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Two-dimensional analogy of space-time distortion. The presence of matter curves the geometry of spacetime, which is interpreted as gravity.

General relativity is the geometrical theory of gravitation published by Albert Einstein in 1916. It unifies Einstein's earlier special relativity with Sir Isaac Newton's law of universal gravitation. This is done with the insight that gravitation is not due to a force but rather is a manifestation of curved space and time. Einstein noticed that a person free falling in an elevator will experience that he is floating, something very similar to an inertial system in space. Einstein postulated that a free falling system is a privileged system, similar to the inertial systems of special relativity, but while an inertial system follows a straight line in spacetime (see special relativity), these free-falling systems follow bent lines.

Einstein further postulated that the presence of the earth bends the space-time inertial paths in some way, making the straight lines of the inertial systems into curves. With this in mind, we can explain ordinary things in a new way. For example, a satellite circling the earth is following an inertial path, with the path being curved by the presence of the earth. Therefore, the gravitational field we feel at the surface of the earth is really a fictitious force like those of other non-inertial frames of reference.

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