Short answer: the strong force is the force that glues quarks together inside protons and neutrons. It is the strongest of the four basic forces in nature. Particles called gluons carry it.
The four basic forces
- Gravity: pulls masses together.
- Electromagnetism: acts between electric charges and magnets.
- Weak force: causes some kinds of radioactive change.
- Strong force: holds quarks together and keeps nuclei from falling apart. At the size of a nucleus, it is about 100 times stronger than electromagnetism.
What are gluons?
Gluons are the particles that carry the strong force between quarks. The name comes from “glue”, because that is what they do. Quarks constantly swap gluons, and that swapping holds them together.
Colour charge in simple words
Electric force works because particles have electric charge, plus or minus. The strong force works because quarks have “colour charge”, with three kinds called red, green and blue. These names are only labels. A proton or neutron is always “colour neutral”, meaning the three colours balance out, a bit like red, green and blue light mixing to make white.
Why can’t we see a single quark?
With most forces, the pull gets weaker as things move apart. The strong force is different. The more you try to pull two quarks apart, the stronger the pull becomes, like stretching a very strong rubber band. At some point, so much energy goes in that the “band” snaps and new quarks appear from the energy. So instead of a free quark, you get new particles made of quarks. This effect is called confinement.
A surprising fact about mass
The three quarks inside a proton add up to only about 1 percent of its weight. Most of the proton’s mass comes from the energy of the strong force that holds the quarks together, and from the gluons moving between them. In other words, a lot of the weight of everyday things is “energy of the glue”.
How does the nucleus stay together?
Protons all have a positive charge and push each other away. What keeps them from flying apart is a leftover bit of the strong force that reaches between protons and neutrons. It is very strong, but it only works over a tiny distance, about the size of a nucleus.
Quick FAQ
Is the strong force the strongest force?
Yes, at very short distances it is stronger than the other three.
What carries the strong force?
Gluons.
Can a quark ever be alone?
Not in normal conditions. Quarks only appear free in extremely hot, dense matter, such as the “quark soup” made briefly in particle accelerators.
Read more: the six quark flavours, quark vs atom and the idea of a quark star. Also see quark-gluon plasma, where quarks and gluons break free. Back to the science section