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# 7.0: Prelude to 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 $$\hbar$$, $$c$$ and the mass of the proton, $$m_p$$. The natural classification is then given in Table $$\PageIndex{1}$$. 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 $$\PageIndex{1}$$: A summary of the four fundamental forces
Force Range Strength Acts on
Gravity $$\infty$$ $$G_N\approx 6\, 10^{-39}$$ All particles (mass and energy)
Weak Force $$<10^{-18}$$m $$G_{F}\approx 1\, 10^{-5}$$ Leptons, Hadrons
Electromagnetism $$\infty$$ $$\alpha \approx 1/137$$ All charged particles
Strong Force $$\approx 10^{-15}$$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 $$\hbar$$ 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{align*} \hbar &= 6.58 \times 10^{-22} \text{ MeV s}\\ \hbar c & = 1.97 \times 10^{-13} \text{ MeV m}\\ m_{\mathrm{p}} & = 938 \text{ MeV}/c^2\end{align*}