# B Quantum communication

1. Jun 23, 2016

### ed777

is it possible to entangle two particles and then separate them buy a large distance, and then interfere with one of the particles and observe this in the other, like a quantum communication system that is faster than light?

2. Jun 23, 2016

Staff Emeritus
No, it is not.

3. Jun 23, 2016

### entropy1

Suppose you have two stone-throwers sitting back-to-back throwing stones at two oppositely positioned targets. Let's call the targets A and B. Now let's assume that the stone-throwers both repeatedly throw stones simultanuously at the targets, and let's suppose they have a chance of exactly 50 percent to hit the target, and 50 percent to miss it. Suppose behind each target stands an observer, checking for each stone if it hits or misses. Let's call them Alice and Bob.

What we can firstly conclude is that Alice and Bob both observe 50 percent of the stones hitting their observed target, and 50 percent missing it.

Now suppose the stone-throwers simultanuously hit or miss their targets. There would still be a 50/50 chance of hitting or missing each target, but if one was hit, the other would also be hit, and similarly for misses. This would be a measureble effect if Alice and Bob would compare their notes! They would measure a 100 percent match between hits and likewise between misses.

Now suppose that in midflight of the stones, Bob would change something to target B that would influence whether it would be hit or missed while keeping the probability ratio of being hit 50/50. Now Alice and Bob still would observe 50 percent hits and 50 percent misses. However, if they would compare their notes, they would notice that there wouldn't be a 100 percent match between hits on one side and misses on the other. Sometimes there would be a miss where the other got a hit.

So, in the latter case, Alice and Bob would still see a 50/50 ratio hits/misses on their own target. Bob didn't change Alice's ratio. The difference between the targets only became apparent when Alice and Bob compared notes.

Note: To be more precise, all we know, is that Alice's and Bob's observations stay the same, and that the hit/miss correlation between them may vary; whether Bob's change in his target really does not at all affect Alice's results, we cannot know. After all, we can't know what 'would have happened' would Bob have decided otherwise. In any case, any 'influence' Bob would pose on Alice would perfectly blend into Alice's results, since both exhibit a purely random 50/50 ratio.

Last edited: Jun 23, 2016
4. Jun 25, 2016

### ed777

what about spooky action at a distance?

5. Jun 25, 2016

### Staff: Mentor

We could have another long thread about whether spooky action at a distance even exists - we already have several of these - but even if it does it cannot be used for communication.

There are many many threads here explaining why.