Spacetime is the unified mathematical model that combines three-dimensional space (length, width, and height) with time as a fourth dimension. This concept, fundamental to Einstein's theory of relativity, revolutionized our understanding of the universe by showing that space and time are not separate entities but are intimately connected. Key aspects of spacetime: 1. Unity of space and time - Events are described by both their location (3D coordinates) and when they occur (time) - Time is not absolute but [[Relativity|relative]] to the observer's motion - The faster an object moves through space, the slower it moves through time 2. Curvature and gravity - Mass and energy curve spacetime, creating what we experience as gravity. Space is not some flat unmoving thing. It can bend, twist, expand, and more. That means instead of being like an empty room it's more like a thick glob of goo. Normally things can move around in goo with no problems, but under certain circumstances goo can bend changing the way that things move through it. - Planets don't orbit the Sun because of a "force" pulling them - they follow the natural curves in spacetime created by the Sun's mass - Think of a heavy ball placed on a stretched rubber sheet - the sheet curves, and smaller objects follow these curves - We know this is true because gravitational waves exist. Like putting your hand in a pool of still water produces waves, gravitational waves are ripples in space caused by accelerating masses. 3. Speed of light - The speed of light is constant for all observers - Nothing with mass can travel faster than light because it would require infinite energy - As objects approach the speed of light, time slows down for them relative to stationary observers Learned this from [[We Have No Idea]]