I've found some similar posts, but as the answers heavily depend on the individual, I decided to present my own case. I want to apply for the physics graduate programs of the big universitie MIT, Harvard, Standord and UC Berkley. For this I need to decide on a specific field of study for my motivational letters, but I'm having trouble deciding.

I've just completed my physics bachelor degree as well as my mathematics bachelors degree. My physics thesis was about a simulation of a quantum many-particle system using statistical physics, and my maths thesis was about mathematical logic, or more precisely type theory. As you might already guess I'm more theoretically inclined. I can already rule out anything experimental since I was always swayed more to the theoretical side of things. That's also the reason why I studied Mathematics, in order to better understand theoretical physics.

My Bachelor's thesis would point in the direction of Condensed Matter, but I'm not quite sure if that direction is really what I want to pursue. My problem is that I really don't have a good overview of the different theoretical fields. I would like something that is as fundamental as possible while still being able to connect it somehow to my bachelor thesis in order to tell a good story in my motivational letters.

Maybe someone can give me some tipps overviews or personal experiences on the matter.

Thank you in advance.