How did Democritus think up atoms?

I’ve been reading about Democritus and how was one of the earlier people to propose the existence of atoms.

I’ve read plenty of detail about him and his ideas, but nowhere does anyone say how he first came up with the idea. There was physically nothing to indicate atoms’ existence, and obviously no experimental work, so how on earth did he come up with atoms? And was his concept of atoms similar to ours today?

I believe it originated from the question of how finely matter can be divided. If you continually divide stuff into smaller and smaller pieces there must be a smallest piece in order for there to be any stuff at all.

And no, Greek atoms had no relation to the atom of today and they didn’t really do anything with the idea.

He didn’t come up with atoms as in the tiny little negative charges orbiting a nucleus which we call atoms today. A-tom just means cannot be cut: he was merely proposing a unit of matter which couldn’t be divided any further, from a parsimonious perspective (he was also one of the first atheists).

In fact, since today’s “atoms” can be divided into electrons, neutrons and protons, which can further be subdivided into quarks and leptons, which might be further divided into superstrings, they shouldn’t really be called atoms at all but “element units” or something. If String Theory is correct, Planck-scale strings are the real “atoms” - the fundamental entities which cannot be further divided.

And apologies to Greek scholars: atomos was the word I was looking for.

The SEP’s article on Democritus is excellent, as ever.

Yet.

I once heard a story about how Democritus was walking along the beach with some other dead Greek guy. He looked down at the sand, picked some up, and was instantly struck with wonder at the way a huge, continguous “beach” could consist of nothing more than tiny grains of sand. Then he thought about it some more, came up with a primitive atomic theory, and died.

Google has a few hits that correspond to this myth, but as with all things concerning decomposed Greeks, it’s hard to be sure.

“Smaller than the Planck length (R)” is rather a meaningless phrase in physics, rather like “before time” or “north of the north pole”. I’m no String Theorist but apparently, from Brian Greene’s excellent The Elegant Universe, the mathematics suggest a kind of reflection-in-a-spoon effect wherein the description at >R is the inverse of 1/R, as though the universe is simultaneously little and large.

How frightening.

Yes, I’m reading Fabric of the Cosmos. Absolutely un-put-downable. Elegant Universe is queued up.

Not the best comparison. “North of the North Pole” and “before time” are inherently meaningless. By contrast, some string models predict that scales smaller than the Planck length should be meaningless, but we still don’t really have any evidence for the truth of any string model (or of string models in general), and it’s quite possible that in the true description of quantum gravity, scales smaller than the Planck scale might be possible. We have no clue what things might be like at such scales, but that’s not to say that we’ll never have a clue.

The (possibly apocryphal) story I’ve always heard (and I’m surprised no one has mentioned it so far) is that Democritus proposed the following thought experiment: say you have a very fine copper wire and a pair of very sharp shears. You use the shears to cut the copper wire in half, and then in half again, and so on. Eventually you will reach a point where it is no longer possible to cut the wire in half; what you have left, then, is an atom of copper: the smallest piece of copper which cannot be further divided into smaller pieces.

I may be wrong, but I was under the impression that the ancient Greek concept of “atoms” was originally geometrically based. The ancient Greeks hated the concept of infinity because it played havoc with their geometry. Take the following example: You have a cone, which smoothly tapers from a circular base to a point. Cut off the tip of the cone parallel to the base at any level, and you have a smaller cone and a frustum. The base of the smaller cone and the top of the frustum are circles of equal size. But if every slice of the cone yields a frustum top and smaller cone base of equal size, then how can the cone taper from base to tip?

The modern answer is that the two circles are not equal, but their difference in size is “infinitesimal”. But the Greeks hadn’t mastered infinitesimals (Archimedes used them but refused to believe they were real). So to dispense with such seeming paradoxes, it was proposed that space simply couldn’t be infinitely dividable; there had to be what we would call a “quantum” of space, and this was developed into the atomos.

The Greeks posited atomic matter as an answer to a Zeno’s Paradox. Infinite subdivision (of time, space, matter, etc) caused them a lot of consternation; a minimal particle nicely prevents further division.

I wonder what they would have made of modern quantum theory and string theory?

Then how do we know about his atomic theory?

He etched it into the sand with the first mininuke, duh.

And I wasn’t kidding about the beach; that’s really the story I was told in college. Hmm.