Inner Lakes

The general theory of relativity allows an abundant variety of possible universes, including ones in
which time has truly bizarre properties. For example, in some universes the possible trajectory of
an observer can loop back upon itself in time, to form what is called a closed timelike curve (CTC).
In these universes time travel is possible, in the sense that an observer traversing such a curve would
return to exactly the same point in spacetime at the “end” of all her exploring. Such curves and
other similarly exotic possible structures illustrate the remarkable flexibility of general relativity
(GR) with regard to the global properties of spacetime.