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I have this one boggling my mind for quite some time. Let's consider a very simple EPR experiment -- pair of entangled particles are sent to Alice and Bob (separated by large distance), who (at the same time) measure its spin along different axis: Alice does the measurement on axis z (and, suppose she reads it points up), while Bob does the measurement along axis x. What will Bob read?

a) Either left or right, but in this case, we know the particle's spin on both axes in the same time, which violates Heisenberg's principle, right?

b) He will not be able to conduct the measurement. What does this mean exactly, what will he see? And if he can know that his measurement failed, wouldn't that make faster than light communication method possible? Example: Alice codes a message as, e.g. 01010011 and for every second, she decides to do the measurement along the z axis (if the next bit is 1) or not to do it (if the next bit is 0). On the other side of the galaxy, Bob does the measurement along x axis _every_ second and if he gets the result, Alice sent him a 0. If he cannot get it (because of the Heisenberg's principle), Alice sent him a 1.

I would really appreciate the explanation of this.

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# EPR paradox

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