Half-Life
A lump of uranium left to itself will gradually decay into various smaller
particles. This rate of decay is measured by how
long it would take for half the
uranium to decay (the half-life). Even though it is utterly random
when a particular atom will decay, we can get a good prediction of the
half-life for a large number of atoms.
Obviously, many people were very unhappy with this idea that chance can
rule physical properties. In response to this theory
Einstein proclaimed "God
doesn't play dice!" (Einstein was wrong).
If you flip a coin two times and get two "heads",
you can't conclude too much about the chance of getting a
"tails" on the next toss because your sample size is too small.
However, if you flip a coin a hundred times and get
51 "heads" and 49 "tails", you can conclude that the chance of
getting a "tails" is about 50%. Even though chance does rule
physical properties, physicists can accurately
predict the overall results of a large sample of random events.
In the case of half-life,
they can predict when half of the atoms in a given
bunch will have randomly decayed.