"In 1992, Stephen Hawking tried to resolve this question about time travel once and for all. Instinctively, he was against time travel; if journeys through time were as common as Sunday picnics, then we should see tourists from the future gawking at us and taking pictures.
But physicists often quote from T. H. White’s epic novel The Once and Future King, where a society of ants declares, “Everything not forbidden is compulsory.” In other words, if there isn’t a basic principle of physics forbidding time travel, then time travel is necessarily a physical possibility. (The reason for this is the uncertainty principle. Unless something is forbidden, quantum effects and fluctuations will eventually make it possible if we wait long enough. Thus, unless there is a law forbidding it, it will eventually occur.) In response, Stephen Hawking proposed a “chronology protection hypothesis” that would prevent time travel and hence “make history safe for historians.” According to this hypothesis, time travel is not possible because it violates specific physical principles ...
Today, Hawking no longer says that time travel is absolutely impossible, only that it is highly unlikely and impractical. The odds are overwhelmingly against time travel. But one cannot rule it out entirely. If one can somehow harness large quantities of positive and negative energy and solve the stability problem, time travel may indeed be possible. (And perhaps the reason we are not flooded by tourists from the future is that the earliest time they can go back to is when the time machine was created, and perhaps time machines haven’t been created yet.)"
–"Parallel Worlds: A Journey Through Creation, Higher Dimensions, and the Future of the Cosmos" by Michio Kaku