Carbon dating meteorites
"This process was still occurring when conditions were unfavorable for carbonation.
By measuring the atoms produced by these breakdowns inside rocks, physicists were able to estimate their ages (right).
And by comparing the ratios of those atoms to atoms from meteorites, they could estimate how long ago it was that the Earth formed along with the rest of the solar system.
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.