Tried looking it up and it got complicated real quick.
In: Physics
The reason is basically that there are LOADS of atoms of stuff in a human-scale lump of something, and we are really good at measuring tiny amounts of stuff too. In a gram of some Uranium salt there are enough atoms (something like 2×10^(21) of them) that hundreds will decay each second despite this incredibly long half-life (I think somewhere around 1000 per second from some rough and maybe wrong calculations), and if we leave a bunch of Uranium salt alone for a few days that is quite a lot of atoms, whose presence we can pick up even at that tiny concentration. By measuring exactly what amount of new elements are produced in what time period, we can work out the decay rate of the Uranium atoms.
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