Escape Velocity

Escape velocity is the speed an object must reach to break free of a body's gravitational pull without any further push. For Earth it is about 11 kilometres per second; for the Moon, with its weaker gravity, far less. The more massive and compact the body, the higher its escape velocity.

This simple idea leads straight to the concept of a black hole. Imagine compressing an object until its gravity is so strong that its escape velocity reaches the speed of light. At that point, not even light, the fastest thing there is, can escape, and the object becomes black. The boundary at which this happens is the event horizon.

Why it mattered to Hawking

The link between escape velocity and black holes is surprisingly old, dating to speculations by John Michell in the eighteenth century about "dark stars." But it took Einstein's general relativity to make the idea rigorous, replacing the simple Newtonian picture with the curving of spacetime. Hawking worked firmly in that relativistic framework, where a black hole is not just an object with very high escape velocity but a region where the structure of spacetime itself folds inward. The everyday notion of escape velocity is the gateway to the much stranger reality he spent his life studying.