General relativity: gravity as curved spacetime

In 1915 Albert Einstein published his general theory of relativity, a new theory of gravity. Instead of treating gravity as a force pulling across empty space, as Newton did, general relativity describes it as the curving of spacetime, the combined fabric of space and time, by mass and energy. Objects moving freely follow the straightest possible paths through this curved spacetime, which is why planets orbit the Sun. A common analogy is a heavy ball resting on a stretched rubber sheet, making a dip that rolling marbles curve around, though the analogy captures only part of the idea. General relativity predicts effects that Newton's theory does not. Light bends as it passes massive objects: this was first measured during a solar eclipse in 1919, and today astronomers observe gravitational lensing, where galaxies bend the light of more distant objects behind them. Clocks run slower deeper in a gravitational field, so clocks on GPS satellites, farther from Earth, run slightly faster than clocks on the ground. Extremely dense objects can curve spacetime so strongly that not even light can escape, forming black holes. Accelerating masses send out ripples in spacetime called gravitational waves, first detected directly by LIGO in 2015 from two merging black holes. For everyday speeds and ordinary gravity, Newton's law of gravitation remains an excellent approximation.

This explanation is AI-generated and can occasionally be wrong. 🚩 Report