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 $\hslash$, $c$ 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.

In order to set the scale we need to express everything in a natural set of units. Three scales are provided by $\hslash$ 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)

$$\begin{array}{rcll}\hslash & =& 6.58\times 10^{-22}\text{ MeV s}& \text{(7.1)}\\ \hslash c& =& 1.97\times 10^{-13}\text{ MeV m}& \text{(7.2)}\\ m_p& =& 938\text{ MeV}∕c^2& \text{(7.3)}\end{array}$$

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