so, i have a few interviews for PhDs in particle physics over the next month and i'm looking for some information on what generally happens at a PhD interview. If anyone would like to share their experiences, interviewers or interviewees, i would greatly appreciate it. I'm not too worried about the general interview stuff - i'm pretty confident and enthusiastic and generally knowledgeable about the subject (compared to my peers) - its the hard physics questions they might ask that concerns me. With my undergrad degree I find it easy enough to learn everything and do well in the exams, and I'm on track for a 1st, but i'm not so good at answering random questions off the top of my head. Basically I'm worried they're going to ask me to derive something or solve a problem and I just won't be able to do it, even though given 5 minutes on my own I would probably work it out. Is this a reasonable to concern to have, and is it common to be asked to solve or derive stuff in a PhD interview?