A note for parents: Lucretius argues that the soul does not survive death and that the gods take no part in the world; these are his views, presented as his. Book IV includes a frank passage on love, and Book VI ends with a grim account of a plague; the lesson leaves out both.
For young readers
About ages 8 to 12. Read it together, or on your own.
About 2,080 years ago, a Roman poet named Lucretius wrote a long poem with a big idea: everything in the world is made of tiny bits, far too small to see, that are always moving. He called them the "first-beginnings" of things. We call them atoms.
How could he know about things nobody could see? He used clues. Wet clothes dry in the sun, but you never see the water leave. A stone step wears down over many years, but you never see a single bit go. So, he said, the bits must be too small to see.
Try this
A smell race. Have someone open a jar of something with a strong smell, like an orange peel or vanilla, at one end of a room. Stand at the other end and time how long until you smell it. Nothing visible crossed the room. What did? Lucretius would say: tiny bodies, moving through the air.Lucretius also noticed dust dancing in a sunbeam in a dark room, and thought invisible bodies were knocking it about. Today we know the dust is mostly pushed by moving air, but very tiny specks seen under a microscope really are jiggled by bumping molecules. That jiggling helped prove atoms exist, about 1,950 years after Lucretius wrote.
For teens and adults
The full lesson.
Who, when and why
Titus Lucretius Carus (c. 99–c. 55 BC) lived in the last years of the Roman Republic, a time of civil wars. Almost nothing is known of his life. His one work, On the Nature of Things (De rerum natura), is a poem in six books. The orator Cicero mentions it in a letter of 54 BC, praising both its brilliance and its skill.
Lucretius followed the Greek philosopher Epicurus (341–270 BC), who taught that the aim of life is peace of mind, and that the greatest threats to it are fear of the gods and fear of death. Lucretius's problem was how to free his readers from those fears. His answer was physics: if people understood that everything happens by natural causes, he argued, they would stop being afraid. These are Lucretius's own views about religion and death; readers, religious and not, have long argued with them.
Key idea
This terror then and darkness of mind must be dispelled not by the rays of the sun and glittering shafts of day, but by the aspect and the law of nature; whose first principle we shall begin by thus stating, nothing is ever gotten out of nothing by divine power.
Evidence for bodies you can't see
Again after the revolution of many years a ring on the finger is thinned on the under side by wearing, the dripping from the eaves hollows a stone, the bent ploughshare of iron imperceptibly decreases in the fields, and we behold the stone-paved streets worn down by the feet of the multitude. … Nature therefore works by unseen bodies.
This is a good argument from evidence: we see the total loss, so something must be leaving, but each piece is too small to see. It still works. A gold ring worn every day for fifty years loses a measurable amount of gold, a few atoms at a time.
Worked example: How small?
- Atoms lost: $0.06 \times 3 \times 10^{21} = 1.8 \times 10^{20}$.
- Seconds in 30 years: $30 \times 365 \times 24 \times 3600 \approx 9.5 \times 10^{8}$.
- Per second: $1.8 \times 10^{20} \div 9.5 \times 10^{8} \approx 1.9 \times 10^{11}$, about 200 billion atoms every second, and still you can't see the ring change from day to day.
Answer: About 200 billion atoms a second
Motes in a sunbeam
Observe whenever the rays are let in and pour the sunlight through the dark chambers of houses: you will see many minute bodies in many ways through the apparent void mingle in the midst of the light of the rays, and as in never-ending conflict skirmish and give battle combating in troops and never halting, driven about in frequent meetings and partings; so that you may guess from this, what it is for first-beginnings of things to be ever tossing about in the great void.
Common mistake
Worked example: A random walk
- For a random walk, the typical distance grows as the square root of the number of steps.
- After 100 steps: $\sqrt{100} \times 1 = 10$ micrometers.
- After 400 steps: $\sqrt{400} \times 1 = 20$ micrometers. Four times the steps, only twice the distance.
Answer: About 10 and 20 micrometers
What else the poem covers
- Book II: atoms come in many shapes but not infinitely many; they fall through the void and sometimes "swerve" slightly, which Lucretius uses to explain free will.
- Book III: the mind and spirit are made of very fine atoms and do not survive death, so death, he argues, is nothing to fear.
- Book IV: how we see, hear and dream.
- Book V: how the world, life and human society began, including an early picture of living things that failed to survive because they were not fitted to their surroundings.
- Book VI: thunder, earthquakes, volcanoes, magnets and disease.
What later science kept, and what it changed
- Kept: conservation (Lavoisier, 1789, showed that mass is neither created nor destroyed in chemical change); atoms (Dalton, 1808); and the habit of explaining the visible by the invisible.
- Changed: atoms are not indivisible. The electron was found in 1897 and the nucleus in 1911. And they are not tiny hooked or smooth pellets, as Lucretius pictured.
- Wrong: following Epicurus, Lucretius said the Sun is about as big as it looks, and he rejected the idea that things fall toward a center, which led him to deny that people could live on the other side of a round Earth.
Going deeper
Context, the big questions, and how later books answered.
Lost and found
By the Middle Ages the poem had nearly vanished. In 1417 the Italian book hunter Poggio Bracciolini found a copy in a German monastery and had it copied. Printed in 1473, it was read by Montaigne (later on this ladder), who quotes it often; by the philosopher Pierre Gassendi, who revived atomism in the 1600s; and by Thomas Jefferson, who owned several copies. Scientists of the 1600s such as Boyle and Newton built on an atomism that owed much to Epicurus and Lucretius, though they put it within a world made by God.
Some recent writers have claimed that this one rediscovery started the modern world. Many historians think that goes too far: the scientific revolution had many causes. But the poem was one of the routes by which ancient atomism reached early modern science.
Science and peace of mind
Lucretius believed that understanding nature brings calm. Many readers since have shared some version of that hope; others have argued that his picture of a purposeless world leaves something out. Dante, Milton and Pascal, later on this ladder, offer very different accounts. The question of what science can and cannot tell us about how to live is one the Great Books keep returning to.
Big questions
- Is it reasonable to believe in things nobody can see? What evidence would convince you?
- Lucretius thought knowing the causes of things would take away fear. Does understanding something make it less frightening?
- A poem taught science for centuries. Can poetry teach science well?
Talk about it together
- Do the smell race. How else could you show that tiny invisible things are moving around you?
- Lucretius said nothing comes from nothing. Can you think of anything that seems to come from nothing? Where does it really come from?
- When a log burns, it seems to disappear. Where does it go?
- Why do you think Lucretius wrote about science as a poem instead of a plain book?
- Lucretius was right about atoms but for some wrong reasons. Is it still fair to give him credit?