What makes it impossible to escape from black holes?

• General relativity describes mass as changing the shape of spacetime, and the shape of spacetime as describing how matter moves through space.

• For objects much less dense than black holes, this results in something similar to Newton's laws of gravity: objects with mass attract each other, but it's possible to define an escape velocity which allows a test object to leave the gravitational field of any large object. For objects as dense as black holes, this stops being the case. The effort required to leave the hole becomes infinite, with no escape velocity defined.

• There are several ways of describing the situation that causes escape to be impossible. The difference between these descriptions is how space and time coordinates are drawn on spacetime (the choice of coordinates depends on the choice of observation point and on additional definitions used).

• One common description, based on the Schwarzschild description of black holes, is to consider the time axis in spacetime to point inwards towards the center of the black hole once the horizon is crossed. Under these conditions, falling further into the hole is as inevitable as moving forward in time..

• A related description is to consider the future light cone of a test object near the hole (all possible paths the object or anything emitted by it could take, limited by the speed of light). As the object approaches the event horizon at the boundary of the black hole, the future light cone tilts inwards towards the horizon. When the test object passes the horizon, the cone tilts completely inward, and all possible paths lead into the hole.