Fernando M. Belchior

In a water vortex swirling toward the drain, the flow speed near the center can exceed the speed of sound. Sound gets trapped: waves cannot escape back, forming a silent region — a sound shadow. Half a century ago, William Unruh realized that this is an exact copy of a black hole, but for sound instead of light.

The fifth coordinate, curled into a tiny loop, could be hiding right under our noses. In such a universe, everything we see would be stuck to a four-dimensional film, the brane, like water droplets on a window pane. But why doesn't matter slip into the fifth dimension? The answer may lie in the twisting of spacetime itself.