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tags: #physics #relativity #gravity

General Relativity redefines gravity not as a force, but as the curvature of spacetime caused by the presence of mass and energy. The famous summary by physicist John Archibald Wheeler is:

"Spacetime tells matter how to move; matter tells spacetime how to curve."

Imagine a stretched rubber sheet. Placing a heavy bowling ball (mass) in the center creates a deep curve. A marble (another mass) rolled nearby will not be "pulled" by the bowling ball directly, but will follow the curve in the rubber sheet created by the ball's presence.

This is analogous to how planets orbit the Sun. The Sun's enormous mass curves the spacetime around it, and the Earth follows a straight line (a geodesic) through this curved spacetime, which we perceive as an orbit.

This theory elegantly explains everything Newton's Law of Universal Gravitation does, while also correctly predicting new phenomena like the bending of starlight around massive objects (gravitational lensing) and the precise orbit of Mercury. It is a direct consequence of the Principle of Equivalence.

Reference

Coursera, "Question Reality: Matter"