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.