[sgt]

Although it seems unlikely now, imagine if we could ever reach 0.9c or maybe even 99.9% of the speed of light.

Perhaps not ships containing humans, but some kind of other tiny ship containing a replicator machine and DNA for humans.

For 0.999c, it would only take around 71 days to reach Alpha Centauri from Earth (as seen from the ship). That's pretty mind boggling.

[tachyonbeam]

Besides the incredible amount of energy you would need for 0.999c, which is physically difficult (impossible?) to bring on a ship, there's other physical constraints that make this difficult, such as the fact that the vacuum is actually not that empty. The faster you go, the more you have small particles hitting your ship at near lightspeed, micrometeorites, etc.