Apparently xenon-124 has a half life of 18 sextillion years for example. How can it be calculated when it’s greater the age of the universe?
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You don’t need to watch it decay by half to calculate the time it will take. You can calculate how long it will take a car to drive 3000 miles across the US even by measuring its speed over 100 feet. If you can calculate how long even a minute amount of xenon-124 in a given sample decays then you can extrapolate the rest.
It’s sort of like how you don’t need to actually wait an entire hour to know how fast your car is going in miles per hour. Half-life is just a convenient point of reference for describing a rate of exponential decay.
For example, if I had 1000 grams of some substance on January 1, and on December 31 there were 999.99 grams left, I could do some math and determine that that substance has a half-life of about 70,000 years (assuming I’ve calculated that correctly). I don’t actually have to wait 70,000 years until there are 500 grams left to know for sure.
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