If you wish to pursue a graduate degree in X, do you have to have an undergraduate degree in X? I read that one of the greatest physicists living today - Ed Witten - was a history major and linguistics minor prior to going to graduate school at Princeton for applied math (after which he switched to physics there). My first thought was how is this possible? I then realized he was a student many years ago, as he's in his 60's now. So, I wonder if this would still be possible today?