Physics

I

Introduction

Physics, major science dealing with the fundamental constituents of the universe, the forces they exert on one another, and the effects of these forces. Sometimes in modern physics a more sophisticated approach is taken that incorporates elements of the three areas listed above; it relates to symmetry and conservation laws, such as those pertaining to energy, momentum, charge, and parity. See Atom; Energy.

See also separate articles on the different aspects of physics and the various sciences mentioned in this article.

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Scope of Physics

Physics is closely related to the other natural sciences and, in a sense, encompasses them. Chemistry, for example, deals with the interaction of atoms to form molecules; much of modern geology is largely a study of the physics of the Earth and is known as geophysics; and astronomy deals with the physics of the stars and outer space. Even living systems are made up of fundamental particles and, as studied in biophysics and biochemistry, they follow the same types of laws as the simpler particles traditionally studied by a physicist.

The emphasis on the interaction between particles in modern physics, known as the microscopic approach, must often be supplemented by a macroscopic approach that deals with larger elements or systems of particles. This macroscopic approach is indispensable to the application of physics to much of modern technology. Thermodynamics, for example, a branch of physics developed during the 19th century, deals with defining and measuring properties of a system as a whole and is useful in other fields of physics; it also forms the basis of much of chemical and mechanical engineering. Such properties as the temperature, pressure, and volume of a gas have no meaning for an individual atom or molecule; these thermodynamic concepts can only be applied directly to a very large system of such particles. A bridge exists, however, between the microscopic and macroscopic approach; another branch of physics, known as statistical mechanics, indicates how pressure and temperature can be related to the motion of atoms and molecules on a statistical basis (seeStatistics).

Even into the 19th century a physicist was often also a mathematician, philosopher, chemist, biologist, or engineer. Today the field has grown to such an extent that with few exceptions modern physicists have to limit their attention to one or two branches of the science. Once the fundamental aspects of a new field are discovered and understood, they become of interest to engineers and other applied scientists. The 19th-century discoveries in electricity and magnetism, for example, are now the province of electrical and communication engineers; the properties of matter discovered at the beginning of the 20th century have been applied in electronics; and the discoveries of nuclear physics, most of them not yet 40 years old, have passed into the hands of nuclear engineers for applications to peaceful or military uses.

III

Early History of Physics

Although ideas about the physical world date from antiquity, physics did not emerge as a well-defined field of study until early in the 19th century.

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Antiquity

The Chinese, Babylonians, Egyptians, and early Mesoamericans observed the motions of the planets and succeeded in predicting eclipses, but they failed to find an underlying system governing planetary motion. The speculations of Greek philosophers introduced two major rival ideas about the fundamental constituents of the universe: atomism, proposed by Leucippus in the 4th century bc, and the theory of the elements, which had been proposed in the 5th century bc. SeePhilosophy, Greek; Philosophy, Western.

Notable progress was made in Alexandria, the scientific centre of Western civilization during the Hellenistic Age. There, the Greek mathematician and inventor Archimedes designed various practical mechanical devices involving levers and screws, and measured the density of solid bodies by submerging them in a liquid. Other important Greek scientists of this time were the astronomer Aristarchus of Samos, who measured the ratio of the distances from the Earth to the Sun and to the Moon; the mathematician, astronomer, and geographer Eratosthenes, who determined the circumference of the Earth and drew up a catalogue of stars; and the astronomer Hipparchus, who discovered the precession of the equinoxes (seeEcliptic). In the 2nd century ad the astronomer, mathematician, and geographer Ptolemy proposed the system of planetary motion that was named after him, in which the Earth was at the centre and the Sun, Moon, and stars moved around it in circular orbits (seePtolemaic System).