Well, it's isn fully dummy talk, but i chatted with friends about speed limits in space one tells that you can accelerate untill you reach speed of light(if you have infinite fuel without mass) second one talks about newton laws and so on, i understand that he meaned that after you go to speed of specific impulse you cant accelerate further(he probably forgot about that rockets on orbit fly on speed far exeeding speed of their isp) well, both suggestions wrong and right in some ways, but i think its wery dumb disscussion for people who really know physics