Is an internship in theoretical physics valuable for me? I am planning to do an internship in a theoretical physics area for a year before applying for PhD. My current masters' thesis advisor (who is a hardcore experimentalist) warned me that I might not actually learn anything new in a theoretical intern, and that I will only end up solving some textbook problems. I was wondering how plausible this is, and if it is a strong possibility, how I can make sure that there is a significant addition to my skillset during my intern in theory. Anyone?

EDIT : This internship will NOT be a part of my M.Sc. I will be doing it to bolster my otherwise ordinary profile. I basically want to publish something or at least get a good recommendation letter before I apply for PhD.
Answering is an experimental physicist who had authored a few theoretical papers. I find the suggestion given to you incorrect (maybe voluntarily, as your MS advisor would like you to be his/her PhD student right away?). As a Master student, the level of work you will do is not limited much by your skills/knowledge, rather from your supervisor's wits and guidance. An excellent supervisor with a brilliant idea waiting to be executed can guide you to a very valuable research publication in three months or so. And of course you can be left in a corner with a textbook and generic instructions which will only leave you confused and isolated. It really depends on your supervisor and how much effort will you both put into it.