String Theory and Gauge Bosons
In the Standard Model of particle physics, the forces can be explained in terms of gauge theories, which possess certain mathematical properties. These forces transmit their influence through particles called gauge bosons. String theory allows gravity to be expressed in terms of a gauge theory, which is one of its benefits.
Throughout the development of the Standard Model, it became clear that all the forces (or, as many physicists prefer, interactions) in physics could be broken down into four basic types:
Weak nuclear force
Strong nuclear force
The electromagnetic force and weak nuclear force were consolidated in the 1960s by Sheldon Lee Glashow, Abdus Salam, and Steven Weinberg into a single force called the electroweak force. This force, in combination with quantum chromodynamics (which defined the strong nuclear force), is what physicists mean when they talk about the Standard Model of particle physics.
One key element of the Standard Model of particle physics is that it’s a gauge theory, which means certain types of symmetries are inherent in the theory; in other words, the dynamics of the system stay the same under certain types of transformations. A force that operates through a gauge field is transmitted with a gauge boson. The following gauge bosons have been observed by scientists for three of the forces of nature:
Electromagnetism — photon
Strong nuclear force — gluon
Weak nuclear force — Z, W+, and W– bosons
In addition, gravity can be written as a gauge theory, which means that there should exist a gauge boson that mediates gravity. The name for this theoretical gauge boson is the graviton. (The discovery of the graviton in the equations of string theory led to its development as a theory of quantum gravity.)