3.3 Forces and interactions

Elementary particles interact with each other in various ways. Right now, we know of four fundamental forces in nature. Only two of these can be experienced directly in everyday life. These two are gravitation (which is described by Newton’s law of gravitation, and more precisely by Einstein’s general theory of relativity) and electromagnetism (which is described by a set of equations known as Maxwell’s equations).

Gravitation is the attraction between any two objects that have mass. This is the force that shapes our universe and holds our solar system together. Electromagnetism covers the electric and magnetic forces between charged or magnetic objects. Both gravitation and electromagnetism also act on particles at the atomic and subatomic scale. For instance, the nucleus and the electrons in an atom are attracting each other due to the electric force because the nucleus has positive charge and the electrons are negatively charged, and opposite charges attract each other. It is this force that holds atoms together, because the gravitational force is tiny.

However, at the subatomic scale, two more forces become relevant. These are known as the weak nuclear force and the strong nuclear force. The strong force keeps the atomic nucleus together – it is strong enough to overcome the repulsion of its positively charged constituents – but it does not affect electrons. The weak force acts on all the elementary particles that constitute an atom, but as the name suggests, it is much weaker than the strong force. The weak force is responsible for a particular type of radioactive decay that is observed in some unstable elements (such as for Carbon-14, used in dating ancient materials). It is the weak force that allows the heavier quarks to decay into the lighter quarks.