system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know the "stable" isotopes don't just have absurdly long half-lives (as in several magnitutes times the age of the universe)?

I know bismuth was formely considered stable until it was discovered it just had an extremely long half-life.
gpt
We don't really, it is theorized that protons even in stable atoms may decay over long enough timescales, in this case no atom would truly be stable. There are also the tiny probabilities that collections of stable atoms may slowly undergo spontaneous fusion events which isn't radioactive decay but would mean that no sample of atoms would remain stable over unlimited periods of time.