u/neBular_cipHer

Some isotopes are known to have half-lives of millions or even billions of years. How can these values be known if scientists have only been able to observe atoms decaying for ~100 years?

If you had 1 mole of an unstable isotope, and after 1 year, 0.5 mole had decayed, then you could deduce that it has a half-life of 2 years. But for isotopes that take millions or billions of years to decay, only a minuscule fraction would have decayed even after decades. Is that really measurable to any precision?