system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: so researchers have observed the radioactive decay of xenon-124 which is extremely rare due to its half-life of 18 sextillion years, so how is it they observed this if the universe isn't even 18 sextillion years old?
gpt
A half life of an element doesn't mean an atom can't decay before that amount of time has elapsed, and it doesn't mean exactly half of all atoms of some group will decay after exactly the half-life has elapsed. The half-life is probabilistic for any individual atom. This means there's a 50% chance any single atom will decay *some time* **BEFORE** it's half life, and a 50% chance it will decay after. So if we have a hypothetical element with a half life of exactly 100 years, there's a 50% probability any individual atom will decay before 100 years has elapsed, but this could occur at any point. It could happen in 1 day 7, weeks, or 63 years. Then there's still a 50% chance it won't decay at all in those 100 years.