The General Theory of Relativity, proposed by Albert Einstein in 1915, is a fundamental theory in physics that describes gravity not as a force, but as a curvature of spacetime caused by mass and energy.

Key Concepts:
Spacetime: A four-dimensional continuum combining the three dimensions of space and one of time.
Mass-Energy Curves Spacetime: Massive objects like stars and planets bend the fabric of spacetime, and this curvature affects the motion of other objects.
Geodesics: Objects move along paths called geodesics, which are the "straightest possible paths" in curved spacetime.
Equivalence Principle: Locally, the effects of gravity are indistinguishable from acceleration. This principle led Einstein to realize that gravity could be described geometrically.
Einstein's Field Equations: These are a set of ten interrelated differential equations that describe how matter and energy determine the curvature of spacetime.
Predictions and Confirmations:
Gravitational Time Dilation: Time runs slower in stronger gravitational fields.
Gravitational Lensing: Light bends around massive objects, confirmed by observing starlight bending around the Sun.
Black Holes: Regions of spacetime with extreme curvature where not even light can escape.
Gravitational Waves: Ripples in spacetime caused by accelerating masses, directly detected in 2015.