" These last few points are key to how quantum mechanics works, so let me summarize them: The wave represents the quantum state. When we leave the system alone, it changes in time deterministically, according to Rule 1. But the quantum state is only indirectly related to what we observe when we make a measurement, and that relation is not deterministic. The relation between the quantum state and what we observe is probabilistic. Randomness enters in a "

― Lee Smolin , Einstein's Unfinished Revolution: The Search for What Lies Beyond the Quantum