strong interactions

• strongest of the four fundamental interactions, range ~10⁻¹⁵ m (the size of a nucleus)
• acts on colour charge, carried by gluons
• binds quarks into hadrons (e.g. protons, neutrons)
• confinement: force between quarks stays roughly constant with distance → no isolated quarks
• asymptotic freedom: weak at very short distances (Nobel Prize 2004)
• theory: quantum chromodynamics (QCD)
• residual strong (nuclear) force: binds protons and neutrons into nuclei, reaches only nearest neighbours
• no stable nuclei heavier than lead: electric repulsion across the whole nucleus wins