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According to Einstein's principle of equivalence, inertial forces in an accelerated reference system are equivalent to the existence of a gravitational field. In order to formulate the gravitational force as well as inertial forces in explicit form, we introduce two conditions into the 4-D line element and transformations. As a consequence, the equation of motion for gravitational force or inertial force has a form similar to the equation of Lorentz force on a charge in electrodynamics. The inertial forces in non-inertial systems are calculated for two special cases: a uniformly accelerated system, and a uniformly rotating system.

Multiple definitions of time in the case of flat space and curved space are discussed. It is found that Einstein time is not the only definition of time in flat space. Other definitions of time are permissible. On the other hand, the definition of time in General Relativity is too arbitrary. Certain constraints must be imposed to set the coordinate conditions. Therefore, the proper way in spacetime theory should adopt multiple concepts of time, which are related to each other by given rules and have corresponding physical meanings. The definitions of the velocity of light are also briefly discussed since it is related to the definitions of time.

We consider the We consider the gravitational mass of a particle as an imaginary charge. A two-dimensionasl sub-space, gravitational-electric (G-E) space, is introduced to combine the imaginary gravitational charge with the real electric charge. By using this new concept, the gravitational field equations for weak gravity and low velocities can then be written in a form analogous to Maxwell's equations. With this arrangement the gravitational and electromagnet fields can be unified in the G-E sub-space, which provides a new approach to study the properties of the gravitational field. As a consequence, the gravitational energy density and energy flux would exhibit new properties with this formation.