EPR pairs:a method of instant communication?

Alnilam · Post by **Alnilam** » 2003-04-03 04:14am

Hi.In the F.Pohl's novel "World at the End of Time" (HIGHLY suggested reading),the aliens described there that live inside stars use EPR pairs -a pair of particles,one located near of the receiver and other more or less far from it that,when one vibrates the other does so- for instant communication.
The narrator comments that,as being that effect a probabilistic event,they send several times the message and make parity and redundance tests to the received messages.The message that more times is received is the true one.
It's that true?.Could we manage to use these particle pairs for instant communications between,let's say Earth and Mars? (remember the delay of radio messages between these two planets due to the speed of radio waves (the speed of the light)).

Kuroneko · Post by **Kuroneko** » 2003-04-03 05:23am

Einstein-Podolsky-Rosen pairs? No, not so. If you have two entangled particles A and B, and you do an operation on one, it does put a constraint on the state of the the other if you observe it, but you can't reproduce this information unless you know the result of the operation. Hence, you need some other mechanism to communicate this result. (Worse yet, without this you can't even be sure whether it was A affected by B or vice versa; it appears differently from different inertial frames). Therefore, this an EPR pair teleport a particle (or, more precisely, produce a copy with the original destroyed), but at a speed no faster than that of light.

However you slice it, the communication limit of relativity refuses to be violated.

Post by **NecronLord** » 2003-04-03 05:42am

Did you just say that matter transmission is possible?

Kuroneko · Post by **Kuroneko** » 2003-04-03 06:22am

NecronLord wrote:Did you just say that matter transmission is possible?

Well, that's one way you can look at it, but it isn't really; the original isn't transported anywhere. A particle can be made to have the same state as another, with the state of the original destroyed, but the process is still limited by lightspeed. I suppose the best you can do is perform this process on particles with positive mass, in which case what you get is a form of transportation at lightspeed, despite the particles having mass.

This is nothing new. Schrödinger presented the EPR argument in mid-1930s (Einstein, et. al. wanted to use this result against quantum mechanics), though it was almost ignored until John Bell resurrected the issue (in the 1960s, IIRC).

Post by **NecronLord** » 2003-04-03 06:24am

Lightspeed matter transmission is fast enough.

Kuroneko · Post by **Kuroneko** » 2003-04-03 06:37am

NecronLord wrote:Lightspeed matter transmission is fast enough.

Perhaps; but I can guarantee you won't see ST-scale transporters in the 24th century. If I'm wrong, you can collect from me any amount of funds I am capable of producing and you are capable of receiving on Jan 1, 2401.

Post by **haas mark** » 2003-04-03 07:09am

Kuroneko wrote:
NecronLord wrote:Lightspeed matter transmission is fast enough.
Perhaps; but I can guarantee you won't see ST-scale transporters in the 24th century. If I'm wrong, you can collect from me any amount of funds I am capable of producing and you are capable of receiving on Jan 1, 2401.

Either way, would that particularly matter?

In any case..

One question, Kuroneko.. You said

If you have two entangled particles A and B, and you do an operation on one, it does put a constraint on the state of the the other if you observe it, but you can't reproduce this information unless you know the result of the operation. Hence, you need some other mechanism to communicate this result. (Worse yet, without this you can't even be sure whether it was A affected by B or vice versa; it appears differently from different inertial frames).

I'm just a bit confused.. are you saying that (a) you need to know the result before you can repeat it, and (b) you need to know the result to be able to communicate with it?

Kuroneko · Post by **Kuroneko** » 2003-04-03 07:40am

verilon wrote:One question, Kuroneko.. I'm just a bit confused.. are you saying that (a) you need to know the result before you can repeat it, and (b) you need to know the result to be able to communicate with it?

I should have put it in a little more detail. If A and B are entangled, it is possible to perform an operation on A and C (some other particle) that constrains the possible states of B. If one knows the result of the A-C operation, one can deduce another operation that puts B's state identical to what C's previously was. So, C's state is copied onto B, but only after the result is communicated to B's location (hence, there is still no superluminal communication). By the way, A and B will no longer be entangled, and thus cannot be reused.

Admiral Valdemar · Post by **Admiral Valdemar** » 2003-04-03 07:48am

Doesn't this instant communication still need a "Heisenberg compensator" as Star Trek would put it? I mean, you can't not examine the particle, thus the concept is flawed if no data can be transmitted.

Post by **haas mark** » 2003-04-03 07:52am

Kuroneko wrote:
verilon wrote:One question, Kuroneko.. I'm just a bit confused.. are you saying that (a) you need to know the result before you can repeat it, and (b) you need to know the result to be able to communicate with it?
I should have put it in a little more detail. If A and B are entangled, it is possible to perform an operation on A and C (some other particle) that constrains the possible states of B. If one knows the result of the A-C operation, one can deduce another operation that puts B's state identical to what C's previously was. So, C's state is copied onto B, but only after the result is communicated to B's location (hence, there is still no superluminal communication). By the way, A and B will no longer be entangled, and thus cannot be reused.

Okay.. so to do what you need to do to B, you need to do to C first. And only after you do that can you communicate with B (or where B is at?). Am I getting that right?

Kuroneko · Post by **Kuroneko** » 2003-04-03 08:21am

verilon wrote:Okay.. so to do what you need to do to B, you need to do to C first.

Except that they won't be the same operation, but essentially yes.

verilon wrote:And only after you do that can you communicate with B (or where B is at?).

Only after that communication occurs can you force B to C's previous state. It should be noted that you don't really know what C's state was, but you get a copy anyway.

Post by **haas mark** » 2003-04-03 08:24am

Kuroneko wrote:
verilon wrote:Okay.. so to do what you need to do to B, you need to do to C first.
Except that they won't be the same operation, but essentially yes.
verilon wrote:And only after you do that can you communicate with B (or where B is at?).
Only after that communication occurs can you force B to C's previous state. It should be noted that you don't really know what C's state was, but you get a copy anyway.

Okay, so you're converting B to C's post-op (so to speak) phase, or pre-op?

Kuroneko · Post by **Kuroneko** » 2003-04-03 08:40am

verilon wrote:Okay, so you're converting B to C's post-op (so to speak) phase, or pre-op?

Pre-op, the original state.

Post by **haas mark** » 2003-04-03 08:44am

Kuroneko wrote:
verilon wrote:Okay, so you're converting B to C's post-op (so to speak) phase, or pre-op?
Pre-op, the original state.

Okay. =) Thanks for explaining that.

Kuroneko · Post by **Kuroneko** » 2003-04-03 09:29am

Admiral Valdemar wrote:Doesn't this instant communication still need a "Heisenberg compensator" as Star Trek would put it? I mean, you can't not examine the particle, thus the concept is flawed if no data can be transmitted.

Not really. Yes, the wavefunction collapses if you scan the particle, so C's state is destroyed. However, since A and B are no longer entangled after the interaction (actually, A and C will be), no amount of scanning will disturb B. Due to the previous entanglement of A and B, this scanning information can be used to reproduce C's state in B, without actually exactly what C's state was. You get a copy, but you still don't know its state.

Post by **haas mark** » 2003-04-03 09:30am

So no matter what you do, you have to use C to make a copy onto B. Hmm. But if A and B were entangled still, and B were scanned, then would B's state be destroyed?

Kuroneko · Post by **Kuroneko** » 2003-04-03 09:36am

verilon wrote:But if A and B were entangled still, and B were scanned, then would B's state be destroyed?

Yes, it would be.

kojikun · Post by **kojikun** » 2003-04-03 04:52pm

If I understand Kuro correctly, hes sayign that because youre only observing a single particle's post-transmission state, you dont have any idea what the probability of it landing in that situation was because you have nothing to compare it to, so in order to know what you got and if its correct you'd need to know before hand what the probability was of getting that.

But would it not be possible to use say 100 or 1000 entangles paairs and "flash" them all with the signal, and then read the entangled particles at the recieving end? Then you'd get a nice number to work with and you'd know what the chances of something happening were versus what you got.