The Earth is 4.54 billion years old; 3.8 billion years ago, the very first life form came into existence; 225 million years ago, dinosaurs came on the scene; and man took his first steps in Africa 200,000 years ago. Carbon-14 and other radioisotopes are used to measure the age of fossils, rocks, and other materials that make up Earth's geologic history. But at any given time, there are trace amounts of carbon-14, or C14, in the atmosphere.
And they contribute to the hundreds of lines of evidence supporting Charles Darwin's theory of evolution, which continues to stand the test of time. See, all living things contain carbon, which has six protons and six neutrons, so in its typical form, we call it carbon-12.
The Earth is 4.54 billion years old; 3.8 billion years ago, the very first life form came into existence; 225 million years ago, dinosaurs came on the scene; and man took his first steps in Africa 200,000 years ago. In 1960, Willard Libby won a Nobel Prize for developing this technology.
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C14 is a radioactive isotope that's made when cosmic rays bombard nitrogen atoms at high altitudes, converting them to this excited form.
When some living things, like plants and algae, make their own food through photosynthesis, they take in carbon dioxide from the air.
Trace amounts of C14 make up a tiny percentage of that carbon dioxide, and it's integrated into the tissues of the organism.
Then creatures that can't make their own food through photosynthesis (like us) eat the ones that can, and that C14 is taken into our bodies as well.
And because there's a constant quantity of C14 in the atmosphere, there's a constant, corresponding quantity of it in the bodies of all living things, at least while they're still alive. That doesn't mean it's dangerous, only that it's unstable. See, when an organism dies, it stops taking in carbon.
And the C14 in the organism's tissues starts to decay at a precise speed, but the amount of carbon-12 stays the same, since it's not radioactive.
We know that it takes 5,730 years for half of the C14 in a sample to decay. And if we compare the amount of C14 in a dead thing to the amount of regular carbon-12, voila! Now, some people who think that the earth is only 6,000 years old may base their claims on words in the Bible, not measurable evidence. If I wanted to find out the age of a dinosaur fossil, I might measure its uranium-235 concentration, which has a half-life of 704 million years.
It takes another 5,730 years for half of what's left to decay, and so on. And one ploy they use to cast doubt on radiocarbon dating is to point out its shortcomings. So, anything older than 50,000 years only has too little C14 left to make an accurate calculation of its age. Radioactive isotopes like potassium-40 and rubidium-87 have half-lives in the billions of years.
Critics also like to point out that over time, the amount of C14 in the Earth's atmosphere may have varied.