Isaac Newton described gravity as a force that reaches instantly across empty space — the Sun pulls on the Earth, the Earth pulls on the Moon, full stop. Einstein's 1915 theory of general relativity replaced that picture entirely. In general relativity, gravity isn't a force pulling objects together; it's the shape of space and time itself. Einstein merged three dimensions of space with one dimension of time into a single four-dimensional fabric called spacetime. Mass and energy bend this fabric, and objects moving through it — planets, light, you — simply follow the straightest possible path through the bent shape. What looks like the Earth being "pulled" toward the Sun is really the Earth coasting along a curved lane in spacetime, the same way a marble rolling across a dented trampoline curves toward the dent without anything reaching out to grab it.

That trampoline image is the standard classroom analogy, and it's useful but limited — it shows a 2D sheet bending under gravity (itself), which is a bit circular, and it leaves out time entirely. The real content of general relativity is captured in a mathematical object called the metric: a rule that tells you how to measure distances and time intervals at every point in spacetime. In flat, empty space far from any mass, the metric is simple and constant — it's the same everywhere. Near a massive object like a star or black hole, the metric changes from point to point; distances and durations get stretched and compressed depending on how close you are to the mass. Curvature is just a name for how much the metric deviates from that flat, simple case. Zero curvature means flat spacetime and no gravity in the Newtonian sense; nonzero curvature means gravity.