In 1909, the prevailing theory of the atom's structure was that atoms were mushy, semi- permeable balls, with bits of charge strewn around them. This theory worked just fine for most experiments about the physical world.

Physics, however, is not only interested in how the world appears to operate, but how it actually works. And so in 1909 a man named Ernest Rutherford set up an experiment to test the validity of the prevailing theory. In doing so he established a way that for the first time physicists could "look into" tiny particles they couldn't see with microscopes.

In Rutherford's experiment, a radioactive source shot a stream of alpha particles at a sheet of very thin gold foil which stood in front of a screen. The alpha particles would make little flashes of light where they hit the screen.

The alpha particles were expected to pass right through the very thin gold foil and make their marks in a small cluster on the screen.