Quote of the Day

The probabilities assigned to events refer to someone’s state of knowledge: before I know the outcome of Jane’s exam I can only say that she has a 70% chance of passing; whereas after I know I must say either 0 or 100%. 

Thus, the traditional view is that the probabilities in quantum mechanics --- and hence the “wavefunction” that encodes them --- refer to the state of knowledge of some “observer”.  (In the words of the famous physicist Sir James Jeans, wavefunctions are “knowledge waves.”)  An observer’s knowledge --- and hence the wavefunction that encodes it --- makes a discontinuous jump when he/she comes to know the outcome of a measurement (the famous “quantum jump”, traditionally called the “collapse of the wave function”). But the Schrödinger equations that describe any physical process do not give such jumps!

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