I originally posted this as a reply to a post in the Michio Kaku forum, but it hasn't been replied to in a while. I would really like this cleared up, so please excuse the double post. Someone else made the argument that accelerating even a human nostril hair beyond 0.9c would make its relativistic mass as much as the entire Universe. Hence it would be impossible, even in theory, to accelerate a ship to close to light speed, because we would just run out of mass in the Universe with which to fuel the craft. But I believe this is wrong. Here's my thinking : Let's say we have some sort of advanced fusion engine on board the spacecraft. At a speed of 0.999c relative to the Earth, the spacecraft would appear to have the mass of a "gazillion" Universes or whatever. But the nuclear fuel woud go up in mass by the same factor and the extractable energy would still be related by E = mc^2. From the rest frame of the ship, the remaining nuclear fuel would only have its rest mass, but the ship would be at rest mass too. As far as the ship itself is concerned, there is no great difficulty in accelerating itself further to 0.9999c, for example. Is my logic totally off ? Is it always going to be impossible to bring a ship to near light speeds ? Thanks.