This is an excerpt from The Cosmic Machine: The Science That Runs Our Universe and the Story Behind It

By Scott Bembenek, PhD

Democritus (c. 460 BC–c. 370 BC) was a native of Abdera in Thrace, located in present-day Greece. He traveled widely, perhaps spending time in Egypt and Persia. He also spent time in Athens: “I went to Athens and no one knew me.”

Indeed, it seems that in Athens Democritus never really fit into the intellectual elite, and his philosophy was ignored for some time. Nonetheless, his wealth of knowledge and exactness of thinking give him a much-deserved place in the history of philosophy. By our present-day standards, he was perhaps the most successful of the ancient Greek philosophers with regards to the remarkable accuracy of his ideas. For example, he considered the Milky Way to be a collection of tiny stars, and the Moon to be very much like the Earth in that it contained mountains and valleys. Regardless, we know him primarily for his atomic theory.

Democritus was a student of Leucippus (fifth century BC), who had an atomic theory of his own. In fact, it’s hard to untangle the atomic theories of Democritus and Leucippus. This is mostly because we know very little about Leucippus, and it has been speculated that he never actually existed, although this seems unlikely since Aristotle and Theophrastus (c. 371 BC–c. 287 BC) mentioned his atomic theory explicitly. It seems more likely that Leucippus set in place some of the fundamentals, and that Democritus built upon them, thereby extending the overall theory.

Democritus considers everything in the universe – including the human mind and soul, and even the gods – to be made up of atomos, which is Greek for indivisible and from which we get the word atom. Indeed, Democritus considered these atoms to be indivisible (contrast this with Anaxagoras, who considered his fundamental pieces to be infinitely divisible). He imagined atoms to occur in a variety of different shapes and sizes, which were responsible for the properties found in the objects they made up. Moreover, he considered atoms to be changeless, eternal, and indestructible, similar to the way Empedocles envisioned his four fundamental elements.

Democritus saw material objects as existing in a temporary state, being created or destroyed as atoms come together or fall apart under the influence of natural forces; all that remains, then, are the atoms constituting those material objects. This is not unlike Empedocles’ view, where he imagined the four elements giving rise to material objects under the influence of the forces Love and Strife. In addition, Democritus also gave motion to his atoms.

Democritus imagined atoms as always in motion, undergoing collision after collision with each other as they moved around. Moreover, this motion was a fundamental property and, like the atoms themselves, was eternal and indestructible, although changeable under certain circumstances.

In order for atoms to be in motion, there must be a space for them to move, and thus Democritus invented the void. According to Democritus, atoms move in the void with a constant random motion (he likened the movement of atoms to the dust particles one sees dancing around in the sunlight when there’s no breeze). This is much like how we imagine them doing so today, as described by modern-day kinetic theory.

Recall that in Parmenides’ philosophy, material things have existence because we are able to think of them. He also considers it impossible to think of nothing, and therefore it can’t exist. Thus, Democritus’ void may seem to be in blatant disregard of this tenet, as for all practical purposes it seems to be nothing. However, Democritus saw the void as something: a place independent of the atoms for the atoms to reside and move around in. The real problem is that Parmenides could only imagine material objects as something, whereas Democritus was able to imagine both a material object (the atom) and the space it lived in as being something. Democritus makes his point clear: “Nothing exists except atoms and empty space; everything else is opinion.”

Democritus made concessions to both Parmenides and Heraclitus, just as Empedocles and Anaxagoras did, by imagining a universe consisting of an infinite number of changeless, eternal, and indestructible atoms, always engaged in random collisions with each other, and capable of comingling to form material objects as we know them.

Aside from its remarkable similarity with modern-day atomic theory, Democritus’ atomic theory is redeeming in itself for the very fact that it offers a “mechanical explanation” for matter: matter is made of atoms that move in a void and undergo collisions (where preceding collisions are determined by previous ones) that are governed by certain physical laws of nature.

He invokes no divine intervention in this atomic process, but quite simply he maintains that atoms have always been and always will be in motion, and that physical laws describe this motion. The beauty of such a construct is that it lends itself to a scientific description. That is to say, one can hope to develop a mathematical theory describing the physical laws and then proceed to perform experiments to test this theory.

Obviously, neither the needed mathematics nor the experimental procedures were available to Democritus. Additionally, Democritus’ theory suffered another blow – namely, Aristotle, who stunted the development of Democritus’ work. On several accounts he mentions Democritus’ atomic theory explicitly, only to attack it. Ironically, it’s in this way that we learn much, perhaps the majority, of what we know of Democritus’ atomic theory.

Dr. Scott Bembenek is a principal scientist in the Computer-Aided Drug Discovery group at Johnson & Johnson Pharmaceutical Research & Development in San Diego. He is also the author of The Cosmic Machine: The Science That Runs Our Universe and the Story Behind It. To learn more about Dr. Bembenek and his work, visit and connect with him on Twitter


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