  Classically, all topologies are allowed as solutions to the Einstein
equations. However, one does not observe any topological structures on medium
range distance scales, that is scales that are smaller than the size of the
observed universe but larger than the microscopic scales for which quantum
gravity becomes important. Recently, Friedman, Schleich and Witt have proven
that there is topological censorship on these medium range distance scales: the
Einstein equations, locally positive energy, and local predictability of
physics imply that any medium distance scale topological structures cannot be
seen. More precisely, we show that the topology of physically reasonable
isolated systems is shrouded from distant observers, or in other words there is
a topological censorship principle.
