A black hole is a true “hole” in space: Anything that crosses the edge of the hole – called the “horizon” of the hole – is swallowed forever. For this reason, black holes are considered an edge of space, a one-way exit door from our universe; nothing inside a black hole can ever communicate with our universe again, even in principle.
However black holes are even stranger than that. As you get closer to a black hole, the flow of time slows down, compared to flow of time far from the hole. (According to Einstein’s theory, any massive body, including the Earth, produces this effect. Earth’s gravity is so weak that the slowing of time is not noticeable, but the effect has been confirmed using sensitive instruments. For example, at sea level you age one-billionth of a second less every year than you would if you lived on top of Mt. Everest.) Near a black hole, the slowing of time is extreme. From the viewpoint of an observer outside the black hole, time stops. For example, an object falling into the hole would appear frozen in time at the edge of the hole.
Inside a black hole is where the real mystery lies. According to Einstein’s theory, time and space, in a way, trade places inside the hole. Inside the black hole, the flow of time itself draws falling objects into the center of the black hole. No force in the universe can stop this fall, any more than we can stop the flow of time.
At the very center of the black hole is where our understanding breaks down. Einstein’s theory of gravity seems to predict that time itself is destroyed at the center of the hole: time comes to an abrupt end there. For this reason, a black hole is sometimes described as the “reverse of creation.” But no one knows how or why time could come to an abrupt end, any more than we know how time was created in the first place. Einstein’s theory of gravity no longer applies at these tiniest scales of distance, and new laws of nature must be found that describe what happens at the center of a black hole.
To understand why, think back on the twin experiment that is often used to explain how time and space work together in Einstein’s theory of general relativity:
One twin stays on Earth while the other one zooms out into space at the speed of light, turns around, and returns home. The twin that travelled through space is significantly younger because the faster you move, the slower time passes for you.
As you reach the event horizon, you are moving at such high speeds due to the strong gravitational force from the black hole, that time will slow down.