Are Democritus' theories of atoms still relevant today?
Well, yes, sort of. Democritus had no instruments to extend the reach of his senses, so all of his experiments were just "mind experiments." The ancient Greeks gave humanity tremendous gifts despite only having their minds to work with. We humans tend to be skeptical and want proof before we believe in most things, so thousands of years passed before we could prove his theories. Also, being only "mind experiments," they really don't quantify or say much about the nature of matter. Their theories tended to be very general. So although he was correct that there was this thing that could not really be cut any further, he could not tell you all the things we know about atoms, like the mass, charge or what they were made of.
Brian Kross, Chief Detector Engineer (Other answers by Brian Kross)