Chapter 4. Strong Force – the “purple” field

Chapter 4. Strong Force – the “purple” field


The discovery of the atomic nucleus led to the discovery of a new force field that binds protons and neutrons tightly together in the atomic nucleus.  It is called the strong field and its quanta are called pions. The strong field has the following characteristics:

  • It is generated by and acts on nucleons.
  • Its force is always attractive, like gravity.
  • It is strong enough to overcome the electric repulsion between protons (hence its name).
  • It has a short range, with an intensity that drops almost to zero at distances greater than the size of a nucleus.
  • It has a mass that causes pions to propagate more slowly through space than photons. The mass and short range arise from the same term in the field equations.
  • It comes in three electrical varieties: positive, negative and neutral.
  • It has zero spin.
  • Because of the mass and charge, the field quanta behave more like particles than do photons.

The three fields presented so far can be summarized as follows:

Field Spin Quantum Interacts with “Color”
Gravity 2 Graviton everything blue
EM 1 Photon (γ) charged fields green
Strong* 0 Pion (π+, π, πo) Nucleons purple

*We will see in Chapter 10 that the strong field is made of more basic but “invisible” fields called quarks and gluons.