▦ Spacetime
Space and time aren't separate stages. They're one geometry, and each observer slices it into 'space' and 'time' in their own way.
One geometry
In 1908 Hermann Minkowski put Einstein's relativity into a picture: "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." Events are points in a four-dimensional spacetime; a thing's whole history is a line through it (its worldline). Established
What everyone agrees on
Observers disagree about distances and durations separately, but they all agree on one combination: the interval, (c × time)² − (distance)². The minus sign is what makes time a different kind of dimension from space. Light's path has an interval of zero. A clock's reading is the interval along its own path — its proper time. Established
Light cones
From any event, light spreads out as a cone into the future (and came in as a cone from the past). Inside your future cone: everything you could ever affect. Inside your past cone: everything that could have affected you. Outside both: the 'elsewhere' — events that can't be connected to you by any signal, and whose timing relative to you depends on who's asking. Established
Curved spacetime
General relativity (1915) goes further: mass and energy curve spacetime, and the curvature is gravity. Objects in free fall follow the straightest possible paths through curved spacetime. Confirmed by starlight bending at the 1919 eclipse, the precision of GPS, Gravity Probe B (2011), gravitational waves (2015) and images of black holes (2019, 2022). Established
Is spacetime fundamental?
Many approaches to quantum gravity suggest spacetime may not be the bottom layer — it might emerge from something else, such as quantum entanglement or discrete building blocks. None is confirmed. Speculative See holes H1 and H11.
See it in the labs
Further exploring
- Terence Tao — Spacetime diagram applet ↗Build events, worldlines and lines of simultaneity in two frames; worked twin-paradox and pole-and-barn scenes.
- Einstein Online (Max Planck Institute for Gravitational Physics) ↗Short, authoritative articles on relativity, black holes and cosmology.
Sources: Minkowski, 'Raum und Zeit' (1908); Einstein (1905, 1915); Dyson, Eddington & Davidson (1920); Everitt et al., Gravity Probe B, PRL 106 (2011); LIGO, PRL 116 (2016); Event Horizon Telescope (2019, 2022).