Few of Albert Einstein’s sayings have been as widely quoted as his remark that God does not play dice with the universe. People have naturally taken his quip as proof that he was dogmatically opposed to quantum mechanics, which views randomness as a built-in feature of the physical world. When a radioactive nucleus decays, it does so spontaneously; no rule will tell you when or why. When a particle of light strikes a half-silvered mirror, it either reflects off it or passes through; the outcome is open until the moment it occurs. You do not need to visit a laboratory to see these processes: lots of Web sites display streams of random digits generated by Geiger counters or quantum optics. Being unpredictable even in principle, such numbers are ideal for cryptography, statistics and online poker.