Definition
Spacetime is the four-dimensional manifold that combines the three dimensions of space and one of time into a single geometric structure.
Hermann Minkowski introduced the concept in 1908 in a lecture in Cologne: "Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality." The union turns special relativity from an algebra of transformations into a geometry of events.
Why it matters
How it works
In Newtonian physics, space and time are absolute and independent. An event happens at a position (x, y, z) at a time t, and the coordinates transform independently under changes of frame (t' = t, x' = x − vt). Two events that happen "at the same time" do so in every frame.
Special relativity broke that independence. The Lorentz transformations mix time and space: t' = γ(t − vx/c²), x' = γ(x − vt). Two observers in relative motion disagree on simultaneity, on time intervals, on lengths. Minkowski's insight was that the disagreement disappears in a four-dimensional picture. The invariant under Lorentz transformations is not Euclidean distance but the spacetime interval s² = −(ct)² + x² + y² + z² — Lorentzian rather than Euclidean signature, with one minus sign separating the temporal and spatial directions.
This signature has consequences. The interval can be negative (timelike separation — one event can causally affect the other), zero (null — connected by a light ray), or positive (spacelike — no signal can connect them). Light cones, world-lines, and proper time emerge naturally as geometric objects. The "twin paradox" becomes the simple statement that two world-lines connecting the same two events generally have different lengths in the Lorentzian metric.
General relativity drops the assumption that spacetime is flat. It replaces the constant Minkowski metric η_μν with a smoothly varying metric g_μν(x) whose curvature is determined by the matter and energy present (Einstein's equations). Free particles follow geodesics — the curved-spacetime generalisation of straight lines. What feels like gravity is just particles travelling on the straightest paths available in a curved geometry.