Carbon fourteen dating
Carbon-14 dates usually appear to be reasonably accurate whenever they can be checked against historical records.
For example, when the Dead Sea Scrolls were dated, three methods could be used: 1) Dates written in the documents themselves (like the date at the start of a letter) 2) Paleography, which uses the style of script used to write documents to date them, and 3) Carbon-14.
Cosmic rays naturally mix with molecules found in our environment.
This can be done very accurately, although some samples may be difficult to work with.
Beyond this, the accuracy of the date depends on the reliability of the assumptions used in interpreting the measurements (see below).
This technique works well for materials up to around 50,000 years old.
Each radioactive isotope decays by a fixed amount, and this amount is called the half-life.