The Epr Paradox

Einstein famously rejected some of the principles of quantum mechanics, and tried to devise thought experiments refuting it. One of these is the “Einstein-Padolsky-Rosen (EPR) Paradox.” Imagine that you have an atom with zero spin, which decays into two particles (e.g., a positron and an electron). See Fig. 75 . Since the original atom had zero spin, the resulting two particles must have zero net spin. So if the spin of one is up, the spin of the other must be down. But given that we are talking about quantum mechanics, we do not know, a priori, which is up or down until we do an experiment.