# Spacetime ## Overview Spacetime is the fundamental fabric of reality and, when talking about our perspective of the universe, involves both space and — you guessed it — time. Time and space are bound together, and changing one will affect the other. For example, as mass creates gravity, gravity stretches physical space and it will also dilate time itself. You need four dimensions (4D) to be able to describe something in spacetime. With them, you can specify both where and when something is. By considering both time and space, you can represent the possible futures that something or someone might have. This gets pretty interesting when talking about [[Black Hole|black holes]], as once you cross the event horizon, all paths within the future light cone point towards the singularity within — there are no other futures. **![[light_cone.png]]** #astrophysics