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- This is a discussion of a present category of science. For the work by Aristotle, see Physics (Aristotle). For a history of the science, see History of physics. For the etymology of the word "physics," see physis (φύσις)..
Physics is the science of matter[1] and its motion,[2][3] as well as space and time.[4][5] It uses concepts such as energy, force, mass, and charge. Physics is an experimental science,[6] creating theories that are tested against observations. Broadly, it is the general scientific analysis of nature, with a goal of understanding how the universe behaves.[7]
Physics is one of the oldest academic disciplines, and through its modern subfield of astronomy, it may be the oldest of all.[8] Experimental physics began in the Middle Ages and eventually emerged as a modern science during the early modern period.[9] Those who work professionally in the field are known as physicists.
Advances in physics often translate to the technological sector, and sometimes influence the other sciences, as well as mathematics and philosophy. For example, advances in the understanding of electromagnetism have led to the widespread use of electrically driven devices (televisions, computers, home appliances etc.); advances in thermodynamics led to the development of motorized transport; and advances in mechanics motivated and benefitted from the development of calculus, quantum chemistry, and the use of instruments such as the electron microscope in microbiology. The "Atomic Age" is also an important concept in philosophy and historical analysis.
Today, physics is a broad and highly developed subject. Research is often divided into four subfields: condensed matter physics; atomic, molecular, and optical physics; high-energy physics; and astronomy and astrophysics. Most physicists also specialize in either theoretical or experimental
research, the former dealing with the development of new theories, and
the latter dealing with the experimental testing of theories and the
discovery of new phenomena. Despite important discoveries during the
last four centuries, there are a number of unsolved problems in physics, and many areas of active research.
Branches of physics
Domains of physics theories
Although physics encompasses a wide variety of phenomena, the fundamental branches of physics are classical mechanics, electromagnetism (including optics), relativity, thermodynamics, and quantum mechanics.
Each of these theories has been tested in numerous experiments and
proven to be an accurate model of nature within its domain of validity.
For example, classical mechanics
correctly describes the motion of objects in everyday experience, but
it breaks down at the atomic scale, where it is superseded by quantum mechanics, and at speeds approaching the speed of light, where relativistic
effects become important. While these theories have long been
well-understood, they continue to be areas of active research — for
example, a remarkable aspect of classical mechanics known as chaos theory
was developed in the 20th century, three centuries after the original
formulation of mechanics by Isaac Newton (1642–1727). The basic
theories form a foundation for the study and research of more
specialized topics.
A table of these theories, along with many of the concepts they employ, is available.
Classical mechanics
-
A
pulley uses the principle of
mechanical advantage so that a small force over a large distance can lift a heavy weight over a shorter distance.
Classical mechanics is a model of the physics of forces acting upon bodies. It is often referred to as "Newtonian mechanics" after Isaac Newton and his laws of motion. Mechanics is subdivided into statics, which models objects at rest, kinematics, which models objects in motion, and dynamics, which models objects subjected to forces. The classical mechanics of continuous and deformable objects is continuum mechanics, which can itself be broken down into solid mechanics and fluid mechanics according to the state of matter being studied. The latter, the mechanics of liquids and gases, includes hydrostatics, hydrodynamics, pneumatics, aerodynamics,
and other fields. Mechanical Statics deals with objects at rest.
Mechanical kinematics deals with objects in motion. Mechanical dynamics
deals with motion of objects subject to forces.
Classical mechanics produces accurate results within the domain of everyday experience. It is superseded by relativistic mechanics for systems moving at large velocities near the speed of light, quantum mechanics for systems at small distance scales, and relativistic quantum field theory
for systems with both properties. Nevertheless, classical mechanics is
still useful, because it is much simpler and easier to apply than these
other theories, and it has a very large range of approximate validity.
Classical mechanics can be used to describe the motion of human-sized
objects (such as tops and baseballs), many astronomical objects (such as planets and galaxies), and certain microscopic objects (such as organic molecules).
An important concept of mechanics is the identification of conserved energy and momentum, which lead to the Lagrangian and Hamiltonian reformulations of Newton's laws. Theories such as fluid mechanics and the kinetic theory
of gases result from applying classical mechanics to macroscopic
systems. A relatively recent result of considerations concerning the
dynamics of nonlinear systems is chaos theory, the study of systems in which small changes in a variable may have large effects. Newton's law of universal gravitation, formulated within classical mechanics, explained Kepler's laws of planetary motion and helped make classical mechanics an important element of the Scientific Revolution.
Electromagnetism
-
- See also: Optics
Magnetic lines of force of a bar magnet shown by iron filings on paper
Electromagnetism describes the interaction of charged particles with electric and magnetic fields. It can be divided into electrostatics, the study of interactions between charges at rest, and electrodynamics, the study of interactions between moving charges and radiation. The classical theory of electromagnetism is based on the Lorentz force law and Maxwell's equations.
Electrostatics is the study of phenomena associated with charged bodies at rest. As described by Coulomb’s law, such bodies exert forces on each other. Their behavior can be analyzed in terms of the concept of an electric field
surrounding any charged body, such that another charged body placed
within the field is subject to a force proportional to the magnitude of
its own charge and the magnitude of the field at its location. Whether
the force is attractive or repulsive depends on the polarity
of the charge. Electrostatics has many applications, ranging from the
analysis of phenomena such as thunderstorms to the study of the
behavior of electron tubes.
Electrodynamics is the study of phenomena associated with charged bodies in motion and varying electric and magnetic fields. Since a moving charge produces a magnetic field, electrodynamics is concerned with effects such as magnetism, electromagnetic radiation, and electromagnetic induction, including such practical applications as the electric generator and the electric motor. This area of electrodynamics, known as classical electrodynamics, was first systematically explained by James Clerk Maxwell, and Maxwell’s equations describe the phenomena of this area with great generality. A more recent development is quantum electrodynamics, which incorporates the laws of quantum theory in order to explain the interaction of electromagnetic radiation with matter. Dirac, Heisenberg, and Pauli were pioneers in the formulation of quantum electrodynamics. Relativistic electrodynamics accounts for relativistic
corrections to the motions of charged particles when their speeds
approach the speed of light. It applies to phenomena involved with particle accelerators and electron tubes carrying high voltages and currents.
Electromagnetism encompasses various real-world electromagnetic phenomena. For example, light is an oscillating electromagnetic field that is radiated from accelerating charged particles. Aside from gravity, most of the forces in everyday experience are ultimately a result of electromagnetism.
The principles of electromagnetism find applications in various allied disciplines such as microwaves, antennas, electric machines, satellite communications, bioelectromagnetics, plasmas, nuclear research, fiber optics, electromagnetic interference and compatibility, electromechanical energy conversion, radar meteorology, and remote sensing. Electromagnetic devices include transformers, electric relays, radio/TV, telephones, electric motors, transmission lines, waveguides, optical fibers, and lasers.
Relativity
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High-precision test of general relativity by the
Cassini space probe (artist's impression):
radio signals sent between the Earth and the probe (green wave) are
delayed by the warping of
space and time (blue lines).
Relativity is a generalization of classical mechanics that describes fast-moving or very massive systems. It includes special and general relativity.
The theory of special relativity was proposed in 1905 by Albert Einstein in his article "On the Electrodynamics of Moving Bodies". The title of the article refers to the fact that special relativity resolves an inconsistency between Maxwell's equations<
Classical
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