spacetime

The four-dimensional fabric unifying three spatial dimensions with time into a single continuum.

• special relativity: spacetime is flat (Minkowski), distances mix space and time depending on observer velocity
• general relativity: mass and energy curve spacetime, and curvature dictates gravity
• the metric tensor field encodes geometry at every point — see field
• light cones define causal structure: nothing exceeds the speed of light
• gravitational waves are ripples propagating through spacetime
• expansion of spacetime drives cosmology — the universe grows, galaxies recede
• at the Planck scale, spacetime may be quantized — frontier of quantum mechanics and gravity
• entropy and the arrow of time emerge from the structure of spacetime — see entropy