A black hole is a one-way exit from our universe.

The idea of a black hole - an object so massive that nothing could escape the grasp of its gravity - dates back to the 1700s. But the modern story of black holes really starts with Einstein's revolutionary theory of gravity, completed in 1917. Over the past century, scientists have used Einstein's theory of gravity to develop a picture of what black holes must be like. Here are some of the amazing properties of black holes that they have predicted.

In principle, any object - even a rock - can be made into a black hole, by squeezing it into a tiny enough volume. Under these conditions, the object continues to collapse under its own weight, crushing itself down to zero size. However, according to Einstein's theory, the object's mass and gravity remain behind, in the form of an extreme distortion of the space and time around it. This distortion of space and time is the black hole.

In nature, the only objects that can form a black hole on their own are large stars - stars several times more massive than our own Sun. At the end of their lives, these stars collapse under their own weight, forming a black hole. While the original star might be millions of miles wide, the resulting black hole is just a few miles across. The hole itself is shaped like a beach ball - spherical.  

The resulting black hole is the darkest black in the universe: No matter how powerful a light you shine on it, no light ever bounces back, because the light is swallowed by the hole. 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.

Does the inside of a black hole lead to another universe, as some scientists have claimed? The truth is, no one really knows. We cannot do a direct experiment to find out, even in principle, since no information or evidence can ever get back out of a black hole. That's what makes it so important to find and study black holes from the outside, while at the same time developing theories that can more confidently predict what might happen on the inside of the black hole.