Unified field theory is sometimes called the Theory of Everything (TOE, for short): the long-sought means of tying together all known phenomena to explain the nature and behavior of all matter and energy in existence. In physics, a field refers to an area under the influence of some force, such as gravity or electricity, for example. A unified field theory would reconcile seemingly incompatible aspects of various field theories to create a single comprehensive set of equations. Such a theory could potentially unlock all the secrets of nature and make a myriad of wonders possible, including such benefits as time travel and an inexhaustible source of clean energy, among many others. According to Michio Katu, a theoretical physicist at City College, City University of New York, those in pursuit of a unified field theory seek "an equation an inch long that would allow us to read the mind of God."
James Clerk Maxwell proposed the first field theory, for electromagnetism, in the middle of the 1800s. Early in the 20th century, Albert Einstein's general theory of relativity - dealing with gravitation - became the second field theory. The term unified field theory was coined by Einstein, who was attempting to prove that electromagnetism and gravity were different manifestations of a single fundamental field. When quantum theory entered the picture, the puzzle became more complex. The theory of relativity explains the nature and behavior of all phenomena on the macroscopic level (things that are visible to the naked eye); quantum theory explains the nature and behavior of all phenomena on the microscopic (atomic and subatomic) level. Perplexingly, however, the two theories are incompatible. Unconvinced that nature would prescribe totally different modes of behavior for phenomena that were simply scaled differently, Einstein sought a theory that would reconcile the two apparently irreconcilable theories that form the basis of modern physics.
Although electromagnetism and the strong and weak nuclear forces have long been explained by a single theory known as the standard model, gravitation does not fit into the equation. The current quest for a unified field theory (sometimes called the holy grail of physicists) is largely focused on superstring theory and, in particular, on an adaptation known as M-theory.