Libby's idea was based on the realisation that all living things contain a special kind of radioactive carbon called carbon-14, which begins to decay at a steady rate as its atoms start to die. Since half of the atoms in carbon 14 decay over a period of 5,600 years, this is known as its half life. So Libby was able to work out how much carbon-14 was left in any dead organism and get a good idea on its age. But there was a fault, Libby could only this for objects up to about 40,000 years old.
In fact there were lots of problems with carbon dating, and with every other technique that followed it to find the final definite age of the earth. Even the best of these couldn't date anything older than about 200,000 years. But most of all, these techniques couldn't date inorganic materials like rocks, which is, of course, what you need to do to if you want to determine the age of your planet.
So it was left to a man called Clair Patterson to come up with the solution. He began work on the project in 1948, making very precise measurements of the lead-uranium ratios in specially selected rocks. These had to be rocks that were extremely ancient and contained lead- and uranium bearing crystals that were about as old as the planet - anything much younger would misleadingly youthful dates. But the problem was that rocks that old were rarely found on earth.
finally it occurred to him that he could get around the rock shortage by using rocks from beyond earth. He turned to meteorites. The assumption he made- rather a large one, but correct was that many meteorites are left over building materials from the early days of the solar system, and thus more or less in their original state. Measure the age of these rocks and you would also have the age of the earth.
It took Patterson 7 years of patient work just to find and measure suitable samples for final testing. By this time, he had his specimens, containing minute quantities of uranium and lead locked up in ancient crystals- and he was able to tell the world that the definite age of the earth was 4,550 million years ( plus or minus 70 million years) in 1953. The earth finally has a age.
