  But fortunes were reversed once again by a result of Kay,  Radzikowski, and Wald (1997). The details of their theorem are too  technical to review here, but the structure of the argument is easy  to grasp. The naïve calculation of  \(\langle \Psi \mid T_{ab}\mid\Psi \rangle\) results in infinities  which have to be subtracted off to produce a renormalized expectation value  \(\langle \Psi \mid T_{ab}\mid\Psi \rangle_R\) with a finite value. The standard renormalization procedure uses a  limiting procedure that is mathematically well-defined if, and only  if, a certain condition   obtains.[30]  The KRW theorem shows that this condition is violated for points on  \(H^+ (\Sigma)\) and, thus, that the expectation value of the stress-energy tensor is not well-defined at the chronology  horizon.