Strong force & gluons
Before you start
An atomic nucleus packs several protons into a tiny space. According to Coulomb’s law, those positive charges should push each other apart with an enormous force, yet many nuclei are perfectly stable. Something stronger must hold the nucleus together. The same strong interaction also acts inside each proton, where three quarks are bound so tightly that a single quark has never been seen on its own. This page looks inside a proton at that interaction and at the gluons that carry it.
A proton contains quarks, so what holds them together? The Standard Model page introduces these particles; if you have not read it, this page starts again from inside the proton. We will use colours to distinguish a property called colour charge; this is not ordinary visible colour. First observe an interaction involving gluons, then try pulling quarks apart. Follow where the work we do goes, and why new hadrons can form instead of leaving a single isolated quark.
The setupA proton has two up valence quarks and one down valence quark. Colour charge is their charge for the strong interaction, distinct from electric charge. Red, green and blue name three states, not visible colours. Flavour (up/down) and colour charge are separate properties; gluons also carry colour charge.
What to do and noticeFollow successive pairs: all three quarks participate. A highlighted exchange involves two quarks, while the third still interacts with the field. The pulse is simplified colour bookkeeping, not a definite trajectory of a virtual gluon.
Predict firstWhat is the cloud around the three markers? Is a proton just three little balls?
Understand why pulling quarks apart normally makes new hadrons instead of freeing a quark.
Prerequisites: Atomic nuclei contain protons and neutrons, which contain quarks. Here u means up quark and d means down quark; a bar denotes an antiparticle, and Q a heavy quark. Watch the animation and the nearby readings first; you do not need to remember the formulae.First, enter a proton
Predict firstWhat is the cloud around the three markers? Is a proton just three little balls?
The u, u, d markers denote the proton’s three valence quarks: its net quark content. A real proton also contains gluons and a dynamic quark–antiquark sea. The cloud suggests spatial field structure and the filaments emphasise concentrated regions; they are not countable gluon orbits. Quarks have no observed surfaces or sizes like these enlarged markers. Drag to orbit and become familiar with the structure.
How to observe
- Follow a two-colour pulse: colour changes, quark flavour does not.
- Stretch the flux tube and follow external work into field energy and new hadrons.
- Explore a three-gluon vertex and weaker coupling at higher energy scales.
What this model simplifies
A colour-flow and effective-string teaching model, not real-time lattice QCD or a photograph of a proton. Positions, sizes, Y-shaped structure, filaments, pulses and seconds of animation are illustrative. Colours label basis components; singlets require quantum superpositions. While stretching, the field energy grows in proportion to the separation and is checked against the external work. The illustrative pair/hadronisation budget is not a bare quark mass; the smooth transition is not a quantum production probability. Further separation of the two final clusters is for display, with no unaccounted kinetic-energy claim. The heavy–light mesons show colour structure, not a predicted hadron spectrum. The three-gluon junction is a vertex within a process, not a free on-shell gluon decay. Fixed-five-flavour one-loop coupling is used only from 10 to 150 GeV, never extrapolated into confinement. Ordinary low-temperature hadron confinement cannot be applied unchanged to quark–gluon plasma. The residual nuclear interaction between nucleons is related to QCD but is not this same flux tube.
