I understand that it works by measuring the amount of carbon 14 which has different amounts depending how old the matter is. But how is it known how much carbon 14 there was that long ago? Is it just that is takes x years for this much to decay so 104 x yea ago they figure it out on a simple graph?
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We know the concentration of carbon-14 in our current atmosphere, and since carbon-14 is created by cosmic rays entering our tropo-/stratosphere which, as far as we know, has not changed significantly in the timeframes we’re measuring, we can be relatively certain that the concentration was the same in those timeframes.
And since we know that there *should* be x% carbon-14, finding y% carbon-14 allows us to calculate the age since something died as carbon-14 decays, assuming that the concentration was similar as it is now.
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