Thursday, October 4, 2012

Democritus: atomic theory



The theory of Democritus and Leucippus held that everything is composed of "atoms", which are physically, but not geometrically, indivisible; that between atoms lies empty space; that atoms are indestructible; have always been, and always will be, in motion; that there are an infinite number of atoms, and kinds of atoms, which differ in shape, and size. Of the mass of atoms, Democritus said "The more any indivisible exceeds, the heavier it is." But his exact position on weight of atoms is disputed.[1]

Democritus, along with Leucippus and Epicurus, proposed the earliest views on the shapes and connectivity of atoms. They reasoned that the solidness of the material corresponded to the shape of the atoms involved. Thus, iron atoms are solid and strong with hooks that lock them into a solid; water atoms are smooth and slippery; salt atoms, because of their taste, are sharp and pointed; and air atoms are light and whirling, pervading all other materials.[28] Democritus was the main proponent of this view. Using analogies from our sense experiences, he gave a picture or an image of an atom that distinguished them from each other by their shape, their size, and the arrangement of their parts. Moreover, connections were explained by material links in which single atoms were supplied with attachments: some with hooks and eyes others with balls and sockets.[29] 

The Democritean atom is an inert solid (merely excluding other bodies from its volume) that interacts with other atoms mechanically. In contrast, modern, quantum-mechanical atoms interact via electric and magnetic force fields and are far from inert.

Lucretius, describing atomism in his De rerum natura gives very clear and compelling empirical arguments for the original atomist theory. He observes that any material is subject to irreversible decay. Through time, even hard rocks are slowly worn down by drops of water. Things have the tendency to get mixed up: mix water with soil and you get mud, that will usually not un-mix by itself. Wood decays. However, we see in nature and technology that there are mechanisms to recreate 'pure' materials like water, air, metals. The seed of an oak will grow out into an oak tree, made of similar wood as historical oak trees, the wood of which has already decayed.

The conclusion is that many properties of materials must derive from something inside, that will itself never decay, something that stores for eternity the same inherent, indivisible properties. The basic question is: why has everything in the world not yet decayed, and how can exactly the same materials, plants, animals be recreated again and again? One obvious solution to explain how indivisible properties can be conveyed in a way not easily visible to human senses, is to hypothesize the existence of 'atoms'. These classical 'atoms' are nearer to our modern concept of 'molecule' than to the atoms of modern science. The other big point of classical atomism is that there must be a lot of open space between these 'atoms': the void. Lucretius gives reasonable arguments that the void is absolutely necessary to explain how gasses and fluids can change shape, flow, while metals can be molded, without changing the basic material properties.

The atomistic void hypothesis was a response to the paradoxes of Parmenides and Zeno, the founders of metaphysical logic, who put forth difficult to answer arguments in favor of the idea that there can be no movement. They held that any movement would require a void—which is nothing—but a nothing cannot exist. The Parmenidean position was "You say there 'is' a void; therefore the void is not nothing; therefore there is not the void."

The atomists agreed that motion required a void, but simply ignored the argument of Parmenides on the grounds that motion was an observable fact. Therefore, they asserted, there must be a void.
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