Physics is the science concerned with the discovery and understanding of the fundamental laws which govern matter, energy, space, and time. Physics deals with the elementary constituents of the universe and their interactions, as well as the analysis of systems best understood in terms of these fundamental principles. Because physics treats the core workings of the universe, including the quantum mechanical details which underpin all atomic interactions, it can be thought of as the foundational science, upon which stands "the central science" of chemistry, and the earth sciences, biological sciences, and social sciences. Discoveries in basic physics have important ramifications for all of science. Classical physics traditionally included the fields of mechanics, optics, electricity, magnetism, acoustics and heat. Modern physics is a term normally used to cover fields which rely on quantum theory, including quantum mechanics, atomic physics, nuclear physics, particle physics and condensed matter physics. The more recent fields of general and special relativity are also usually placed within this category. Although this distinction can be commonly found in older writings, it is of limited current significance as quantum effects are now understood to be of importance even in fields previously considered purely classical. Physics research is divided into two main branches: experimental physics and theoretical physics. Experimental physics focuses mainly on empirical research, and on the development and testing of theories against practical experiment. Theoretical physics is more closely related to mathematics, and involves generating and working through the mathematical implications of systems of physical theories, even where experimental evidence of their validity may not be immediately available.