Half-Life Applications

Carbon-14 dating works because living things constantly take in carbon-14 from the air. When they die, that carbon-14 decays with a half-life of 5,730 years, so the amount left reveals how long ago they lived.

For older rocks, geologists use uranium-lead dating. Uranium-238 has a half-life of 4.5 billion years, ideal for dating Earth's oldest minerals — this method puts Earth's age at about 4.54 billion years.

In medicine, short-lived tracers like fluorine-18 (half-life: 110 minutes) power PET scans that spot cancer by revealing cells with unusually high sugar use.