Science › Fields & particles
The strong force and quarks
1973· Theory
ProvenTested many times; part of working physics.
The strongest force, which binds quarks into protons and neutrons and holds atomic nuclei together.
See it on the Science map →How it works
Protons and neutrons are each made of three quarks, glued by particles called gluons. The theory, quantum chromodynamics (1973), says the force between quarks grows as they separate, like a rubber band, so a lone quark is never seen. A leftover of this force binds protons and neutrons into nuclei despite the protons' electric repulsion. About 99% of a proton's mass is the energy of this binding, not the quarks themselves.
Why it matters
It explains why nuclei exist and where most of the mass of ordinary matter comes from, via E = mc².
How we know
Quarks proposed by Gell-Mann and Zweig in 1964; seen inside protons at Stanford in 1968. Gross, Wilczek and Politzer's 1973 theory (Nobel 2004) matches collider data precisely.
Still a mystery
Nobody has mathematically proven that quarks must stay confined. A closely related problem (the Yang-Mills 'mass gap') carries a $1 million Clay Millennium Prize.
Connections (4)
▸The strong force and quarks binds the nuclei of The first elements: hydrogen and heliumProven
The relationship
The strong force's leftover pull glues protons and neutrons into nuclei, overpowering the electric repulsion between protons. In the first minutes, it first had to bind a proton and neutron into fragile deuterium, which hot radiation kept breaking apart. Only once the universe cooled, about three minutes in, could deuterium survive and build helium.
How the link was found
Hideki Yukawa proposed a theory of the nuclear force in 1935 (Nobel 1949). Its role in early-universe element building was worked out from 1948 onward.
Why it's proven
Nuclear binding energies measured in labs, plugged into Big Bang calculations, give the observed helium and deuterium amounts.
Open questions and mysteries
If the strong force were slightly different, deuterium or carbon might not exist. Why it has this exact strength is unknown.
▸The strong force and quarks is one part of The Standard ModelProven
The relationship
Quantum chromodynamics, the theory of quarks and gluons, is the strong-force piece of the Standard Model. It uses the same kind of symmetry-based field theory as the electroweak part but with three types of 'color' charge. Combined with electroweak theory, it covers every particle interaction seen in a lab.
How the link was found
Murray Gell-Mann, Harald Fritzsch and Heinrich Leutwyler (1973), with the key discovery of Gross, Wilczek and Politzer the same year.
Why it's proven
Particle jets at colliders, gluons seen at DESY in 1979, and precise calculations of proton collisions all match.
Open questions and mysteries
The strong and electroweak forces sit side by side but are not truly unified. Grand unified theories that would merge them remain unconfirmed.
▸The strong force and quarks binds quarks into protons in Quarks bind into protons and neutronsProven
The relationship
The strong force between quarks weakens at high energy, so above about 2 trillion degrees quarks and gluons were not confined inside particles. As the universe cooled, typical collision energies fell to where the force becomes strong and confining. Quarks could no longer stay apart and were locked into protons, neutrons and short-lived particles, within a few millionths of a second.
How the link was found
The 1973 discovery that the force weakens at high energy implied a free quark plasma when hot. John Collins and Malcolm Perry, and separately Nicola Cabibbo and Giorgio Parisi, drew that conclusion in 1975.
Why it's proven
Lattice simulations of the strong force, precise by 2014, put the transition near 155 MeV. Heavy-ion collisions at RHIC and the LHC produce and measure the plasma, consistent with them.
Open questions and mysteries
Is there a 'critical point' where the smooth change turns sharp? Not in the early universe, with its near-even matter and antimatter, but perhaps in denser matter. RHIC's Beam Energy Scan (2010–2021) was inconclusive; neutron-star cores may hold it.
▸Axions: ultralight particles would fix a symmetry puzzle in The strong force and quarksSpeculative
The relationship
The strong force's equations allow a setting, an angle, that would make neutrons electrically lopsided, slightly positive at one end. Experiments find neutrons balanced at least ten billion times better than a random setting would give. Peccei and Quinn proposed that the setting is a field that relaxes to zero by itself, like a ball settling in a bowl. Its tiny wobble is the axion.
How the link was found
Roberto Peccei and Helen Quinn, at Stanford, 1977. Steven Weinberg and Frank Wilczek realized independently in 1978 that the mechanism requires a new particle.
Is it proven?
The puzzle is solid: the best measurement, at the Paul Scherrer Institute in Switzerland (2020), finds no lopsidedness. The axion fix is elegant but untested until an axion is found.
Open questions and mysteries
The original axion was quickly ruled out; today's 'invisible' versions interact far more weakly. Other fixes exist. Why nature chose the zero setting is unsettled.