User can create gravitational singularities, where matter is compressed to a point of zero size and space-time curvature is infinite. This usually creates a black hole, where the singularity is "cloaked" by an event horizon (the boundary where the escape velocity equals the speed of light). Black holes produce a strong gravitational field that destroys and devours matter, with many offensive applications; however, rotating black holes have ring-shaped singularities, allowing some in-falling matter to pass through unharmed and emerge from a white hole in a different place, time, or universe.