Physics departments have their research professors decide on candidates primarily on their field of interest, i.e. experimental vs theory, and fields such as solid-state, optics, nuclear, gravitational, string theory, etc. Professors need to fill their labs, and groups like string theory get filled the fastest. A candidate interested in condensed matter physics (experimental) is more like to get accepted into the program, compared to a string theory candidate. If you have already accepted the PhD offer, they will most like try to recruit to into a different group other than string theory. I did experimental physics research for a number of years, and that was the predominant factor that got me accepted into a graduate program.
My post above was probably down-voted because it is skeptical, doesn't directly address the original question; but it is important advice for a candidate in my humble opinion.
This post imported from StackExchange Physics at 2014-04-01 13:17 (UCT), posted by SE-user acedittutoring