Around the turn of the fifth century B.C. the followers of Parmenides and Heraclitus got into a rather lively debate about whether the universe was characterized by static being or constant flux—but those guys have been dead for a long time now, and no one much cares anymore what they had to say. The issue has, however, proved to be stubborn. In just the last century, physicists had to confront the puzzling fact that certain tests revealed light itself to be composed of discrete particles, while others showed it to constitute a wave: a controversy as yet unresolved.

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