Are you talking about your statement of purpose? If so, this is more general than a "research objectives" essay. Your research objectives are just one part of this essay. However when writing about your research objectives, I would recommend a little of the general and a little of the specific. Talk about your general interest in theoretical physics and if you can, include very specific examples that show your aptitude in this area (classes you took and did well in, theoretical research that you have already been involved in if there is any, or anything that you undertook independently that shows initiative and skill). Then, mention a few particular research groups you are interested in working with at the University you are applying to (of course this section changes slightly in each essay for each different school). If you have a particular interest in a certain subject, say so, and say why it interests you, and what leads you to believe that you want to study this area in grad school.