In 1991, David Deutsch proposed a method to explain how quantum systems interact with closed timelike curves (CTCs) using time evolution equations. This method aims to address paradoxes like the grandfather paradox, which suggest that a time traveler who stops their own birth would create a contradiction. One interpretation of Deutsch's approach is that it implies the time traveler might end up in a parallel universe rather than their own, although the formalism itself does not explicitly require the existence of parallel universes.