The original post: /r/askscience by /u/will_scc on 2025-02-17 18:04:08.

I understand what a half-life is (the time after which half the sample of an element decays into some other element), but let’s say the half-life of something is 2 millions years… How do we know that, without waiting 2 million years and checking if half has gone?

Presumably we could wait a shorter period and see the change, but how would you know if it was “half” decayed yet, or not?