When a cosmic particle gets inside the earth’s atmosphere, it does not go far before colliding with a nucleus of nitrogen or oxygen. Out of this nuclear event emerge all the fundamental particles mentioned so far, and some others known as mesons. Mesons are particles which may be charged or neutral, and which have a weight a few hundred times that of the electron. Some of these particles are believed to be connected with the forces that hold the nucleus together. The nuclear debris from the collision will itself be very energetic and will further disrupt other nitrogen and oxygen nuclei. There soon develops a cascade of electrons, positrons, mesons, neutrons, protons, and electromagnetic radiation moving toward the surface of the earth. About once a second every square inch of the earth’s atmosphere receives such an energetic particle from outer space. The cascade that results carries penetrating radiations to the surface of the earth. All living organisms are constantly subjected to this radiation background. It is an important fact that the intensity of this radiation is reduced in its passage through the air, and inhabitants of Denver or Lima receive more cosmic radiation than the inhabitants of Los Angeles or New York. Some neutrons made by collisions of the primary cosmic particles in the atmosphere may collide with nuclei of nitrogen. When this happens, the following reaction occurs: nitrogen¹⁴ + neutron → carbon¹⁴ + proton (7 protons) (6 protons) (7 neutrons) (8 neutrons) Carbon¹⁴ is a radioactive electron emitter with a half-life of 5,600 years. This half-life is long enough so that much of the carbon¹⁴ in the world today was probably made ten to twenty thousand years ago. Willard Libby studied this process in a careful and quantitative way, traced the history of the radioactive carbon from the atmosphere into living beings, and, by measuring the carbon¹⁴ content in historical remains, opened up a whole new branch of archeology.