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Physics is the branch of scientific investigation that focuses its attention on fundamental concepts, patterns, and relationships involving matter, energy, space, and time. Other natural sciences, such as chemistry, biology, geology, and astronomy, also deal with these categories in their investigation of material systems like atoms, molecules, life processes, organisms, planets, stars, and galaxies, but physics is concerned with the most basic and universal principles that apply to all of these diverse systems.

It is sometimes convenient to divide physics into several different arenas of concern, such as mechanics (the study of motion), electromagnetism and optics, thermodynamics, quantum physics, atomic physics, nuclear physics, particle physics, and relativity (the study of space, time, and gravity).

Classical mechanics is the study of motion in the manner established by Isaac Newton in the seventeenth century. Among its major contributions is a fruitful method for describing the cause-effect relationship for motion in a quantifiable manner. A force, like the familiar push or pull, functions as the cause of acceleration (any change in the speed or direction of motion), which is its effect. Another major contribution of Newton was his concept and description of the force of gravity that is experienced and exerted by every object possessing the quality of mass. The gravitational force that causes apples to fall earthward is also the kind of kind of force that steers the moon in its orbit around the Earth and the planets in their orbits around the sun.

Electromagnetism encompasses all phenomena in which electric and magnetic fields play a role. In classical physics, fields may be thought of as qualities of space that lead objects with certain properties to experience a force. Any object possessing the property of electric charge, for example, will experience a force in the presence of an electric field. Electromagnetic radiation (light, X-rays, radio waves) may be understood as variations in electric and magnetic fields that travel at the characteristic speed of three hundred thousand meters per second through space.

Thermodynamics is concerned with the manner in which energy, especially heat energy, affects the state of a system and its interaction with its environment. Energy, often characterized as the capacity to do work, appears in a diversity of forms and may be changed in either form or location as a consequence of some physical process. In all processes, however, the sum of the energy possessed by a system and its environment remains constant. This principle, called the First Law of Thermodynamics, or the conservation of energy, is thought to apply without exception to all physical phenomena.

Quantum theory describes the structure and behavior of systems like atoms, atomic nuclei, and molecules. Extremely small structures behave in a manner different from the predictions of classical mechanics. The quantities of energy possessed by a system or exchanged between systems, for instance, is restricted to certain values only. Furthermore, the outcome of many processes is open to diverse options, each outcome having a calculable probability of occurrence.

Relativity theory provides a framework for speaking of the interactive relationships among space, time, mass, and gravity. Special Relativity describes the way in which the experience of time and space are interrelated, while General Relativity focuses its attention on the interrelationships among mass, space, gravity, and motion.

See also COSMOLOGY, PHYSICAL ASPECTS

HOWARD J. VAN TILL