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Strong interaction

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The strong interaction (or strong nuclear force) is a force that acts between particles in the nucleus of an atom. It is what holds the nucleus together. It is one of the four basic forces in physics.

Under the theory of quantum chromodynamics (QCD), the strong force represents the interactions between quarks and gluons. Quantum chromodynamics is the theory that explains different colours. The strong force is the fundamental force mediated by gluons, acting upon quarks, antiquarks, and the gluons themselves.

Although the strong force only acts upon elementary particles directly, the force is observed between hadrons as the nuclear force. As has been shown by many failed free quark searches, the elementary particles affected are unobservable directly. This phenomenon is called confinement, a theory which allows only hadrons to be seen.

The strong force is about 167 trillion trillion trillion times as strong as gravity and works over 1 trillionth of a millimeter. There are two types of strong force: residual (left over) and fundamental (basic).

Fundamental/ Color Strong Force

Fundamental, or color strong force is the nuclear force that acts between the three quarks that a proton or neutron is made of. It is called the color strong force because, like the electomagnetic force, the strong force has charges. The major difference, though, is that while the electromagnetic force has two types of charges, the strong force has three. These three types of charges are named after colors, they are red, blue, and green. Like the electromagnetic force, opposite colors attract, and the same colors repel. Some particles that have color charge are quarks and antiquarks. The type of quark is not related to that quark's color charge at all. Quarks are one of the smallest particles currently known to humans; they take up no space because they are points, and the only particles that we have not been able to break apart from other particles yet. This is in fact because the nature of the strong force between particles is that it becomes stronger the further away the particles are. The force carrier of the strong force is the gluon. Gluons also have color charge. Both quarks and gluons have properties that make them unique from other particles.


  • There are six types of quarks: up, down, charm, strange, top and bottom.
  • All quarks have an electric charge charge of either 2/3 or -1/3.
  • All quarks have a spin of 1/2.
    residual strong force acts on protons and neutrons


  • Gluons have a mass and electric charge of 0.
  • Gluons have a spin of 1.

Residual Strong Force

Residual strong force is the type of strong force that acts between hadrons (stuff made of two or three quarks, e.g. protons and neutrons). It is what holds the nucleus of an atom together. Alpha decay is a result of the residual strong force. This force is carried by pions, which are made of one quark and one antiquark. Protons and neutrons are some of the particles that experience it. Each of these three particles has unique properties as well.


  • Pions have a charge of -1, 0, or 1.
  • Pions have a spin of 0.


  • Protons have a charge of 1.
  • Protons have a spin of 1/2.


  • Neutrons are neutral, or have a charge of 0.
  • Neutrons have a spin of 1/2.

Related pages


  • David J. Griffiths, 1987. Introduction to Elementary Particles. John Wiley & Sons. ISBN 0-471-60386-4
  • Gordon L. Kane (1987). Modern Elementary Particle Physics. Perseus Books. ISBN 0-201-11749-5
  • Richard Morris, 2003. The Last Sorcerers: The Path from Alchemy to the Periodic Table. Washington DC: Joseph Henry Press. ISBN 0-309-50593-3

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