The Big Picture: On the Origins of Life, Meaning, and the Universe Itself

by Sean Carroll

This quantum probability is very different from ordinary classical uncertainty. Think once again of playing poker. At the end of a certain hand, your opponent makes a big bet, and you need to decide whether your hand can beat theirs. You don’t know what their hand is, but you know what the possibilities are: nothing, a pair, three of a kind, and so forth. Given their behavior so far in the hand, and the odds that they received certain cards to start, you can be a good Bayesian and assign different probabilities to the various hands they could have. Quantum states sound kind of like that, but they are crucially different. In the (classical) game of poker, you don’t know what your opponent has, but they have something definite. When we say that a quantum state is a superposition, we don’t mean “it could be any one of various possibilities, we’re not sure which.” We mean “it is a weighted combination of all those possibilities at the same time.” If you could somehow play “quantum poker,” your opponent would really have some combination of each of the possible hands all at once, and their hand would become one specific alternative only once they turned over the cards for you to look at them.