It's actually that there is no amount of thrust that would propel you to the speed of light - no 'almost' about it. From the perspective of an outside observer

It's actually that there is no amount of thrust that would propel you to the speed of light - no 'almost' about it. From the perspective of an outside observer, a constantly-accelerating spaceship will approach the speed of light, but never quite get there. This will happen regardless of the level of thrust. Alcubierre drives are pretty sweet - the only trick is they seem to require negative mass [0]... at least they've gotten down the requirement from a negative Jupiter mass to a mere -700 kg! [0] - https://en.wikipedia.org/wiki/Alcubierre_drive#Mass%E2%80%93energy_requirement https://en.wikipedia.org/wiki/Alcubierre_drive#Mass%E2%80%93...