What is now proved was once only imagin’d – William Blake
Really important ideas in science are not the work of a single individual or even a single generation. The idea of an atom, for instance, was developed by ancient Greek philosophers, revived by eighteenth century chemists to make sense of their discoveries about the composition of matter, and used by nineteenth century physicists to explain the effect of temperature and pressure on gases. Our modern idea of molecules, formed with definite shapes by joining atoms together according to definite rules, was developed by chemists studying naturally occurring substances in the late nineteenth century. In the early twentieth century, the structure of the atom itself was explained in terms of more fundamental particles, while the last half century has seen advances that make it possible for us to directly sense, and even move around, individual atoms.
Atomic theory dates back to the pre-Socratic philosophers, especially Leucippus and Democritus, who wrote and taught more than four hundred years BCE. The works of the pre-Socratics survive only in fragments, and in quotations by later authors. For example, Epicurus, some 130 years later, built this theory into his unified view of the world and morality. The views of Epicurus were beautifully expressed by the Roman poet Lucretius, who lived at the same time as Julius Caesar, in his great work De Rerum Natura (On the Nature of the Universe). According to this ancient atomic theory, atoms are eternal and indestructible. All forms of matter are built up from a relatively small number of kinds of atoms.
What led the early atomists to theories so remote from simple appearances? Greek philosophers were greatly puzzled by the phenomena of change and motion. If something is real, how can it be transformed into something that it is not? If something is in one place, how can it move, since that would imply that it was no longer in that place? Besides, how can anything move without displacing something (if only the air) that is already there, in which case which one moves first? There are serious problems here, that were not properly solved until the mathematics of fluid flow and the theory of limits were developed in the 18th and 19th Centuries.
One radical approach to the problems posed by change is to say that change itself is an illusion, and that the world of experience, in which we live and act, grow and die, is in some important sense unreal. Plato was influenced by this approach when he compared knowledge gained through the senses to a mere shadow-play on the walls of a cave. Such a view is deeply hostile to science, which relies on observation, and the influence of Plato and his followers was to greatly hinder the development of scientific thinking.
An alternative view, that taken by the atomists, is that nature consists of two components; atoms, and the void. The atoms are eternal, since nothing can be created out of nothing. Everything that happens, happens according to natural laws, otherwise nature would not be predictable. The atoms have room to move because there is a void between them. The individual atoms are unchanging, but their arrangement can change, and the forces between atoms explain why objects have a definite shape. All natural processes are caused by collisions and rearrangements of atoms. The force of the wind, for instance, is attributed to the impact of the “atoms” (we would now say “molecules”) of air on objects. As Lucretius points out, the theory is consistent with common experience – for instance, the smell of a perfume diffuses through a room, so the perfume must be giving off invisible particles. Sheepskins hung up by the sea become wet overnight; this shows that “atoms” of water have found their way from the water through the air to the sheepskin’s surface. A pit dug a little inland from the sea fills up with drinkable water. This is because sea water contains more than one kind of atoms, and the jagged ones (which are also responsible for its sharp taste) stick to the ground through which the drinkable water flows (this is remarkably similar to the modern view, in which the ions that make up salt stick to clay). He could also have mentioned the way that things dissolve in water and can then be recovered unchanged when the water evaporates.
But let us allow Lucretius to speak for himself:
The headlong force of the wind lashes the body, overthrows great ships and scatters the clouds, then swiftly whirling strews the fields with tall trees … So there must be unseen particles in the wind, sweeping the seas, the land, the clouds of the sky.
Without seasonal storms, the Earth would not be able to give rise to the produce that gladdens us, nor could living things sustain themselves and propagate. So it is better to think that the same components occur in different things, much as the same elements occur in different words, rather than that anything could exist without an origin.
Some things are primary objects, while others are assemblages, but there is no force that can destroy the primary objects, for in the end they overcome through their own sheer solidity.
In other words, rain, plants and animals must be made of similar atoms in different arrangements, in order to explain the facts of growth and feeding (Lucretius, of course, had no idea that the plants needed carbon dioxide from the air, as well as water and minerals from the soil, but otherwise his account is surprisingly modern). The force of the wind, and its ability to carry clouds, is due to the impulse of the “atoms” in the air. All the changes that we observe in matter are due to the movement of atoms and their combining together in new ways. For Lucretius, as for Epicurus and Democritus before him, the atoms themselves are eternal and indestructible.*
I should mention that this is part of a moral, as well as a natural, world view. Lucretius was writing at a time of great civil disorder. The constitution of the Roman republic, originally designed for a small city-state, was breaking down even as the territory under Roman control grew. The central authority was increasingly unable to control its own generals, and the civil liberties enjoyed by the old governing class, to which Lucretius belonged, were being trampled underfoot. War, external along the expanding frontiers or internal between one ambitious general and another, seemed endless. At such times, individuals of spirit concentrate all the more keenly on the inner freedoms of thought and feeling. Thus Lucretius wrote to free the mind of superstitious fears through the light of understanding:
This fear and darkness of the mind cannot be dissipated by the rays of the Sun, nor by the clear shafts of day, but only by the perusal and understanding of nature.
His purpose, in short, was to demystify.
Why was there no progress from such a promising beginning? Bertrand Russell (History of Western Philosophy, Chapters IX, XXVII) suggests two types of possibility, one deriving from the way society was structured, the other from political circumstances.
In the world of Greece and Rome, there was a total lack of contact between the useful arts (cookery and brewing, dyeing, metallurgy, tanning, ceramics, even medicine), which were delegated to the laboring classes, and the speculations of gentlemen. When labor is cheap, there is little pressure to improve technology, and when thinkers despise manual activity, they will not develop any experimental technique. Lucretius himself often appeals to observation, but experiment involves more than simply observing what presents itself. It is the deliberate setting up of situations in order to observe them, and no Greek or Roman gentleman would be likely to soil his hands in such a business.
Secondly, there was what Russell calls a failure of nerve. Leucippus and Democritus were great intellectual innovators; Epicurus and Lucretius were not. Leucippus and Democritus wrote as citizens of free city states at their most confident, during and immediately after Greek successes in the Persian Wars. By the time of Epicurus, these city states had been subdued by Philip of Macedon and his son Alexander. Lucretius wrote when the Roman Republic was degenerating by way of civil war into a despotic empire. Later centuries saw the decline and fall of Rome, the chaos of Europe’s Dark Ages, and the subsequent medieval reverence for authority and verbal argument rather than experience.
As Lucretius repeatedly reminds us, if everything happens according to natural laws, there is no need to fear the gods. Indeed, he regarded religious belief as a source of evil, and gave as an example how Agamemnon had sacrificed his own daughter to the gods in order to secure a fair wind for the Greeks on their way to attack Troy. He also regarded the mind as the product of subtle atoms within the body, rather than a separate immaterial entity. As for the murderous political struggles of his time, these were the expressions of misplaced ambition, itself the product of incomplete understanding.
Such views did not endear him to the early Church. Thus St. Jerome, translator of the Bible into Latin, claimed, four centuries after the event, that Lucretius’s death was suicide, that he had been driven insane by a love potion, and that his books, written in lucid intervals, had been reworked and corrected by Cicero. I have found no reason to believe that any of this is true.
.Further south and east, in the Islamic and Hindu worlds, science and mathematics continued to flourish. Muslim theologians developed their own extreme version of atomic theory, in which matter consisted of dimensionless atoms, while space was atomised into separate points and time into instants, but the scientists of the Islamic world generally followed the continuous matter theories of Aristotle. There were major advances in the practical applications of chemistry, while the alchemists made important discoveries of how different substances react. However, a proper understanding of molecules, atoms, and the relationships between them requires an understanding of the mechanics of moving particles, and quantitative understanding of chemical reactions. We would have to wait until the 17th century for the first of these, and another hundred years for the second.
*But it would be easy to see Lucretius as more modern than he really is. He believes that he can discover facts about the world using purely verbal arguments, and many of his explanations amount to nothing more than “This is how things behave, because such is the nature of the atoms that compose them.” Such reasoning gives the illusion of understanding, and is the enemy of scientific enquiry. He describes a fantastical cosmology, in which the Sun is composed of lighter atoms than the Earth, squeezed out into space when Earth first cohered. Here we should not blame him for being wrong, but should blame him for being too easily satisfied. And while Democritus regarded all directions in space as equivalent, Lucretius, following Epicurus, thought that there was an absolute direction of “downwards”, in which direction atoms were eternally falling. This although Plato, centuries earlier, had taught that the Earth was round, at the centre of things, and stationary.
Reposted from 3 Quarks Daily. Translations by the present author. An earlier version of this material appeared in the author’s From Stars to Stalagmites, World Scientific, 2012.