  While the KRW theorem is undoubtedly of fundamental importance for  semi-classical quantum gravity, it does not serve as an effective  no-go result for Thornian time machines. In the first place, the  theorem assumes, in concert with Hawking’s chronology protection  theorem, that \(H^+ (\Sigma)\) is compactly generated, and we repeat that it is far from clear that this assumption is  necessary for seeing a Thornian time machine in operation. A second,  and more fundamental, reservation applies even if a compactly  generated \(H^+ (\Sigma)\) is accepted as a necessary  condition for time machines. The KRW theorem functions as a no-go  result by providing a reductio ad absurdum with a dubious  absurdity: roughly, if you try to operate a Thornian time machine,  you will end up invalidating semi-classical quantum gravity. But  semi-classical quantum gravity was never viewed as anything more than a stepping stone to a genuine quantum theory of gravity, and its  breakdown is expected to be manifested when Planck-scale physics  comes into play. This worry is underscored by Visser’s (1997, 2003)  findings that in chronology violating models trans-Planckian physics  can be expected to come into play before  \(H^+ (\Sigma)\) is reached.