THE ANATOMY OF A BLACK HOLE
Once a massive star has shrunk to form a black hole, nothing is left (as far as we know) but a bottomless pit of space- time. A black hole is surrounded by an event horizon, an imaginary membrane that marks the point of no return for in-falling matter and light. Inside the event horizon, and at the heart of the black hole, Einstein’s General Relativity predicts the existence of a point of infinite density called a ‘singularity’. Yet once you reach the singularity, Einstein’s theory – and all of physics as we know it – breaks down. Perhaps a new, quantum theory of gravity is needed to tell us what really exists there.
Imagine an astronaut falling feet first into a black hole. When they are at a circumference corresponding to 1.5 times the circumference of the black hole, gravity is so strong it bends light into a circle around the hole, so they can see the back of their head! Near a stellar-mass black hole, the huge difference in gravity between the astronaut’s head and feet will tear them apart before they reach the event horizon. However, this tidal effect is negligible near a supermassive black hole, and the astronaut can cross the event horizon with no ill- effect.
Einstein’s theory predicts that time flows more slowly in strong gravity. So, if you were to observe the astronaut falling down to the black hole from a safe distance, they would appear to move in ever slower motion, and stop altogether on reaching the event horizon. Although they would fall through into the hole, never to appear again, their image would be frozen on the event horizon, gradually fading as light from the image struggled to climb out.
In the case of a rotating, or ‘Kerr’, black hole, there is a twist. In effect, these have two horizons. When the astronaut crosses the outer one, and enters the ‘ergosphere’, they are dragged around by a tornado of space-time. They can still gain energy from the hole’s rotation and be ejected from the black hole. However, once they cross the inner event horizon, there is no going back.
Nobody knows what the inside of a black hole looks like. However, space and time are so distorted that they swap places. The singularity therefore exists not across an interval of space but in the astronaut’s future. Consequently, they can no more avoid reaching it and being crushed to death than you can avoid tomorrow.