Swerve, String and Chimera
Two thousand years ago, Lucretius built a whole world from falling atoms and one tiny swerve. Modern particle physics keeps more of his picture than you might expect, and overturns much of it. This essay follows the thread from his atoms to quarks and mesons, and ends with an illustration that imagines the swerve at work in the mind.
Lucretius: atoms, swerve and images
In De rerum natura, Lucretius rests everything on one principle: nothing comes from nothing, and nothing returns to nothing. Things are made and unmade, but only by assembling and dispersing atoms that are never themselves destroyed. Division must stop somewhere, he argues, or the world would long ago have crumbled to nothing.
The atoms have a lower limit in a second sense. Each, he says, contains “smallest parts” that can never exist on their own, only bound together in the atom.
The atoms fall through the void like parallel rain. Left to themselves they would never meet, so at no fixed time or place one swerves by the smallest possible amount. This is the clinamen. Collisions follow, atoms hook together, and worlds form. The same swerve, Lucretius claims, breaks the chain of fate and makes free will possible.
Objects also shed thin films of atoms from their surfaces, the simulacra. These travel to our eyes and minds and are how we see. The finest of them drift through the air and merge. An image of a man meets an image of a horse, and the mind sees a centaur. Even imagination, for Lucretius, is particles in motion.
Particle physics: what survives and what changes
Some of Lucretius’ picture has a modern counterpart. The light we see from an object is emitted and scattered by electrons in its outer atoms, and it carries the object’s shape to the eye, much like his films. The difference is that his films were made of the object’s own matter. Photons are not; they are created at the surface and absorbed in the eye.
His smallest parts have a striking echo too. The proton turned out to be divisible into quarks, but quarks can never be isolated. Pull two apart and the energy you put in makes a new quark and antiquark, so you get two particles instead of a free quark. Like his smallest parts, quarks exist only bound to others.
The largest difference is conservation. Matter is not conserved: particles appear and vanish all the time, and mass converts into energy and back. What is conserved is mass and energy together, along with a handful of exact counts: electric charge, baryon number and lepton number. These counts are the modern form of his principle. No reaction ever makes them from nothing or destroys them into nothing. Particles change; the counts do not.
How particles transform
Three interactions govern how particles change, and each has its own job. I explored them through twenty equations in my Particle Equation Builder, built for A level physics: decays such as n → p + e⁻ + ν̄ₑ, and reactions such as p + p → p + p + K⁺ + K⁻.
| Interaction | Carrier | What it does | Example |
|---|---|---|---|
| Weak | W⁺, W⁻ bosons | The only one that changes one kind of quark or lepton into another | K⁺ → π⁺ + π⁰: an anti-strange quark becomes an anti-up |
| Strong | Gluons (pions between nucleons) | Binds quarks into hadrons; makes and destroys matching quark–antiquark pairs | p + p → p + p + K⁺ + K⁻: a strange pair made from collision energy |
| Electromagnetic | Photons | Acts on charge; destroys matching pairs into light, or makes them from it | π⁰ → γ + γ |
In the equations, decays of baryons and most mesons are weak. The neutral pion is the exception: a quark with its own antiquark can annihilate into photons, a hundred million times faster. The strong interaction appears in collisions, where it binds, creates and rearranges. A meson is matter and antimatter bound together, so it can vanish entirely, leaving only photons, leptons and neutrinos.
The strong force between quarks behaves like a string. Its tension stays roughly constant as the quarks separate, about 1 GeV per femtometre, or some 160,000 newtons. So the stored energy climbs steadily until it can pay for a new quark–antiquark pair. Then the string snaps into two shorter strings, each starting again with little energy stored. The force holds steady while the energy rises and falls in a sawtooth.
Which pair appears at each break is a matter of chance: up, down, or less often strange. The laws decide what is allowed; chance decides which allowed outcome happens. It is the nearest thing in modern physics to a clinamen.
The swerve in the mind
This illustration imagines particles cascading through the mind. At its centre is a green form labelled PLANT, a seed or a starting thought. Around it, blue links loop and break like the meson string, each snap making something new. “AND” repeats around the edges, the joining word, like Lucretius’ atoms hooking together or his images merging into a centaur. Numbered markers suggest a sequence of moves, and a red double arrow measures a gap that is about to be crossed.
The sequence I have in mind runs from swerve, to particle conversion, to an unexpected thought. A tiny undetermined event tips the balance; a cascade of lawful transformations follows; and something arrives in the mind that was not there before. Lucretius would call the result a chimera, a new image built from old parts. I would call it free will at work.
I have left out the quantum wave function, which is the mathematics behind all of this. It decides how often and when each allowed transition happens, while the conservation laws decide which transitions are allowed at all. In short, the laws set the menu and chance chooses from it.
The illustration is an analogy, not a mechanism. The brain works with chemical energies millions of times smaller than these particle conversions. Your body does host about 4,000 weak-force decays every second, mostly from potassium-40, but nothing suggests they shape our thoughts. Whether quantum chance plays any part in the mind is still disputed.
What the analogy keeps is the structure Lucretius saw: lawful motion, a minimal deviation, and a world, or a thought, that could have been otherwise.