As best we can tell, the behavior of all matter in the universe—from elementary particles to clusters of galaxies—is ruled by just four (or fewer) basic forces, which are fundamental to everything in the universe. In a sense, the search to understand the nature of the universe is the quest to understand the nature of these forces.

The gravitational force is probably the best known. Gravity binds galaxies, stars, and planets together and holds humans on the surface of Earth. As we saw in Chapter 2, its magnitude decreases with distance, according to an inverse-square law. (Sec. 2.7) Its strength is also proportional to the masses of each of the two objects involved. Thus, the gravitational field of an atom is extremely weak, but that of a galaxy, consisting of huge numbers of atoms, is very powerful. Gravity is by far the weakest of the forces of nature, but its effect accumulates as we move to larger and larger volumes of space, and nothing can cancel its attractive pull. As a result, gravity is the dominant force in the universe on all scales larger than that of Earth.

The electromagnetic force is another of nature's basic agents. Any particle having a net electric charge, such as an electron or a proton in an atom, exerts an electromagnetic force on any other charged particle. The everyday things we see around us are held together by this force. Like gravity, its strength also decreases with distance according to an inverse-square law. (Sec. 3.2) However, for subatomic particles, electromagnetism is much stronger than gravity. For example, the electromagnetic force between two protons exceeds their gravitational attraction by a factor of about 10³⁶. Unlike gravity, electromagnetic forces can repel (between like charges) as well as attract (between opposite charges). Positive and negative charges tend to neutralize each other, greatly diminishing their net

A third fundamental force of nature is simply termed the weak nuclear force. It is much weaker than electromagnetism, and its influence is somewhat more subtle. The weak force governs the emission of radiation from some radioactive atoms; the emission of a neutrino during the first stage of the proton—proton reaction is also the result of a weak interaction. It is now known that the weak force is not really a separate force at all but just a form of the electromagnetic force. Thus, physicists often speak of the "electroweak force." However, when acting in its "weak" mode, the electroweak force does not obey the inverse-square law. Its effective range is less than the size of an atomic nucleus, about 10^-15 m.

Strongest of all the forces is the strong nuclear force. It binds atomic nuclei together and governs the generation of energy in the Sun and all other stars. Like the weak force, and unlike the forces of gravity and electromagnetism, the strong force operates only at very close range. It is unimportant outside a distance of a hundredth of a millionth of a millionth (10^-14) of a meter. However, within this range (for example, in atomic nuclei), it binds particles with enormous strength. In fact, it is the range of the strong force that determines the typical sizes of atomic nuclei. Only when two protons are brought within about 10^-15 m of one another can the attractive strong force overcome their electromagnetic repulsion.

Not all particles are subject to all types of force. All particles interact through gravity because all have mass. However, only charged particles interact electromagnetically. Protons and neutrons are affected by the strong force, but electrons are not. Under the right circumstances, the weak force can affect any type of subatomic particle, regardless of its charge.