General relativity from first principles – Adam Brown

General relativity redefines gravity not as a traditional force, but as the curvature of spacetime caused by matter and energy. Einstein’s equivalence principle—the observation that inertial and gravitational mass are identical—serves as the theory's cornerstone, explaining why objects in free fall follow straight lines within curved geometry. Black holes emerge as extreme consequences of this framework, where gravitational time dilation and infinite acceleration at the event horizon prevent escape. Empirical validation, ranging from the anomalous orbit of Mercury and light deflection during solar eclipses to modern detections of gravitational waves by LIGO and imaging of Sagittarius A*, confirms the theory's accuracy across vast scales. This model successfully describes phenomena from planetary motion to the expansion of the universe, demonstrating the power of theoretical consistency in predicting physical reality.