According to this The Register article, faster-than-light travel has another obstacle besides relativity: the lowly hydrogen atom. Since that article is quite brief, this post will be all the more brief. As a craft approaches light speed, it compresses what would ordinarily be the sparse hydrogen in space, resulting in incredibly high voltages...more than 1000 times the lethal dose of ionizing radiation. (That's bad.) As the author points out, they'll think of something. SF authors will, for sure.