Classical unified field theories since antisymmetry seemed essential for electromagnetism, the symmetry requirement was relaxed for one or both fields. Einstein's proposed unified-field equations (fundamental laws of physics) were generally derived from a variational principle expressed in terms of the Riemann curvature tensor for the presumed space-time manifold. In field theories of this kind, particles appear as limited regions in space-time in which the field strength or the energy density are particularly high. Einstein and coworker Leopold Infeld managed to demonstrate that, in Einstein's final theory of the unified field, true singularities of the field did have trajectories resembling point particles. However, singularities are
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