Take a high-energy particle released by a radioactive atom like uranium in Earth’s crust. At first this particle exists as a wave function, spreading in every possible direction, not quite real until it interacts with, say, a piece of quartz. When that happens, one of its many possible trajectories condenses. The interaction with the quartz transforms what might have happened into what did happen when the uranium atom decayed. Within any given branch of history, this process shows up as a frozen accident in the form of an array of atoms affected by the high-energy particle, the tracks of which
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