Physics deals with the combination of matter and energy. It also deals with a wide variety of systems, about which theories have been developed that are used by physicists. In general, theories are experimentally tested numerous times before they are accepted as correct as a description of Nature (within a certain domain of validity). For instance, the theory of
classical mechanics accurately describes the motion of objects, provided they are much larger than
atoms and moving at much less than the
speed of light. These theories continue to be areas of active research: for instance, a remarkable aspect of classical mechanics known as
chaos was discovered in the 20th century, three centuries after the original formulation of classical mechanics by
Isaac Newton (1642–1727). These "central theories" are important tools for research in more specialized topics, and any physicist, regardless of his or her specialization, is expected to be literate in them.