The fundamental forces are normally divided in four groups, of the four socalled “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.




Force  Range  Strength  Acts on 








Gravity  $\infty $  ${G}_{N}\approx 6\phantom{\rule{0.3em}{0ex}}1{0}^{39}$  All particles (mass and energy) 




Weak Force  $<1{0}^{18}$ m  ${G}_{F}\approx 1\phantom{\rule{0.3em}{0ex}}1{0}^{5}$  Leptons, Hadrons 




Electromagnetism  $\infty $  $\alpha \approx 1\u2215137$  All charged particles 




Strong Force  $\approx 1{0}^{15}$ m  ${g}^{2}\approx 1$  Hadrons 




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)
$$\begin{array}{rcll}\hslash & =& 6.58\times 1{0}^{22}\text{MeVs}& \text{(7.1)}\text{}\text{}\\ \hslash c& =& 1.97\times 1{0}^{13}\text{MeVm}& \text{(7.2)}\text{}\text{}\\ {m}_{p}& =& 938\text{MeV}\u2215{c}^{2}& \text{(7.3)}\text{}\text{}\end{array}$$