See Inside September 2009


Their origin is one of the deepest questions in modern physics

Sundials and water clocks are as old as civilization. Mechanical clocks—and, with them, the word “clock”—go back to 13th-century Europe. But these contraptions do nothing that nature did not already do. The spinning Earth is a clock. A dividing cell is a clock. Radioactive isotopes are clocks. So the origin of clocks is a question not for history but for physics, and there the trouble begins.

You might innocently think of clocks as things that tell time, but according to both of the pillars of modern physics, time is not something you can measure. Quantum theory describes how the world changes in time. We observe those changes and infer the passage of time, but time itself is intangible. Einstein’s theory of general relativity goes further and says that time has no objective meaning. The world does not, in fact, change in time; it is a gigantic stopped clock. This freaky revelation is known as the problem of frozen time or simply the problem of time.

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