"The Wick rotation idea is now so commonly used in quantum field theory that it is often taken as an automatic procedure in numerous different kinds of situation, with barely a mention, and its validity is hardly ever questioned. It does, in fact, have a broad applicability, but it is not a universally valid procedure. Most particularly. it is highly questionable in the context of the curved space-times arising in general relativity, when in normal circumstances the procedure cannot even be applied, because there is no natural time coordinate. In string theory, this is a problem both in the 10-dimensional space-time, in general curved-space situations, and also in the string world-sheet ..."