Two atoms with the same number of protons may sometimes have different numbers of neutrons. Such atoms are called isotopes. Over the course of millions of years, the proportion of the two isotopes of carbon, carbon-12 and carbon-14, in the atmosphere has remained constant. Carbon-14 is radioactive, and its concentration decreases by half every 5,730 years, a process known as "half-life". When organisms are alive, they continuously exchange matter with the outside world, so the concentration of carbon-14 in their bodies is the same as that in the natural environment at that time. However, once organisms die, they no longer exchange matter with the outside world, and the carbon-14 in their bodies will continue to decay and decrease. There is no force in nature that can slow down or speed up this process. Therefore, by measuring the concentration of carbon-14 in the remains of organisms, we can determine the time of their death. This method of dating the age of fossils by measuring the concentration of carbon-14 and using its half-life is called the "carbon clock". The carbon clock can verify the "ages" of fossils within 20,000 years.
Chinese archaeologists used this method to measure the fir coffin lid in the No.1 Han tomb of the Mawangdui site and determined that the tomb was 2,130 years old. Historical document verification also confirmed that the ancient tomb dates back to the early Western Han Dynasty about 2,100 years ago. American chemist Libby developed this method and won the Nobel Prize in Chemistry in 1960.