Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....


I was brought up to look at the atom as a nice, hard fellow, red or grey in color according to taste. Rutherford

The Greeks have been kicked around a lot in the last couple of millennia: dominated by the Romans, bullied during the crusades by warlords going to and from the Holy Land, and occupied by Turkey until recently. It's no wonder they prefer to remember their salad days, when their best thinkers came up with concepts like democracy and atoms. Greece is democratic again after a period of military dictatorship, and an atom is proudly pictured on one of their coins. That's why it hurts me to have to say that the ancient Greek hypothesis that matter is made of atoms was pure guesswork. There was no real experimental evidence for atoms, and the 18th-century revival of the atom concept by Dalton owed little to the Greeks other than the name, which means "unsplittable." Subtracting even more cruelly from Greek glory, the name was shown to be inappropriate in 1899 when physicist J.J. Thomson proved experimentally that atoms had even smaller things inside them, which could be extracted. (Thomson called them "electrons.") The "unsplittable" was splittable after all.

But that's getting ahead of our story. What happened to the atom concept in the intervening two thousand years? Educated people continued to discuss the idea, and those who were in favor of it could often use it to give plausible explanations for various facts and phenomena. One fact that was readily explained was conservation of mass. For example, if you mix 1 kg of water with 1 kg of dirt, you get exactly 2 kg of mud, no more and no less. The same is true for the a variety of processes such as freezing of water, fermenting beer, or pulverizing sandstone. If you believed in atoms, conservation of mass made perfect sense, because all these processes could be interpreted as mixing and rearranging atoms, without changing the total number of atoms. Still, this is nothing like a proof that atoms exist.

If atoms did exist, what types of atoms were there, and what distinguished the different types from each other? Was it their sizes, their shapes, their weights, or some other quality? The chasm between the ancient and modern atomisms becomes evident when we consider the wild speculations that existed on these issues until the present century. The ancients decided that there were four types of atoms, earth, water, air and fire; the most popular view was that they were distinguished by their shapes. Water atoms were spherical, hence water's ability to flow smoothly. Fire atoms had sharp points, which was why fire hurt when it touched one's skin. (There was no concept of temperature until thousands of years later.) The drastically different modern understanding of the structure of atoms was achieved in the course of the revolutionary decade stretching 1895 to 1905. The main purpose of chapters 1 and 2 is to describe those momentous experiments.

Are you now or have you ever been an atomist?

You are what you eat. The glib modern phrase more or less assumes the atomic explanation of digestion. After all, digestion was pretty mysterious in ancient times, and premodern cultures would typically believe that eating allowed you to extract some kind of mysterious "life force" from the food. Myths abound to the effect that abstract qualities such as bravery or ritual impurity can enter your body via the food you eat. In contrast to these supernatural points of view, the ancient atomists had an entirely naturalistic interpretation of digestion. The food was made of atoms, and when you digested it you were simply extracting some atoms from it and rearranging them into the combinations required for your own body tissues. The more progressive medieval and renaissance scientists loved this kind of explanation. They were anxious to drive a stake through the heart of Aristotelian physics (and its embellished, Church-friendly version, scholasticism), which in their view ascribed too many occult properties and "purposes" to objects. For instance, the Aristotelian explanation for why a rock would fall to earth was that it was its "nature" or "purpose" to come to rest on the ground.

The seemingly innocent attempt to explain digestion naturalistically, however, ended up getting the atomists in big trouble with the Church. The problem was that the Church's most important sacrament involves eating bread and wine and thereby receiving the supernatural effect of forgiveness of sin. In connection with this ritual, the doctrine of transubstantiation asserts that the blessing of the eucharistic bread and wine literally transforms it into the blood and flesh of Christ. Atomism was perceived as contradicting transubstantiation, since atomism seemed to deny that the blessing could change the nature of the atoms. Although the historical information given in most science textbooks about Galileo represents his run-in with the Inquisition as turning on the issue of whether the earth moves, some historians believe his punishment had more to do with the perception that his advocacy of atomism subverted transubstantiation. (Other issues in the complex situation were Galileo's confrontational style, Pope Urban's military problems, and rumors that the stupid character in Galileo's dialogues was meant to be the Pope.) For a long time, belief in atomism served as a badge of nonconformity for scientists, a way of asserting a preference for natural rather than supernatural interpretations of phenomena. Galileo and Newton's espousal of atomism was an act of rebellion, like later generations' adoption of Darwinism or Marxism.

Another conflict between scholasticism and atomism came from the question of what was between the atoms. If you ask modern people this question, they will probably reply "nothing" or "empty space." But Aristotle and his scholastic successors believed that there could be no such thing as empty space, i.e. a vacuum. That was not an unreasonable point of view, because air tends to rush in to any space you open up, and it wasn't until the renaissance that people figured out how to make a vacuum.

Last Update: 2009-06-21