The general theory of relativity, which Albert Einstein completed in 1915, is a theory of gravity and motion. His special theory of relativity challenged the classical ideas of space, time, and simultaneity, and the general theory went further to declare that the properties of space-time are neither homogeneous (the same at all locations) nor isotropic (the same in every direction regardless of orientation), and do not necessarily remain the same as time passes. Whereas special relativity deals with frames of reference in relative motion at constant velocity, general relativity also considers accelerated frames of reference and gravity. Measurements of space and time intervals depend not only on the relative motion between observers, but also on the strength of the local gravitational fields.

The central concept in general relativity is the equivalence principle, which states that the effects of gravity and acceleration are indistinguishable and equivalent. Using this...