Radioactive isotopes carbon 14 dating
When the half-life was corrected in 1950, the year was taken as a base date from which to calculate all resulting dates.
Therefore, any expression of “before present” will mean “before 1950”.
This does not mean that we have a precise year of 3780BC, it means we then need to calibrate through other methods that will show us how atmospheric concentrations of the C isotope has changed - most typically through the dendrochronology records (tree ring data) (10).
Very old trees such as North American Bristlecone Pine are ideal for constructing long and accurate records of the state of the atmosphere.
The sample passes through several accelerators in order to remove as many atoms as possible until the C pass into the detector.
These latter atoms are used as part of the calibration process to measure the relative number of isotopes (9).
If a radioactivity level comes back as half of what would have been expected if the organism had died in 1950, then it is presumed to be 5,730 years before 1950.
There are three carbon isotopes that occur as part of the Earth's natural processes; these are carbon-12, carbon-13 and carbon-14.
The unstable nature of carbon 14 (with a precise half-life that makes it easy to measure) means it is ideal as an absolute dating method.
Radiocarbon dating is simply a measure of the level of C isotopes in the atmosphere can vary.
This is why calibration against objects whose age is known is required (14).