First principles of quantum mechanics
1. The probability of an event is given by the square of the absolute value of the probability amplitude for that event
P ? probability
? ? probability amplitude (complex)
P ? |? |2
2. When an event can occur several ways, the probability amplitude for the event is the sum of the individual probability amplitudes (superposition)
? ? ?1 + ?2
P ? |?1 + ?2 |2
3. If you can determine which of the possible alternatives an experiment takes, then the probability of the event is the sum of the probabilities for each alternative
P12 = P1 + P2 = |?1|2 + |?2|2