PHYSICS. - From the Greek φύσις (nature). This is the name given to the science that studies the generic properties of matter and of the elementary particles that constitute it, and the manifestations and transformations of energy in all bodies, insofar as they are material.
The other mathematical-physical-natural sciences also deal with material bodies. But 1) mathematics studies them under a most general aspect (number, quantity, form, dimensions), 2) chemistry studies their specific properties and substantial transformations, 3) the natural sciences (astronomy, geology, biology) do indeed deal with generic properties (like physics) and specific ones (like chemistry), but only with certain categories of bodies and under an aspect peculiar to each of them (as a star, as a living being, etc.), neglecting more or less whatever already falls within the province of mathematics, physics and chemistry.
I. HISTORICAL DEVELOPMENT
The first attempts at study date back to the schools of Greek philosophy, scattered along the shores of the entire eastern basin of the Mediterranean. Anaximander and Anaximenes of Miletus (6th century B.C.) interpret the cosmos as due to the Νοῦς (mind), an ordering element. Leucippus of Miletus and Democritus of Abdera (5th-4th centuries B.C.) formulate theories clearly derived from the preceding ones. Democritus in particular develops concepts of atoms, movements and interferences among them that seem to foreshadow modern conceptions.
The word "physics" is encountered for the first time in the works of Aristotle of Stagira (4th century B.C.), where it indicates the observation and study of sensible things, separately from non-sensible things, which are designated as metaphysics. In the great mass of Aristotle's works the only question of physics considered is motion. While in other fields, such as logic, the subjects are set out and developed in so exact a manner as to remain still the basis of that discipline, and, in natural history, the description and classification of animals and plants demonstrate an accurate spirit of observation, the question of motion is treated with a purely deductive method, deriving laws from it on the basis of principles of a general character. Nevertheless, because of the great value they have in other fields and because scholasticism, the dominant philosophy of the Middle Ages, was predominantly metaphysical in tendency, Aristotle's works exercised undisputed influence also in the field of physics for at least 15 centuries.
To Archimedes of Syracuse (3rd century B.C.) is due the discovery of the law of the buoyant force that a body immersed in a liquid receives and that of the lever, the only exact laws that were handed down by Greek civilization.
Aristotle's physics was the object of criticism from the thirteenth century onward through the work of Roger Bacon and subsequently by Nicholas of Cusa, Leonardo da Vinci, William Gilbert and others, as a call for a more realistic conception of facts based on observation. To Galileo Galilei (1564-1642) is due the exact formulation of the laws of motion and of mechanics, and as such he is recognized as the founder of the experimental method. These laws then found confirmation and were linked with the heliocentric system advocated by Nicholas Copernicus and with the laws of planetary motion found by Johannes Kepler, through the work of Isaac Newton (1642-1727), who showed, with the aid of calculus, their dependence on the single law of universal gravitation.
Evangelista Torricelli in the middle of the 17th century carried out the mercury barometer experiment. From this derived new conceptions of the gaseous state, for which Robert Boyle and Daniel Bernoulli in the 18th century then formulated laws of the free movement of molecules (kinetic theory of gases).
Robert Mayer in the 19th century determined the fixed relation between work and heat when one is transformed into the other, enunciating the first principle of thermodynamics, the beginning of the more general law of conservation of energy. Shortly afterward Sadi Carnot and Rudolf Clausius enunciated the second principle of thermodynamics, of the incomplete reversibility in transformations between heat and work.
Luminous phenomena became of interest after the inventions of lenses and telescopes due to craftsmen (Florentine, Venetian, Dutch, from the 17th to the 18th century). The law of reflection was known from remote times; those of refraction and diffraction date back to 1600.
Certain electrical and magnetic phenomena were already known from remote times. It appears that the Chinese knew the compass as early as 2000 B.C., and the navigators of Amalfi were using it in the 12th century. In the 18th century the first electrostatic machines and the Leyden jar were built, and the lightning rod was invented (Benjamin Franklin). Coulomb, at the end of that same century, extended to electric charges the law of attraction determined by Newton for matter.
Electrodynamic phenomena were observed for the first time by Luigi Galvani at the end of the 18th century and by Alessandro Volta, who was the first to construct an apparatus capable of producing electric currents (the pile). In the first twenty years of the 19th century Biot, Savart and Ampère established laws of interdependence between electric currents and magnetic fields.
The study of physics has been greatly aided by the application of mathematics, especially of infinitesimal calculus, due to Wilhelm Leibniz in the 18th century. With this method (theoretical physics) almost all subjects have been treated, seeking to unify them into ever more general theories, sometimes succeeding in predicting new phenomena. Joseph Fourier treated heat, Carl Gauss and Louis Cauchy elasticity, Heinrich Hertz and Clerk Maxwell electricity and magnetism, in relation also to light. Recently Anton Lorentz, Albert Einstein and Max Planck, with the revision of the basic concepts of physics, have developed more general theories that concern and pervade all its fields.
II. METHOD OF STUDY
Methods are distinguished into two: inductive and deductive. The first method is followed when, through the observation of a certain number of phenomena, necessarily limited, with regard also to other concomitant ones as well as to the conditions under which such phenomena occur, a judgment of a general character is arrived at (law of relations of causes and effects). The second method follows an opposite path. As a consequence of postulates (which are therefore admitted and not demonstrated), facts are deduced which, however, must be in agreement with experiment as a verification of the admitted postulate.
Physics treated by the first method takes the name of experimental, by the second method of theoretical and also mathematical, because of the means predominantly used.
III. CLASSIFICATION
For convenience of study, physical phenomena are classified into five categories: mechanics (v.); heat (v. TERMOLOGIA); acoustics (v. SUONO); optics (v.); electricity and magnetism (v. ELETTROLOGIA).