The Equivalence Principle
Einstein's starting point — his 'happiest thought' — was the equivalence principle: a person in free fall cannot tell the difference between falling in a gravitational field and floating in empty space. Locally, gravity and acceleration are identical. From this simple observation, he spent 8 years developing the mathematics of curved spacetime.
What Is Curved Spacetime?
In Newtonian physics, space is flat and three-dimensional; time is separate and absolute. In general relativity, space and time are unified into four-dimensional spacetime, and massive objects curve this fabric. The Earth curves spacetime around it; the Moon follows a curved path through this curved spacetime — what we call orbit. There is no gravitational force; there is only curved geometry.
The Einstein Field Equations
The Einstein field equations (Gμν + Λgμν = 8πG/c⁴ Tμν) relate the curvature of spacetime (left side) to the distribution of energy and matter (right side). John Wheeler summarized them: 'Spacetime tells matter how to move; matter tells spacetime how to curve.' The equations are 10 coupled, nonlinear partial differential equations — among the most mathematically challenging in physics.
Confirmations
Eddington's 1919 solar eclipse observations confirmed that starlight bends around the Sun as GR predicts. Mercury's orbital precession (unexplained by Newton) is exactly explained by GR. GPS satellites must correct for GR time dilation — without the correction, GPS would drift by ~10 km per day. Gravitational waves (predicted in 1916) were first detected by LIGO in 2015. Black holes, predicted by GR's equations, have been directly imaged by the Event Horizon Telescope (2019).
Frequently Asked Questions
What is general relativity in simple terms?
General relativity is Einstein's theory that gravity is not a force but the curvature of spacetime caused by mass and energy. Massive objects curve the fabric of spacetime; other objects follow the curves, which we experience as gravitational attraction. Think of a bowling ball on a rubber sheet causing nearby marbles to roll toward it.
How was general relativity confirmed?
General relativity has been confirmed by: Eddington's 1919 observation of starlight bending around the Sun during a solar eclipse; GPS satellite corrections for time dilation (without which GPS would be inaccurate by ~10km/day); gravitational wave detection by LIGO (2015); and the Event Horizon Telescope's black hole images (2019). It is the most precisely tested theory in the history of physics.
What is the difference between special and general relativity?
Special relativity (1905) applies to observers in uniform motion (no acceleration) and deals with time dilation, length contraction, and E=mc². General relativity (1915) extends this to include gravity and acceleration, describing gravity as spacetime curvature. General relativity reduces to special relativity locally in the absence of gravity.
