How did they figure out how to make the atom bomb work?
They spent a lot of money :-)
Actually, I have always wondered why it was so difficult to
make the bomb work - the main principle is really simple. The
idea is that a nucleus that is splitting gives off some neutrons.
Some nuclei do this once in a while just naturally, but if
there are lots of neutrons around they have a much higher
tendency to split, and then produce more neutrons, which can
cause more nuclei to split, in a chain reaction leading to
an explosion... As soon as you have collected a critical mass
of the material (Uranium or Plutonium, for example) you should
get an explosion. I guess the hard problems are:
(1) NOT getting an explosion until you actually want it
(2) NOT getting a wimpy little explosion that blows your
material apart before the chain reaction goes very far.
Also, it was a tough problem because it is not something you
really want to do lots of trial and error on... There are
a bunch of interesting books on the beginning of the nuclear
age that you could probably find in the library. The U.S. effort
was called the "Manhattan project".
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Update: June 2012