The Four Basic Forces

 

All of the known forces in the universe can be grouped into four basic types. In order of increasing strength, these are: 

1. The gravitation force
2. The weak force
3. Electromagnetic force
4. The strong force. 

These forces have important roles not only in the interactions between particles, but also in the decay of one particle into other particles. 

The gravitational force

Gravity is of course exceedingly important in our daily lives, but on the scale of fundamental interactions between particles in the subatomic realm, it is of no importance at all. To give a relative figure, the gravitational force between two protons just touching at their surfaces is about 10^-38 of the strong force between them. The principal difference between gravitation and the other forces is that, on the practical scale, gravity is cumulative and infinite in range. For example, your weight is the cumulative effect of the gravitational force exerted by each atom of the Earth on each atom of your body.

The weak force

The weak force is responsible for nuclear beta decay and other similar decay processes involving fundamental particles. It does not play a major role in the binding of nuclei. The weak force between two neighboring protons is about 10^-7 of the strong force between them, and the range of the weak force is smaller than 1 fm. That is, at separations greater than about 1 fm, the weak force between particles is negligible. Nevertheless, the weak force is important in understanding the behavior of fundamental particles, and it is critical in understanding the evolution of the universe.

The electromagnetic force

Electromagnetism is important in the structure and the interactions of the fundamental particles. For example, some particles interact or decay primarily through this mechanism. Electromagnetic forces are of infinite range, but shielding generally diminishes their effect for ordinary objects. The properties of  atoms and molecules are determined by electromagnetic forces, and many common macroscopic forces (such as friction, air resistance, drag, and tension) are ultimately due to the electromagnetic force. The electromagnetic force between neighboring protons is about 10^-2 of the strong force, but within the nucleus the electromagnetic forces can act cumulatively because there is no shielding. As a result, the electromagnetic force can compete with the strong force in determining the stability and the structure of nuclei.

The strong force

The strong force, which is responsible for the binding of nuclei, is the dominant one in the reactions and decays of most of the fundamental particles.

However, as we shall see, some particles (such as the electron) do not feel this force at all. It has a relatively short range, on the order of 1 fm. The relative strength of a force determines the time scale over which it acts. If we bring two particles close enough together for any of these forces to act, then a longer time is required for the weak force to cause a decay or reaction than for the strong force. As we shall see, the mean lifetime of a decay process is often a signal of the type of interaction responsible for the process, with strong forces being at the shortest end of the time scale (often down to 10^-23 s). The characteristic time for each force gives a typical range of time intervals observed for systems in which each force acts. Usually this is the typical lifetime of a particle that decays through that force.