QM students study the singlet state, (|u>-|d>)/SQRT(2). Particles in the singlet state can be separated by any distance, and remain in the singlet state. That leads to the EPR paradox, Bell's Theorem and the more. My question has more to do with entering the singlet state. Leonard Susskind, in a video lecture, said that the singlet state has lower energy than other states for a pair of spin 1/2 particles. OK, that suggests that such a pair, initially in some other state, would emit a photon and enter the singlet state. But common sense says that the probability of such an event must be a function of the proximity of the two particles. My question: how would I write an expression for the probability of this event as a function of proximity? I'm hoping that you can point me to a source where I can study it.