For a long time I thought that aliens could have not visited us due to the immense distances between star systems.
Yesterday however, I started thinking about the twin paradox and realized that it could be used to send creatures from one star system to another and have them travel the distance in a very short time. This idea crossed my mind a long time ago, but now, after reading a bit on my favourite 'book', Wikipedia do I see that I was in fact correct.
Ever heard of the "Twin Paradox"? It's a famous example of special relativity: say you have 2 identical twins both 20 years old. One stays on Earth (let's call him Andrew) and the other (named Benjamin) flies in a space-ship close to the speed of light. When he returns to Earth, from his point of view, 5 years later, he finds that his brother aged 60 years. Now how is this possible? During the trip Andrew aged only 5 years, while his twin 60. The interesting part is actually managed to cover in 5 years a distance that light would have traversed in say 50. Note that we consider two frames of reference, so Andrew didn't actually went faster than the speed of light.
The paradox is related to relative motion, so basically why did the stationary brother age faster? Shouldn't the fact that speed is relative mean that we could've consider Andrew stationary but in that case he would have aged, and we know that Benjamin is the one aged... contradiction. This isn't important here (a possible explanation if you must, could be given by accelerated motion).
Now, with a little help from Wikipedia, let's give a more accurate example: Say an alien race (let's call them Zorgonians), found our planet 1 billion years ago. I would recommend you pay attention to the time-line because we're talking in relative terms here. After analysing the constituents of Earth's atmosphere they found out that it could support life. Zorgonia, their home planet is located some 100,000 light years from us (100000 ly is about the diameter of the Milky Way), so what they were seeing was actually the Earth 1,000,100,000 years ago. They sent a ship equipped with the Faztzorg-9800 engine that can reach a speed of 98% of the speed of light, carrying a crew of Scientzorgs. Given it's speed, the space ship should reach Earth in 102040 years, from our and from the Zorgonians' point of view.
But what do the Scientzorgs think of their trip? Because of distance contraction at very high speeds they only have to travel a fraction of the 100000 ly, more exactly for them, the trip takes about 20000 light years. But what about, if we use after-burners? Increasing our speed to 99.5%? Then the trip would last 10000 years, a unbelievable speed of 99.99998% of the speed of light gives us a trip time of only 63 years from the Scientzorgs' point of view. However, as the speed increases the mass of the space ship for also increases. So, a 1,000 ton space-ship, at 99.99998% of light-speed has a mass of 15 million tons. Which is quite heavy.
Truth is, that now, once I've done that math, a space ship travelling at 99.99998% of the speed of light seems unlikely, however, the point of this article was to show, that a sufficiently advanced civilization, could send scientists to another planet without having them die of old age before they reach their destination.
Still, the Zorgonians on Zorgonia would have to wait 2 * 100,000 years to hear from their Scientzorgs...
NOTE: A quick insight on the math / equations I used to get my results can be found on my blog's helper wiki: http://neduard.wikidot.com/specialrelativity