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It’s the first Friday of the month, and that means another installment in the series about the theory of atoms from physics professor (and my dad) Dean Zollman.–Kim

By Dean Zollman

Dean ZollmanWhat happens internally when a plant or animal grows? How do we smell an aroma almost as soon as someone walks into a room with flowers? Where does the “stuff” go when a plant decays? These questions may have occurred to earlier civilizations, but the Greeks were the first to record answers them.

We cannot see what is happening in these and many other situations, so we must build a scientific model in any attempt to understand it. Greek philosophers in the sixth and fifth centuries BCE had many ideas about substance and how thing change. However, Leucippus and Democritus were the first to develop a model that is similar, in some ways, to our present ideas of atoms.

They concluded that all matter was made of small indivisible and indestructible objects. These objects were called atoms, based on the Greek word atomos, or indivisible. Leucippus and Democritus hypothesized that atoms came in a variety of sizes and shapes. The atoms combined in different ways to form the difference that we see.


Democritus, as depicted in 1628 by Hendrick ter Brugghen (public domain image via Wikimedia Commons).

Democritus, who is sometimes called the laughing philosopher, developed the concept further and stated that these atoms were in constant motion, frequently colliding with each other. With this kind of motion, he could explain the aroma from flowers. The atoms from the flowers move across the room colliding with air atoms and spreading out through the room along the way. When the reach our nose, we smell the roses.

For atoms to move through the room Democritus needed to postulate that empty space existed. Atoms cannot move unless there is space to move in. This empty space was called the void.

Two very important philosophers of the generations that followed Democritus did not like these ideas. The void did not exist. Plato never mentions Democritus.  Plato’s student, Aristotle, was somewhat more direct in his criticism.

Aristotle’s description of forces and motion required that all space be filled with matter. So, he felt the need to refute Democritus’s ideas. Aristotle wrote at length describing Democritus’ concepts about atoms and what was wrong with them.

We are fortunate that Aristotle did this. Most of Democritus’ writings have been lost. If Aristotle had not described Democritus’ ideas so he could tell the world why Democritus was wrong, we might not today know how close to right Democritus was.

On the other hand, Aristotle’s science and philosophy dominated Western thought for most of the next 2,000 years. So interest in Democritus’ atoms, except in a few cases, was limited for a long time. We will look at one of those cases, an epic poem by Lucretius, next time.

Previously: An overview of the history of the theory of atoms.

Dean Zollman is university distinguished professor of physics at Kansas State University, where he has been a faculty member for more than 40 years. During his career he has received three major awards—the National Science Foundation Director’s Award for Distinguished Teacher Scholars (2004), the Carnegie Foundation for the Advancement of Teaching Doctoral University Professor of the Year (1996), and American Association of Physics Teachers’ Robert A. Millikan Medal (1995). His present research concentrates on the teaching and learning of physics and on science teacher preparation.