Physics: Tiny Tracks to Ancient Ages

When was the earth formed? What are the ancient milestones in man's development? In recent years, scientists have tackled such mysteries by means of radioactive dating. By comparing the amount of radioactive carbon 14 in a fossil with the amount contained in a living counterpart, for example, paleontologists determine when the fossil was part of a functioning organism. Using similar methods, scientists date meaningful objects as old as 3.5 billion years.

For all its startling applications, radioactive dating is inadequate for objects that are very small or contain only tiny traces of radioactive matter. Hence the significance of two recent...

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