Physics, major science, dealing with the fundamental constituents of the universe, the forces they exert on one another, and the results produced by these forces. Sometimes in modern physics a more sophisticated approach is taken that incorporates elements of the three areas listed above; it relates to the laws of symmetry and conservation, 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.



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 the elucidation and measurement of properties of a system as a whole and remains 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 (see Statistics).

Physics emerged as a separate science only in the early 19th century; until that time a physicist was often also a mathematician, philosopher, chemist, biologist, engineer, or even primarily a political leader or artist. 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 the domain of 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.



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.




A Greek mathematician and inventor, Archimedes is credited with important contributions to the development of physics. He is known for applying science to everyday life, developing practical inventions such as the lever and the screw. These simple machines have found uses as diverse as warfare and irrigation. Archimedes supposedly discovered the principle of water displacement while taking a bath, shouting “Eureka!” when he realized why his body caused the level of the water to rise.

The 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. Little was added by the Greek civilization, partly because the uncritical acceptance of the ideas of the major philosophers Plato and Aristotle discouraged experimentation.

Some progress was made, however, notably in Alexandria, the scientific center of Greek civilization. There, the Greek mathematician and inventor Archimedes designed various practical mechanical devices, such as levers and screws, and measured the density of solid bodies by submerging them in a liquid. Other important Greek scientists were the astronomer Aristarchus of Sámos, who measured the ratio of the distances from the earth to the sun and the moon; the mathematician, astronomer, and geographer Eratosthenes, who determined the circumference of the earth and drew up a catalog of stars; the astronomer Hipparchus, who discovered the precession of the equinoxes (see Ecliptic); and the astronomer, mathematician, and geographer Ptolemy, who proposed the system of planetary motion that was named after him, in which the earth was the center and the sun, moon, and stars moved around it in circular orbits (see Ptolemaic System).


Middle Ages

Saint Thomas Aquinas

During the 13th century, Saint Thomas Aquinas sought to reconcile Aristotelian philosophy with Augustinian theology. He employed both reason and faith in the study of metaphysics, moral philosophy, and religion, but he suggested that the truths of reason and those of faith applied to different realms. Aquinas’ work allayed some of the fears that officials of the Roman Catholic church had regarding the study and development of science.

Little advance was made in physics, or in any other science, during the Middle Ages, other than the preservation of the classical Greek treatises, for which the Arab scholars such as Averroës and Al-Quarashi, the latter also known as Ibn al-Nafīs, deserve much credit. The founding of the great medieval universities by monastic orders in Europe, starting in the 13th century, generally failed to advance physics or any experimental investigations. The Italian Scholastic philosopher and theologian Saint Thomas Aquinas, for instance, attempted to demonstrate that the works of Plato and Aristotle were consistent with the Scriptures. The English Scholastic philosopher and scientist Roger Bacon was one of the few philosophers who advocated the experimental method as the true foundation of scientific knowledge and who also did some work in astronomy, chemistry, optics, and machine design.


16th and 17th Centuries


Italian physicist and astronomer Galileo maintained that the earth revolved around the sun, disputing the belief held by the Roman Catholic church that the earth was the center of the universe. He refused to obey orders from Rome to cease discussions of his theories and was sentenced to life imprisonment. It was not until 1984 that a papal commission acknowledged that the church was wrong.

The advent of modern science followed the Renaissance and was ushered in by the highly successful attempt by four outstanding individuals to interpret the behavior of the heavenly bodies during the 16th and early 17th centuries. The Polish natural philosopher Nicolaus Copernicus propounded the heliocentric system that the planets move around the sun. He was convinced, however, that the planetary orbits were circular, and therefore his system required almost as many complicated elaborations as the Ptolemaic system it was intended to replace (see Copernican System). The Danish astronomer Tycho Brahe, believing in the Ptolemaic system, tried to confirm it by a series of remarkably accurate measurements. These provided his assistant, the German astronomer Johannes Kepler, with the data to overthrow the Ptolemaic system and led to the enunciation of three laws that conformed with a modified heliocentric theory. Galileo, having heard of the invention of the telescope, constructed one of his own and, starting in 1609, was able to confirm the heliocentric system by observing the phases of the planet Venus. He also discovered the surface irregularities of the moon, the four brightest satellites of Jupiter, sunspots, and many stars in the Milky Way. Galileo's interests were not limited to astronomy; by using inclined planes and an improved water clock, he had earlier demonstrated that bodies of different weight fall at the same rate (thus overturning Aristotle's dictums), and that their speed increases uniformly with the time of fall. Galileo's astronomical discoveries and his work in mechanics foreshadowed the work of the 17th-century English mathematician and physicist Sir Isaac Newton, one of the greatest scientists who ever lived.