Chapter 7
The fundamental forces

The fundamental forces are normally divided in four groups, of the four so-called “fundamental” forces. These are often naturally classified with respect to a dimensionless measure of their strength. To set these dimensions we use , c and the mass of the proton, m p. The natural classification is then given in table 7.1 . Another important property is their range: the distance to which the interaction can be felt, and the type of quantity they couple to. Let me look a little closer at each of these in turn.


Table 7.1: A summary of the four fundamental forces




Force Range Strength Acts on








Gravity G N 6 1 0 3 9 All particles (mass and energy)




Weak Force < 1 0 1 8 m G F 1 1 0 5 Leptons, Hadrons




Electromagnetism α 1 1 3 7 All charged particles




Strong Force 1 0 1 5 m g 2 1 Hadrons





In order to set the scale we need to express everything in a natural set of units. Three scales are provided by and c and e– actually one usually works in units where these two quantities are 1 in high energy physics. For the scale of mass we use the mass of the proton. In summary (for e= 1 we use electron volt as natural unit of energy)

= 6 . 5 8 × 1 0 2 2  MeV s (7.1) c = 1 . 9 7 × 1 0 1 3  MeV m (7.2) m p = 9 3 8  MeV c 2 (7.3)