User blog comment:PitaKang/Funday Monday 3: What would an interstellar civilization look like?/@comment-32656-20130501080354/@comment-1375165-20130505185046

The Lorentz factor at 0.5c is 1.15, which would produce noticeable relativistic effects for a human observer. I wouldn't say 0.5c isn't fast enough "to feel significant effects of relativity".

The kinetic energy of a hydrogen atom moving at 0.5c (or hitting a ship travelling at 0.5c) is only ~2x10^-11 J - even less than the energy of speck of paint in the wind. You'd need a trillion of those atoms to even approach the kinetic energy of a baseball. The pressure could be higher than the analogy suggests but that would only be if they were concentrated in space, which, if you look out there, is extremely unlikely.

Beauty of getting from A to B with an Alcubierre Drive is that the ship using it isn't locally moving. Free floating particles in space wouldn't be hitting the ship with a relative velocity above the speed of light, even if the "warp" bubble was travelling relative to Earth at a rate faster than the speed of light. If they fell inside the between the contracting and expanding regions of space-time, then they would move no faster relative to the ship than they were while floating freely in outer space.