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Life is made of carbon, hydrogen, oxygen, and smaller amounts of the other chemical elements. Carbon comes mainly in one variety, C-12. This is an atom with 6 protons, 6 neutrons, and 6 electrons. All carbon has 6 protons and 6 electrons. The 6 electrons make it carbon, and give it its chemical properties. Some carbon has more or fewer neutrons. Almost all of the mass (weight) of an atom comes from its protons and neutrons, both of which have almost exactly the same mass. A C-12 atom has a mass of 12, 6 protons + 6 neutrons.
Up in the sky, most of the air is nitrogen, N-14. N-14 has 7 protons and 7 neutrons. Every once in a while an atom of N-14 is struck by radiation from space and becomes C-14. One of its protons became a neutron. There is a fairly constant amount of radiation from space, mainly from the sun. And so, C-14 is being produced at a fairly constant rate. Plants take in carbon dioxide and water, and produce carbohydrates and oxygen. This process is called photosynthesis. A certain percentage of these carbohydrates contain C-14 instead of the usual C-12. You and I eat the carbohydrates, and get a small amount of C-14.
C-14 is radioactive. This means that it gives off radiation. That also means that C-14 automatically changes into something else. It changes from C-14 back into N-14. This is called radioactive decay. Starting with a quantity of C-14, half of it has changed into N-14 after 5730 years. And the amount of the radiation would also drop to half in 5730 years. C-14 is being created out of N-14, and it is decaying into N-14. Over time, the atmosphere has reached an equilibrium, C-14 is being created at the same rate that it is decaying. And the amount of C-14 in the atmosphere remains mostly constant.
Things that are alive continually take in C-14, either directly from the air, or by consuming food that contain C-14. Once these things die, they no longer take in C-14, and the amount of C-14 begins to decrease, by radioactive decay. And so, C-14 allows scientists to deduce roughly how old something is. They measure the amount of radiation coming from something containing carbon, and they can estimate the age. This method of dating objects was discovered in 1949, and is called "carbon dating."
Perhaps there was more or less radiation coming from outer space in the distant past. How can we know? Well there are some very old trees. Some bristlecone pines are about 4600 years old. And there are pieces of wood that can be accurately dated to be older than that. Tree rings have been matched (narrow rings during droughts and wide rings during rainy years) backwards through over 8000 years. And this shows that in the distant past (over 4000 years ago), C-14 dating underestimates ages by from 500 to 1000 years. And so there would seem to have been somewhat more radiation from space back then. This corrects C-14 dates, and give an accuracy to within a few hundred years.