If your dissertation is like most, it will only be read by your committee and some other . candidates seeking to build on your work. As such, it does not need to be a masterwork of literature, nor does it need to solve a long-standing problem in computing. It merely needs to be correct, to be significant in the judgement of your committee, and it needs to be complete. We will all applaud when you change the world after graduation. And at that you will find that many well-known scientists in CS have made their careers in areas different from their dissertation topic. The dissertation is proof that you can find and present original results; your career and life after graduation will demonstrate the other concerns you might have about making an impact.