General Relativity and Gravity
Newton's law of gravity works beautifully in most situations, but it has a critical flaw: it implies gravity propagates instantaneously. If you moved the Sun suddenly, every planet would feel the change at the same moment, no matter how distant — which would allow signaling faster than light. Einstein showed in 1905 that nothing can propagate faster than light. Therefore Newton's law must be wrong, even if it appears correct at the speeds and field strengths encountered in ordinary astronomy.
Einstein's correction is general relativity (1915). Its central claim: gravity is not a force but a curvature of spacetime. Mass and energy curve the four-dimensional geometry of spacetime, and objects follow the straightest possible paths (called geodesics) through that curved geometry. What we call a gravitational force is the effect of following a geodesic in curved spacetime.
One Striking Consequence
Light has energy. Since energy is equivalent to mass (E = mc²), and gravity attracts mass, gravity attracts light. During a solar eclipse, stars near the apparent edge of the Sun should appear slightly displaced from their known positions — their light bends as it passes through the Sun's gravitational field. Arthur Eddington measured this bending in 1919 and confirmed Einstein's prediction to high precision. This was the first major experimental confirmation of general relativity.
Gravity Propagates at the Speed of Light
In general relativity, changes in a gravitational field propagate as gravitational waves traveling at c. The LIGO detectors confirmed this directly in 2015 by detecting gravitational waves from two merging black holes — ripples in the geometry of spacetime itself, traveling 1.3 billion light-years before arriving on Earth.
The Equivalence Principle as Foundation
General relativity rests on the equivalence principle: gravitational mass equals inertial mass. If you're in a sealed box, you cannot tell whether you're accelerating or in a gravitational field. Einstein took this local indistinguishability as a clue that gravity and acceleration are the same phenomenon — not merely similar. The mathematics of curved spacetime makes this precise.
Where It Fails
General relativity is enormously successful but is a classical (non-quantum) theory. It breaks down in regions where quantum effects are strong — inside black holes, and at the first instant of the Big Bang. Merging general relativity with quantum mechanics is the outstanding open problem of theoretical physics.
Connections
- universal-gravitation — Newton's law that general relativity corrects and contains as a limit
- equivalence-principle — the physical foundation that general relativity is built on
- e-equals-mc-squared — energy has mass; this is why light is affected by gravity
- gravity-has-no-mechanism — even with general relativity, the deep question of what gravity is remains open