If you've ever drifted so close to a waterfall that you could no longer swim fast enough to get away, then you pretty much know what it's like to fall into a black hole. Researchers have now created a laboratory analog of such a point of no return.
"Space-time really behaves like a river," says Ulf Leonhardt of the University of St. Andrews in Scotland. "Gravity can be represented as if space were a medium that is flowing." A swimmer's best efforts correspond to nature's ultimate speed limit, which is the speed of light in empty space.
Black holes are regions where gravity curls space-time so much that nothing inside can escape—think of a waterfall that would trap all swimmers, no matter how fast. Both a spaceship approaching a black hole (or a swimmer edging toward a waterfall) will cross a point of no return called an event horizon. That's where space-time flows into a black hole's region so fast that even light cannot escape.