Hard science fiction, or "hard SF", is special because it uses true facts and theories from sciences. These sciences are very important in Hard SF: physics, astrophysics, and chemistry. Also, Hard SF can show worlds that more advanced technology may make possible. Many correct predictions of the future come from the hard science fiction subgenre. However, there have been many incorrect ideas about the future, too. Some hard SF writers have also worked as professional scientists. A few of these scientist/writers are Gregory Benford and Geoffrey A. Landis, while mathematician authors include Rudy Rucker and Vernor Vinge. Other noteworthy hard SF authors include Arthur C. Clarke, Hal Clement, Isaac Asimov, Greg Bear, Stanislav Lem, Larry Niven, Robert J. Sawyer, Stephen Baxter, Alastair Reynolds, Charles Sheffield, and Greg Egan.