Physics

I / INTRODUCTION

Physics,majorscience,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.

Seealsoseparatearticles on the different aspects of physics and the various sciences mentioned in this article.

II / SCOPE OF PHYSICS

Physicsiscloselyrelated 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.

Theemphasisontheinteraction 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).

Physicsemergedasaseparate 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.

III / EARLY HISTORY OF PHYSICS

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

A / Antiquity

TheBabylonians,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.

Someprogresswasmade, 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).

B / Middle Ages

Littleadvancewasmade 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.

C / 16th and 17th Centuries

Theadventofmodernscience 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.

IV / NEWTON AND MECHANICS

Startingabout1665,at the age of 23, Newton enunciated the principles of mechanics, formulated the law of universal gravitation, separated white light into colors, proposed a theory for the propagation of light, and invented differential and integral calculus. Newton's contributions covered an enormous range of natural phenomena: He was thus able to show that not only Kepler's laws of planetary motion but also Galileo's discoveries of falling bodies follow a combination of his own second law of motion and the law of gravitation, and to predict the appearance of comets, explain the effect of the moon in producing the tides, and explain the precession of the equinoxes.

A / The Development of Mechanics

Thesubsequentdevelopment of physics owes much to Newton's laws of motion (see Mechanics), notably the second, which states that the force needed to accelerate an object will be proportional to its mass times the acceleration. If the force and the initial position and velocity of a body are given, subsequent positions and velocities can be computed, although the force may vary with time or position; in the latter case, Newton's calculus must be applied. This simple law contained another important aspect: Each body has an inherent property, its inertial mass, which influences its motion. The greater this mass, the slower the change of velocity when a given force is impressed. Even today, the law retains its practical utility, as long as the body is not very small, not very massive, and not moving extremely rapidly. Newton's third law, expressed simply as “for every action there is an equal and opposite reaction,” recognizes, in more sophisticated modern terms, that all forces between particles come in oppositely directed pairs, although not necessarily along the line joining the particles.

B / Gravity

Newton'smorespecific contribution to the description of the forces in nature was the elucidation of the force of gravity. Today scientists know that in addition to gravity only three other fundamental forces give rise to all observed properties and activities in the universe: those of electromagnetism, the so-called strong nuclear interactions that bind together the neutrons and protons within atomic nuclei, and the weak interactions between some of the elementary particles that account for the phenomenon of radioactivity. Understanding of the force concept, however, dates from the universal law of gravitation, which recognizes that all material particles, and the bodies that are composed of them, have a property called gravitational mass. This property causes any two particles to exert attractive forces on each other (along the line joining them) that are directly proportional to the product of the masses, and inversely proportional to the square of the distance between the particles. This force of gravity governs the motion of the planets about the sun and the earth's own gravitational field, and it may also be responsible for the possible gravitational collapse, the final stage in the life cycle of stars. See Black Hole; Gravitation; Star.

Oneofthemostimportant observations of physics is that the gravitational mass of a body (which is the source of one of the forces existing between it and another particle), is effectively the same as its inertial mass, the property that determines the motional response to any force exerted on it (see Inertia). This equivalence, now confirmed experimentally to within one part in 1013, holds in the sense of proportionality—that is, when one body has twice the gravitational mass of another, it also has twice the inertial mass. Thus, Galileo's demonstrations, which antedate Newton's laws, that bodies fall to the ground with the same acceleration and hence with the same motion, can be explained by the fact that the gravitational mass of a body, which determines the forces exerted on it, and the inertial mass, which determines the response to that force, cancel out.

Thefullsignificance of this equivalence between gravitational and inertial masses, however, was not appreciated until Albert Einstein, the theoretical physicist who enunciated the theory of relativity, saw that it led to a further implication: the inability to distinguish between a gravitational field and an accelerated frame of reference (see the Modern Physics: Relativity section of this article).

Theforceofgravityis the weakest of the four forces of nature when elementary particles are considered. The gravitational force between two protons, for example, which are among the heaviest elementary particles, is at any given distance only 10-36 the magnitude of the electrostatic forces between them, and for two such protons in the nucleus of an atom, this force in turn is many times smaller than the strong nuclear interaction. The dominance of gravity on a macroscopic scale is due to two reasons: (1) Only one type of mass is known, which leads to only one kind of gravitational force, which is attractive. The many elementary particles that make up a large body, such as the earth, therefore exhibit an additive effect of their gravitational forces in line with the addition of their masses, which thus become very large. (2) The gravitational forces act over a large range, and decrease only as the square of the distance between two bodies.

Bycontrast,theelectric charges of elementary particles, which give rise to electrostatic and magnetic forces, are either positive or negative, or absent altogether. Only particles with opposite charges attract one another, and large composite bodies therefore tend to be electrically neutral and inactive. On the other hand, the nuclear forces, both strong and weak, are extremely short range and become hardly noticeable at distances of the order of 1 million-millionth of an inch.

Despiteitsmacroscopic importance, the force of gravity remains so weak that a body must be very massive before its influence is noticed by another. Thus, the law of universal gravitation was deduced from observations of the motions of the planets long before it could be checked experimentally. Not until 1771 did the British physicist and chemist Henry Cavendish confirm it by using large spheres of lead to attract small masses attached to a torsion pendulum, and from these measurements also deduced the density of the earth.

Inthetwocenturiesafter Newton, although mechanics was analyzed, reformulated, and applied to complex systems, no new physical ideas were added. The Swiss mathematician Leonhard Euler first formulated the equations of motion for rigid bodies, while Newton had dealt only with masses concentrated at a point, which thus acted like particles. Various mathematical physicists, among them Joseph Louis Lagrange of France and Sir William Rowan Hamilton of Ireland extended Newton's second law in more sophisticated and elegant reformulations. Over the same period, Euler, the Dutch-born scientist Daniel Bernoulli, and other scientists also extended Newtonian mechanics to lay the foundation of fluid mechanics.

C / Electricity and Magnetism

AlthoughtheancientGreeks were aware of the electrostatic properties of amber, and the Chinese as early as 2700 bc made crude magnets from lodestone, experimentation with and the understanding and use of electric and magnetic phenomena did not occur until the end of the 18th century. In 1785 the French physicist Charles Augustin de Coulomb first confirmed experimentally that electrical charges attract or repel one another according to an inverse square law, similar to that of gravitation. A powerful theory to calculate the effect of any number of static electric charges arbitrarily distributed was subsequently developed by the French mathematician Siméon Denis Poisson and the German mathematician Carl Friedrich Gauss.

Apositivelychargedparticle attracts a negatively charged particle, tending to accelerate one toward the other. If the medium through which the particle moves offers resistance to that motion, this may be reduced to a constant-velocity (rather than accelerated) motion, and the medium will be heated up and may also be otherwise affected. The ability to maintain an electromotive force that could continue to drive electrically charged particles had to await the development of the chemical battery by the Italian physicist Alessandro Volta in 1800. The classical theory of a simple electric circuit assumes that the two terminals of a battery are maintained positively and negatively charged as a result of its internal properties. When the terminals are connected by a wire, negatively charged particles will be simultaneously pushed away from the negative terminal and attracted to the positive one, and in the process heat up the wire that offers resistance to the motion. Upon their arrival at the positive terminal, the battery will force the particles toward the negative terminal, overcoming the opposing forces of Coulomb's law. The German physicist Georg Simon Ohm first discovered the existence of a simple proportionality constant between the current flowing and the electromotive force supplied by a battery, known as the resistance of the circuit. Ohm's law, which states that the resistance is equal to the electromotive force, or voltage, divided by the current, is not a fundamental and universally applicable law of physics, but rather describes the behavior of a limited class of solid materials. See Electric Circuit.

Thehistoricalconcepts of magnetism, based on the existence of pairs of oppositely charged poles, had started in the 17th century and owe much to the work of Coulomb. The first connection between magnetism and electricity, however, was made through the pioneering experiments of the Danish physicist and chemist Hans Christian Oersted, who in 1819 discovered that a magnetic needle could be deflected by a wire nearby carrying an electric current. Within one week after learning of Oersted's discovery, the French scientist André Marie Ampère showed experimentally that two current-carrying wires would affect each other like poles of magnets. In 1831 the British physicist and chemist Michael Faraday discovered that an electric current could be induced (made to flow) in a wire without connection to a battery, either by moving a magnet or by placing another current-carrying wire with an unsteady—that is, rising and falling—current nearby. The intimate connection between electricity and magnetism, now established, can best be stated in terms of electric or magnetic fields, or forces that will act at a particular point on a unit charge or unit current, respectively, placed at that point. Stationary electric charges produce electric fields; currents—that is, moving electric charges—produce magnetic fields. Electric fields are also produced by changing magnetic fields, and vice versa. Electric fields exert forces on charged particles as a function of their charge alone; magnetic fields will exert an additional force only if the charges are in motion.