that any alternative timelines (such as those postulated by the many-worlds interpretation of quantum mechanics) are not accessible. Given these assumptions, the constraint that time travel must not lead to inconsistent outcomes could be seen merely as a tautology, a self-evident truth that cannot possibly be false. However, the Novikov self-consistency principle is intended to go beyond just the statement that history must be consistent, making the additional nontrivial assumption that the universe obeys the same local laws of physics in situations involving time travel that it does in regions of space-time that lack closed timelike curves. This is clarified in the above-mentioned "Cauchy problem in spacetimes with closed timelike curves", where the authors write: Implications for time travelers The assumptions of the self-consistency principle can be extended to hypothetical scenarios involving intelligent time travelers as well as unintelligent objects such as billiard balls. The authors of "Cauchy problem in spacetimes with closed timelike curves" commented on the issue in the paper's conclusion, writing: Similarly, physicist and astronomer J. Craig Wheeler concludes that: Time-loop logic Time-loop logic, coined by roboticist and futurist Hans Moravec, is a hypothetical system of computation that exploits the Novikov self-consistency principle to compute answers much faster than possible with the standard model of computational complexity using Turing machines. In this system, a computer sends a result of a computation backwards through time and relies upon the self-consistency principle to force the sent result to be correct, provided the machine can reliably receive information from the future and provided the algorithm and the underlying mechanism are formally correct. An incorrect result or no result can still be produced if the time travel mechanism or algorithm are not guaranteed to be accurate. A simple example is an iterative method algorithm. Moravec states: Quantum computation with a negative delay Physicist David Deutsch showed in 1991 that this model of computation could solve NP problems in polynomial time, and Scott Aaronson later extended this result to show that the model could also be used to solve PSPACE problems in polynomial time. Deutsch shows that quantum computation with a negative delay—backwards time travel—produces only self-consistent solutions, and the chronology-violating region imposes constraints that are not apparent through classical reasoning. Researchers published in 2014 a simulation in which they claim to have validated Deutsch's model with photons. However, it was shown in an article by Tolksdorf and Verch that Deutsch's self-consistency