Radiocarbon dating is a process for determining the age of a prehistoric object by measuring its radiocarbon content. The technique was developed by an American chemist, Willard F. Libby (1908-1980), in the late 1940s. Radiocarbon dating involves the analysis of a radioactive carbon isotope, carbon 14 (also called radiocarbon), which is produced in small amounts continuously in the atmosphere by cosmic rays (invisible, high-energy particles that bombard Earth from space). Radiocarbon, like normal carbon, becomes absorbed into green plants through photosynthesis and into animals when they eat the green plants.
After an animal or plant dies, it no longer absorbs radiocarbon and the radiocarbon present in the organism begins to decay (break down by releasing atomic particles) at an exact and uniform rate. Its half-life (the time it takes a sample to lose half its radioactivity) of 5,730 years makes it useful for measuring prehistory and events occurring within the past 35,000 to 50,000 years.
A newly developed instrument called the Accelerated Mass Spectrometer is able to separate and detect atomic particles of different masses. As a result, it's now possible to establish more accurate dates using smaller samples than ever before. The radiocarbon in a fossilized sample can be measured and compared to that of a living sample. Using this process, it's possible to determine the age of an animal or plant that lived less than 50,000 years ago.
Since Libby's work, other isotopes having longer half-lives have been used as "geologic clocks" to date very old rocks. The isotope uranium-238 (decaying to lead-206) has a half-life of 4.5 billion years; uranium-235 (decaying to lead-207) has a value of 704 million years; thorium-232 (decaying to lead-278) has a half-life of 14 billion years; rubidium-87 (decaying to strontium-87) has a half-life value of 48.8 billion years; potassium-40 (decaying to argon-40) has a value of 1.25 billion years; and samarium-147 (decaying to neodymium-143) has a value of 106 billion years.
Another way to determine a past date is by counting tree rings. Trees add new cells to the center of their trunk each year. These new cells force the previous years' cells outward, forming concentric rings with the oldest ring on the outer edge.
A third dating technique is the analysis of ice cores. Ice accumulates in layers, one year at a time, so ice cores drilled from deep within the two-mile-thick polar ice caps provide information about the past 250,000 years or more.
Sources: Engelbert, Phillis. The Complete Weather Resource, pp. 494-499; Gray, Peter. The of the Biological Sciences, p. 141; Porter, Roy, ed. The Biographical Dictionary of Scientists, 2nd ed., p. 430.