Spacetime

Spacetime is the idea that space and time are not separate things but a single, unified, four-dimensional fabric. Rather than treating the three dimensions of space and the one dimension of time independently, modern physics treats them as different directions within one continuous structure.

The concept comes from Einstein. His theory of general relativity describes gravity not as a force pulling objects together but as the bending of spacetime by mass and energy. A heavy object like the Sun curves the spacetime around it, and other objects, like planets, simply follow the straightest available paths through that curved fabric. The common image is a heavy ball resting on a stretched rubber sheet, with smaller balls rolling toward it.

Why it mattered to Hawking

Spacetime is the stage on which all of Hawking's physics takes place. Black holes are regions where spacetime is curved so steeply that paths fold back on themselves; singularities are points where its curvature becomes infinite; and the singularity theorems are, at heart, rigorous arguments about the global shape of spacetime. His most speculative idea, the no-boundary proposal, even reimagines time near the Big Bang as behaving like another dimension of space, blurring the line between the two entirely.

See how physicists map spacetime causally with the Penrose diagram explainer.