How binding is a PhD application for theoretical vs. experimental physics?
I've heard that I should apply as if I'm pursuing an experimental physics track, though, because it's less competitive, and then transition towards a theory group once admitted. Is this sound advice?


Yes, it is, in general. A very high percentage of people applying to physics PhD programs say that they want to do theory, especially particle theory, but in reality only a very small percentage of grad students end up doing theory.

However, the number of people saying they want to do condensed matter theory is probably relatively small, so the usual advice may not apply to you if you're applying to schools that have a strong, established program in condensed matter theory.


  Still, I am wary about misrepresenting myself and also concerned about getting stuck in a path that I don't intend to be in.


Frankly, at this stage you simply have no experience on which to base any such expectations. That's why nobody really takes statements like this on grad school applications very seriously. That kind of statement is not binding in any way, and I doubt that anyone even remembers it once you start grad school. It may be relevant to your application mainly as an indication of whether you were serious enough about that school to find out what their research areas are. E.g., if you apply and say you want to do quantum gravity, but nobody at that school does quantum gravity, they'll probably circular-file your app.

What will typically happen to people who think they want to do theory is that they start their graduate coursework, and they find that there's stiff competition in courses such as field theory and general relativity. If you want to do a thesis in relativity, for example, you probably need to be the top student in the GR class. For condensed matter theory, the relevant course would probably be field theory, since a lot of the same methods are used.

I don't think it's exactly the same at all universities in the US, but at the grad school I went to, there was this initial sorting process based on performance in coursework, and after that students spent the summer as research assistants, which was a further vetting process.

There's just no way to predict the outcome of this process in advance. As an undergrad, you're comparing yourself to other undergrads at school A, while as a grad student, you're comparing yourself to other grad students at school B. Often it's the experience of moving from being a big fish in a small pond to being just one more fish. That's how it was for me, at least.