Chapter 33 Particle Physics
33.2 The Four Basic Forces
Summary
- State the four basic forces.
- Explain the Feynman diagram for the exchange of a virtual photon between two positive charges.
- Define QED.
- Describe the Feynman diagram for the exchange of a between a proton and a neutron.
As first discussed in Chapter 4.6 Problem-Solving Strategies and mentioned at various points in the text since then, there are only four distinct basic forces in all of nature. This is a remarkably small number considering the myriad phenomena they explain. Particle physics is intimately tied to these four forces. Certain fundamental particles, called carrier particles, carry these forces, and all particles can be classified according to which of the four forces they feel. The table given below summarizes important characteristics of the four basic forces.
| Force | Approximate relative strength | Range | +/−1 | Carrier particle |
|---|---|---|---|---|
| Gravity | [latex]{10^{-38}}[/latex] | [latex]{\infty}[/latex] | + only | Graviton (conjectured) |
| Electromagnetic | [latex]{10^{-2}}[/latex] | [latex]{\infty}[/latex] | +/− | Photon (observed) |
| Weak force | [latex]{10^{-13}}[/latex] | [latex]{10^{-18} \;\text{m}}[/latex] | +/− | [latex]{W^+}[/latex], [latex]{W^-}[/latex], [latex]{Z^0}[/latex] (observed2) |
| Strong force | 11 | [latex]{10^{-15} \;\text{m}}[/latex] | +/− | Gluons (conjectured3) |
| Table 1: Properties of the Four Basic Forces |
Although these four forces are distinct and differ greatly from one another under all but the most extreme circumstances, we can see similarities among them. (In Chapter 33.6 GUTs: the Unification of Forces, we will discuss how the four forces may be different manifestations of a single unified force.) Perhaps the most important characteristic among the forces is that they are all transmitted by the exchange of a carrier particle, exactly like what Yukawa had in mind for the strong nuclear force. Each carrier particle is a virtual particle—it cannot be directly observed while transmitting the force. Figure 1 shows the exchange of a virtual photon between two positive charges. The photon cannot be directly observed in its passage, because this would disrupt it and alter the force.
Figure 2 shows a way of graphing the exchange of a virtual photon between two positive charges. This graph of time versus position is called a Feynman diagram, after the brilliant American physicist Richard Feynman (1918–1988) who developed it.
Figure 3 is a Feynman diagram for the exchange of a virtual pion between a proton and a neutron representing the same interaction as in Chapter 33.1 Figure 1. Feynman diagrams are not only a useful tool for visualizing interactions at the quantum mechanical level, they are also used to calculate details of interactions, such as their strengths and probability of occurring. Feynman was one of the theorists who developed the field of quantum electrodynamics (QED), which is the quantum mechanics of electr