Half Life

Radioactivity obeys a very interesting rule: every radioactive atom has a "half life." For Potassium-40, the half life is 1.26 billion years. That means that if you start with 64 atoms of Potassium-40, and wait 1.26 billion years, that half of them will decay (i.e. half of them will explode, each time emitting an electron and a neutrino). That will leave 32. (The square-root rule applies. It will usually not be exactly 32, but 32 ± 6.) If you wait another 1.26 billion years, the rest will not disappear, but only half of them! So after a total time of 2.52 billion years (2 half-lives) you will be left with 16. After another 1.26 billion years, you expect to have only 8. And so on.

Notice how this rule is exactly the opposite of the doubling rule. Instead of the amount doubling with each generation, it is halved at every half life. This is very mysterious behavior. Try to imagine why it works. You will have trouble. It is as if the atoms which lived to one half-life have not aged at all. They don't behave like old particles. In fact, they behave exactly the same as they did at the earlier time. There is no difference between the initial particles, and the few that have remained after several half lives.

There are half lives in every range you can think of. The half life for radiocarbon (also known as carbon-14 or C-14) is 5730 years. The half life for tritium (hydrogen with two neutrons in the nucleus as well as one proton) is 12 years. Tritium is what is in my watch: the electron emitted in the decay causes the phosphor to glow. Lithium-8 has a half life of about a tenth of a second.

I would like you to know that the half life of K-40 is about a billion years, and that the half-life of C-14 is about 6,000 years. I will not hold you responsible for the others.