TECHNOLOGY

Counting Atoms to Tell Time

1949 CE

Phanerozoic · Cenozoic · Quaternary · Holocene · Meghalayan

Living things take in a radioactive form of carbon for as long as they are alive, and stop at death; what is already in them then decays at a fixed rate. What is left measures how long ago that was. The method was first tried on wood whose age was already known from written history, including Egyptian pieces, because a clock is only worth having once it has been checked against something.

Why it matters

Archaeology stopped arguing about which site came before which and started giving dates. Most ages on this timeline within the last fifty thousand years rest on it, and the ceiling is the method's own: beyond it, too little of the carbon is left to count, which is why everything older here is dated by other clocks.

Dating & uncertainty

The year is that of the published checks: the method was measured against objects of known age, Egyptian wood among them, before being trusted on unknowns. Its useful reach is about fifty thousand years.

Sources

1. Arnold & Libby (1949), Science · Age determinations by radiocarbon content: checks with samples of known age
2. Currie (2004), Journal of Research of the National Institute of Standards and Technology · The remarkable metrological history of radiocarbon dating

Earth history as one day

23:59:59.99851.5 ms before midnight

if Earth’s 4.54 billion years were compressed into a single day