Gravity

Gravity is the phenomenon by which mass and energy determine the curvature of spacetime, and curved spacetime determines the motion of matter.

Newton (1687) described gravity as a force between masses: proportional to the product of the masses, inversely proportional to the square of the distance. This predicts planetary orbits, tides, and projectile trajectories with extraordinary accuracy, and remains adequate for most terrestrial and solar-system calculations.

Einstein (1915) replaced Newton’s force with geometry. In general relativity, there is no gravitational force. Objects in free fall follow the straightest possible paths – geodesics – through curved spacetime. What we perceive as gravitational attraction is the convergence of geodesics near massive bodies. Mass tells spacetime how to curve; curved spacetime tells matter how to move.

Gravity is the dominant force at cosmological scales. Electromagnetism is stronger but cancels out in bulk matter (positive and negative charges balance). The strong and weak nuclear forces have extremely short ranges. Gravity alone shapes the large-scale structure of the universe: the formation of stars, galaxies, galaxy clusters, and the cosmic web.