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After one half life, it would have had 1/2 the carbon.And then after another half life, half of that also turns into a nitrogen-14.And so this would involve two half lives, which is the same thing as 2 times 5,730 years. You'd say this thing is 11,460 years old, give or take.Many rocks and organisms contain radioactive isotopes, such as U-235 and C-14. So carbon by definition has six protons, but the typical isotope, the most common isotope of carbon is carbon-12. And then that carbon dioxide gets absorbed into the rest of the atmosphere, into our oceans. When people talk about carbon fixation, they're really talking about using mainly light energy from the sun to take gaseous carbon and turn it into actual kind of organic tissue. But what's interesting is that a small fraction of carbon-14 forms, and then this carbon-14 can then also combine with oxygen to form carbon dioxide.But as more dates became available, Egyptologists, who had hieroglyphic records back thousands of years, began to recognize that C-14 dates were generally too young.They proved this by showing that C-14 dates of wooden artifacts with cartouches (dated royal names) did not agree.
But what's interesting is as soon as you die and you're not ingesting anymore plants, or breathing from the atmosphere if you are a plant, or fixing from the atmosphere. Once a plant dies, it's no longer taking in carbon dioxide from the atmosphere and turning it into new tissue. And this carbon-14 does this decay at a specific rate. And you say, hey, that bone has one half the carbon-14 of all the living things that you see right now.The C-14 method cannot be used on material more than about 50,000 years old because of this short half-life.Other isotopes are used by geologists to date older material.And then you can use that rate to actually determine how long ago that thing must've died. It would be a pretty reasonable estimate to say, well, that thing must be 5,730 years old.So the rate at which this happens, so the rate of carbon-14 decay, is essentially half disappears, half gone, in roughly 5,730 years. Even better, maybe you dig a little deeper, and you find another bone. And you say, wow, you know this thing right over here has 1/4 the carbon-14 that I would expect to find in something living. Well, if it only has 1/4 the carbon-14 it must have gone through two half lives.