For every subfield of physics, there are physicists who focus on the problems with those fields. Some physicists try to identify the basic principles governing the universe of matter and energy – the structures, behavior, generation, transfer, motion and interactions that underlie the phenomena of reality, while others explore the higher level effects of these principles, working towards more "practical" goals – developing better materials and more useful devices. Across this spectrum, there are physicists working with all possible degrees of "application" from abstract research that is virtually pure mathematics to very applied technology research that is barely distinguishable from engineering.
Typically physicist's interests and work are defined by two choices: their field and whether they choose to pursue experimental or theoretical research. Most physicists concentrate on a single field, one of the many subfields from: Acoustics, astronomy astrophysics, atomic physics, biophysics, chaos, chemical physics, computational physics, cosmology, crystallography, electromagnetism, electronics, fluid dynamics, general relativity, geophysics, high energy physics, high pressure physics, laser physics, light physics, low temperature physics/cryophysics/cryogenics, mathematical physics, mechanics, meteorology/weather physics, molecular physics, nanotechnology, nuclear physics, optics, particle physics, plasma physics, quantum physics, quantum optics, quantum field theory, quantum gravity, statistical mechanics, string theory and thermodynamics.
While many physicists will remain in the same field throughout their careers, just as many work in a number of areas – following their interests as they evolve with time.
Normally, physicists further specialize in the experiments or the theory related to those problems (Enrico Fermi, one of the great minds of Nuclear Physics is the last physicist to be generally acknowledged to be a master of both experiments and theory). These two types of physicist – the theoretician and the experimentalist have many differences and commonalities.
The theoreticians work with pencil and paper (or, more recently with keyboard and mathematics software), working through the physics of the phenomena they are interested in from the equations.
The experimentalists perform another crucial task: They seek to confirm or contradict the theories of physics through comparison with actual results. Detailed, arduous experiments are often required to explore the details of theories at the forefront of physics, sometimes requiring very large teams of researchers working with multimillion dollar apparatus (especially in the field of high energy physics).
While research is the core of the typical physicist's job, most will have to take teaching work as part of their positions, instructing potential new physicists (and other students who need to take physics for various reasons – like getting into medical school) in the facts of the universe. In addition to teaching classes, they usually have research students – typical graduate and postdoctoral students who are essentially apprenticed to the professor, learning the details of their specific fields as they help with their supervisor's research, gaining more independence in their research as their experience progresses.Most of all, physicists follow their natural curiosity and interests as they explore the ways of the universe.