132 The Four Basic Forces

Despite the enormous variety of physical phenomena observed in nature, all known interactions can be described in terms of only four fundamental forces: gravity, electromagnetism, the weak interaction, and the strong interaction. These forces govern processes ranging from the motion of planets to the structure of atomic nuclei and the radioactive transformations used in medical imaging and cancer treatment.

Particle physics is closely connected to these four interactions. Each force is associated with one or more carrier particles, also called gauge bosons. Particles can be classified partly according to which interactions they experience. For example, electrically charged particles experience the electromagnetic interaction, while particles containing quarks experience the strong interaction.

The relative strengths shown in Table 132.1 are approximate and depend on the energy and distance scales involved. The strong interaction is assigned a relative strength of 1, and the other forces are compared with it. Gravity is extraordinarily weak at the scale of individual particles, although it dominates the motion of planets, stars, and galaxies because it is always attractive and acts over unlimited distances.

Forces as Particle Exchanges

One of the central ideas of modern particle physics is that interactions can be represented through the exchange of carrier particles. This resembles Yukawa's earlier proposal that the nuclear force between protons and neutrons could be described through the exchange of pions.

In quantum field theory, forces are not best imagined as tiny objects being thrown back and forth in the ordinary mechanical sense. Instead, particle exchange is a mathematical description of how interacting quantum fields transfer energy, momentum, and other properties.

Figure 132.1 provides a pictorial representation of a virtual photon exchanged between two positively charged particles. The exchange transfers momentum, and the two charges move apart. Because the exchanged photon is virtual, it is not directly detected during the interaction. Only the measurable consequences of the interaction are observed.

Feynman Diagrams

A Feynman diagram is a visual and mathematical representation of a particle interaction. These diagrams were developed by physicist Richard Feynman and are widely used in quantum field theory.

A Feynman diagram does not show a literal path photographed in space and time. Instead, it organizes the initial particles, final particles, exchanged particles, and interaction points involved in a process. The diagram can then be translated into mathematical expressions used to calculate the probability that the interaction will occur.

In Figure 132.2, the two charged particles are represented by the outer lines. The internal line represents the virtual photon exchanged between them. The points where the lines meet are called vertices. Each vertex represents an elementary interaction between charged matter and the electromagnetic field.

Quantum Electrodynamics

Quantum electrodynamics, abbreviated QED, is the quantum field theory of electromagnetic interactions. It describes how electrically charged particles interact with photons and with one another.

QED has been tested with extraordinary precision. It successfully explains phenomena such as the scattering of electrons, the emission and absorption of photons, small shifts in atomic energy levels, and the magnetic properties of electrons.

Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga independently developed major parts of the modern formulation of QED. They shared the 1965 Nobel Prize in Physics for this work.

Pion Exchange Between Nucleons

Feynman diagrams can also represent the effective interaction between protons and neutrons inside a nucleus. In Yukawa's model, the residual nuclear force can be described through the exchange of virtual pions.

Consider the process shown in Figure 132.3. A proton emits a positively charged pion and becomes a neutron:

The other neutron absorbs the pion and becomes a proton:

The identities of the two nucleons change, but the overall system still contains one proton and one neutron. Electric charge, energy, momentum, and other required quantities are conserved in the complete interaction.

Gluons and the Fundamental Strong Interaction

If pions can represent the force between protons and neutrons, why are gluons listed as the carriers of the strong interaction? The answer is that these descriptions apply at different structural levels.

Protons, neutrons, and pions are not fundamental particles. They are composite particles made of quarks. The fundamental strong interaction acts between quarks and is carried by gluons. The theory describing quarks and gluons is called quantum chromodynamics, or QCD.

The force between color-charged quarks is the fundamental strong interaction. The force between color-neutral protons and neutrons is a remaining or residual strong force. Pion exchange provides an effective description of this residual interaction at nuclear distance scales.

This relationship is somewhat analogous to the connection between electromagnetic forces inside atoms and the weaker intermolecular forces that remain between electrically neutral molecules. The underlying interaction is electromagnetic, but its residual effects can still act between composite neutral systems.

Gluons are considered fundamental particles and have no known internal structure. They are massless, but unlike photons, they carry the type of charge associated with the strong interaction, called color charge. Because gluons themselves carry color charge, they can interact with one another.

Carrier-Particle Mass and Interaction Range

There is a general relationship between the mass of an exchanged particle and the effective range of an interaction. A heavier carrier particle is associated with a shorter-range force.

The photon has zero rest mass, allowing the electromagnetic interaction to extend over unlimited distances. Its strength decreases with distance, but the interaction does not have a fixed maximum range.

The hypothetical graviton is also expected to be massless, which would be consistent with gravity's unlimited range. However, a complete experimentally verified quantum theory of gravity has not yet been developed.

The [latex]W^+[/latex], [latex]W^-[/latex], and [latex]Z^0[/latex] bosons are extremely massive. Their large masses are associated with the very short range of the weak interaction. Consequently, weak processes usually occur over distances much smaller than the diameter of a proton.

The strong interaction requires a more careful explanation. Although gluons are massless, quarks and gluons are confined inside composite particles called hadrons. The effective residual interaction between nucleons is short-ranged because it is commonly represented by the exchange of massive mesons, especially pions.

Interaction Strength at High Energies

The relative strengths of the four forces are not fixed constants at every energy. Their effective strengths change with the energy scale at which particles interact. At ordinary laboratory energies, the forces appear dramatically different. At very high energies, some of these differences become smaller.

The electromagnetic and weak interactions have already been successfully described as two aspects of a unified electroweak interaction. Physicists continue to investigate whether the strong interaction can also be unified with the electroweak interaction at still higher energies. A successful quantum unification of gravity with the other interactions remains an open problem.

Section Summary

• All known interactions are described by four fundamental forces: gravity, electromagnetism, the weak interaction, and the strong interaction.
• Each fundamental interaction is associated with one or more carrier particles.
• The photon carries the electromagnetic interaction, while the [latex]W^+[/latex], [latex]W^-[/latex], and [latex]Z^0[/latex] bosons carry the weak interaction.
• Gluons carry the fundamental strong interaction between quarks.
• Pion exchange provides an effective description of the residual strong force between protons and neutrons.
• Feynman diagrams represent particle interactions and can be translated into mathematical expressions used to calculate interaction probabilities.
• Quantum electrodynamics is the quantum field theory describing electromagnetic interactions between charged particles and photons.
• Massless carrier particles are associated with long-range interactions, while massive carrier particles generally produce short-range interactions.
• The electromagnetic and weak interactions are understood as different aspects of a unified electroweak interaction at sufficiently high energies.