Over time, certain kinds of rocks and organic material, such as coral and teeth, are very good at trapping electrons from sunlight and cosmic rays pummeling Earth. Researchers can measure the amount of these trapped electrons to establish an age. But to use any trapped charge method, experts first need to calculate the rate at which the electrons were trapped. This includes factoring in many variables, such as the amount of radiation the object was exposed to each year. These techniques are accurate only for material ranging from a few thousand to 500,000 years old — some researchers argue the accuracy diminishes significantly after 100,000 years.
Thermoluminescence: Silicate rocks, like quartz, are particularly good at trapping electrons. Researchers who work with prehistoric tools made from flint — a hardened form of quartz — often use thermoluminescence (TL) to tell them not the age of the rock, but of the tool. After shaping flint, toolmakers typically dropped the rocks into a fire. Shea explains: “The rock gets heated, and the heat frees up the electrons; after that event, however, the rock starts absorbing the electrons again via cosmic rays,” essentially resetting the rock’s clock. Archaeologists also frequently use TL to date ceramics, which are also exposed to high temperatures during manufacture.
Optically stimulated luminescence: Similar to TL, optically stimulated luminescence measures when quartz crystals in certain kinds of rock last saw sunlight. Exposure to sunlight resets the crystals’ clock to zero, but, once buried, the trapped electrons accumulate what’s called a luminescence signal, which can be measured in the lab. Researchers expose a sample to certain light wavelengths that briefly “free” the electrons, just enough for each of them to emit a photon. That emitted light, the signal, can be used to calculate when the sample was last exposed to sunlight.
Electronic spin resonance: ESR, which measures trapped electrons using magnetic fields, is related to magnetic resonance imaging, the medical technique that allows doctors to look for tumors or peek inside your creaking knee. Because ESR essentially tracks the activity — the “spin” — of the electrons without freeing them, the sample can be subjected to repeated dating attempts. ESR also has a longer range — some researchers claim up to 1 million years — but it’s more complicated than other trapped charge methods, leaving it more susceptible to error.
[This article originally appeared in print as "Scientific Dating Methods."]