Radiometric dating human history
He came up with that figure by estimating how long it had taken for the planet to cool down to its current temperature from its molten infancy.But Kelvin didn't, and couldn't, know that radioactive atoms such as uranium were breaking down and keeping the planet warmer than it would be otherwise.Darwin had argued that the Earth was immensely old which gave his gradual process of evolution plenty of time to unfold.The great physicist Lord Kelvin had countered that the planet was actually relatively young perhaps 20 million years old.Geochronologists do not claim that radiometric dating is foolproof (no scientific method is), but it does work reliably for most samples.
Atoms can fuse together to create new elements; they can also spontaneously break down, firing off subatomic particles and switching from one element to another in the process (see figure, right).
While some physicists used these discoveries for applications ranging from nuclear weapons to nuclear medicine, others applied them to understanding the natural world.
The sun was once thought to burn like a coal fire, but physicists showed that it actually generates energy by slamming atoms together and creating new elements.
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