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n; and Einstein [28] entirely concurred with this view. As Pauli [3] pointed out "The general covariant formulation of the physical laws acquires a physical content only through the principle of equivalence in consequence of which gravitation is described solely by the (metric) gki •••." In Einstein’s theory, the principle of general relativity is the physical basis of covariance. However, in order to eliminate the term general relativity form a theory of gravity, Fock [11] defined a different “principle for relativity” as follows: “When speaking of the relativity of a frame of reference or simply of relativity, one usually means that there exist identical physical processes in different frames of reference. According to the generalized Galilean principle of relativity identical processes are possible in all inertial frames of reference related by Lorentz transformations. On the other hand, Lorentz transformations characterized the uniformity of Galilean space-time. Thus, the principle of relativity is directly related to uniformity. This also shows that the nomenclature introduced in Einstein’s first papers, by which the theory of uniform Galilean space is named “Theory of Relativity” can to some extent be justified.” Fock’s “principle of relativity” is based on identical processes; whereas Einstein’s principle is based on the covariance of physical laws. Thus it is, at most, a matter of personal preference (i.e., without any scientific value) that Fock claimed Einstein’s principle of relativity to be invalid. But, such a denial of Einstein’s principle of relativity would leave the requirement of mathematical covariance without any physical basis. Thus, one may wonder should Fock be considered as a real physicist. Einstein’s principle of relativity is also the theoretical basis for the geodesic equation to be the equation of motion for gravity; whereas Fock’s “principle of relativity” does not seem to serve a useful scientific purpose. Since the motion of a particle in an initial system is a straight line, the shortest line between two points, the corresponding equation of motion for system K’ is the geodesic equation in Riemann geometry. Thus, gravity is due to ten metric elements that include the velocity-dependent force. But, the time-time metric component gtt still plays the dominating role of a potential that provides the acceleration to a resting particle. In other words, the rotating disk shows not only that general relativity requires a Riemannian Space, but also that the equation of motion involves more than just a potential. 5. The Geodesic Equation, Mathematical Theorems, and Einstein Space Since the equivalence principle is applicable only in a physical space, where a geodesic representing a free falling particle, it would be useful to discuss the mathematical theorems related to a geodesic. The principle of relativity and the equivalence principle imply that the physical space-time is a Riemannian space with a space-time metric function gmn. Einstein [9] believes, “what characterizes the existence of a gravitational field, from the empirical standpoint, is the non-vanishing of the Glik (field strength), not the non-vanishing of the Riklm.” For an idealized point-like classical massive particle (which has no spin, charge, or other attributions), the equation of motion under gravity is independent of the mass, and is the geodesic equation, = 0, &nb sp;