The EPR paradox is often described this way: (from Wikipedia) A common presentation of the paradox is as follows: two particles interact and fly off in opposite directions. Even when the particles are so far apart that any classical interaction would be impossible (see principle of locality), a measurement of one particle nonetheless determines the corresponding result of a measurement of the other. My question is that when the particles "fly off in opposite directions", are they not already spinning opposite each other? Said differently, how do we know the difference between these two scenarios: 1) The particles each have unknowable and indeterminate spin until we measure one of the particles, and then we know the spin of the other. 2) The particles each have unknowable, but definite spin, and knowing one particle's spin can inform us about the other. In the first case, we must invoke a nonlocal mechanism to ensure the integrity of the system. But in the second, the locality of the particles during the incident which sends them off in opposite directions is what actually caused the opposite spin. Measuring one of the particles doesn't create a "causation" of the second particle physically, it merely reveals the information which was previously unknowable. I assume there is some experiment which rules out the second scenario?