What is gravity in space time?

Newton described gravity as an attraction acting instantly across empty space, a picture that works superbly for planning a lunar trajectory. Einstein replaced the mechanism. In general relativity, presented to the Prussian Academy of Sciences in November 1915, mass and energy determine the curvature of spacetime, and that curvature determines how matter moves. A planet orbiting the Sun is not being tugged; it is coasting along a geodesic, the closest thing to a straight line that the local geometry permits.

The key insight came earlier, from what Einstein called his happiest thought: a person in free fall feels no weight. Being in a windowless box accelerating uniformly is indistinguishable from standing in a gravitational field. If gravity can be made to disappear by choosing a falling reference frame, it cannot be a force like electromagnetism — it is a property of the frame, and therefore of geometry.

The spacetime in question includes the time axis, and the temporal effect dominates in weak fields. Clocks deeper in a gravitational well tick more slowly than clocks higher up. This is not a measurement artefact: GNSS satellites must correct for it, because the combination of their altitude and speed shifts their onboard clocks by tens of microseconds per day relative to the ground, enough to ruin positioning within hours if ignored.

The theory's predictions have survived a century of increasingly sharp tests. Light bends around the Sun by twice the Newtonian value, confirmed during the 1919 eclipse expeditions. Mercury's perihelion advances at the rate Einstein calculated. In September 2015, the LIGO detectors recorded gravitational waves from merging black holes — ripples in spacetime geometry, travelling at the speed of light, precisely as the field equations require.