For many people, the prospect of time travel holds great appeal, with possible reasons running the gamut from movingly selfless to vilely greedy. The "Back to the Future" films are a good case study in all that might go wrong, though time travel is a purely hypothetical endeavor, with widely varying interpretations of how it might actually work. In any case, in that film series, Doc Brown wants to travel through time for the thrill of adventure, because it hasn't been done, to discover what lies ahead and what was there before. On the other hand, Biff Tannen, the movies' principal villain, uses time travel to enrich himself, giving a latter-day sports almanac to his younger self, who then uses it get rich on sports betting.
Perhaps the most lofty -- and potentially dangerous -- reason to travel through time is to change history. Whether it's to stop Hitler from assuming power or to stave off a financial collapse, time travel is synonymous with meddling with a larger historical narrative. The problem, of course, is that no one knows what the consequences might be. Nonetheless, pondering the effects of time travel, and their attendant paradoxes, is a popular topic of discussion.
The most widely discussed time-travel conundrum is the "grandfather paradox," which asks: If you went back in time and killed your grandfather, would you cease to exist? Would your trip back in time (and subsequent killing of your grandfather) then be impossible? It's a recursive line of thinking that leads perpetually back on itself, but it also represents a fundamental problem with time travel: predicting the future from even small changes to the past. For example, if you went back in time and prevented a plane from crashing, could that lead to a chain of events ensuring that all of human history -- including your very existence -- is vastly different? One can never know.
Some theorists have tried to confront time-travel paradoxes by creating quantum-physics-based models that prevent paradoxical events from occurring. There is, however, one other way of avoiding these paradoxes: go to the future.
The idea of time travel, going back into the past or forward into the future, has fascinated science fiction writers, readers, moviegoers and dreamers for centuries. Whether it involves using a time machine to travel back into the distant past to meet historical personalities, wanting to be present at major events like the signing of the Declaration of Independence or curiosity about what the future will look like, until Albert Einstein's theory of special relativity, it seemed like time travel was merely a fantasy.
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