Unified field theory classical field theory and, two years later, that of Theodor Kaluza, who extended General Relativity to five dimensions. Continuing in this latter direction, Oscar Klein proposed in 1926 that the fourth spatial dimension be curled up into a small, unobserved circle. In Kaluza–Klein theory, the gravitational curvature of the extra spatial direction behaves as an additional force similar to electromagnetism. These and other models of electromagnetism and gravity were pursued by Albert Einstein in his attempts at a classical unified field theory. By 1930 Einstein had already considered the Einstein–Maxwell–Dirac System [Dongen]. This system is (heuristically) the super-classical [Varadarajan] limit
A Brief History of Time to make a theory that combines the electromagnetic force, the weak nuclear force, and the strong nuclear force. This theory is called a "grand unified theory" (or a GUT). This theory tries to explain these forces in one big unified way or theory. Black holes are talked about in this chapter. Black holes are stars that have collapsed into one very small point. This small point is called a "singularity". Black holes suck things into their center because they have very strong gravity. Some of the things it can suck in are light and stars. Only very large stars, called