Half-Life

[FuBar5]

Browsing for answers online, I've found many complicated references to what it means.

Can someone summarize it's meaning? Why 'half'? Why not double the answer and get the whole thing? Why 'life'? What's alive?

btw, I don't mean the game(s) by VALVe. though that IS where the question originated.
Something about 'decay of radioactive isotopes' or something, right?

[fo3nix]

Certain isotopes decay. This takes a certain amount of time, but technically it's a falling exponential, meaning that it'll never be 100% decayed. Stuff just keeps decaying and decaying, mathematically to infinity.

However, it's easy to measure the time it takes to decay to half the amount of stuff there is right now. This is called the half life.

[Panic Button]

No its because if you ingest radio active stuff you only live half a life. That Russian guy just discovered that

[buildersmate]

The atomic nucleus of some variants of elements (isotopes) is unstable and decays over time. Atomic particles and/or electromagnetic radiation are given off as the nucleus decays. The rate at which the decay occurs depends on the isotope but the rate of decay is always exponential (see above). One measure used to define the rate of decay is the half-life - the time taken for one half of the nucleus to decay to another isotope. In the next equal period of time (half-life), one half of the remaining nucleus will decay, meaning only one quarter of the original is left after two half-live time periods. And so on.
The 'life' bit is nothing more than the term for emission of neutrons, electrons and electromagnetic (gamma) radiation.

[FuBar5]

Thanx a lot, it makes more sense now! :-))

[parkesquay]

You can see from these answers that in practical terms there are some radioactive elements that could take a 1,000 years to decay to a point where they were no longer life threatening to mankind. That is why you hear of agencies trying to dispose of nuclear waste products a 1,000 feet undergound so there are no harmful effects at surface level.
Places like Chernobal still have no--go areas because it is impossible to declare the area safe for human habitation.

[BillyBB]

Plutonium 239 has a half-life of 24000 years and so will remain dangerous for a period many times longer than homo sapiens have existed. The more stable Uranium 238 has a half-life over 4 billion years.
One analogy would be to imagine a 500 mph wind for 24000 years then a 250 mph wind for 24000 years then a 125 mph wind for 24000 years. So in 70000 years time you'd still be blown away!