Remember that 1960's sci-fi masterpiece—the original Planet of the Apes
? In that now cult classic film a group of astronauts travel to Earth's distant future where "apes rule!" While there, Charlton Heston chews up the scenery and, more importantly,
shows us a pretty good example of how Einstein's theory of special relativity makes space travel a challenge. In the film the astronauts have been traveling for one and a half years in their frame of reference; meanwhile 2000 years have elapsed on Earth. According to the rules of special relativity this is a realistic scenario. But how can this be? It's actually a natural consequence of the observation that the speed of light (c) is constant regardless of the frame of reference from which you measure it. Through a series of logical steps Einstein reasoned that this means that the amount of time that passes between events depends on the frame of reference from which you observe them.
According to special relativity anything with any mass will always travel less than the speed of light, but if you get close enough then time dilation starts to rear its (ugly?) head. The time dilation equation can be expressed as follows:
Tship = Tearth(1-v2/c2)1/2
(Using the above equation you can show that the ship must have been traveling at an average speed of v = 0.9999997c relative to Earth to achieve this amount of time dilation.)
So while it's theoretically possible to get places somewhat quickly in your spaceship if you travel close enough to c, when you arrive at your destination you're hardly going to agree on what time it is, what year it is, or who's the president. Your children may well be older than you when you get back. This could be extremely inconvenient, to say the least.