So calculating the age range of a once-living sample involves measuring the 14C/12C ratio, and using this the known half-life to estimate the length of time since the sample died.
That age range is then compared with known 14C/12C ratios from the tree ring/marine record to find the best match, and the result is a calibrated age range you can be 95 per cent sure of.
So the proportion of carbon-14 inside living things is the same as the proportion of carbon-14 in the atmosphere at that time.
All radioactive atoms eventually decay into something more stable, and carbon-14 decays into nitrogen.And nuclear reactions have seen a leap in carbon-14 activity since 1945.Luckily for us we have a record of atmospheric carbon-14 levels for every one of the last 12,000 years.From the moment we die the proportion of carbon-14 compared to non-radioactive carbon-12 in what's left of our bodies starts to drop as it gradually turns to nitrogen.And the longer dead things lie around, the lower the carbon-14 levels get.
And that something else starts where Earth meets space.