Abstract
Some recent ideas about topology and signature changing spacetimes are described. If spacetime is everywhere Lorentzian but non-orientable, one can sometimes avoid closed timelike curves, but one must must consider pinors rather than spinors. One finds that there is now an important distinction between signature (+ + + −) and (− − − +). In some cases one signature may be excluded and the other allowed. Topology changing spacetimes with domains of non-Lorentzian signature are considered. These domains may be Riemannian or Kleinian (+ + − −). It is argued that our present signature, together with the idea of time must have arisen as the consequence of physical processes. This emergence of the idea of time is also connected with the origin of the complex numbers in Quantum Mechanics which should also be regarded as the consequence of the evolution of the universe.
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