Hello, before I start off, I apologize for asking a question which I am sure has been asked hundreds of times before: but I felt there is just way too much information out there which is a little confusing, so I am here with the hope of getting some personalized suggestions. I am currently a PhD student in engineering, robotics, to be specific. I've always had a lot of interest towards theoretical physics, although I wasn't really brave enough to start getting to it, but I've decided I have to start somewhere with self study. I don't really have any ideas of making it my career or anything like that. All I am looking for is a rigorous understanding of all the fundamentals that will let me keep up with and understand current research advances etc. As to my background, I am pretty comfortable with classical mechanics, optics, atomic physics, basics of electromagnetism (my undergraduate study was in electrical engineering), calculus (limits, differentiation, integration, ODEs, PDEs), vectors, matrices, linear algebra, Fourier analysis, Laplace and z transforms etc. My only knowledge in the advanced areas of physics comes from books like The Elegant Universe. It just feels like there's a weird gap between undergraduate mathematics and the exotic sounding concepts like Riemannian geometry, group theory etc., on which most theoretical physics concepts seem to be based. It'd be really great if anyone can let me know what's the best step to start off with: do I just get something like Griffith's QM and wade through the theory? Do I start studying the math first and get a good grasp? Do I study general relativity and special relativity before QM? I have checked out Gerard t'Hooft's website for self-study, and although that gave me a good list of topics, I am having trouble connecting the dots for which one relates to what, and what is a good, not too over-ambitious choice for someone like me. Thanks for your time!