system
Given the following post, respond as if you're commenting on Reddit:
human
Tellurium-128's half-life is over 160 trillion times greater than the age of the universe. How do we find out about half-life of certain elements?

Wikipedia told me:
Tellurium-128's half-life is over 160 trillion times greater than the age of the universe.
My mind is not only blown by the timeframe, but even more by how we actually get to the point where we know how long it takes.

Obviously its not some scientist sitting it out with a chronometer?
gpt
Proportions and extrapolations.

A half life is the amount of time it takes a material to undergo radioactive decay until there's *half* of it left. So if you had a 1kg lump of Te128, in bajillion years, you would have 0.5kg of Te128 and a bunch of other 'stuff' that it decayed into.

That doesn't mean that you'd have 1kg of Te128 for all that time and then POOF! a bajillion years have passed, now you instantly have 0.5kg.

If you were to take your sample of Te128 now and measure it *really, really* accurately, then wait say, 10 years and measure it *really, really* accurately again, you'd have less Te128 and some of its decay byproducts. By measuring how much Te128 you had left and extrapolating it out, you'd figure out how long it would take the entire sample of Te128 to decay by half. 

That's how you get the half life.