|
|
For many the term science refers to the organized body of knowledge concerning the physical world, both animate and inanimate, but a proper definition would also have to include the attitudes and methods through which this body of knowledge is formed; thus, a science is both a particular kind of activity and also the results of that activity.
The Beginnings of Science
The State of Saxony
Science as it is known today is of relatively modern origin, but the traditions out of which it has emerged reach back beyond recorded history. The roots of science lie in the technology of early toolmaking and other crafts, while scientific theory was once a part of philosophy and religion. This relationship, with technology encouraging science rather than the other way around, remained the norm until recent times. Thus, the history of science is essentially intertwined with that of technology. Practical Applications in the Ancient Middle East
The early civilizations of the Tigris-Euphrates valley and the Nile valley made advances in both technology and theory, but separate groups within each culture were responsible for the progress. Practical advances in metallurgy, agriculture, transportation, and navigation were made by the artisan class, such as the wheelwrights and shipbuilders. The priests and scribes were responsible for record keeping, land division, and calendar determination, and they developed written language and early mathematics for this purpose. The Babylonians devised methods for solving algebraic equations, and they compiled extensive astronomical records from which the periods of the planets\' revolution and the eclipse cycle could be calculated; they used a year of 12 months and a week of 7 days, and also originated the division of the day into hours, minutes, and seconds. In Egypt there were also developments in mathematics and astronomy and the beginnings of the science of medicine. Wheeled vehicles and bronze metallurgy, both known to the Sumerians in Babylonia as early as 3000 b.c. , were imported to Egypt c.1750 b.c. . Between 1400 b.c. and 1100 b.c. iron smelting was discovered in Armenia and spread from there, and alphabets were developed in Phoenicia. Early Greek Contributions to Science
The early Greek, or Hellenic, culture marked a different approach to science. The Ionian natural philosophers removed the gods from the personal roles they had played in the cosmologies of Babylonia and Egypt and sought to order the world according to philosophical principles. Thales of Miletus (6th cent. b.c. ) was one of the earliest of these and contributed to astronomy, geometry, and cosmology. He was followed by Anaximander, who extended Thales\' ideas and proposed that the universe is composed of four basic elements, i.e., earth, air, fire, and water; this theory was also taught by Empedocles (5th cent. b.c. ) in Sicily. The philosophers Leucippus and Democritus (both 5th cent. b.c. ) held that everything is composed of tiny, indivisible atoms. In the school founded at Croton, S Italy, by the Greek philosopher Pythagoras of Samos (6th cent. b.c. ) the principal concept was that of number. The Pythagoreans tried to explain the workings of the universe in terms of whole numbers and their ratios; in addition to contributions to mathematics and philosophy, they also made notable studies in the area of biology and anatomy, e.g., by Alcmaeon of Croton (fl. c.500 b.c. ). The most important developments in medicine were made by Hippocratesof Cos (4th cent. b.c. ), known as the Father of Medicine, who formulated the science of diagnosis based on accurate descriptions of the symptoms of various diseases. The greatest figures of the earlier Greek period were the philosophers Plato (427-347 b.c. ) and Aristotle (384-322 b.c. ), each of whom exerted an influence that has extended down to modern times. Influence of the Alexandrian Schools
The later Greek, or Hellenistic, culture was centered not in Greece itself but in Greek cities elsewhere, particularly Alexandria, Egypt, which was founded in 332 b.c. by Alexander the Great. The so-called first Alexandrian school included Euclid (fl. c.300 b.c. ), who organized the axiomatic system of geometry that has served as the model for many other scientific presentations since then; Eratosthenes (3d cent. b.c. ), who made a remarkably accurate estimate of the size of the earth; and Aristarchus (3d cent. b.c. ), who showed that the sun is larger than the earth and suggested a heliocentric model for the solar system. Archimedes (287-212 b.c. ) worked at Syracuse, Sicily, and made contributions to mathematics and mechanics that were surprisingly modern in spirit. The second Alexandrian school flourished in the first centuries of the Christian era, after Rome had become the leading power in the Mediterranean; it included Ptolemy (2d cent. a.d. ), who presented the geocentric system of the universe that was to dominate astronomical thought for 1400 years, and his contemporary Heron, who contributed to geometry and pneumatics. Galen (2d cent. a.d. ) studied at Pergamum and Alexandria and later practiced medicine and made important anatomical studies at Rome. The Romans assimilated the more practical scientific accomplishments of the Greeks but added relatively little of their own. With the collapse of the Roman Empire in the 5th cent. and the coming of the Dark Ages, science ceased to develop in the West. Scientific Progress in China and India
In the East some accomplishments in science had been made paralleling the early developments in the West. However, although many societies were quick to adopt the fruits of technology, they tended to discourage the development of science on the classical model, which is based on the unbiased interaction of theory and experiment.
In China scientific theories were largely subservient to the main schools of philosophy and theology, particularly those of Confucianism, Taoism, and, later, Buddhism. The agricultural society, which endured until modern times, encouraged the separation of theory and experiment, the former falling to the educated, scholar classes and the latter to the lower, craftsman classes. Astronomy and mathematics were used for practical purposes, such as calendar determination, and there was little interest in theory in these fields. Theories of metallurgy, alchemy, and medicine were all tied to the prevailing religious and philosophical doctrines. Nevertheless, many important practical discoveries were made. Paper was invented in the 2d cent. a.d. ; block printing was known in the 7th cent. a.d. , with movable clay type by the 11th cent. and cast metal type in Korea by the beginning of the 15th cent.; gunpowder was invented in the 3d cent. a.d. and firearms were in use by the 13th cent.; and the magnetic compass came into use during the 11th and 12th cent.
In India an alphabetic script was developed, as well as a numeral system based on place value and including a zero; this latter Hindu contribution was adopted by the Arabs and combined with their numeral system. Important Hindu scientists flourished in the 6th and 7th cent. a.d. and also in the 12th cent., making contributions to astronomy and mathematics. Many of these early Indian works showed the influence of Greek science, as in the geocentric systems of astronomy, or of Babylonian science, as in their development of algebraic methods for solving many problems.
Science in the Middle Ages Muslim Preservation of Learning
With the eclipse of the Greek and Roman cultures, many of their works passed into the hands of the Muslims, who by the 7th and 8th cent. a.d. had extended their influence through much of the world surrounding the Mediterranean. All of the Greek works were translated into Arabic, and commentaries were added. Important developments from the East were also transmitted, and the Hindu numeral system was introduced, as well as the manufacture of paper and gunpowder, learned from the Chinese. Scholars gathered at cities like Damascus, Baghdad, and Cairo, at one end of the Mediterranean, and at Cordova and Toledo, in Spain, at the other end. Many astronomical observations were made at different locations, but there was little effort to improve or modify the Greek model of Ptolemy. In medicine important contributions were made by Al-Razi (Rhazes, 865-925) and Ibn-Sina (Avicenna, 980-1037), and in alchemy and pharmacology by Jabir (Geber, 9th cent.), whose work was expanded in the 10th cent. by a mystical sect aligned with the Sufi tradition. At Cairo, Al-Hazen (965-1038) studied optics, particularly the properties of lenses, and Maimonides (1135-1204), the Jewish philosopher, came there from Spain to practice medicine as physician to Saladin, the Sultan. The Arabs thus preserved the scientific works of the Greeks and added to them, and also introduced other contributions from Asia. This body of learning first began to be discovered by Europeans in the 11th cent. The Craft Tradition and Early Empiricism in Europe
Certain technical innovations during the Dark Ages, e.g., development of the heavy plow, the windmill, and the magnetic compass, as well as improvements in ship design, had increased agricultural productivity and navigation and contributed to the rise of cities, with their craft guilds and universities. These changes were more pronounced in N Europe than in the south. The introduction of papermaking (12th cent.) and printing (1436-50) made possible the recording of craft traditions that had been handed down orally in previous centuries. This served to reduce the gap between the artisan classes and the scholar classes and contributed to the development of certain individuals who combined elements of both traditionsthe artist-engineers such as Leonardo da Vinci, whose studies of flight and other technological problems were far beyond their time, and the artist-mathematicians, such as Albrecht Drer, who examined the laws of perspective and wrote a textbook on geometry. Many artists came to study anatomy in detail.
Beginning in the 12th cent. the Arabic versions of Greek works were translated into Latin, an edition of Ptolemy\'s Almagest being translated at Toledo, and one of Aristotle\'s biological works in Sicily. Leonardo da Pisa (Fibonacci) presented some of the new Hindu-Arabic mathematics in the early 13th cent., and the medical and alchemical works were also translated. Also in the 13th cent., a trend toward empiricism was promoted by Roger Bacon and others, but this was short-lived. The dominant philosophy of science and other fields was the Christianized version of Aristotelian philosophy created by Albertus Magnus and Thomas Aquinas in the 13th cent. This view tended to treat scientific theories as extensions of philosophy and, for example, postulated the existence of angelic agents to account for the movements of the heavenly bodies. Even so, the craft traditions continued to develop in an independent manner, particularly medieval alchemy, and certain schools grew up that were not dominated by the main scholastic philosophy. The rebirth, or Renaissance, of learning spread throughout the West from the 14th to the 16th cent. and was further enhanced by the great voyages of discovery that began in the 15th cent.
The Scientific Revolution The Craft Tradition and Early Empiricism in Europe
Science, in the modern sense of the term, came into being in the 16th and 17th cent., with the merging of the craft tradition with scientific theory and the evolution of the scientific method. The feeling of dissatisfaction with the older philosophical approach had begun much earlier and had produced other results, such as the Protestant Reformation, but the revolution in science began with the work of Copernicus, Paracelsus, Vesalius, and others in the 16th cent. and reached full flower in the 17th cent. The Rejection of Traditional Paradigms
Copernicus broke with the traditional belief, supported by both scientists and theologians, that the earth was at the center of the universe; his work, finally published in the year of his death (1543), proposed that the earth and other planets move in circular orbits around the sun. Paracelsus rejected the older alchemical and medical theories and founded iatrochemistry, the forerunner of modern medical chemistry. Andreas Vesalius, like Paracelsus, turned away from the medical teachings of Galen and other early authorities and through his anatomical studies helped to found modern medicine and biology. The philosophical basis for the scientific revolution was expressed in the writings of Francis Bacon, who urged that the experimental method plays the key role in the development of scientific theories, and of Ren Descartes, who held that the universe is a mechanical system that can be described in mathematical terms. The science of mechanics was established by Galileo, Simon Stevin, and others. The astronomical system of Copernicus gained support from the accurate observations of Tycho Brahe; the modification of Johannes Kepler, who used Tycho\'s work to show that the planetary orbits are elliptical rather than circular; and the writings of Galileo, who based his arguments on his own mechanical theories and observations with the newly invented telescope. Other instruments were also of major importance in the discoveries of the scientific revolution. The microscope extended human knowledge of living things just as the telescope had extended human knowledge of the heavens. The mechanical clock was perfected in the late 16th cent. by Christian Huygens, who also made improvements in the telescope, and thus events, both celestial and terrestrial, could be timed with greater precisionan essential factor in the development of the exact sciences, such as mechanics. The 17th cent. also saw the discovery of the circulation of the blood by William Harvey and the founding of modern chemistry by Robert Boyle. Improved Communication of Scientific Knowledge
Another important factor in the scientific revolution was the rise of learned societies and academies in various countries. The earliest of these were in Italy and Germany and were short-lived. More influential were the Royal Society in England (1660) and the Academy of Sciences in France (1666). The former was a private institution in London and included such scientists as Robert Hooke, John Wallis, William Brouncker, Thomas Sydenham, John Mayow, and Christopher Wren (who contributed not only to architecture but also to astronomy and anatomy); the latter, in Paris, was a government institution and included as a foreign member the Dutchman Huygens. In the 18th cent. important royal academies were established at Berlin (1700) and at St. Petersburg (1724). The societies and academies provided the principal opportunities for the publication and discussion of scientific results during and after the scientific revolution. The Impact of Sir Isaac Newton
The greatest figure of the scientific revolution, Sir Isaac Newton, was a fellow of the Royal Society of England. To earlier discoveries in mechanics and astronomy he added many of his own and combined them in a single system for describing the workings of the universe; the system is based on the concept of gravitation and uses a new branch of mathematics, the calculus, that he invented for the purpose. All of this was set forth in his Philosophical Principles of Natural Philosophy (1687), the publication of which marked the beginning of the modern period of mechanics and astronomy. Newton also discovered that white light can be separated into a spectrum of colors, and he theorized that light is composed of tiny particles, or corpuscles, whose behavior can be described by the laws of mechanics. A rival theory, holding that light is composed of waves, was proposed by Huygens about the same time. However, Newton\'s influence was so great and the acceptance of the mechanistic philosophy of Descartes and others so widespread that the corpuscular philosophy was the dominant one for more than a century.
The Scientific Method
The scientific method has evolved over many centuries and has now come to be described in terms of a well-recognized and well-defined series of steps. First, information, or data, is gathered by careful observation of the phenomenon being studied. On the basis of that information a preliminary generalization, or hypothesis, is formed, usually by inductive reasoning, and this in turn leads by deductive logic to a number of implications that may be tested by further observations and experiments (see induction; deduction). If the conclusions drawn from the original hypothesis successfully meet all these tests, the hypothesis becomes accepted as a scientific theory or law; if additional facts are in disagreement with the hypothesis, it may be modified or discarded in favor of a new hypothesis, which is then subjected to further tests. Even an accepted theory may eventually be overthrown if enough contradictory evidence is found, as in the case of Newtonian mechanics, which was shown after more than two centuries of acceptance to be an approximation valid only for speeds much less than that of light.
Branches of Specialization
Science may be roughly divided into the physical sciences, the earth sciences, and the life sciences. mathematics, while not a science, is closely allied to the sciences because of their extensive use of it. Indeed, it is frequently referred to as the language of science, the most important and objective means for communicating the results of science. The physical sciences include physics, chemistry, and astronomy; the earth sciences (sometimes considered a part of the physical sciences) include geology, paleontology, oceanography, and meteorology; and the life sciences include all the branches of biologysuch as botany, zoology, genetics, and medicine. Each of these subjects is itself divided into different branchese.g., mathematics into arithmetic, algebra, geometry, and analysis; physics into mechanics, thermodynamics, optics, acoustics, electricity and magnetism, and atomic and nuclear physics. In addition to these separate branches, there are numerous fields that draw on more than one branch of science, e.g., astrophysics, biophysics, biochemistry, geochemistry, and geophysics.
All of these areas of study might be called pure sciences, in contrast to the applied, or engineering, sciences, i.e., technology, which is concerned with the practical application of the results of scientific activity. Such fields include mechanical, civil, aeronautical, electrical, architectural, chemical, and other kinds of engineering; agronomy, horticulture, and animal husbandry; and many aspects of medicine. Finally, there are distinct disciplines for the study of the history and philosophy of science.
Role of Measurement and Experiment
All of the activities of the scientific method are characterized by a scientific attitude, which stresses rational impartiality. measurement plays an important role, and when possible the scientist attempts to test his theories by carefully designed and controlled experiments that will yield quantitative rather than qualitative results. Theory and experiment work together in science, with experiments leading to new theories that in turn suggest further experiments. Although these methods and attitudes are generally shared by scientists, they do not provide a guaranteed means of scientific discovery; other factors, such as intuition, experience, good judgment, and sometimes luck, also contribute to new developments in science.
Revolutions in Modern Science Science and the Industrial Revolution
The enormous growth of science during the classical period engendered an optimistic attitude on the part of many that all the major scientific discoveries had been made and that all that remained was the working out of minor details. Faith in the absolute truth of science was in some ways comparable to the faith of earlier centuries in such ancient authorities as Aristotle and Ptolemy. This optimism was shattered in the late 19th and early 20th cent. by a number of revolutionary discoveries. These in turn attracted increasing numbers of individuals into science, so that whereas a particular problem might have been studied by a single investigator a century ago, or by a small group of scientists a few decades ago, today such a problem is attacked by a virtual army of highly trained, technically proficient scholars. The growth of science in the 20th cent. has been unprecedented.
In much of modern science the idea of progressive change, or evolution, has been of fundamental importance. In addition to biological evolution, astronomers have been concerned with stellar and galactic evolution, and astrophysicists and chemists with nucleosynthesis, or the evolution of the chemical elements. The study of the evolution of the universe as a whole has involved such fields as non-Euclidean geometry and the general theory of relativity. Geologists have discovered that the continents are not static entities but are also evolving; according to the theory of plate tectonics, some continents are moving away from each other while others are moving closer together. The Impact of Elementary Particles
Physics in particular was shaken to the core around the turn of the century. The atom had been presumed indestructible, but discoveries of X rays (1895), radioactivity (1896), and the electron (1897) could not be explained by the classical theories. The discovery of the atomic nucleus (1911) and of numerous subatomic particles in addition to the electron opened up the broad field of atomic and nuclear physics. Atoms were found to change not only by radioactive decay but also by more dramatic processesnuclear fission and fusionwith the release of large amounts of energy; these discoveries found both military and peaceful applications. Quantum Theory and the Theory of Relativity
The explanation of atomic structure required the abandonment of older, commonsense, classical notions of the nature of space, time, matter, and energy in favor of the new view of the quantum theory and the theory of relativity. The first of these two central theories of modern physics was developed by many scientists during the first three decades of the 20th cent.; the latter theory was chiefly the product of a single individual, Albert Einstein. These theories, particularly the quantum theory, revolutionized not only physics but also chemistry and other fields. Advances in Chemistry
Knowledge of the structure of matter enabled chemists to synthesize a sweeping variety of substances, especially complex organic substances with important roles in life processes or with technological applications. Radioactive isotopes have been used as tracers in complicated chemical and biochemical reactions and have also found application in geological dating. Chemists and physicists have cooperated to create many new chemical elements, extending the periodic table beyond the naturally occurring elements. Biology Becomes an Interdisciplinary Science
In biology the modern revolution began in the 19th cent. with the publication of Charles Darwin\'s theory of evolution (1859) and Gregor Mendel\'s theory of genetics, which was largely ignored until the end of the century. With the work of Hugo de Vries around the turn of the century biological evolution came to be interpreted in terms of mutations that result in a genetically distinct species; the survival of a given species was thus related to its ability to adapt to its environment through such mutations. The development of biochemistry and the recognition that most important biological processes take place at the molecular level led to the rapid growth of the field of molecular biology, with such fundamental results as the discovery of the structure of deoxyribonucleic acid (DNA), the molecule carrying the genetic code. Modern medicine has profited from this explosion of knowledge in biology and biochemistry, with new methods of treatment ranging from penicillin, insulin, and a vast array of other drugs to pacemakers for weak hearts and implantation of artificial or donated organs. The Abstraction of Mathematics
In mathematics a movement toward the abstract, axiomatic approach began early in the 19th cent. with the discovery of two different types of non-Euclidean geometries and various abstract algebras, some of them noncommutative. While there has been a tendency to consolidate and unify under a few general concepts, such as those of group, set, and transformation, there has also been considerable research in the foundations of mathematics, with a close examination of the nature of these and other concepts and of the logical systems underlying mathematics. Astronomy beyond the Visual Spectrum
In astronomy ever larger telescopes have assisted in the discovery that the sun is a rather ordinary star in a huge collection of stars, the Milky Way, which itself is only one of countless such collections, or galaxies, that in general are expanding away from each other. The study of remote objects, billions of light-years from the earth, has been carried out at all wavelengths of electromagnetic radiation, some of the most notable results being made in radio astronomy, which has been used to map the Milky Way, study quasars, pulsars, and other unusual objects, and detect relatively complex organic molecules floating in space, raising new questions about the origin of life and the possible existence of intelligent life elsewhere in the universe. Modern Science and Technology
The technological advances of modern science, which in the public mind are often identified with science itself, have affected virtually every aspect of life. The electronics industry, born in the early 20th cent., has advanced to the point where a complex device, such as a computer, that once might have filled an entire room can now be carried in an attach case. The electronic computer has become one of the key tools of modern industry. Electronics has also been fundamental in developing new communications devices (radio, television, laser). In transportation there has been a similar leap of astounding range, from the automobile and the early airplane to the modern supersonic jet and the giant rocket that has taken astronauts to the moon. Perhaps the most overwhelming aspect of modern science is not its accomplishments but its magnitude in terms of money, equipment, numbers of workers, scope of activity, and impact on society as a whole. Never before in history has science played such a dominant role in so many areas.
Promise and Problems of Modern Science
Modern science holds out a number of promises, as well as a number of problems. In the foreseeable future researchers may solve the riddle of life and create life itself in a test tube. Most diseases may be brought under control. Science is also working toward control over the environment, e.g., dispersing hurricanes before they can endanger life or property. New sources of energy are being developed, and these together with the capacity to manipulate alien environments may make life possible on the moon or other planets.
Among the challenges faced by modern science are practical ones such as the production and distribution of enough energy to meet increased demands and the elimination or reduction of pollutants in the environment. Some of these problems are political and sociological as well as scientific, as are such problems as control over nuclear and other forms of weapons (biological, chemical) and regulation of the use of computers and other electronic devices that may seriously infringe on individual privacy and freedom. Some have profound ethical implications, e.g., those associated with gene manipulation, organ transplantation, and the capacity to sustain life beyond the point at which it once would have ended. There are also philosophical problems raised by science, as in the uncertainty principle of the quantum theory, which places an absolute limit on the accuracy of certain physical measurements and thus on the predictions that may be made on the basis of such measurements; in the quantum theory itself, with its suggestion that at the atomic level much depends on chance; and in certain paradoxical discoveries in mathematics and mathematical logic. Even a detailed account of the history of science cannot be complete, for scientific activity is not isolated but takes place within a larger matrix that also includes, for example, political and social events, developments in the arts, philosophy, and religion, and forces within the life of the individual scientist. In other words, science is a human activity and is affected by all that affects human beings in any way.
Bibliography
See Henri Poincar, Science and Hypothesis (1902, tr. 1905, repr. 1952); Jacob Bronowski, The Common Sense of Science (1953); Ernest Nagel, The Structure of Science (1961); Alexandre Koyr, Metaphysics and Measurement (1968); George Sarton, Introduction to the History of Science (3 vol., 1927-48; repr. 1968); Barry Commoner, Science and Survival (1966, repr. 1969); N. R. Hanson, Perception and Discovery: An Introduction to Scientific Inquiry (1969); T. S. Kuhn, The Structure of Scientific Revolutions (2d ed. 1970); Jacques Monod, Chance and Necessity (tr. 1971); L. P. Williams and H. J. Steffens, The History of Science in Western Civilization (3 vol., 1978-79); C. A. Ronan, Science (1982); J. Ziman, An Introduction to Science Studies (1985). See also R. J. Blackwell, ed., A Bibliography of the Philosophy of Science, 1945-1981 (1983).
Physics
A branch of sciencetraditionally defined as the study of matter, energy, and the relation between them; it was called natural philosophy until the late 19th cent. and is still known by this name at a few universities. Physics is in some senses the oldest and most basic pure science; its discoveries find applications throughout the natural sciences, since matter and energy are the basic constituents of the natural world. The other sciences are generally more limited in their scope and may be considered branches that have split off from physics to become sciences in their own right. Physics today may be divided loosely into classical physics and modern physics.
Evolution of Physics
Greek Contributions
The earliest history of physics is interrelated with that of the other sciences. A number of contributions were made during the period of Greek civilization, dating from Thales and the early Ionian natural philosophers in the Greek colonies of Asia Minor (6th and 5th cent. b.c. ). Democritus (c.460-370 b.c. ) proposed an atomic theory of matter and extended it to other phenomena as well, but the dominant theories of matter held that it was formed of a few basic elements, usually earth, air, fire, and water. In the school founded by Pythagoras of Samos the principal concept was that of number; it was applied to all aspects of the universe, from planetary orbits to the lengths of strings used to sound musical notes.
The most important philosophy of the Greek period was produced by two men at Athens, Plato (427-347 b.c. ) and his student Aristotle (384-322 b.c. ); Aristotle in particular had a critical influence on the development of science in general and physics in particular. The Greek approach to physics was largely geometrical and reached its peak with Archimedes (287-212 b.c. ), who studied a wide range of problems and anticipated the methods of the calculus. Another important scientist of the early Hellenistic period, centered in Alexandria, Egypt, was the astronomer Aristarchus (c.310-220 b.c. ), who proposed a heliocentric, or sun-centered, system of the universe. However, just as the earlier atomic theory had not become generally accepted, so too the astronomical system that eventually prevailed was the geocentric system proposed by Hipparchus (190-120 b.c. ) and developed in detail by Ptolemy (85 a.d. -165 a.d. ). Preservation of Learning
With the passing of the Greek civilization and the Roman civilization that followed it, Greek learning passed into the hands of the Muslim world that spread its influence from the E Mediterranean eastward into Asia, where it picked up contributions from the Chinese (papermaking, gunpowder) and the Hindus (the place-value decimal number system with a zero), and westward as far as Spain, where Islamic culture flourished in Crdoba, Toledo, and other cities. Little specific advance was made in physics during this period, but the preservation and study of Greek science by the Muslim world made possible the revival of learning in the West beginning in the 12th and 13th cent. The Scientific Revolution
The first areas of physics to receive close attention were mechanics and the study of planetary motions. Modern mechanics dates from the work of Galileo and Simon Stevin in the late 16th and early 17th cent. The great breakthrough in astronomy was made by Nicolaus Copernicus, who proposed (1543) the heliocentric model of the solar systemthat was later modified by Johannes Kepler (using observations by Tycho Brahe) into the description of planetary motions that is still accepted today. Galileo gave his support to this new system and applied his discoveries in mechanics to its explanation.
The full explanation of both celestial and terrestrial motions was not given until 1687, when Isaac Newton published his Principia (Mathematical Principles of Natural Philosophy). This work, the most important document of the Scientific Revolution of the 16th and 17th cent., contained Newton\'s famous three laws of motion and showed how the principle of universal gravitation could be used to explain the behavior not only of falling bodies on the earth but also planets and other celestial bodies in the heavens. To arrive at his results, Newton invented one form of an entirely new branch of mathematics, the calculus(also invented independently by G. W. Leibniz), which was to become an essential tool in much of the later development in most branches of physics.
Other branches of physics also received attention during this period. William Gilbert, court physician to Queen Elizabeth I, published (1600) an important work on magnetism, describing how the earth itself behaves like a giant magnet. Robert Boyle (1627-91) studied the behavior of gases enclosed in a chamber and formulated the gas law named for him; he also contributed to physiology and to the founding of modern chemistry.
Newton himself discovered the separation of white light into a spectrum of colors and published an important work on optics, in which he proposed the theory that light is composed of tiny particles, or corpuscles. This corpuscular theory was related to the mechanistic philosophy presented early in the 17th cent. by Ren Descartes, according to which the universe functioned like a mechanical system describable in terms of mathematics. A rival theory of light, explaining its behavior in terms of Waves, was presented in 1690 by Christian Huygens, but the belief in the mechanistic philosophy together with the great weight of Newton\'s reputation was such that the wave theory gained relatively little support until the 19th cent. Development of Mechanics and Thermodynamics
During the 18th cent. the mechanics founded by Newton was developed by several scientists and received brilliant exposition in the Analytical Mechanics (1788) of J. L. Lagrange and the Celestial Mechanics (1799-1825) of P. S. Laplace. Daniel Bernoulli made important mathematical studies (1738) of the behavior of gases, anticipating the kinetic theory of gases developed more than a century later, and has been referred to as the first mathematical physicist.
The accepted theory of heat in the 18th cent. viewed heat as a kind of fluid, called caloric; although this theory was later shown to be erroneous, a number of scientists adhering to it nevertheless made important discoveries useful in developing the modern theory, including Joseph Black (1728-99) and Henry Cavendish (1731-1810). Opposed to this caloric theory, which had been developed mainly by the chemists, was the less accepted theory dating from Newton\'s time that heat is due to the motions of the particles of a substance. This mechanical theory gained support in 1798 from the cannon-boring experiments of Count Rumford (Benjamin Thompson), who found a direct relationship between heat and mechanical energy.
In the 19th cent. this connection was established quantitatively by J. R. Mayer and J. P. Joule, who measured the mechanical equivalent of heat in the 1840s. This experimental work and the theoretical work of Sadi Carnot, published in 1824 but not widely known until later, together provided a basis for the formulation of the first two laws of thermodynamics in the 1850s by William Thomson (later Lord Kelvin) and R. J. E. Clausius. The first law is a form of the law of conservation of energy, stated earlier by J. R. von Mayer and Hermann Helmholtz on the basis of biological considerations; the second law describes the tendency of energy to be converted from more useful to less useful forms.
The atomic theory of matter had been proposed again in the early 19th cent. by the chemist John Dalton and became one of the hypotheses of the kinetic-molecular theory of gases developed by Clausius and James Clerk Maxwell to explain the laws of thermodynamics. The kinetic theory in turn led to the statistical mechanics of Ludwig Boltzmann and J. W. Gibbs. Advances in Electricity. Magnetism, and Thermodynamics
The study of electricity and magnetism also came into its own during the 18th and 19th cents. C. A. Coulomb had discovered the inverse-square laws of electrostatics and magnetostatics in the late 18th cent. and Alessandro Volta had invented the electric battery, so that electric currents could also be studied. In 1820, H. C. Oersted found that a current-carrying conductor gives rise to a magnetic force surrounding it, and in 1831 Michael Faraday (and independently Joseph Henry) discovered the reverse effect, the production of an electric potential or current through magnetism; these two discoveries are the basis of the electric motor and the electric generator, respectively.
Faraday invented the concept of the field of force to explain these phenomena and Maxwell, from c.1856, developed these ideas mathematically in his theory of electromagnetic radiation. He showed that electric and magnetic fields are propagated outward from their source at a speed equal to that of light and that light is one of several kinds of electromagnetic radiation, differing only in frequency and wavelength from the others. Experimental confirmation of Maxwell\'s theory was provided by Heinrich Hertz, who generated and detected electric waves in 1886 and verified their properties, at the same time foreshadowing their application in radio, television, and other devices. The wave theory of light had been revived in 1801 by Thomas Young and received strong experimental support from the work of A. J. Fresnel and others; the theory was widely accepted by the time of Maxwell\'s work on the electromagnetic field, and afterward the study of light and that of electricity and magnetism were closely related. Birth of Modern Physics
By the late 19th cent. most of classical physics was complete, and optimistic physicists turned their attention to what they considered minor details in the complete elucidation of their subject. Several problems, however, provided the cracks that eventually led to the shattering of this optimism and the birth of modern physics. On the experimental side, the discoveries of X rays by Wilhelm Roentgen (1895), radioactivity by A. H. Becquerel (1896), the electron by J. J. Thomson (1897), and new radioactive elements by Marie and Pierre Curie raised questions about the supposedly indestructible atom and the nature of matter. Ernest Rutherford identified and named two types of radioactivity and in 1911 interpreted experimental evidence as showing that the atom consists of a dense, positively charged nucleus surrounded by negatively charged electrons. Classical theory, however, predicted that this structure should be unstable. Classical theory had also failed to explain successfully two other experimental results that appeared in the late 19th cent. One of these was the demonstration by A. A. Michelson and E. W. Morley that there did not seem to be a preferred frame of reference, at rest with respect to the hypothetical luminiferous ether, for describing electromagnetic phenomena. Relativity and Quantum Mechanics
In 1905, Albert Einstein showed that the result of the Michelson-Morley experiment could be interpreted by assuming the equivalence of all inertial (unaccelerated) frames of reference and the constancy of the speed of light in all frames; Einstein\'s special theory of relativity eliminated the need for the ether and implied, among other things, that mass and energy are equivalent and that the speed of light is the limiting speed for all bodies having mass. Hermann Minkowski provided (1908) a mathematical formulation of the theory in which space and time were united in a four-dimensional geometry of space-time. Einstein extended his theory to accelerated frames of reference in his general theory (1916), showing the connection between acceleration and gravitation. Newton\'s mechanics was interpreted as a special case of Einstein\'s, valid as an approximation for small speeds compared to that of light.
Although relativity resolved the electromagnetic phenomena conflict demonstrated by Michelson and Morley, a second theoretical problem was the explanation of the distribution of electromagnetic radiation emitted by a black body; experiment showed that at shorter wavelengths, toward the ultraviolet end of the spectrum, the energy approached zero, but classical theory predicted it should become infinite. This glaring discrepancy, known as the ultraviolet catastrophe, was solved by Max Planck\'s quantum theory (1900). In 1905, Einstein used the quantum theory to explain the photoelectric effect, and in 1913 Niels Bohr again used it to explain the stability of Rutherford\'s nuclear atom. In the 1920s the theory was extensively developed by Louis de Broglie, Werner Heisenberg, Wolfgang Pauli, Erwin Schrdinger, P. A. M. Dirac, and others; the new quantum mechanics became an indispensable tool in the investigation and explanation of phenomena at the atomic level. Particles, Energy, and Contemporary Physics
Dirac\'s theory, which combined quantum mechanics with the theory of relativity, also predicted the existence of antiparticles. During the 1930s the first antiparticles were discovered, as well as other particles. Among those contributing to this new area of physics were James Chadwick, C. D. Anderson, E. O. Lawrence, J. D. Cockcroft, E. T. S. Walton, Enrico Fermi, and Hideki Yukawa.
The discovery of nuclear fission by Otto Hahn and Fritz Strassmann (1938) and its explanation by Lise Meitner and Otto Frisch provided a means for the large-scale conversion of mass into energy, in accordance with the theory of relativity, and triggered as well the massive governmental involvement in physics that is one of the fundamental facts of contemporary science. The growth of physics since the 1930s has been so great that it is impossible in a survey article to name even its most important individual contributors.
Among the areas where fundamental discoveries have been made more recently are solid-state physics, plasma physics, and cryogenics, or low-temperature physics. Out of solid-state physics, for example, have come many of the developments in electronics (e.g., the transistor and microcircuitry) that have revolutionized much of modern technology. Another development is the maser and laser (in principle the same device), with applications ranging from communication and controlled nuclear fusion experiments to atomic clocks and other measurement standards.
Classical Physics
Classical physics includes the traditional branches and topics that were recognized and fairly well developed before the beginning of the 20th cent.mechanics, sound, light, heat, and electricity and magnetism. Mechanics is concerned with bodies acted on by forces and bodies in motion and may be divided into statics (study of the forces on a body or bodies at rest), kinematics (study of motion without regard to its causes), and dynamics (study of motion and the forces that affect it); mechanics may also be divided into solid mechanics and fluid mechanics, the latter including such branches as hydrostatics, hydrodynamics, aerodynamics, and pneumatics. acoustics, the study of sound, is often considered a branch of mechanics because sound is due to the motions of the particles of air or other medium through which sound waves can travel and thus can be explained in terms of the laws of mechanics. Among the important modern branches of acoustics is ultrasonics, the study of sound waves of very high frequency, beyond the range of human hearing. Optics, the study of light, is concerned not only with visible light but also with infrared and ultraviolet radiation, which exhibit all of the phenomena of visible light except visibility, e.g., reflection, refraction, interference, diffraction, dispersion, and polarization of light. Heat is a form of energy, the internal energy possessed by the particles of which a substance is composed; thermodynamics deals with the relationships between heat and other forms of energy. Electricity and magnetism have been studied as a single branch of physics since the intimate connection between them was discovered in the early 19th cent.; an electric current gives rise to a magnetic field and a changing magnetic field induces an electric current. Electrostatics deals with electric charges at rest, electrodynamics with moving charges, and magnetostatics with magnetic poles at rest.
Modern Physics
Most of classical physics is concerned with matter and energy on the normal scale of observation; by contrast, much of modern physics is concerned with the behavior of matter and energy under extreme conditions or on the very large or very small scale. For example, atomic and nuclear physics studies matter on the smallest scale at which chemical elements can be identified. The physics of elementary particles is on an even smaller scale, being concerned with the most basic units of matter; this branch of physics is also known as high-energy physics because of the extremely high energies necessary to produce many types of particles in large particle accelerators. On this scale, ordinary, commonsense notions of space, time, matter, and energy are no longer valid.
The two chief theories of modern physics present a different picture of the concepts of space, time, and matter from that presented by classical physics. The quantum theory is concerned with the discrete, rather than continuous, nature of many phenomena at the atomic and subatomic level, and with the complementary aspects of particles and waves in the description of such phenomena. The theory of relativity is concerned with the description of phenomena that take place in a frame of reference that is in motion with respect to an observer; the special theory of relativity is concerned with relative uniform motion in a straight line and the general theory of relativity with accelerated motion and its connection with gravitation. Both the quantum theory and the theory of relativity find applications in all areas of modern physics.
Bibliography
See R. P. Feynman, The Character of Physical Law (1967); K. W. Ford, Basic Physics (1968); Isaac Asimov, Understanding Physics (3 vol. 1966, repr. separately 1969); L. N. Cooper, An Introduction to the Meaning and Structure of Physics (1970); V. F. Weisskopf, Physics in the Twentieth Century (1972); I. M. Freeman, Physics (2d ed. 1973); J. B. Marion, Physics: the Foundation of Modern Science (1973). Matter
Anything that has mass and occupies space. Matter is sometimes called koinomatter (Gr. koinos=common) to distinguish it from antimatter, or matter composed of antiparticles. Mass
In physics, the quantity of matter in a body regardless of its volume or of any forces acting on it. The term should not be confused with weight, which is the measure of the force of gravity acting on a body. Under ordinary conditions the mass of a body can be considered to be constant; its weight, however, is not constant, since the force of gravity varies from place to place. There are two ways of referring to mass, depending on the law of physics defining it: gravitational mass and inertial mass. The gravitational mass of a body may be determined by comparing the body on a beam balance with a set of standard masses; in this way the gravitational factor is eliminated. The inertial mass of a body is a measure of the body\'s resistance to acceleration by some external force. One body has twice as much inertial mass as another body if it offers twice as much force in opposition to the same acceleration. All evidence seems to indicate that the gravitational and inertial masses of a body are equal, as demanded by Einstein\'s equivalence principle of relativity; so that at the same location equal (inertial) masses have equal weights. Because the numerical value for the mass of a body is the same anywhere in the world, it is used as a basis of reference for many physical measurements, such as density and heat capacity. According to the special theory of relativity, mass is not strictly constant but increases with the speed according to the formula m=m0/√1−v2/c2, where m0 is the rest mass of the body, v is its speed, and c is the speed of light in vacuum. This increase in mass, however, does not become appreciable until very great speeds are reached. The rest mass of a body is its mass at zero velocity. The special theory of relativity also leads to the Einstein mass-energy relation, E=mc2, where E is the energy, and m and c are the (relativistic) mass and the speed of light, respectively. Because of this equivalence of mass and energy, the law of conservation of energy was extended to include mass as a form of energy.
Early Theories of Matter
In ancient times various theories were suggested about the nature of matter. Empedocles held that all matter is made up of four elements;earth, air, fire, and water. Leucippus and his pupil Democritus proposed an atomic basis of matter, believing that all matter is built up from tiny particles differing in size and shape. Anaxagoras, however, rejected any theory in which matter is viewed as composed of smaller constituents, whether atoms or elements, and held instead that matter is continuous throughout, being entirely of a single substance.
Theory of Matter In the 20th Century
The modern theory of matter dates from the work of John Dalton at the beginning of the 19th cent. The atomis considered the basic unit of any element, and atoms may combine chemically to form molecules, the moleculebeing the smallest unit of any substance that possesses the properties of that substance. An elementin modern theory is any substance all of whose atoms are the same (i.e., have the same atomic number), while a compound is composed of different types of atoms together in molecules. Physical and Chemical Changes
The difference between a mixture and a compound helps to illustrate the difference between a physical change and a chemical change. Different atoms may also be present together in a mixture, but in a mixture they are not bound together chemically as they are in a compound. In a physical change, such as a change of state (e.g., from solid to liquid), the substance as a whole changes, but its underlying structure remains the same; water is still composed of molecules containing two hydrogen atoms and one oxygen atom whether it is in the form of ice, liquid water, or steam. In a chemical change, however, the substance participates in a chemical reaction, with a consequent reordering of its atoms. As a result, it becomes a different substance with a different set of properties.
Many of the physical properties and much of the behavior of matter can be understood without detailed assumptions about the structure of atoms and molecules. For example, the kinetic-molecular theory of gas laws provides a good explanation of the nature of temperature and the basis of the various gas laws and also gives insight into the different states of matter. Substances in different states vary in the strength of the forces between their molecules, with intermolecular forces being strongest in solids and weakest in gases. The force holding like molecules together is called cohesion, while that between unlike molecules is called adhesion (see adhesion and cohesion). Among the phenomena resulting from intermolecular forces are surface tension and capillarity. An even larger number of aspects of matter can be understood when the nature and structure of the atom are taken into account. The quantum theory has provided the key to understanding the atom, and most basic problems relating to the atom have been solved. The Relationship of Matter and Energy
The atomic theory of matter does not answer the question of the basic nature of matter. It is now known that matter and energy are intimately related. According to the law of mass-energy equivalence, developed by Albert Einstein as part of his theory of relativity, a quantity of matter of mass m possesses an intrinsic rest mass energy E given by E = mc2, where c is the speed of light. This equivalence is dramatically demonstrated in the phenomena of nuclear fission and fusion, in which a small amount of matter is converted to a rather large amount of energy. The converse reaction, the conversion of energy to matter, has been observed frequently in the creation of many new elementary particles. The study of elementary particles has not solved the question of the nature of matter but only shifted it to a smaller scale.
The States of Matter
Matter is ordinarily observed in three different states, or phases, and scientists distinguish still a fourth state. Matter in the solid state has both a definite volume and a definite shape; matter in the liquid state has a definite volume but no definite shape, assuming the shape of whatever container it is placed in; matter in the gaseous state has neither a definite volume nor a definite shape and expands to fill any container. The properties of a plasma, or hot, ionized gas, are sufficiently different from those of a gas at ordinary temperatures for scientists to consider plasma a fourth state of matter.
The Properties of Matter
The general properties of matter result from its relationship with mass and space. Because of its mass, all matter has inertia(the mass being the measure of its inertia) and weight, if it is in a gravitational field. Because it occupies space, all matter has volume and impenetrability, since two objects cannot occupy the same space simultaneously.
The special properties of matter, on the other hand, depend on internal structure and thus differ from one form of matter, i.e., one substance, to another. Such properties include ductility, elasticity, hardness, malleability, porosity (ability to permit another substance to flow through it), and tenacity (resistance to being pulled apart).
Gravitation
The attractive force existing between any two particles of matter.
The Force of Gravity
The term gravity is commonly used synonymously with gravitation, but in correct usage a definite distinction is made. Whereas gravitation is the attractive force acting to draw any bodies together, gravity indicates that force in operation between the earth and other bodies, i.e., the force acting to draw bodies toward the earth. The force tending to hold objects to the earth\'s surface depends not only on the earth\'s gravitational field but also on other factors, such as the earth\'s rotation. The measure of the force of gravity on a given body is the weight of that body; although the mass of a body does not vary with location, its weight does vary. It is found that at any given location, all objects are accelerated equally by the force of gravity, observed differences being due to differences in air resistance, etc. Thus, the acceleration due to gravity, symbolized as g, provides a convenient measure of the strength of the earth\'s gravitational field at different locations. The value of g varies from about 9.832 meters per second per second (m/sec2) at the poles to about 9.780 m/sec2 at the equator. Its value generally decreases with increasing altitude. Because variations in the value of g are not large, for ordinary calculations a value of 9.8 m/sec2, or 32 ft/sec2, is commonly used.
The Law of Universal Gravitation Other Gothic Arts
Since the gravitational force is experienced by all matter in the universe, from the largest galaxies down to the smallest particles, it is often called universal gravitation. Sir Isaac Newton was the first to fully recognize that the force holding any object to the earth is the same as the force holding the moon, the planets, and other heavenly bodies in their orbits. According to Newton\'s law of universal gravitation, the force between any two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. The constant of proportionality in this law is known as the gravitational constant; it is usually represented by the symbol G and has the value 6.670 10−11 N-m2/kg2 in the meter-kilogram-second (mks) system of units. Very accurate early measurements of the value of G were made by Henry Cavendish. The Relativistic Explanation of Gravitation
Newton\'s theory of gravitation was long able to explain all observable gravitational phenomena, from the falling of objects on the earth to the motions of the planets. However, as centuries passed, very slight discrepancies were observed between the predictions of Newtonian theory and actual events, most notably in the motions of the planet Mercury. The general theory of relativityproposed in 1916 by Albert Einstein explained these differences and provided a geometric explanation for gravitational phenomena, holding that matter causes a curvature of the space-time framework in its immediate neighborhood. The Search for Gravity Waves
Tantalizing evidence for the existence of gravity waves, which are predicted by Einstein\'s general theory of relativity and would be analogous to electromagnetic waves, comes from astronomical observations of a binary pulsar designated 1913 + 16. The rate at which the two neutron stars in the binary rotate around each other is changing in a manner that is consistent with the emission of gravity waves. A hypothetical particle, given the name graviton, has been suggested as the mediator of the gravitational force; it is analogous to the photon, the particle embodying the quantum properties of electromagnetic waves. The search for gravity waves continues with plans to build large interferometers that would be sensitive enough to detect the faint waves directly. Weight
The measure of the force of gravity on a body. Since the weights of different bodies at the same location are proportional to their masses, weight is often used as a measure of mass. However, the two are not the same; mass is a measure of the amount of matter present in a body and thus has the same value at different locations, and weight varies depending upon the location of the body in the earth\'s gravitational field (or the gravitational field of some other astronomical body). A given body will have the same mass on the earth and on the moon, but its weight on the moon will be only about 16% of the weight as measured on the earth. The distinction between weight and mass is further confused by the use of the same units to measure boththe pound, the gram, or the kilogram. One pound of weight, or force, is the force necessary at a given location to accelerate a one-pound mass at a rate equal to the acceleration of gravity at that location (about 32 ft per sec per sec). Similar relationships hold between the gram of force and the gram of mass and between the kilogram of force and the kilogram of mass. Field
In physics, region throughout which a force may be exerted; examples are the gravitational, electric, and magnetic fields that surround, respectively, masses, electric charges, and magnets. The field concept was developed by M. Faraday based on his investigation of the lines of force that appear to leave and return to a magnet at its poles Fields are used to describe all cases where two bodies separated in space exert a force on each other. The alternative to postulating a field is to assume that physical influences can be transmitted through empty space without any material or physical agency. Such action-at-a-distance, especially if it occurs instantaneously, violates both common sense and certain modern theories, notably relativity, which posits that nothing can travel faster than light. In a field description, rather than body A directly exerting a force on body B, body A (the source) creates a field in every direction around it and body B (the detector) experiences the field that exists at its position. If a change occurs at the source, its effect propagates outward through the field at a constant speed and is felt at the detector only after a certain delay in time. The field is thus a kind of middleman transmitting forces. Each type of force (electric, magnetic, nuclear, or gravitational) has its own appropriate field; a body experiences the force due to a given field only if the body itself it also a source of that kind of field. The reciprocity implied by Newton\'s third law of motion (equal action and reaction) is thus preserved. If two bodies exert a mutual force, they possess potential energy that depends on their relative positions; it is natural to regard this energy as residing in the field the bodies create. Motion
The change of position of one body with respect to another. The rate of change is the speed of the body. If the direction of motion is also given, then the velocity of the body is determined; velocity is a vector quantity, having both magnitude and direction, while speed is a scalar quantity, having only magnitude.
Bibliography
See A. S. Eddington, Space, Time and Gravitation (1920); John Archibald Wheeler, A Journey into Gravity and Spacetime (1990). Energy
In physics, the ability or capacity to do work or to produce change. Forms of energy include heat, light, sound, electricity, and chemical energy. Energy and work are measured in the same unitsfoot-pounds, joules, ergs, or some other, depending on the system of measurement being used. When a force acts on a body, the work performed (and the energy expended) is the product of the force and the distance over which it is exerted.
Potential and Kinetic Energy
Potential energy is the capacity for doing work that a body possesses because of its position or condition. For example, a stone resting on the edge of a cliff has potential energy due to its position in the earth\'s gravitational field. If it falls, the force of gravity (which is equal to the stone\'s weight) will act on it until it strikes the ground; the stone\'s potential energy is equal to its weight times the distance it can fall. A charge in an electric field also has potential energy because of its position; a stretched spring has potential energy because of its condition. Chemical energy is a special kind of potential energy; it is the form of energy involved in chemical reactions. The chemical energy of a substance is due to the condition of the atoms of which it is made; it resides in the chemical bonds that join the atoms in compound substances.
Kinetic energy is energy a body possesses because it is in motion. The kinetic energy of a body with mass m moving at a velocity v is one half the product of the mass of the body and the square of its velocity, i.e., KE = 1 2 mv2. Even when a body appears to be at rest, its atoms and molecules are in constant motion and thus have kinetic energy. The average kinetic energy of the atoms or molecules is measured by the temperature of the body.
The difference between kinetic energy and potential energy, and the conversion of one to the other, is demonstrated by the falling of a rock from a cliff, when its energy of position is changed to energy of motion. Another example is provided in the movements of a simple pendulum. As the suspended body moves upward in its swing, its kinetic energy is continuously being changed into potential energy; the higher it goes the greater becomes the energy that it owes to its position. At the top of the swing the change from kinetic to potential energy is complete, and in the course of the downward motion that follows the potential energy is in turn converted to kinetic energy.
Conversion and Conservation of Energy
It is common for energy to be converted from one form to another; however, the law of conservation of energy, a fundamental law of physics, states that although energy can be changed in form it can be neither created nor destroyed. The theory of relativity shows, however, that mass and energy are equivalent and thus that one can be converted into the other. As a result, the law of conservation of energy includes both mass and energy.
Many transformations of energy are of practical importance. combustion of fuels results in the conversion of chemical energy into heat and light. In the electric storage battery chemical energy is converted to electrical energy and conversely. In the photosynthesis of starch, green plants convert light energy from the sun into chemical energy. Hydroelectric facilities convert the kinetic energy of falling water into electrical energy, which can be conveniently carried by wires to its place of use. The force of a nuclear explosion results from the partial conversion of matter to energy. Temperature
The measure of the relative warmth or coolness of an object. Temperature is measured by means of a thermometer or other instrument having a scale calibrated in units called degrees. The size of a degree depends on the particular temperature scale being used. A temperature scale is determined by choosing two reference temperatures and dividing the temperature difference between these two points into a certain number of degrees. The two reference temperatures used for most common scales are the melting point of ice and the boiling point of water. On the Celsius temperature scale, or centigrade scale, the melting point is taken as 0 C and the boiling point as 100 C, and the difference between them is divided into 100 degrees. On the Fahrenheit temperature scale, the melting point is taken as 32 F and the boiling point as 212 F, with the difference between them equal to 180 degrees. The Raumur scale, used in some parts of Europe, also sets the melting point at zero, but it has an 80-degree temperature difference between 0 R and the boiling point at 80 R. The temperature of a substance does not measure its heat content but rather the average kinetic energy of its molecules resulting from their motions. A one-pound block of iron and a two-pound block of iron at the same temperature do not have the same heat content. Because they are at the same temperature the average kinetic energy of the molecules is the same; however, the two-pound block has more molecules than the one-pound block and thus has greater heat energy. A temperature scale can be defined theoretically for which zero degree corresponds to zero average kinetic energy. Such a scale is known as an absolute temperature scale. The Kelvin temperature scale is an absolute scale having degrees the same size as those of the Celsius scale (see gas laws). The relationship between absolute temperature and average molecular kinetic energy is one result of the kinetic-molecular theory of gases. See heat; thermodynamics. Heat
Internal energy of a substance associated with the positions and motions of the individual molecules (or atoms or ions) composing the substance rather than with the position or motion of the substance as a whole.
Measures of Heat Statehood
The average kinetic energy of the molecules, which is due to their motions, is measured by the temperature of the substance, while the potential energy is associated with the state, or phase, of the substance. Heat energy is commonly expressed in either of two units: the calorie, a metric unit, is used by scientists everywhere, and the British thermal unit (Btu), an English unit, is used by engineers in the United States. It has also become common for scientists to express heat energy in the unit joule, a unit used to express all forms of energy. Specific Heat
As heat is added to a substance in the solid state, the molecules of the substance gain kinetic energy and the temperature of the substance rises. The amount of heat needed to raise a unit of mass of the substance one degree of temperature is called the specific heat of the substance. Because of the way in which the calorie and the Btu are defined, the specific heat of any substance is the same in either system of measurement. For example, the specific heat of water is 1 calorie per gram per degree Celsius; i.e., 1 calorie of heat is needed to raise the temperature of 1 gram of water by 1 degree Celsius; it is also 1 Btu per pound per degree Fahrenheit. Heat of Fusion
When a solid reaches a certain temperature, it changes to a liquid. This temperature is a particular property of the substance and is called its melting point. While the solid-liquid transition is taking place, there is no change in temperature. All of the heat being added is being converted to the internal potential energy associated with the liquid state. The amount of heat needed to convert one unit of mass of a substance from a solid to liquid is called the heat of fusion, or latent heat of fusion, of the substance. Like specific heat, latent heat is also a property of the particular substance. The latent heat of fusion for the ice-to-water transition is 80 calories per gram. Heat of Vaporization
After a substance is completely changed from a solid to a liquid, further addition of heat again causes the temperature to rise until it reaches the boiling point, the particular temperature at which the given substance changes from a liquid to a gas. During the liquid-gas transition, the temperature remains constant until the change is completed. The heat of vaporization, or latent heat of vaporization, is the heat that must be added to convert one unit of mass of the substance from a liquid to a gas.
Transfer of Heat
Heat may be transferred from one substance to another by three meansconduction, convection, and radiation. Conduction involves the transfer of energy from one molecule to adjacent molecules without the substance as a whole moving. Convection involves the movement of warmer parts of a substance away from the source of heat and takes place only in fluids, i.e., liquids and gases. Radiation is the transfer of heat energy in the form of electromagnetic radiation, principally in the infrared radiation portion of the spectrum.
Study and Analysis of Heat
The study of heat and its relationship to useful work is called thermodynamics and involves macroscopic quantities such as pressure, temperature, and volume without regard for the molecular basis of these quantities. low-temperature physics is concerned with phenomena that occur at extremely low temperatures. The analysis of heat on the basis of the structure of matter is considered in the kinetic-molecular theory of gases and provides an explanation for the various gas laws. The gas laws in turn serve to define an absolute temperature scale based on theoretical considerations (see Kelvin temperature scale). See M. C. Mott-Smith, Heat and Its Workings (1933, repr. 1962); Richard Becker, Theory of Heat (tr. 1967). Light
A visible electromagnetic radiation. Of the entire electromagnetic spectrum, the human eye is sensitive to only a tiny part, the part that is called light. The wavelengths of visible light range from about 350 or 400 nm to about 750 or 800 nm. The term light is often extended to adjacent wavelength ranges that the eye cannot detectto infrared radiation, which has a frequency less than that of visible light, and to ultraviolet radiation and black light, which have a frequency greater than that of visible light.
If white light, which contains all visible wavelengths, is separated, or dispersed, into a spectrum, each wavelength is seen to correspond to a different color. Light that is all of the same wavelength and phase (all the waves are in step with one another) is called coherent; one of the most important modern applications of light has been the development of a source of coherent lightthe laser. Electromagnetic Radiation
The energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an electric field. These interacting electric and magnetic fields are at right angles to one another and also to the direction of propagation of the energy. Thus, an electromagnetic wave is a transverse wave. If the direction of the electric field is constant, the wave is said to be polarized (see polarization of light). Electromagnetic radiation does not require a material medium and can travel through a vacuum. The theory of electromagnetic radiation was developed by James Clerk Maxwell and published in 1865. He showed that the speed of propagation of electromagnetic radiation should be identical with that of light, about 186,000 mi (300,000 km) per sec. Subsequent experiments by Heinrich Hertz verified Maxwell\'s prediction through the discovery of radio waves, also known as hertzian waves. Light is a type of electromagnetic radiation, occupying only a small portion of the possible spectrum of this energy. The various types of electromagnetic radiation differ only in wavelength and frequency; they are alike in all other respects. The possible sources of electromagnetic radiation are directly related to wavelength: long radio waves are produced by large antennas such as those used by broadcasting stations; much shorter visible light waves are produced by the motions of charges within atoms; the shortest waves, those of gamma radiation, result from changes within the nucleus of the atom. In order of decreasing wavelength and increasing frequency, various types of electromagnetic radiation include: electric waves, radio waves (including AM, FM, TV, and shortwaves), microwaves, infrared radiation, visible light, ultraviolet radiation, X rays, and gamma radiation. According to the quantum theory, light and other forms of electromagnetic radiation may at times exhibit properties like those of particles in their interaction with matter. (Conversely, particles sometimes exhibit wavelike properties.) The individual quantum of electromagnetic radiation is known as the photon and is symbolized as &ggr;, the Greek letter gamma. Quantum effects are most pronounced for the higher frequencies, such as gamma rays, and are usually negligible for radio waves at the long-wavelength, low-frequency end of the spectrum.
The Nature of Light The Qaddafi Regime
The scientific study of the behavior of light is called optics and covers reflection of light by a mirror or other object, refraction by a lens or prism, diffraction of light as it passes by the edge of an opaque object, and interference patterns resulting from diffraction. Also studied is the polarization of light. Any successful theory of the nature of light must be able to explain these and other optical phenomena. The Wave, Particle, and Electromagnetic Theories of Light
The earliest scientific theories of the nature of light were proposed around the end of the 17th cent. In 1690, Christian Huygens proposed a theory that explained light as a wave phenomenon. However, a rival theory was offered by Sir Isaac Newton in 1704. Newton, who had discovered the visible spectrum in 1666, held that light is composed of tiny particles, or corpuscles, emitted by luminous bodies. By combining this corpuscular theory with his laws of mechanics, he was able to explain many optical phenomena.
For more than 100 years, Newton\'s corpuscular theory of light was favored over the wave theory, partly because of Newton\'s great prestige and partly because not enough experimental evidence existed to provide an adequate basis of comparison between the two theories. Finally, important experiments were done on the diffraction and interference of light by Thomas Young (1801) and A. J. Fresnel (1814-15) that could only be interpreted in terms of the wave theory. The polarization of light was still another phenomenon that could only be explained by the wave theory. Thus, in the 19th cent. the wave theory became the dominant theory of the nature of light.
The wave theory received additional support from the electromagnetic theory of James Clerk Maxwell (1864), who showed that electric and magnetic fields were propagated together and that their speed was identical with the speed of light. It thus became clear that visible light is a form of electromagnetic radiation, constituting only a small part of the electromagnetic spectrum. Maxwell\'s theory was confirmed experimentally with the discovery of radio waves by Heinrich Hertz in 1886. Modern Theory of the Nature of Light
With the acceptance of the electromagnetic theory of light, only two general problems remained. One of these was that of the luminiferous ether, a hypothetical medium suggested as the carrier of light waves, just as air or water carries sound waves. The ether was assumed to have some very unusual properties, e.g., being massless but having high elasticity. A number of experiments performed to give evidence of the ether, most notably by A. A. Michelson in 1881 and by Michelson and E. W. Morley in 1887, failed to support the ether hypothesis. With the publication of the special theory of relativity in 1905 by Albert Einstein, the ether was shown to be unnecessary to the electromagnetic theory.
The second main problem, and the more serious of the two, was the explanation of various phenomena, such as the photoelectric effect, that involved the interaction of light with matter. Again the solution to the problem was proposed by Einstein, also in 1905. Einstein extended the quantum theory of thermal radiation proposed by Max Planck in 1900 to cover not only vibrations of the source of radiation but also vibrations of the radiation itself. He thus suggested that light, and other forms of electromagnetic radiation as well, travel as tiny bundles of energy called light quanta, or photons. The energy of each photon is directly proportional to its frequency.
With the development of the quantum theory of atomic and molecular structure by Niels Bohr and others, it became apparent that light and other forms of electromagnetic radiation are emitted and absorbed in connection with energy transitions of the particles of the substance radiating or absorbing the light. In these processes, the quantum, or particle, nature of light is more important than its wave nature. When the transmission of light is under consideration, however, the wave nature dominates over the particle nature. In 1924, Louis de Broglie showed that an analogous picture holds for particle behavior, with moving particles having certain wavelike properties that govern their motion, so that there exists a complementarity between particles and waves known as particle-wave duality. The quantum theory of light has successfully explained all aspects of the behavior of light.
The Speed of Light
An important question in the history of the study of light has been the determination of its speed and of the relationship of this speed to other physical phenomena. At one time it was thought that light travels with infinite speedi.e., it is propagated instantaneously from its source to an observer. Olaus Rmer showed that it was finite, however, and in 1675 estimated its value from differences in the time of eclipse of certain of Jupiter\'s satellites when observed from different points in the earth\'s orbit. More accurate measurements were made during the 19th cent. by A. H. L. Fizeau (1849), using a toothed wheel to interrupt the light, and by J. B. L. Foucault (1850), using a rotating mirror. The most accurate measurements of this type were made by Michelson. Modern electronic methods have improved this accuracy, yielding a value of 2.99792458 108 m (c.186,000 mi) per sec for the speed of light in a vacuum, and less for its speed in other media. The theory of relativity predicts that the speed of light in a vacuum is the limiting velocity for material particles; no particle can be accelerated from rest to the speed of light, although it may approach it very closely. Particles moving at less than the speed of light in a vacuum but greater than that of light in some other medium will emit a faint blue light known as Cherenkov radiation when they pass through the other medium. This phenomenon has been used in various applications involving elementary particles.
Luminous and Illuminated Bodies
In general, vision is due to the stimulation of the optic nerves in the eye by light either directly from its source or indirectly after reflection from other objects. A luminous body, such as the sun, another star, or a light bulb, is thus distinguished from an illuminated body, such as the moon and most of the other objects one sees. The amount and type of light given off by a luminous body or reflected by an illuminated body is of concern to the branch of physics known as photometry. Illuminated bodies not only reflect light but sometimes also transmit it. Transparent objects, such as glass, air, and some liquids, allow light to pass through them. Translucent objects, such as tissue paper and certain types of glass, also allow light to pass through them but diffuse (scatter) it in the process, so that an observer cannot see a clear image of whatever lies on the other side of the object. Opaque objects do not allow light to pass through them at all. Some transparent and translucent objects allow only light of certain wavelengths to pass through them and thus appear colored. The colors of opaque objects are caused by selective reflection of certain wavelengths and absorption of others. See W. L. Bragg, The Universe of Light (1959); John Rublowsky, Light (1964); H. Haken, Light (1981). Vision
Physiological sense of sight by which the form, color, size, movements, and distance of objects are perceived.
Color and Stereoscopic Vision
Color vision is based on the ability to discriminate between the various wavelengths that constitute the spectrum. The Young-Helmholtz theory, developed in 1802 by Thomas Young and H. L. F. Helmholtz, is based on the assumption that there are three fundamental color sensationsred, green, and blueand that there are three different groups of cones in the retina, each group particularly sensitive to one of these three colors. Light from a red object, for example, stimulates the cones that are more sensitive to red than the other cones. Other colors (besides red, green, and blue) are seen when the cone cells are stimulated in different combinations. Only in recent years has conclusive evidence shown that the Young-Helmholtz theory is, indeed, accurate. The sensation of white is produced by the combination of the three primary colors, and black results from the absence of stimulation.
Humans normally have binocular vision, i.e., separate images of the visual field are formed by each eye; the two images fuse to form a single impression. Because each eye forms its own image from a slightly different angle, a stereoscopic effect is obtained, and depth, distance, and solidity of an object are appreciated. Stereoscopic color vision is found primarily among the higher primates, and it developed fairly late on the evolutionary scale.
Vision in Humans Desegregation
The human eye functions somewhat like a camera; that is, it receives and focuses light upon a photosensitive receiver, the retina. The light rays are bent and brought to focus as they pass through the cornea and the lens. The shape of the lens can be changed by the action of the ciliary muscles so that clear images of objects at different distances and of moving objects are formed on the retina. This ability to focus objects at varying distances is known as accommodation. The Role of the Retina
The retinathe embryonic outgrowth of the brainis a very complex tissue. Its most important elements are its many light-sensitive nerve cells, the rods and cones. The cones secrete the pigment iodopsin and are most effective in bright light; they alone provide color vision. The rods, which secrete a substance called visual purple, or rhodopsin, provide vision in dim light or semidarkness; since rods do not provide color vision, objects in such light appear in shades of gray.
Light rays brought to focus on the rods and cones produce a chemical reaction in those cells, in which the two pigments are broken down to form a protein and a vitamin A compound. This chemical process stimulates an electrical impulse that is sent to the brain. The structural change of pigment is normally balanced by the formation of new pigment through the recombination of the protein and vitamin A compound; thus vision is uninterrupted.
The division of function between rods and cones is a result of the different sensitivity of their pigments to light. The iodopsin of cone cells is less sensitive than rhodopsin, and therefore is not activated by weak light, while in bright light the highly sensitive rhodopsin of rod cells breaks down so rapidly that it soon becomes inactive. There is a depression near the center of the retina called the fovea that contains only cone cells. It provides the keenest possible vision when an object is viewed directly in bright light. In dim light objects must be viewed somewhat to one side so the light rays fall on the area of the retina that contains rod cells. The Role of the Optic Nerve and Brain
The nerve impulses from the rods and cones are transmitted by nerve fibers across the retina to an area where the fibers converge and form the optic nerve. The area where the optic nerve passes through the retina is devoid of rods and cones and is known as the blind spot. The optic nerve from the left eye and that from the right eye meet at a point called the optic chiasma. There each nerve separates into two branches. The inner branch from each eye crosses over and joins the outer branch from the other eye. Two optic tracts exit thereby from the chiasma, transferring the impulses from the left side of each eye to the left visual center in the cerebral cortex and the impulses from the right half of each eye to the right cerebral cortex. The brain then fuses the two separate images to form a single image. The image formed on the retina is an inverted one, because the light rays entering the eye are refracted and cross each other. However, the mental image as interpreted by the brain is right side up. How the brain corrects the inverted image to produce normal vision is unknown, but the ability is thought to be acquired early in life, with the aid of the other senses.
Defects of Vision
Defects of vision include astigmatism, color blindness, farsightedness, and nearsightedness. The absence of rods causes a condition known as night blindness; an absence of cones constitutes legal blindness. See A. Hughes, The Visual System in the Evolution of Vertebrates (1977); Gerald S. Wasserman, Color Vision: An Historical Introduction (1978); Mark Fineman, The Inquisitive Eye (1981); David H. Hubel, Eye, Brain, and Vision (1988). Color
The effect produced on the eye and its associated nerves by light waves of different wavelength or frequency. Light transmitted from an object to the eye stimulates the different color cones of the retina, thus making possible perception of various colors in the object.
Properties of Colors
The scientific description of color, or colorimetry, involves the specification of all relevant properties of a color either subjectively or objectively. The subjective description gives the hue, saturation, and lightness or brightness of a color. Hue refers to what is commonly called color, i.e., red, green, blue-green, orange, etc. Saturation refers to the richness of a hue as compared to a gray of the same brightness; in some color notation systems, saturation is also known as chroma. The brightness of a light source or the lightness of an opaque object is measured on a scale ranging from dim to bright for a source or from black to white for an opaque object (or from black to colorless for a transparent object). In some systems, brightness is called value. A subjective color notation system provides comparison samples of colors rated according to these three properties. In an objective system for color description, the corresponding properties are dominant wavelength, purity, and luminance. Much of the research in objective color description has been carried out in cooperation with the Commission Internationale de l\'Eclairage (CIE), which has set standards for such measurements. In addition to the description of color according to these physical and psychological standards, a number of color-related physiological and psychological phenomena have been studied. These include color constancy under varying viewing conditions, color contrast, afterimages, and advancing and retreating colors.
Apparent Color of Objects Mid-Century to the Present
Color is a property of light that depends on wavelength. When light falls on an object, some of it is absorbed and some is reflected. The apparent color of an opaque object depends on the wavelength of the light that it reflects; e.g., a red object observed in daylight appears red because it reflects only the waves producing red light. The color of a transparent object is determined by the wavelength of the light transmitted by it. An opaque object that reflects all wavelengths appears white; one that absorbs all wavelengths appears black. Black and white are not generally considered true colors; black is said to result from the absence of color, and white from the presence of all colors mixed together. Additive Colors
Colors whose beams of light in various combinations can produce any of the color sensations are called primary, or spectral, colors. The process of combining these colors is said to be additive; i.e., the sensations produced by different wavelengths of light are added together. The additive primaries are red, green, and blue-violet. White can be produced by combining all three primary colors. Any two colors whose light together produces white are called complementary colors, e.g., yellow and blue-violet, or red and blue-green. Subtractive Colors
When pigments are mixed, the resulting sensations differ from those of the transmitted primary colors. The process in this case is subtractive, since the pigments subtract or absorb some of the wavelengths of light. Magenta (red-violet), yellow, and cyan (blue-green) are called subtractive primaries, or primary pigments. A mixture of blue and yellow pigments yields green, the only color not absorbed by one pigment or the other. A mixture of the three primary pigments produces black.
Symbolic Uses of Color
Color has long been used to represent affiliations and loyalties (e.g., school or regimental colors) and as a symbol of various moods (e.g., red with rage) and qualities (e.g., worthy of a blue ribbon). A well-known use of the symbolism of color is in the liturgical colors of the Western Church, according to which the color of the vestments varies through the ecclesiastical calendar; e.g., purple (i.e., violet) is the color of Advent and Lent; white, of Easter; and red, of the feasts of the martyrs.
The Visible Spectrum
Since the colors that compose sunlight or white light have different wavelengths, the speed at which they travel through a medium such as glass differs; red light, having the longest wavelength, travels more rapidly through glass than blue light, which has a shorter wavelength. Therefore, when white light passes through a glass prism, it is separated into a band of colors called a spectrum. The colors of the visible spectrum, called the elementary colors, are red, orange, yellow, green, blue, indigo, and violet (in that order).
Prism
In optics, a piece of translucent glass or crystal used to form a spectrum of light separated according to colors. Its cross section is usually triangular. The light becomes separated because different wavelengths or frequencies are refracted (bent) by different amounts as they enter the prism obliquely and again as they leave it (see refraction). The shorter wavelengths, toward the blue or violet end of the spectrum, are refracted by the greatest amount; the longer wavelengths, toward the red end, are refracted the least. The Nicol prism is a special type of prism made of calcite; it is used for polarization of light. See Gnter Wyszecki and W. S. Stiles, Color Science (1967); M. W. Levine and J. M. Shefner, Fundamentals of Sensation and Perception (1991). Sound
Any disturbance that travels through an elastic medium such as air, ground, or water to be heard by the human ear. When a body vibrates, or moves back and forth, nee vibration, the oscillation causes a periodic disturbance of the surrounding air or other medium that radiates outward in straight lines in the form of a pressure wave. The effect these waves produce upon the ear is perceived as sound. From the point of view of physics, sound is considered to be the waves of vibratory motion themselves, whether or not they are heard by the human ear. Waves
In physics, the transfer of energy by the regular vibration, or oscillatory motion, either of some material medium or by the variation in magnitude of the field vectors of an electromagnetic field (see electromagnetic radiation). Many familiar phenomena are associated with energy transfer in the form of waves. sound is a longitudinal wave that travels through material media by alternatively forcing the molecules of the medium closer together, then spreading them apart. light and other forms of electromagnetic radiation travel through space as transverse waves; the displacements at right angles to the direction of the waves are the field intensity vectors rather than motions of the material particles of some medium. With the development of the quantum theory, it was found that particles in motion also have certain wave properties, including an associated wavelength and frequency related to their momentum and energy. Thus, the study of waves and wave motion has applications throughout the entire range of physical phenomena.
Generation of Sound Waves
Sound waves are generated by any vibrating body. For example, when a violin string vibrates upon being bowed or plucked, its movement in one direction pushes the molecules of the air before it, crowding them together in its path. When it moves back again past its original position and on to the other side, it leaves behind it a nearly empty space, i.e., a space with relatively few molecules in it. In the meantime, however, the molecules which were at first crowded together have transmitted some of their energy of motion to other molecules still farther on and are returning to fill again the space originally occupied and now left empty by the retreating violin string. In other words, the vibratory motion set up by the violin string causes alternately in a given space a crowding together of the molecules of air (a condensation) and a thinning out of the molecules (a rarefaction). Taken together a condensation and a rarefaction make up a sound wave; such a wave is called longitudinal, or compressional, because the vibratory motion is forward and backward along the direction that the wave is following. Because such a wave travels by disturbing the particles of a material medium, sound waves cannot travel through a vacuum.
Characteristics of Sound Waves
Sounds are generally audible to the human ear if their frequency (number of vibrations per second) lies between 20 and 20,000 vibrations per second, but the range varies considerably with the individual. Sound waves with frequencies less than those of audible waves are called subsonic; those with frequencies above the audible range are called ultrasonic.
A sound wave is usually represented graphically by a wavy, horizontal line; the upper part of the wave (the crest) indicates a condensation and the lower part (the trough) indicates a rarefaction. This graph, however, is merely a representation and is not an actual picture of a wave. The length of a sound wave, or the wavelength, is measured as the distance from one point of greatest condensation to the next following it or from any point on one wave to the corresponding point on the next in a train of waves. The wavelength depends upon the velocity of sound in a given medium at a given temperature and upon the frequency of vibration. The wavelength of a sound can be determined by dividing the numerical value for the velocity of sound in the given medium at the given temperature by the frequency of vibration. For example, if the velocity of sound in air is 1,130 ft per second and the frequency of vibration is 256, then the wave length is approximately 4.4 ft.
The velocity of sound is not constant, however, for it varies in different media and in the same medium at different temperatures. For example, in air at 0 C. it is approximately 1,089 ft per second, but at 20 C. it is increased to about 1,130 ft per second, or an increase of about 2 ft per second for every centigrade degree rise in temperature. Sound travels more slowly in gases than in liquids, and more slowly in liquids than in solids. Since the ability to conduct sound is dependent on the density of the medium, solids are better conductors than liquids, liquids are better conductors than gases.
Sound waves can be reflected, refracted (or bent), and absorbed as light waves can be. The reflection of sound waves can result in an echoan important factor in the acoustics of theaters and auditoriums. A sound wave can be reinforced with waves from a body having the same frequency of vibration, but the combination of waves of different frequencies of vibration may produce so-called beats or pulsations or may result in other forms of interference.
Characteristics of Musical Sounds
Musical sounds are distinguished from noises in that they are composed of regular, uniform vibrations, while noises are irregular and disordered vibrations. Composers, however, frequently use noises as well as musical sounds. One musical tone is distinguished from another on the basis of pitch, intensity, or loudness, and quality, or timbre. Pitch describes how high or low a tone is and depends upon the rapidity with which a sounding body vibrates, i.e., upon the frequency of vibration. The higher the frequency of vibration, the higher the tone; the pitch of a siren gets higher and higher as the frequency of vibration increases. The apparent change in the pitch of a sound as a source approaches or moves away from an observer is described by the Doppler effect. The intensity or loudness of a sound depends upon the extent to which the sounding body vibrates, i.e., the amplitude of vibration. A sound is louder as the amplitude of vibration is greater, and the intensity decreases as the distance from the source increases. Loudness is measured in units called decibels. The sound waves given off by different vibrating bodies differ in quality, or timbre. A note from a saxophone, for instance, differs from a note of the same pitch and intensity produced by a violin or a xylophone; similarly vibrating reeds, columns of air, and strings all differ. Quality is dependent on the number and relative intensity of overtones produced by the vibrating body and these in turn depend upon the nature of the vibrating body. Doppler effect
change in the wavelength (or frequency) of energy in the form of waves, e.g., sound or light, as a result of motion of either the source or the receiver of the waves; the effect is named for the Austrian scientist Christian Doppler, who demonstrated the effect for sound. If the source of the waves and the receiver are approaching each other (because of the motion of either or both), the frequency of the waves will increase and the wavelength will be shortenedsounds will become higher pitched and light will appear bluer. If the sender and receiver are moving apart, sounds will become lower pitched and light will appear redder. A common example is the sudden drop in the pitch of a train whistle as the train passes a stationary listener. The Doppler effect in reflected radio waves is employed in radar to sense the velocity of the object under surveillance. In astronomy, the Doppler effect for light is used to measure the velocity and rotation of stars and galaxies along the direction of sight. In the spectrum of nearly every star there are wavelengths, characteristic of atoms, that lie near but not quite coincident to the same wavelengths as measured in the laboratory. The small deviations or shifts are generally due to the relative motion of the celestial object and the earth. Both blue shifts and red shifts are observed for various objects, indicating relative motion both towards and away from the earth. Such shifts have been used to measure the orbital velocity of the earth, to detect binary stars and variable stars, and to detect rotation of other galaxies. The Doppler effect is responsible for the red shifts of distant galaxies, and also of quasars, and thus provides the best evidence for the expansion of the universe, as described by Hubble\'s law. In addition to observations of visible light, the Doppler effect for radio waves is utilized by astronomers to determine the velocities of dust clouds in the spiral arms of the Milky Way galaxy. These observations provided the first direct proof that our own galaxy is rotating. The Doppler shift in radar pulses reflected from the surfaces of Venus and Mercury have been analyzed to obtain new values for their periods of rotation about their axes. Decibel
Abbr. dB, unit used to measure the loudness of sound. It is one tenth of a bel (named for A. G. Bell), but the larger unit is rarely used. The decibel is a measure of sound intensity as a function of power ratio, with the difference in decibels between two sounds being given by dB=10 log10(P1/P2), where P1 and P2 are the power levels of the two sounds. The faintest audible sound, corresponding to a sound pressure of about 0.0002 dyne per sq cm, is arbitrarily assigned a value of 0 dB. The loudest sounds that can be tolerated by the human ear are about 120 dB. The level of normal conversation is about 50 to 60 dB. The decibel is also used to measure certain other quantities, such as power loss in telephone lines. Acoustics
[Gr.,=the facts about hearing], the science of sound, including its production, propagation, and effects. Various branches of acoustics that deal with different aspects of sound and hearing include bioacoustics, physical acoustics, ultrasonics, and architectural acoustics. Unlike electromagnetic radiation, which can travel in the vacuum of free space, sound Waves require a medium (solid, liquid, or gas) in which to travel. Another important difference is that sound travels much slower than electromagnetic radiation; the speed of sound in air at sea level is approximately 1000 ft/sec (300 m/sec), which is roughly a millionth the speed of light in air. Sound waves are longitudinal, which means that the material particles transmitting the waves oscillate in the direction of propagation. Important factors to be considered in working with sound include reverberation and interference. Reverberation is the persistence of sound in an enclosed space caused by repeated reflections. Reflection of sound sometimes causes an echo. Depending on the location of the listener and the frequency of the sound, varying degrees of interference between the primary sound and its reflections will be produced. Reflection can be reduced by the use of sound-absorbent materials, which are usually soft and porous, such as draperies, upholstery, carpets, acoustic tile, or plaster. In a room, reflection is decreased by the presence of people and open windows and doors. See John Backus, The Acoustical Foundations of Music (1969); R. B. Lindsay, Acoustics (1973); A. D. Pierce, Acoustics (1981, repr. 1989). Echo
The reflection of a sound wave back to its source in sufficient strength and with a sufficient time lag to be separately distinguished. If a sound wave returns within 1 10 sec, the human ear is incapable of distinguishing it from the original one. Thus, since the velocity of sound is c.344 m (1,130 ft) per sec at a normal room temperature of about 20 C (68 F), a reflecting wall must be more than 16.2 m (56 1 2 ft) from the sound source at this temperature for an echo to be heard by a person at the source. In this case the sound requires 1 20 sec to reach the reflecting surface and the same time to return. Bats navigate by listening for the echo of their high-frequency cry. Sonar and depth sounders work by analyzing electronically the echo time lag of sound waves, generally between 10 and 50 kilohertz, produced by underwater transducers. Radar sets broadcast radio waves, usually between 100 and 10,000 megahertz, pick up the portion reflected back by objects, and electronically determine the distance and direction of the objects. A sound echo that is reflected again and again from different surfaces, as by parallel walls in a tunnel, is called reverberation. When a surface reflects sound it partially absorbs and partially reflects the energy. As the process is repeated the sound becomes weaker and weaker and eventually ceases. Ultrasonics
The study and application of the energy of sound waves vibrating at frequencies greater than 20,000 cycles per second, i.e., beyond the range of human hearing. The application of sound energy in the audible range is limited almost entirely to communications, since increasing the pressure, or intensity, of sound waves increases loudness and therefore causes discomfort to human beings. Ultrasonic waves, however, being inaudible, have little or no effect on the ear even at high intensities. They are produced, commonly, by a transducer containing a piezoelectric substance, e.g., a quartz-crystal oscillator that converts high-frequency electric current into vibrating ultrasonic waves. Ultrasonics has found wide industrial use. For nondestructive testing an object is irradiated with ultrasonic waves; variation in velocity or echo of the transmitted waves indicates a flaw. Fine machine parts, ball bearings, surgical instruments, and many other objects can be cleaned ultrasonically. They are placed in a liquid, e.g., a detergent solution or a solvent, into which ultrasonic waves are introduced. By a phenomenon called cavitation, the vibrations cause large numbers of invisible bubbles to explode with great force on the surfaces of the objects. Film or dirt is thus removed even from normally inaccessible holes, cracks, and corners. Radioactive scale is similarly removed from nuclear reactor fuel and control rods. In medicine ultrasonic devices are used to examine internal organs without surgery and are safer to genetic material than X rays. The waves with which the body is irradiated are reflected and refracted; these are recorded by a sonograph for use in diagnosis. Metals can be welded together by placing their surfaces in contact with each other and irradiating the contact with ultrasound. The molecules are stimulated into rearranged crystalline form, making a permanent bond. Ultrasonic whistles, which cannot be heard by human beings, are audible to dogs and are used to summon them. See Graham Chedd, Sound (1970).
|
|
| |
Home
