2.3.1 The Four Fundamental Interactions

• There are four fundamental interactions, or fundamental forces, that exist. These are:
  • Gravity
  • Electromagnetism
  • Strong Nuclear (or Strong Interaction)
  • Weak Nuclear

• Gravity is the weakest of these forces, whilst the strong interaction is the strongest (hence the name)
• They also have different ranges:
  • The electromagnetic and gravitational interactions have an infinite range
  • The weak force has a range of up to 10^–18 m
  • The strong force has a range of ~ 10^–15 m

The fundamental interactions in order of strength

• The gravitational interaction only affects particles with mass
• The electromagnetic interaction only affects particles with charge
• The weak interaction affects all particles
• The strong interaction only affects hadrons

• When two particles interact, there cannot be instantaneous action at a distance
  • This means one particle needs to "know" that the other is there

• This is the idea behind exchange (or virtual) particles
• When two particles exert a force on each other, a virtual particle is created
• Virtual particles only exist for a short amount of time and carry the fundamental force between each particle
• A force can be attractive or repulsive. An analogy of exchange particles would be:
  • Two people are on skateboards and a ball is passed between them. Due to this, they start to move away from each other. The ball represents an exchange particle creating repulsion
  • However, if one person throws a boomerang to the other, they will start to move closer together. The boomerang represents an exchange particle creating attraction

Both skateboarders can create repulsion or attraction by throwing a ball or boomerang between them. The ball and boomerang represent exchange particles.

• Each fundamental interaction is transmitted by its own exchange particle
  • These are also called gauge bosons

Table of Exchange Particles

• Since gravity is so weak, it only has a noticeable effect on large masses, therefore, gravity does not play a part in particle interactions
• The theorised exchange particle for the gravitational force is the graviton, however, this has not yet been discovered