| General Theory of Relativity:
Sometimes known as the Theory of General Relativity, this wasAlbert Einstein’s refinement (published in 1916) of his earlierSpecial Theory of Relativity and Sir Isaac Newton’s much earlierLaw of Universal Gravitation. The theory holds that acceleration and gravity are indistinguishable - the Principle of Equivalence - and describes gravity as a property of the geometry (more specifically a warpage) of space-time. Among other things, the theory predicts the existence of black holes, an expanding universe, time dilation, length contraction, gravitational light bending and the curvature of space-time. Although classical physics can be considered a good approximation for everyday purposes, the predictions of general relativity differ significantly from those of classical physics. They have become generally accepted in modern physics, however, and have been confirmed by all observations and experiments to date.
Geodesic:
The shortest path between two points in curved space. It originally meant the shortest route between two points on the Earth's surface (namely a segment of a great circle) but, since its application in general relativity, it has come to mean the generalization of the notion of a straight line as applied to all curved spaces. In non-curved three-dimensional space, the geodesic is a straight line. In general relativity, a free falling body (on which only gravitational forces are acting) follows a geodesic in curved four-dimensional space-time.
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