eli5 / How do we know the half life of (or even that it is radioactive) Bismuth-209 when it is literally longer than the age of the universe by several orders of magnitude
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A half life is the rate of decay. You can measure that rate by counting the decays and knowing the number of atoms you have. Since the number of atoms can be very high. Like 10^20 or more. There can be a good number of decays to measure even if the half life is very long. Also you are free to measure for a long time to get good statistics.
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