# Fundamental Interactions

All forces in nature can be classified into four types:

• Gravity. Gravity is a property of matter and space which holds together the universe at large. Isaac Newton formulated gravity as a force between two objects as directly proportional to their masses and inversely proportional to the square of the distance between them. In his general theory of relativity, Albert Einstein envisioned gravity as the distortion of space caused by mass. Einstein showed that Newton’s ideas were a special case for objects moving at low speeds in a weak gravitational field.
• Electromagnetism. The electromagnetic interaction is responsible for the repulsion of like and the attraction of unlike electric charges. It also explains the properties of light and the atomic phenomena such as binding electrons to atoms, and atoms to one another to form molecules and compounds (the chemical behavior of matter). The formulation of the electromagnetic theory was provided by James Clerk Maxwell in the 19th century
• Strong. The strong interaction holds protons and neutrons together in the atomic nucleus. In the 1970s investigators formulated a theory for the strong force known as quantum chromodynamics.
• Weak. The weak interaction is responsible for nuclear beta decay and other similar decay processes.

The four forces are often described according to their relative strengths. The strong force is the most powerful force in nature followed by the electromagnetic, weak, and gravitational forces in descending order of relative strength. Despite its strength, the effect of strong force cannot be felt in the macroscopic universe because of its extremely limited range, being confined to an operating distance of about 10-15 meter (1 fm)—about the diameter of a proton. The range of the weak force is even shorter, about 10-17 meter. By contrast, the ranges for gravitational and electromagnetic forces are infinite. That has the implication that gravity acts between all objects of the universe, no matter how far apart they are, and an electromagnetic wave, such as the light from a distant star, travels undiminished through space until it encounters some particle capable of absorbing it.

 FORCE STRENGTH QUANTUM MASS(GeV/c) RANGE(m) Gravitational 10-45 Graviton (g0)? 0 Infinite ∝ 1/r2 Weak 10-8 W±, Z 80, 91 < 2 x 10-18 Electromagnetic 10-2 Photon (γ) 0 Infinite ∝ 1/r2 Strong 1 (π meson) Gluon (g) (0.14) 0 (10-15) Infinite

The manifestation of all four fundamental forces is via the exchange of one or more particles called the gauge bosons. Each fundamental force has its own gauge boson. The strong force is carried by the gluon (g0), the electromagnetic force is transmitted by the photon, while W± and Z0 bosons are responsible for the weak force. Although still not found experimentally, nevertheless the graviton (g), the force-carrying particle of gravity, is expected to exist.

The Standard Model of particle physics includes the electromagnetic, strong and weak forces and all their carrier particles. However, attempts to incorporate gravitons into the Standard Model have run into serious theoretical difficulties at high energies because of infinities arising due to quantum effects (in technical terms, gravitation is not renormalizable). Classical general relativity and quantum mechanics are incompatible at such energies. Some proposed models of quantum gravity attempt to address these issues, but these are speculative theories.

In spite with the problem of incorporating gravity into the Standard Model, physicists have long sought to show that the four basic forces are simply different manifestations of the same fundamental force. The most successful attempt at such a unification is the electroweak theory, proposed during the late 1960s by Steven Weinberg, Abdus Salam, and Sheldon Lee Glashow. Their theory incorporates quantum electrodynamics (the quantum field theory of electromagnetism) and treats the electromagnetic and weak forces as two aspects of a more-basic electroweak force that is transmitted by four carrier particles. Not surprisingly, one of these carrier particles is the photon of electromagnetism, while the other three—the electrically charged W+ and W particles and the neutral Z0 particle—are associated with the weak force. Unlike the photon, these weak gauge bosons are massive, and it is the mass of these carrier particles that severely limits the effective range of the weak force.

As mentioned, the strong force is transmitted between quarks by gauge bosons called gluons. Like photons, gluons are massless and travel at the speed of light. But they differ from photons in one important respect: they carry what is called “color” charge, a property analogous to electric charge. Gluons are able to interact together because of color charge, which at the same time limits their effective range.