geodesic

A geodesic is the straightest possible path through a curved space, the natural generalization of a straight line. On a flat plane the geodesics are ordinary straight lines, but on a curved surface they bend to follow the lay of the land. On the surface of the Earth the geodesics are great circles, which is why long-haul flights between distant cities trace curving arcs rather than the straight lines you might draw on a flat map.

In general relativity, a body in free fall travels along a geodesic of curved spacetime. It is not being pushed or pulled; it is simply going as straight as the warped geometry allows. A thrown ball, an orbiting planet, and a falling apple all follow geodesics, and they curve through space only because spacetime around the massive Earth is itself curved.

Crucially, light follows geodesics too, the special ones that a beam of light traces. Because spacetime near a heavy mass is curved, starlight grazing the Sun is deflected from a straight line, an effect first confirmed during the 1919 solar eclipse. Two travelers who set off on parallel geodesics can later find themselves drifting together or apart, which is how curvature reveals itself as the tidal pull of gravity.