On the other hand, if tons of half-lives have passed, there is almost none of the sample carbon 14 left, and it is really hard to measure accurately how much is left.
Since physics can't predict exactly when a given atom will decay, we rely on statistical methods in dealing with radioactivity, and while this is an excellent method for a bazillion atoms, it fails when we don't have good sample sizes.
Carbon 14 dating is not great for dating things like a year old because if much less than 1 half-life has passed, barely any of the carbon 14 has decayed, and it is difficult to measure the difference in rates and know with certainty the time involved.
Voila, now you can tell how old a sample of organic matter is.
Some notes: 1) Obviously, this technique only works for dead organic material.
Thus, the amount of ionium in sediments can be used as a rough measure of the age of sediment.
Accurate dating by measurement of ionium alone requires that the rate of sedimentation of ionium be constant with time, an assumption that does not hold for many sediments; any thorium-232 present in seawater will also precipitate, and the decay of the ratio of ionium to thorium-232 can be used as a measure of time.
Asked by: William Baker Carbon 14 (C14) is an isotope of carbon with 8 neutrons instead of the more common 6 neutrons.