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Why does general relativity include gravity?

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Brian Greene sits in a living room, holding a pen, with handwritten equations m = E/c^2 and E = mc^2 on a whiteboard beside him, highlighting c^2 as the speed of light. — relativity Brian Greene kicking off his *Your Daily Equation* video series with Albert Einstein's famous equation E = mc².

General relativity includes gravity because Albert Einstein sought a theory of gravitation that was conceptually consistent with special relativity. While Isaac Newton’s law of gravity worked for solar system dynamics, it relied on "action at a distance," which implied instantaneous interaction—a concept precluded by the speed-of-light limit in special relativity.1

Einstein resolved this by treating gravity not as a force, but as a geometric phenomenon arising from the curvature of space-time. This approach was guided by the equivalence principle, which posits that gravitational and inertial forces are locally indistinguishable.2345

In this framework, massive objects cause space-time to curve, and this curvature dictates the motion of matter and light. As physicist John Wheeler famously summarized: "Matter tells space-time how to curve, and space-time tells matter how to move." This geometric interpretation explains why all objects fall at the same rate and provides a consistent, experimentally verified description of gravity that aligns with the fundamental principles of relativity.352