Because they were created by the transition (of, say, an electron in atom) with angular momentum change 0. So,their total spin should always be 0.

Now, remember that spin is unobservable quantinty due to Heizenberg uncertainty principle (only its projection on some direction is observable), so you can only measure its component along some experimental direction (wich can be a result of different spins).

As far as I know, statistics of measuring spin components for two entangled (by angular momentum conservation) particles completely agrees with "total spin =0" conservation law.

• If so, where is the paradox - yanniru - May 8, 2001 - 12:44 UTC