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) is only 5,730 years—that is, every 5,730 years, half of it decays away.
After two half lives, a quarter is left; after three half lives, only an eighth; after 10 half lives, less than a thousandth is left.
Our planet was pegged at a youthful few thousand years old by Bible readers (by counting all the "begats" since Adam) as late as the end of the 19th century, with physicist Lord Kelvin providing another nascent estimate of 100 million years.
Kelvin defended this calculation throughout his life, even disputing Darwin's explanations of evolution as impossible in that time period.
This week, Cherry Lewis of the University of Bristol presented a talk about the history of dating the Earth as part of the BA Festival of Science in York, England.
Before so-called radiometric dating, Earth's age was anybody's guess.
In 1898, Marie Curie discovered the phenomenon of radioactivity, in which unstable atoms lose energy, or decay, by emitting radiation in the form of particles or electromagnetic waves.