When it comes to space travel, Albert Einstein's theory of relativity is a major inconvenience. With the laws of physics (currently) prohibiting faster-than-light travel, velocities are limited to a mere 300,000,000 ms-1. By any Earthbound measure that's very quick indeed, but it'll still take you 4.2 years to reach our sun's nearest neighbor, Proxima Centauri.
That sort of timeframe clearly isn't conducive to all-action space opera — imagine if the Millennium Falcon had taken a decade to travel from Tatooine to Alderaan in "Star Wars: A New Hope" — so sci-fi writers have come up with numerous ingenious ways to jet around the cosmos at quite ludicrous speeds.
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