But the actual message is transmitted by standard means and is limited to c. ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [December 8, 2011, 12:57pm UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/18 "2011-12-08T12:57:19Z")
> [@rockystone](#): > > Our first task is to send a zero, so we just keep generating particle-pairs until we generate a zero, then we stop generating for, say, ten milliseconds, then start generating again until we send a 1, pause again, then restart to send the third Bit, and so on. That ten-millisecond pause is the signal to the recieving end that “the last bit we sent is the one we wanted to send, so write it down”. I’m not sure what you mean by “keep generating particle pairs”. To generate an entangled particle pair with the particles a light year apart takes at least a year (assuming one end is generating them) so if changes in the way you’re generating particle pairs (e.g. pauses) form part of the message, then you’re already communicating slower than light. The possibility of superluminal communication comes from entangled particles that have \*already \*been sent before the message is even conceived of. E.g. if I send create entangled particles in 2015, send one of each pair out into deep space then, and in 2016 try to measure the particles I kept in such a way as to encode the results of the 2016 World Series in the particles I’d already sent out. That is also impossible, but more subtly so. ---
**Author:** ![Gyrate](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gyrate/32/3588_2.png) [@Gyrate](https://boards.straightdope.com/u/Gyrate)\ **Post date:** [December 8, 2011, 12:57pm UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/19 "2011-12-08T12:57:47Z")
[There is another way…](http://xkcd.com/660/) ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [December 8, 2011, 1:39pm UTC](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366/20 "2011-12-08T13:39:46Z")
> [@The\_Second\_Stone](#): > > The thing is, if I know you are going to send either a “yes” or a “no” at a certain time and I don’t check until that time, how is it that I cannot send even that bit of information, given we have a few qubits set up to try it? What I think you’re missing is that you can’t look at a one photon (of an entangled pair) and figure out whether anyone has measured its partner. All you can do is measure the photon, and know that anyone who has or will measure the other one will get the opposite answer. So in your scenario there’s no way for you to know when I stopped measuring. [Next page](https://boards.straightdope.com/t/is-superluminal-communication-via-quantum-entanglement-impossible/605366.md?page=2)