Anything that can be cut can also be sewn. It is mathematically possible to take two black hole geometries, and sew them together along their "tears". This gives rise to wormhole solutions to Einstein's equations, in which two otherwise separate "universes" are connected by a throat, or tunnel, as shown in the top figure above. Such wormholes could also connect different regions of the same universe, as in the bottom picture. In principle this would enable us to take shortcuts to distant parts of the Universe just like they do in Star Trek. The problem is that within Einstein's theory such wormholes are very unstable. The throats tend to collapse in a much shorter time than it would take to get through to the other side, so that traversing such wormholes is in practice impossible.