PHYSICS

science that deals with the structure of matter and the interactions between the fundamental constituents of the observable universe. In the broadest sense physics, which was long called natural philosophy (from the Greek physikos), is concerned with all aspects of nature on both the macroscopic and submicroscopic levels. Its scope of study encompasses not only the behaviour of objects under the action of given forces but also the nature and origin of gravitational, electromagnetic, and nuclear force fields. Its ultimate objective is the formulation of a few comprehensive principles that bring together and explain all such disparate phenomena. Physics is treated in a number of articles. For the history and principal treatment of the discipline, see physical science. For primary areas of study and divisions, see acoustics; atom; electromagnetic radiation; electromagnetism; gravitation; light; mechanics; phase; radiation; subatomic particle. For fundamental principles and theories, see relativity; thermodynamics; optics. For methodology and instrumentation, see analysis; particle accelerator. For general approach, see physical science, principles of. science that deals with the structure of matter and the interactions between the fundamental constituents of the observable universe. In the broadest sense, physics (from the Greek physikos) is concerned with all aspects of nature on both the macroscopic and submicroscopic levels. Its scope of study encompasses not only the behaviour of objects under the action of given forces but also the nature and origin of gravitational, electromagnetic, and nuclear force fields. Its ultimate objective is the formulation of a few comprehensive principles that bring together and explain all such disparate phenomena. Physics is the basic physical science. Until rather recent times the terms physics and natural philosophy were used interchangeably for the science whose aim is the discovery and formulation of the fundamental laws of nature. As the modern sciences developed and became increasingly specialized, physics came to denote that part of physical science not included in astronomy, chemistry, geology, and engineering. Physics plays an important role in all the natural sciences, however, and all such fields have branches in which physical laws and measurements receive special emphasis, bearing such names as astrophysics, geophysics, biophysics, and even psychophysics. Physics can, at base, be defined as the science of matter, motion, and energy. Its laws are typically expressed with economy and precision in the language of mathematics. Both experiment, the observation of phenomena under conditions that are controlled as precisely as possible, and theory, the formulation of a unified conceptual framework, play essential and complementary roles in the advancement of physics. Physical experiments result in measurements, which are compared with the outcome predicted by theory. A theory that reliably predicts the results of experiments to which it is applicable is said to embody a law of physics. However, a law is always subject to modification, replacement, or restriction to a more limited domain, if a later experiment makes it necessary. The ultimate aim of physics is to find a unified set of laws governing matter, motion, and energy at small (microscopic) subatomic distances, at the human (macroscopic) scale of everyday life, and out to the largest distances (e.g., those on the extragalactic scale). This ambitious goal has been realized to a notable extent. Although a completely unified theory of physical phenomena has not yet been achieved (and possibly never will be), a remarkably small set of fundamental physical laws appears able to account for all known phenomena. The body of physics developed up to about the turn of the 20th century, known as classical physics, can largely account for the motions of macroscopic objects that move slowly with respect to the speed of light and for such phenomena as heat, sound, electricity, magnetism, and light. The modern developments of relativity and quantum theory modify these laws insofar as they apply to higher speeds, very massive objects, and to the tiny elementary constituents of matter, such as electrons, protons, and neutrons. Additional reading I. Bernard Cohen, The Birth of a New Physics, rev. ed. (1985), is an account of the work of Galileo, Newton, and other 17th-century scientists. See also Emilio Segr, From Falling Bodies to Radio Waves: Classical Physicists and Their Discoveries (1984), and From X-Rays to Quarks: Modern Physicists and Their Discoveries (1980; originally published in Italian, 1976). Henry A. Boorse and Lloyd Motz (eds.), The World of the Atom, 2 vol. (1966), is a comprehensive anthology of historical sources on 19th- and 20th-century developments in atomic physics. For more recent history, see Stephen G. Brush, Resource Letter HP-1: History of Physics, American Journal of Physics, 55:683-691 (August 1987). An interesting collection of writings by physicists is presented in Jefferson Hane Weaver (ed.), The World of Physics: A Small Library of the Literature of Physics from Antiquity to the Present, 3 vol. (1987).Reference works surveying the scope and methodology of physics include Robert M. Besanon (ed.), The Encyclopedia of Physics, 3rd ed. (1985); and Cesare Emiliani, Dictionary of the Physical Sciences: Terms, Formulas, Data (1987). Other works include David Halliday and Robert Resnick, Fundamentals of Physics, 3rd extended ed., 2 vol. (1988), a good standard text; Gerald Holton, Introduction to Concepts and Theories in Physical Science, 2nd ed., rev. by Stephen G. Brush (1973, reprinted 1985), analyzing the physical theories from a historical standpoint; Richard P. Feynman, Robert B. Leighton, and Matthew Sands, The Feynman Lectures on Physics, 3 vol. (196365), and Richard P. Feynman, QED: The Strange Theory of Light and Matter (1985), works by a modern master; Frank Close, Michael Marten, and Christine Sutton, The Particle Explosion (1987), a discussion of the latest developments in fundamental physics, written for the general reader; Steven Weinberg, The Discovery of Subatomic Particles (1983), and The First Three Minutes: A Modern View of the Origin of the Universe, updated ed. (1988); P.C.W. Davies, Space and Time in the Modern Universe (1977); and Peter G. Bergmann, The Riddle of Gravitation, rev. ed. (1987). An unusual social history of the U.S. scientific community is presented in Daniel J. Kevles, The Physicists (1978, reprinted 1987).