In this post, I’m going to give an overview of the start of a career in theoretical physics, with a focus on what happens before you find an advisor. This is going to be inherently biased, based as it will be on my experiences. In particular, each country’s education system is different, so much of this will only be relevant for students in the US.
Let’s start at the beginning.
If you want to become a theoretical physicist, you’d better start by taking physics and math courses in high school. Unfortunately, this is where socioeconomic status has a big effect. Some schools have Advanced Placement or International Baccalaureate courses that let you get a head-start on college, many don’t. Some schools don’t even have physics courses at all anymore. My only advice here is to get what you can, when you can. If you can take a physics course, do it. If you can take calculus, do it. If you can take classes that will give you university credit, take them.
After high school, you go to college for a Bachelor’s degree in physics. Getting into college these days is some sort of ridiculous popularity contest, and I don’t pretend to be able to give advice on that. What I can say is that once you’re in college, coursework is important, but research is more important. Graduate schools will look at how well you did in your courses and how advanced those courses were, but they will pay special attention to who you get recommendations from, and whether you did research with them. Whether or not your college has anyone who you can research with, you should consider doing summer research somewhere interesting. With programs like the NSF’s Research Experience for Undergraduates (or REU) you can apply to get hooked up with interesting projects and mentors. In addition to looking good on an application to grad school, doing research helps boost your self-confidence: knowing that you can do something real really helps you start feeling like a scientist. Research also teaches you specialized skills much faster than coursework can: I’ve learned much more about programming from having to use it on projects than from any actual programming course.
That said, coursework is also useful. You want courses that will familiarize you with basic tools of your field, physics courses on classical mechanics and quantum mechanics and electromagnetism and math courses on linear algebra and differential equations. You want to take a math course on group theory, but only if it’s taught by a physicist, as mathematicians focus on different aspects. More than any of that, though, you want to try to take at least a few graduate-level courses in while you’re still in college.
That’s important, because grad school in theoretical physics is kind of a mess. You’ll be there for around five years in total (I was in at the low end with four, some people take six or seven). However, you take most if not all of your courses in the first two years. In general, during that time you are paid as a Teaching Assistant. The school pays your tuition and a livable (if barely) wage, and in return you lead lab sections or grade papers. Teaching experience can be a positive thing, but you don’t want to keep doing it for too long, because the point of grad school isn’t teaching or courses, it’s research. Your goal is to find an advisor who is willing to pay you out of one of their (usually government) grants, so that you can transition from Teaching Assistant to Research Assistant. This is hard to do while you’re still taking courses: you won’t have time, and worse, you won’t know everything you need. Theoretical physics requires a lot of background, and much of it gets taught in grad school. Here at Stony Brook, you’d be taking graduate-level quantum mechanics, quantum field theory, and string theory. Until recently, each one of those was a one-year course, and the most logical way to take them was one after the other. Add that up, and that’s three years…kind of a problem when you want to start research after two. That’s why getting ahead in courses, however and whenever you can, is so important: not so much for the courses themselves, but so you can get past them and do research.
Research is what you do for the rest of your time in grad school. It’s what you do after you graduate, when you become a postdoc. It (and teaching) are what you do as a professor, what you are judged on when they decide whether or not you get tenure. Working through research is going to teach you more than any other experience you will have, so get as much of it as you can. And good luck!