Strong interaction Kids Encyclopedia

An animation showing how the strong force is moved between a proton and neutron through gluons.

The strong interaction (or strong nuclear force) is one of the four fundamental forces in particle physics, which are forces which can't be made even simpler. Despite being the most powerful fundamental force, being 10³⁸ (1 followed by 38 zeros) times as strong as gravity, it only works over lengths of a few femtometres (fm), which is about 10^-15 (0.000000000000001) metres long. Scientists often divide it into two parts: the colour force and nuclear force. At distances of 0.8 fm and less, the colour force is responsible for holding subatomic particles, like protons and neutrons together. At distances of 1 to 3 fm, the nuclear force binds subatomic particles into atom nuclei.

The strong interaction is often thought to be controlled by gluons, which 'glue' quarks together through the strong force. Gluons can be exchanged (moved) between quarks, antiquarks and other gluons. All of those particles are said to carry a 'colour charge', a property of elementary particles which is like electric charge. Particles with colour charge exchange gluons, like how particles with electric charge exchange electrons.

In the theory of quantum chromodynamics (QCD), the strong force is the interactions between quarks and gluons. Quantum chromodynamics is the theory that explains different colours. The strong force is the basic force controlled by gluons, affecting quarks, antiquarks, and the gluons themselves.

The strong force only affects quarks directly. Between hadrons, the strong force is known as the nuclear force. The strong force is the reason why we can't see free quarks, i.e. quarks that are by themselves. This event is called colour confinement, a theory which says you can only see hadrons.

Colour strong force

The colour strong force is the nuclear force that acts between the three quarks that a proton or neutron is made of. It is called the colour strong force because, like the electromagnetic force, the strong force has charges. The major difference is that the electromagnetic force has only one charge (magnetic charges are just slow-moving electric charges), and the strong force has three. These three types of charges are named after colours: red, blue, and green. They also have anti-colours: anti-red, anti-blue and anti-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 called the gluon. Gluons also have color charge. Both quarks and gluons have properties that make them unique from other particles.

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  The three quark colors (red, green, blue). They combine to be white, or colorless

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  The three quark anticolors (antired, antigreen, antiblue). They also combine to be colorless; black in a case referring to physical matter or pigments.

Nuclear force

The nuclear force, or residual (leftover) strong force is the strong force as it acts between hadrons (particles made of two or three quarks, e.g. protons and neutrons). It is what holds the nucleus of an atom together.

Related pages

• Particle physics
• Isotope
• Nuclear physics