Physics

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Central theories

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While physics deals with a wide variety of systems, there are certain theories that are used by all physicists. Each of these theories were experimentally tested numerous times and found correct as an approximation of Nature (within a certain domain of validity). For instance, the theory of classical mechanics accurately describes the motion of objects, provided they are much larger than atoms and moving at much less than the speed of light. These theories continue to be areas of active research; for instance, a remarkable aspect of classical mechanics known as chaos was discovered in the 20th century, three centuries after the original formulation of classical mechanics by Isaac Newton (1642—1727). These "central theories" are important tools for research into more specialized topics, and any physicist, regardless of his or her specialization, is expected to be literate in them.

Classical mechanics is a model of the physics of forces acting upon bodies. It is often referred to as "Newtonian mechanics" after Newton and his laws of motion. Classical mechanics is subdivided into statics (which models objects at rest), kinematics (which models objects in motion), and dynamics (which models objects subjected to forces). See also mechanics.

Electromagnetism, or electromagnetic theory, is the physics of the electromagnetic field: a field, encompassing all of space, which exerts a force on those particles that possess the property of electric charge, and is in turn affected by the presence and motion of such particles. The term electrodynamics is sometimes used to refer to the combination of electromagnetism with mechanics, and deals with the effects of the electromagnetic field on the dynamic behavior of electrically charged particles. Electromagnetism encompasses various real-world electromagnetic phenomena. Optics, a sub-field of electromagnetism, is a branch of physics that describes the behavior and properties of light, an electromagnetic wave, and the interaction of light with matter. Optics explains and is illuminated by optical phenomena.

Thermodynamics is the branch of physics that deals with the action of heat and the conversions from one to another of various forms of energy. Thermodynamics is particularly concerned with how these affect temperature, pressure, volume, mechanical action, and work. Historically, it grew out of efforts to construct more efficient heat engines — devices for extracting useful work from expanding hot gases. Statistical mechanics, a related theory, is the branch of physics that analyzes macroscopic systems by applying statistical principles to their microscopic constituents. Statistical physics relates these principles to the thermodynamic (and other macroscopic) properties of the system and, thus, can be used to calculate the thermodynamic properties of bulk materials from the spectroscopic data of individual molecules.

Quantum mechanics is the branch of mathematical physics treating atomic and subatomic systems and their interaction with radiation in terms of observable quantities. It is based on the observation that all forms of energy are released in discrete units or bundles called quanta. Quantum theory typically permits only probable or statistical calculation of the observed features of subatomic particles, understood in terms of wave functions.

The theory of relativity, or relativity theory, refers to two theories concerning the relationship of time, space, and the motion of objects that account for certain anomalies in the concept of relative motion in Newtonian principles. The first of these two theories, the special theory of relativity (or special relativity), was formulated by Albert Einstein (1879—1955) of Berne, Hendrik Lorentz (1853—1928) of Leiden, and Henri Poincaré (1854—1912) of Paris, and is based on the principle of special relativity, which states that all observers moving at constant velocities with respect to each other should find the same laws of Nature operating in their frames of reference. It follows from this principle that the speed of light would have to appear to be the same to every observer. This, in turn, implies that speed has an upper bound; nothing can pass the speed of light. The theory predicts that moving clocks will appear to run slower than stationary ones (see time dilation), that moving objects will appear shorter and heavier than stationary ones (see Lorentz contraction), and that energy and mass are equivalent (see E = mc²) — as a result, mass can be converted into huge amounts of energy, and vice versa, according to the formula E=mc². Thus, the theory states that time and distance measurements are not absolute but are instead relative to the observer's frame of reference. Space and time are viewed as aspects of a single phenomenon, called spacetime. Energy and momentum are similarly linked. There is abundant experimental confirmation of these predictions. Although special relativity makes relative some quantities, such as time, that we would have imagined to be absolute based on everyday experience, it also makes absolute some others that we would have thought were relative. The theory was called "special" because it applies the principle of relativity only to inertial frames. Special relativity doesn't account for gravity, but it can deal with accelerations. An extension of the special theory of relativity to include gravitational effects and related acceleration phenomena, the general theory of relativity (or general relativity) is a geometrical theory of gravitation also proposed by Einstein, in which gravity is explained via the curvature of spacetime produced by the mass-energy and momentum content of the spacetime. General relativity is distinguished from other metric theories of gravitation by its use of the Einstein field equations to relate spacetime content and spacetime curvature.

Major fields of research

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Contemporary research in physics is divided into several distinct fields that study different aspects of the material world. Condensed matter physics, by most estimates the largest single field of physics, is concerned with how the properties of bulk matter, such as the ordinary solids and liquids we encounter in everyday life, arise from the properties and mutual interactions of the constituent atoms. The field of atomic, molecular, and optical physics deals with the behavior of individual atoms and molecules, and in particular the ways in which they absorb and emit light. The field of particle physics, also known as "high-energy physics", is concerned with the properties of submicroscopic particles much smaller than atoms, including the elementary particles from which all other units of matter are constructed. Finally, the field of astrophysics applies the laws of physics to explain celestial phenomena, ranging from the Sun and the other objects in the solar system to the Universe as a whole.

Since the 20th century, the individual fields of physics have become increasingly specialized, and nowadays it is not uncommon for physicists to work in a single field for their entire careers. "Universalists" like Albert Einstein (1879—1955) and Lev Landau (1908—1968), who were comfortable working in multiple fields of physics, are now very rare.

Fringe theories

• Cold fusion
• Dynamic theory of gravity
• Luminiferous aether
• Steady state theory