Short answer: as far as anyone can tell, nothing is smaller than a quark. Experiments have never found parts inside a quark. Some scientists have ideas about what might be inside, but none of those ideas has been proven.
Where quarks sit in the “size ladder”
It helps to climb down the ladder one step at a time:
- Atom: about one ten-billionth of a metre across (10-10 m).
- Nucleus: the core of the atom, roughly 100,000 times smaller (about 10-15 m). It is made of protons and neutrons.
- Proton and neutron: each one is made of three quarks.
- Quark: smaller still. We have no measured size for it.
Past that step, the ladder stops. When scientists smash particles together at very high energy, quarks behave like single points. They do not break apart, and they show no inside structure.
Do quarks have a size?
Nobody has measured a size for a quark. All experiments can say is that, if a quark has a size, it must be smaller than about one tenth of a billionth of a billionth of a metre (around 10-19 m). That is at least about 2,000 times smaller than a proton. The newest record comes from the Large Hadron Collider at CERN, and stronger machines may push the limit lower.
So a quark may be a true point with no size at all, or it may be so small that our machines cannot see it yet. This is why the question is hard: you cannot say what is inside something you cannot look at closely enough.
Ideas about what could be smaller
Preons
A preon would be a tiny building block that quarks and electrons are made from. The idea is neat, because it could explain why there are six kinds of quarks. But experiments show no sign of preons, so most scientists do not think they exist.
Strings
String theory says that quarks and other particles are not points at all. Instead, each one is a tiny vibrating string, and different ways of vibrating give different particles. This is a mathematical idea, and nobody has found a way to test it directly yet.
Small and light are not the same
Some particles, like neutrinos, are very light, but they are not parts of a quark. Mass and size are also different things. The top quark, for example, weighs about as much as a whole gold atom, yet no one has found any size for it. When people ask “what is smaller than a quark”, they usually mean “what is a quark made of”. The honest answer is that we do not know it is made of anything.
Why this matters
Every time a stronger machine is built, scientists look again. If quarks ever show signs of inside structure, it would be one of the biggest discoveries in physics. Until then, quarks stay at the bottom of the ladder.
Quick recap
- Quarks are the smallest known building blocks of protons and neutrons.
- No experiment has found anything inside a quark.
- Preons and strings are ideas, not proven facts.
Keep reading: compare quark vs atom, meet the strange quark, learn about the idea of a quark star, or go back to the basics of what a quark is. Back to the Quark Magazine home page.