Question

What causes strong interaction force?

Answer 1

Actually there is no strong nuclear force. It is now referred to as the residual strong nuclear force.

Explanation:

In the 20th century is was thought that there was a strong nuclear force which bound protons and neutrons in an atomic nucleus. The force carrier was the `pi` meson.

It was later discovered that protons and neutrons and in fact also `pi` mesons are not fundamental particle but are made up of quarks.

Quarks are bound by the colour force propagated by guons.

The strong nuclear force is now referred to as the residual strong force which is the effect of the colour force acting outside of protons and neutrons. The `pi` meson is a quark anti-quark pair which binds protons and neutrons.