ta’s original three were supplanted by five in the 8th century bc.
phyllite Vfi-.lItX Fine-grained metamorphic rock formed by the recrystal¬ lization of fine-grained, parent sedimentary rocks, such as mudstones or shales. Phyllite has a marked tendency to split into sheets or slabs; it may have a sheen on its surfaces due to tiny plates of micas. Its grain size is larger than that of slate but smaller than that of schist.
phylloxera V.fi-.lak-'sir-o, fs-Tak-so-roX Any of numerous, chiefly North American, insect species (genus Phylloxera, order Homoptera), many of which are serious pests of plants. Phylloxera form galls on and can defoliate trees, especially hickory and pecan. See also grape phylloxera.
phylogenetic tree \,fi-lo-j3-'ne-tik\ Diagram showing the evolution¬ ary interrelations of a group of organisms that usually originated from a shared ancestral form. The ancestor is in the tree trunk; organisms that have arisen from it are placed at the ends of tree branches. The distance of one group from the other groups indicates the degree of relationship; that is, closely related groups are located on branches close to one another. Though phylogenetic trees are speculative, they provide a convenient method for studying phylogenetic relationships and evolution. See also phylogeny.
phytogeny \fi-'la-j3-ne\ History of the evolution of a species or group, especially lines of descent and relationships among broad groups. The fundamental proposition is that plants or animals of different species descended from common ancestors. Because the evidence for such rela¬ tionships is almost always incomplete, most judgments of phylogenicity are based on indirect evidence and cautious speculation. Modern tax¬ onomy, the science of classifying organisms, is based on phylogeny. Early taxonomic systems had no theoretical basis; organisms were grouped according to apparent similarity. Biologists who propose a phylogeny obtain evidence from the fields of paleontology, comparative anatomy, comparative embryology, biochemistry, and molecular biology. The data and conclusions of phylogeny indicate that today’s living creatures are the product of a historical process of evolution and that degrees of resem¬ blance within and between groups correspond to degrees of relationship by descent from common ancestors. See also phylogenetic tree.
physiatry See physical medicine and rehabilitation
physical anthropology Branch of anthropology concerned with the study of human evolution and human biological variation. Research on human evolution involves the discovery, analysis, and description of fos¬ silized human remains. Two key goals are the identification of differences between humans and their human and nonhuman ancestors, and the clari¬ fication of the biological emergence of humankind. A variety of quanti¬ tative methods are used, including the comparative analysis of genetic codes. Research on biological variation among contemporary humans once relied heavily on the concept of race, but today principles of genet¬ ics and the analysis of such factors as blood type have largely eliminated race as a scientific category.
physical chemistry Branch of chemistry concerned with interactions and transformations of materials. Unlike other branches, it deals with the principles of physics underlying all chemical interactions (e.g., gas laws), seeking to measure, correlate, and explain the quantitative aspects of reac¬ tions. Quantum mechanics has clarified much for physical chemistry by modeling the smallest particles ordinarily dealt with in the field, atoms and molecules, enabling theoretical chemists to use computers and sophis¬ ticated mathematical techniques to understand the chemical behaviour of matter. Chemical thermodynamics deals with the relationship between heat and other forms of chemical energy, kinetics with chemical reaction rates. Subdisciplines of physical chemistry include electrochemistry, pho¬ tochemistry (see photochemical reaction), surface chemistry, and catalysis.
physical education Training in physical fitness and in skills requir¬ ing or promoting it. In the U.S. it is required in most primary and sec¬ ondary schools. It generally includes calisthenics, gymnastics, various sports, and some study of health. College majors in physical education have been available since the early 20th century. Most teaching is done in gymnasiums, though outdoor sports are also emphasized.
physical medicine and rehabilitation or physiatry \,fiz-e-'a-tre\ or physical therapy or rehabilitation medicine
Medical specialty treating chronic disabilities through physical means to help patients return to a comfortable, productive life despite a medical problem. Its objectives are pain relief, functional improvement or main¬ tenance, training in essential activities, and functional testing of areas such as strength, mobility, breathing capacity, and coordination. Physical medi¬ cine may use diathermy, hydrotherapy, massage, exercise, and functional training. The last can mean learning to work with a guide dog or a pros¬ thesis or learning new ways to carry out everyday activities with a limb missing, sometimes by using assistive devices. Physician specialists head rehabilitation teams including a physical therapist, rehabilitation engineer, rehabilitation nurse, psychological counselor, and sometimes a respiratory or speech therapist. See also occupational therapy; orthopedics.
physics Science that deals with the structure of matter and the interac¬ tions between the fundamental constituents of the observable universe. Long called natural philosophy (from the Greek physikos ), physics is concerned with all aspects of nature, covering the behaviour of objects under the action of given forces and the nature and origin of gravitational, electromagnetic, and nuclear force fields. The goal of physics is to for¬ mulate comprehensive principles that bring together and explain all dis¬ cernible phenomena. See also aerodynamics; astrophysics; atomic physics; biophysics; mechanics; nuclear physics; particle physics; quantum mechanics;
SOLID-STATE PHYSICS; STATISTICAL MECHANICS.
physics, mathematical See mathematical physics
Mahogany side chair designed by Phyfe, 1807; in The Henry Francis du Pont Winterthur Museum, Delaware
COURTESY OF THE HENRY FRANCIS DU PONT WINTERTHUR MUSEUM, DELAWARE
© 2006 Encyclopaedia Britannica, Inc.
physics ► Piazzetta I 1495
physics, philosophy of Philosophical investigation of the concepts, problems, and methods of physics and related sciences. The philosophy of physics traditionally has been concerned with clarifying the logical struc¬ ture, ontological commitments, and intertheoretic relations of fundamen¬ tal physical theories, including relativity, quantum mechanics, and statistical mechanics. The field also addresses various metaphysical and epistemo¬ logical aspects of problems encountered in the foundations of theoretical physics. Examples include the apparent “directed” nature of time, the meaning of probabilistic descriptions of the initial state of the universe in statistical mechanics, the measurement problem in quantum mechan¬ ics (the problem of stating in precise terms the conditions in which proba¬ bilistic as opposed to deterministic laws apply to the motion of an elementary particle), and the problem of reconciling quantum mechanics with special, and ultimately general, relativity. See also uncertainty prin¬ ciple; UNIFIED FIELD THEORY.
physiocrat Member of a school of economics, founded in 18th-century France, that held that government should not interfere with the operation of natural economic laws. Generally regarded as the first scientific school of economics, the physiocratic school (the name refers to the “rule of nature”) was founded by Francois Quesnay, who demonstrated the economic rela¬ tion between a workshop and a farm and asserted that the farm alone added to a nation’s wealth. Land and agriculture were therefore believed to be the source of all wealth. The physiocrats envisaged a society in which written law would be in harmony with NATURAL law. They pictured a predominantly agricultural society, attacking mercantilism for its emphasis on manufactur¬ ing and foreign trade and its mass of economic regulations. Quesnay’s dis¬ ciples included Victor Riqueti, count de Mirabeau, and Pierre Samuel du Pont de Nemours (1739-1817). The school was in decline by 1768, and after the dismissal of a sympathetic comptroller general in 1776 the leading physiocrats were exiled. Though many of their theories, notably their theory of wealth, were later demolished, their introduction of scientific method to economics had a permanent effect on the discipline.