Strong interaction

In nuclear physics and particle physics, the strong interaction, also called the strong force or strong nuclear force, is one of the four known fundamental interactions. It is what holds tiny particles called quarks together to form larger particles like protons and neutrons, which are parts of atoms.

The strong interaction is very powerful. At a very small distance, about 10⁻¹⁵ m, it is about 100 times stronger than the force that makes magnets work, and much stronger than other forces that we usually think about.

This force also helps hold together the center of atoms, called the nucleus, by binding protons and neutrons together. This binding allows stars like our Sun to shine by creating energy through a process called nuclear fusion. It also plays a role in nuclear power and nuclear weapons.

History

Before 1971, scientists were unsure why the atomic nucleus stayed together. They knew it was made of protons, which have a positive electric charge, and neutrons, which have no charge. Normally, positive charges push each other away, so they wondered why the nucleus didn’t fall apart.

To explain this, they suggested a very strong attractive force, called the strong force, that acted on the protons and neutrons in the nucleus. In 1964, Murray Gell-Mann and George Zweig proposed that protons, neutrons, and other particles are made of tiny building blocks called quarks. The strong force that holds the nucleus together works because it binds these quarks together inside protons and neutrons. This idea is explained by a theory called quantum chromodynamics, which says that quarks have a special property called a color charge, and particles called gluons carry this force between quarks.

Behavior of the strong interaction

The strong interaction is one of the basic forces in nature. It works in two ways. First, it holds tiny particles called quarks together to make bigger particles like protons and neutrons. This happens at very small distances, even smaller than the width of a hair.

When protons and neutrons come close together, the strong interaction also helps bind them to form the center of an atom, called the nucleus. This binding force is a bit weaker than the force that holds quarks together, but it is still very strong. It is this force that keeps the nucleus of an atom from falling apart.

Because of the strong interaction, most of the mass of protons and neutrons comes from the energy of this force, not just from the quarks themselves. This force is so strong that scientists study it using special tools and experiments.

Unification

Some theories, called Grand Unified Theories, try to explain the strong interaction and another force called the electroweak interaction as parts of one bigger force. This is similar to how two other forces were combined into the electroweak interaction. The strong interaction has a special property where it becomes weaker at very high energies or temperatures. Scientists think there might have been a time when the strong force and the electroweak force were together, but this is still just a theory and has not been proven yet.

If these theories are true, after the Big Bang, the electroweak force may have split from the strong force. This idea suggests there was a time called the grand unification epoch before this split happened.