you have to take into account that the power source would have to be able to handle an exponentially increasing mass as it get closer to the light barrier. unless we somehow find a way to lessen or nullify the weight issue i don't forsee a safe power source that could handle such a load and move living cargo. also, on that same note, we sould have to find a way to make the acceleration to said speeds survivable. the gravitational force of the craft alone, moving at such speeds would tear our frail human bodies asunder.