Section 10.2 The Four Fundamental Forces of Nature
Throughout the year, we have discussed a wide variety of forces: tension forces, normal forces, friction forces, air and fluid drag, spring forces, gravitational forces, electric and magnetic forces, etc. But most of these forces are just different manifestations of the same fundamental force. For instance, a normal force between two objects “in contact with each other” 1 is just electrostatic repulsion between the electrons at the surface of one object and those at the surface of the other object.
During the past century, physicists have typically talked about four “fundamental” forces: the strong force, the electromagnetic force, the weak force, and the gravitational force. You are already familiar with the electromagnetic force (which affects any particle with an electric charge) and gravitational force (which affects any particle with mass). The strong force is the force that holds protons and neutrons together in the nuclei of atoms. This is no small matter here: the electric force is extremely repulsive between protons packed together in a nucleus, so there clearly must be a force that is even stronger pulling them together. But the strong force is only active over short distances, those comparable to the sizes of atomic nuclei. For separations greater than that, the strong force is ineffective. As discussed in the previous chapter, the strong force affects all of the hadrons. Actually, since the hadrons are all made up of quarks, it can be said that the strong force is the force that is associated with quarks.
The weak force is somewhat of a “miscellaneous/etc” force; in fact, all of the elementary particles can experience the weak force. For quarks or charged particles, though, the weak force is subordinate to the strong and/or electromagnetic force(s). Only for uncharged leptons (i.e., neutrinos) is the weak force dominant. But since the weak force is so weak, neutrinos can pass through other objects with only minimal interactions. Right now, over \(10^7\) neutrinos pass through you every second. In fact, the vast majority of neutrinos that arrive at the Earth (from the Sun) pass through the entire planet without interacting with any other matter!
The weak force is also important in a lot of decay processes. For example, \(\beta\)-decay by which a neutron decays (\(n \rightarrow p + e^- + \nu\)) is a weak interaction. In fact, the weak interaction was first proposed because this decay process couldn't be explained with any of the other fundamental interactions.