Classical unified field theories a survey of classical relativistic field theories of gravitation that have been motivated by theoretical concerns other than unification, see Classical theories of gravitation. For a survey of current work toward creating a quantum theory of gravitation, see quantum gravity. The early attempts at creating a unified field theory began with the Riemannian geometry of general relativity, and attempted to incorporate electromagnetic fields into a more general geometry, since ordinary Riemannian geometry seemed incapable of expressing the properties of the electromagnetic field. Einstein was not alone in his attempts to unify electromagnetism and gravity; a large number of mathematicians and
Unified combatant command the space-related responsibilities previously assigned to the Commander, United States Strategic Command; and (3) the responsibilities of Joint Force Provider and Joint Force Trainer for Space Operations Forces". A sub-unified command, or, subordinate unified command, may be established by combatant commanders when authorized to do so by the Secretary of Defense or the president. They are created to conduct a portion of the mission or tasking of their parent geographic or functional command. Sub-unified commands may be either functional or geographic, and the commanders of sub-unified commands exercise authority similar to that of combatant commanders. Examples of current and former