after watching a program yesterday. i saw the possibility of instant communication at any distance. using a pair of particals and quantum entanglment. in theory you should be able to vibrate or rotate the poles of one particle, and its paired particle should also vibrate or rotate but in the oposite direction. as particles can be manipulated with magnetism this should be entirley possible. if this change is as instant as the theorys suggest then it should be easy enough to sent a digital message betweein the 2 this is just an idea... i leave it to the scientists with the math to see if its at all possible or just junk.
People have considered that. Although the effect is instantaneous in order to extract the signal from the state the receiver measures you need a piece of information from the sender about what they measured when they interacted with the entangled particles at their end. Since they do not know this in advance they can only tell you after they have done the interaction. This information must be sent in normal ways, which are no faster than light. Since the sender doesn't know the outcome of their actions until they do them they cannot agree anything before hand, when they are standing next to the receiver (before the receiver goes far away), so it cannot be gotten around using a pre-agreed method. To be a bit more specific suppose Alice and Bob get together and make an entangled state of say 2 electrons. Alice takes one electron and flies off to Pluto while Bob waits with the other electron a while for her to do the journey. Alice then wants to send a message to Bob about the weather on Pluto. Your approach would be for Alice to bounce a photon off the electron, making a change in Bob's electron which he can read. Except the states of the electrons individually before Alice fires the photon aren't known. To measure them would destroy the entanglement. For example, the spins of the electrons might sum to 0, \(s_{1}+s_{2}=0\). The spins could say be +1 or -1 (not really true for electrons but that's beside the point). Whose got which isn't known. So how can Bob work out the change in the electron he has if he doesn't know the starting state? Well Alice bounces the photon off her electron and measures the photon's new property. That'll tell her whether her spin was +1 or -1, then she'll immediately know Bob had the opposite. So if she can tell Bob which is which Bob can work out what changed in his electron due to the entangled signal. So how does Alice tell Bob? Well she can't use an entangled communicator, you get the same problem. So instead she must send the result the old fashioned way, perhaps by a radio wave. Radio waves don't go faster than light so until Bob gets that he can't work out Alice's signal. So really he can't get the information until, at best, light has had time to go from Alice to Bob. Now while this means there's no really quick communication it does have an advantage (many in fact but never mind that...). Suppose Eve is at Jupiter and wants to know what Alice said about Pluto's weather but Alice and Bob don't want her to know. Eve can intercept Alice's broadcast about her electron spin result, Alice and Bob can't really keep that secret. However, Eve has no access to the electrons, she cannot actually use the information Alice radios to Bob, so the weather is kept secret from her! What you now have is an encrypted communication channel! Alice and Bob send the secret data via entanglement and broadcast spin results to anyone and everyone without fear they can 'decode' the communications which are to be kept private! This has already been implemented in the lab! This thread isn't really about alternative theories, you raise an interesting point about what quantum entanglement can or can't do so it might be worth moving this thread to the physics subforum. I'd do it but I don't have moderator powers in this forum. I'll ask the relevant mod to do it.
ah, i didnt know that the act of measuring breaks the entanglement.. i thought 1s the state was known you could then separate them... ah well thanx for your quick reply.
You know something about the system as a whole but the system is then split into pieces and carried apart by Alice, Bob and anyone else you care to consider, you can make entanglement, in principle, between as many particles as you like. You then can't say anything specific about individual pieces. You can say things like "The sum of all the spins is...." because that's what you can design into the entanglement generator. If you knew what a particular piece was then it wouldn't be part of the entanglement. This is one of the problems with communication channels which use entanglement in the manner I described. To send a bit of information via a qubit (quantum bit) requires you 'consume' the qubit, reducing it to a normal classical state. So in order to have an actual communication channel you need to have lots of qubits to hand. There's been work in qubit generators, where a source between Alice and Bob generates pairs of entangled photons and sends one to Alice and one to Bob. They then don't have to ever meet one another to share entanglement but they do rely on the source providing them with 'good' photons. It's hard to send photons long distance without the entanglement collapsing. I think they are up to something like 160km through air now, it was done somewhere like the Canary Islands. That's almost far enough to do satellite communication. The ISS is about 200 miles up, which is about twice that distance. Geostationary satellites are something like 36,000 km up but the atmosphere doesn't extend that far so it gets easier to maintain the entanglement. I do know that there's already been bank transfers which use this method of encryption. It's quite rare though, more a proof of concept thing, but perhaps as criminals get more competent or someone works out a quick way to break RSA encryption banks will have to migrate to quantum communications eventually. If RSA is broken we all will!
