Kaluza-Klein Theory: 1921

Independent of each other, Kaluza and Klein found that electromagnetism can be derived from gravity in a unified theory if there are four space dimensions instead of three, and the fourth is curled into a tiny circle.

String Theory is Born: 1971

Simultaneously, three particle theorists realize that the dual theories developed in 1968 to describe the particle spectrum also describe the quantum mechanics of oscillating strings. This marks the official birth of string theory.

Gravitations: 1974

String theory using closed strings fails to describe hadronic physics, making it a perfect candidate for the missing theory of quantum gravity. This marks the advent of string theory as a proposed unified theory of all four observed forces in Nature.

“The Big Year”: 1984

Inconsistencies that threatened to make the theory futile were discovered to cancel each other when the underlying symmetries in the theory belong to special groups. Finally string theory is accepted by the mainstream physics community as an actual candidate theory uniting particle physics, quantum mechanics, and gravity.

The Standard Model (a collection of theories that describes the four fundamental forces at energies currently accessible to experiments. The theories are the SU(2) x U(1) electroweak theory, the SU(3) (QCD) theory of strong interactions and the classical theory of general relativity for gravitation) has been very successful in describing most of the phenomemon that we can experimentally investigate with the current generation of particle acceleraters. However, it leaves many unanswered questions about the fundamental nature of the universe.

Modern theoretical physics aims to find a "unified" description of the universe. For example Einstein-Maxwell theory unifies the forces of electricity and magnetism into the electromagnetic force.

Electromagnetic force: One of the four fundamental forces it includes the familiar forces of electricity and magnetism. It is described by a U(1) gauge theory called "quantum electrodynamics" (QED). It is mediated by the photon.

Glashow, Salam, and Weinberg successfully showed that the electromagnetic and weak forces can be unified into a single electroweak force.

Electroweak force

The electromagnetic and weak forces are known to unify into one electroweak force. It is also known as SU(2)xU(1).

Weak force

One of the four fundamental forces, it is responsible for weak interactions like radioactive beta decay (neutron -> proton + electron + neutrino). It is mediated by W and Z bosons and is described by a spontaneously broken SU(2) gauge theory.

Strong force

One of the four fundamental forces, the strong force is responsible for binding quarks into nucleons like the proton and neutron. The strong force actually gets weaker at high energies. It is also known as the color force. It is described by an SU(3) gauge theory called "quantum chromodynamics" (QCD). It is mediated by gluons.