# EPR Paradox resolution: the spin is fixed at creation but its measurement isn't?

The Wikipedia article on the EPR paradox uses the example of an electron and positron created from a common source, each moving in an opposite direction to the other. Detector A is used to measure the spin of the electron, B measuring the spin of the positron.

The whole article gives the impression of something "paradoxical" being involved whereas I don't and so perhaps I'm misinterpreting the paradox:

At the instant of creation, the electron and positron have opposite but definite spins along an axis, but we can only use probabilities when predicting the outcome of measuring it?

Opposite spins yes, definite no. They are in a superposition of states. In one state, particle A is up while particle B is down, while in the other state, particle A is down and particle B is up. When the pair is created, they are in an entangled state, and only after measurement do they assume definite outcomes.

The crux of the matter is that this particular state is rotationally invariant. So we should get the same results whether we measure spin about the z axis or the x axis or any other. Now, if the two spacelike separated detectors each measure spin along the z axis and repeat the experiment many times, they find perfect correlation, even though their separate readings are random. On the other hand, if they take readings along orthogonal axes, they get zero correlation, while still getting individually random readings. It would seem that the two particles must conspire somehow. How is the measurement axis of one detector communicated to the other particle in order to achieve the correct correlation?

Edit:

What people seem to miss out on is that somehow the choice of measurement axis needs to be communicated from one detector to the other, if we are to hold onto a classical or local hidden variable description. This, of course, breaks a lot of other things.

• But aren't you using "state" here to mean the probability of what's measured? Nov 25 '14 at 23:54
• By state I mean wave function. Nov 26 '14 at 0:07
• So these states are probabilities of it being measured to have some spin; this doesn't mean the spin upon creation is in a superposition of spin states, does it? Nov 26 '14 at 0:37
• States are not probabilities. The word state has a very precise meaning in quantum mechanics, and means a wave function, which is the complete information that there is to know about a system or particle. You get the probability of measuring a certain outcome by squaring one component of the wave function. E.g. if you wanted to know what the probability of measuring A up/B down is , you would square $\psi_{\uparrow\downarrow}$ and in the EPR case, you'd get $1/2$. Nov 26 '14 at 10:04
• What a nasty reply to someone who is merely trying to share knowledge. And you still haven't provided for a reference to your statement "Apparently it is not true". Nov 29 '14 at 11:04

Take two balls from different color and put them into a box with two traps. In the period between the balls are trapped and one open the box there is a mixed state for each trap with 50% probability that in the trap is the white ball or the black ball. This state collapses in the moment one open the box. Does that mean that until one open the box, the balls are half white and half black? YES in mathematical expression and of course NO in the reality. If to be honest this experiment can be described in QM expressions.

The argument against the experiment with balls is that this are not quantum particles. The important point is, that for each EPR experiment we produce always pairs of entangled quantum particles such as spin correlated or momentum correlated particles. After they were produced they are in a mathematical sense in a mixed state. Until we do not measure one of the states we could not know the second state. The mixed state collapse means that our ignorance collapses. The two particles already have got their states in the moment of coexistence.

EPR-experiments were realized after the solution of two problems. The thirst was the production of pairs of entangled particles and the second was the protection of this particles from external influence until they get measured. After this it is not magic that if we measure one particel we know the state of the second one.

• I think this answer is confusing because it makes it sound like quantum states and probabilities stem from observer ignorance, which is not the whole story. Nov 26 '14 at 10:06