» 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”.
Physics is the
science of
matter
and its motion, as well as
space and time
—the
science that deals with concepts such as
force,
energy,
mass, and
charge. As an
experimental science, its goal is to understand the natural world. For the etymology of the word
physics, see
physis (φύσις).
In one form or another, physics is one of the oldest academic disciplines; through its modern subfield of
astronomy, it may be the oldest of all.
Sometimes synonymous with
philosophy,
chemistry and even certain branches of
mathematics and
biology during the last two millennia, physics
emerged as a modern science in the 17th century
and these disciplines are now generally distinct, although the boundaries remain difficult to define.
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 led to the development of the
calculus,
quantum chemistry, and the use of instruments like the
electron microscope in
microbiology.
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 open questions in physics, and many areas of active research.
Core theories
Although physics encompasses a wide variety of phenomena, all competent physicists are familiar with the basic theories of
classical mechanics,
electromagnetism,
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's 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, can be found
here.
Classical mechanics
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.
Classical mechanics produces very 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 very useful, because it's 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
Electromagnetism describes the interaction of charged particles with electric and magnetic
fields. It can be divided into
electrostatics, the study of interactions between electric
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's 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
Relativity is a generalization of classical mechanics that describes fast-moving or very massive systems. It remains consistent with
Maxwell's equations and 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". It is based on
two postulates: (1) that the mathematical forms of the
laws of physics are invariant in all
inertial systems; and (2) that the
speed of light in a
vacuum is constant and independent of the source or observer. Reconciling the two postulates requires a unification of
space and
time into the frame-dependent concept of
spacetime.
Special relativity has a variety of surprising consequences that seem to violate common sense, but all have been experimentally verified. It overthrows
Newtonian notions of absolute space and time by stating that distance and
time depend on the observer, and that time and space are perceived differently, depending on the observer. The theory leads to the assertion of change in
mass,
dimension, and
time with increased
velocity. It also yields the equivalence of
matter and
energy, as expressed in the
mass-energy equivalence formula
E =
mc², where
c is the speed of light in a vacuum. Special relativity and the
Galilean relativity of Newtonian mechanics agree when velocities are small compared to the speed of light. Special relativity doesn't describe gravitation; however, it can handle accelerated motion in the absence of gravitation.
General relativity is the
geometrical theory of
gravitation published by
Albert Einstein in 1915/16. It unifies
special relativity,
Newton's law of universal gravitation, and the insight that gravitation can be described by the
curvature of
space and
time. In general relativity, the curvature of
space-time is produced by the
energy of matter and radiation. General relativity is distinguished from other
metric by its use of the
Einstein field equations to relate space-time content and space-time curvature. Local
Lorentz Invariance requires that the manifolds described in GR be 4-dimensional and Lorentzian instead of
Riemannian. In addition, the principle of
general covariance forces that mathematics be expressed using
tensor calculus.
The first success of general relativity was in explaining the anomalous
perihelion precession of
Mercury. Then in 1919,
Sir Arthur Eddington announced that observations of
stars near the
eclipsed
Sun confirmed general relativity's prediction that massive objects bend
light. Since then, many other
observations and experiments have confirmed many of the
predictions of general relativity, including
gravitational time dilation, the
gravitational redshift of light,
signal delay, and
gravitational radiation. In addition, numerous observations are interpreted as confirming one of general relativity's most mysterious and exotic predictions, the existence of
black holes.
Thermodynamics and statistical mechanics
Thermodynamics studies the effects of changes in
temperature,
pressure, and
volume on
physical systems at the
macroscopic scale, and the transfer of energy as
heat. Historically, thermodynamics developed out of need to increase the
efficiency of early
steam engines.
The starting point for most thermodynamic considerations are the
laws of thermodynamics, which postulate that
energy can be exchanged between physical systems as heat or
work. They also postulate the existence of a quantity named
entropy, which can be defined for any system. In thermodynamics, interactions between large ensembles of objects are studied and categorized. Central to this are the concepts of
system and
surroundings. A system is composed of particles, whose average motions define its properties, which in turn are related to one another through
equations of state. Properties can be combined to express
internal energy and
thermodynamic potentials, which are useful for determining conditions for
equilibrium and
spontaneous processes.
Statistical mechanics analyzes
macroscopic systems by applying
statistical principles to their microscopic constituents. It provides a framework for relating the microscopic properties of individual atoms and molecules to the macroscopic or bulk properties of materials that can be observed in everyday life.
Thermodynamics can be explained as a natural result of statistics and mechanics (classical and quantum) at the microscopic level. In this way, the
gas laws can be derived, from the assumption that a gas is a collection of individual particles, as hard spheres with
mass. Conversely, if the individual particles are also considered to have
charge, then the individual accelerations of those particles will cause the emission of
light. It was these considerations which caused
Max Planck to formulate his law of
blackbody radiation, but only with the assumption that the spectrum of radiation emitted from these particles isn't continuous in frequency, but rather quantized.
Quantum mechanics
Quantum mechanics is the branch of 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". Remarkably, quantum theory typically permits only
probable or
statistical calculation of the observed features of subatomic particles, understood in terms of
wavefunctions. The
Schrödinger equation plays the role in quantum mechanics that
Newton's laws and
conservation of energy serve in classical mechanics—for example, it predicts the future behavior of a
dynamic system—and is a
wave equation in terms of the wavefunction which predicts analytically and precisely the probability of events or outcomes.
According to the older theories of
classical physics, energy is treated solely as a continuous phenomenon, while matter is assumed to occupy a very specific region of space and to move in a continuous manner. According to the quantum theory, energy is held to be emitted and absorbed in tiny, discrete amounts. An individual bundle or packet of energy, called a quantum (pl. quanta), thus behaves in some situations much like particles of matter; particles are found to exhibit certain
wavelike properties when in motion and are no longer viewed as localized in a given region but rather as spread out to some degree. For example, the light or other radiation given off or absorbed by an atom has only certain
frequencies (or
wavelengths), as can be seen from the
line spectrum associated with the chemical element represented by that atom. The quantum theory shows that those frequencies correspond to definite energies of the light quanta, or
photons, and result from the fact that the
electrons of the atom can have only certain allowed energy values, or levels; when an electron changes from one allowed level to another, a quantum of energy is emitted or absorbed whose frequency is directly proportional to the energy difference between the two levels.
The formalism of quantum mechanics was developed during the 1920s. In 1924,
Louis de Broglie proposed that not only do light waves sometimes exhibit particle-like properties, as in the photoelectric effect and atomic spectra, but particles may also exhibit wavelike properties. Two different formulations of quantum mechanics were presented following de Broglie’s suggestion. The
wave mechanics of
Erwin Schrödinger (1926) involves the use of a mathematical entity, the wave function, which is related to the probability of finding a particle at a given point in space. The
matrix mechanics of
Werner Heisenberg (1925) makes no mention of wave functions or similar concepts but was shown to be mathematically equivalent to Schrödinger’s theory. A particularly important discovery of the quantum theory is the
uncertainty principle, enunciated by Heisenberg in 1927, which places an absolute theoretical limit on the accuracy of certain measurements; as a result, the assumption by earlier scientists that the physical state of a system could be measured exactly and used to predict future states had to be abandoned. Quantum mechanics was combined with the theory of relativity in the formulation of
P. A. M. Dirac (1928), which, in addition, predicted the existence of
antiparticles. Other developments of the theory include
quantum statistics, presented in one form by Einstein and
S. N. Bose (the
Bose-Einstein statistics) and in another by Dirac and
Enrico Fermi (the
Fermi-Dirac statistics);
quantum electrodynamics, concerned with interactions between charged particles and electromagnetic fields; its generalization,
quantum field theory; and
quantum electronics. The discovery of quantum mechanics in the early 20th century revolutionized physics, and quantum mechanics is fundamental to most areas of current research.
Research
Theory and experiment
The culture of physics research differs from most sciences in the separation of
theory and
experiment. Since the
twentieth century, most individual physicists have specialized in either
theoretical physics or
experimental physics. The great
Italian physicist
Enrico Fermi (
1901–
1954), who made fundamental contributions to both theory and experimentation in
nuclear physics, was a notable exception. In contrast, almost all the successful theorists in
biology and
chemistry (for example American
quantum chemist and
biochemist Linus Pauling) have also been experimentalists, although this is changing as of late.
Theorists seek to develop
mathematical models that both agree with existing experiments and successfully predict future results, while experimentalists devise and perform experiments to test theoretical predictions and explore new phenomena. Although theory and experiment are developed separately, they're strongly dependent upon each other. Progress in physics frequently comes about when experimentalists make a discovery that existing theories can't explain, or when new theories generate experimentally testable predictions. Theorists working closely with experimentalists frequently employ
phenomenology.
Theoretical physics is closely related to
mathematics, which provides the language of physical theories, and large areas of mathematics, such as
calculus, have been invented specifically to solve problems in physics.
Theorists may also rely on
numerical analysis and
computer simulations, which play an ever richer role in the formulation of physical models. The fields of
mathematical and
computational physics are active areas of research. Theoretical physics has historically rested on
philosophy and
metaphysics; electromagnetism was unified this way. Thus physicists may speculate with multidimensional spaces and
parallel universes, and from this, hypothesize theories.
Experimental physics informs, and is informed by,
engineering and
technology. Experimental physicists involved in
basic research design and perform experiments with equipment such as
particle accelerators and
lasers, whereas those involved in
applied research often work in industry, developing technologies such as
magnetic resonance imaging (MRI) and
transistors. Feynman has noted that experimentalists may seek areas which are not well explored by theorists.
Research fields
Contemporary research in physics can be broadly divided into
condensed matter physics;
atomic, molecular, and optical physics;
particle physics; and
astrophysics. Since the
twentieth century, the individual fields of physics have become increasingly
specialized, and today most physicists work in a single field for their entire careers. "Universalists" such as
Albert Einstein (
1879–
1955) and
Lev Landau (
1908–
1968), who worked in multiple fields of physics, are now very rare. A table of the major fields of physics, along with their subfields and the theories they employ can be found
here.
Condensed matter
Condensed matter physics is the field of physics that deals with the macroscopic physical properties of
matter. In particular, it's concerned with the "condensed"
phases that appear whenever the number of constituents in a system is extremely large and the interactions between the constituents are strong. The most familiar examples of condensed phases are
solids and
liquids, which arise from the bonding and
electromagnetic force between
atoms. More exotic condensed phases include the
superfluid and the
Bose-Einstein condensate found in certain atomic systems at very low
temperatures, the
superconducting phase exhibited by
conduction electrons in certain materials, and the
ferromagnetic and
antiferromagnetic phases of
spins on
atomic lattices.
Condensed matter physics is by far the largest field of contemporary physics. Much progress has also been made in theoretical condensed matter physics. By one estimate, one third of all American
physicists identify themselves as condensed matter physicists. Historically, condensed matter physics grew out of
solid-state physics, which is now considered one of its main subfields. The term
condensed matter physics was apparently coined by
Philip Anderson when he renamed his research group—previously
solid-state theory—in 1967. In 1978, the Division of Solid State Physics at the
American Physical Society was renamed as the Division of Condensed Matter Physics. Condensed matter physics has a large overlap with
chemistry,
materials science,
nanotechnology and
engineering.
Atomic, molecular, and optical
Atomic,
molecular, and
optical physics (AMO) is the study of
matter-matter and
light-matter interactions on the scale of single
atoms or structures containing a few atoms. The three areas are grouped together because of their interrelationships, the similarity of methods used, and the commonality of the
energy scales that are relevant. All three areas include both
classical and
quantum treatments; they can treat their subject from a microscopic view (in contrast to a macroscopic view).
Atomic physics studies the
electron hull of
atoms. Current research focuses on activities in quantum control, cooling and trapping of atoms and ions, low-temperature collision dynamics, the collective behavior of atoms in weakly interacting gases (Bose-Einstein Condensates and dilute Fermi degenerate systems), precision measurements of fundamental constants, and the effects of electron correlation on structure and dynamics. Atomic physics is influenced by the
nucleus (see, for example,
hyperfine splitting), but intra-nuclear phenomenon such as
fission and
fusion are considered part of
high energy physics.
Molecular physics focuses on multi-atomic structures and their internal and external interactions with matter and light.
Optical physics is distinct from
optics in that it tends to focus not on the control of classical light fields by macroscopic objects, but on the fundamental properties of
optical fields and their interactions with matter in the microscopic realm.
High energy/Particle Physics
Particle physics is the study of
elementary constituents of
matter and
radiation, and the interactions between them. It is also called "high energy physics", because many elementary particles don't occur under normal circumstances in nature, but can be created and detected during energetic
collisions of other particles, as is done in
particle accelerators.
The current state of the classification of elementary particles is the Standard Model. It describes the
strong,
weak, and
electromagnetic fundamental forces, using mediating
gauge bosons. The species of gauge bosons are the
gluons,
W- and W+ and
Z bosons, and the
photon, respectively. The model also contains 24
fundamental particles (12 particle/anti-particle pairs), which are the constituents of
matter. Finally, it predicts the existence of a type of
boson known as the
Higgs boson, which has yet to be discovered.
Astrophysics
Astrophysics and
astronomy are the application of the theories and methods of physics to the study of
stellar structure,
stellar evolution, the origin of the
solar system, and related problems of
cosmology. Because astrophysics is a very broad subject, astrophysicists typically apply many disciplines of physics, including mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics.
Astrophysics developed from the ancient science of astronomy. Astronomers of early civilizations performed methodical observations of the night sky, and astronomical artifacts have been found from much earlier periods. After centuries of developments by Babylonian and Greek astronomers, western astronomy lay dormant for fourteen centuries until
Nicolaus Copernicus modified the
Ptolemaic system by placing the sun at the center of the universe.
Tycho Brahe's detailed observations led to
Kepler's laws of planetary motion, and
Galileo's
telescope helped the discipline develop into a modern science. Isaac Newton's theory of universal gravitation provided a physical, dynamic basis for Kepler's laws. By the early 19th cent., the science of celestial mechanics had reached a highly developed state at the hands of
Leonhard Euler,
J. L. Lagrange,
P. S. Laplace, and others. Powerful new mathematical techniques allowed solution of most of the remaining problems in classical gravitational theory as applied to the solar system. At the end of the
19th century, the discovery of
spectral lines in sunlight proved that the chemical elements found in the Sun were also found on Earth. Interest shifted from determining the positions and distances of stars to studying their physical composition (see
stellar structure and
stellar evolution). Because the application of physics to astronomy became increasingly important throughout the
20th century, the distinction between
astronomy and
astrophysics has faded.
Physical cosmology is the study of the formation and evolution of the universe on its largest scales. Albert Einstein’s theory of relativity plays a central role in all modern cosmological theories. In the early
20th century,
Hubble's discovery that the universe was expanding, as shown by the
Hubble diagram, prompted rival explanations known as the
steady state universe and the
Big Bang. The Big Bang was confirmed by the success of
Big Bang nucleosynthesis and the discovery of the
cosmic microwave background in 1964. The Big Bang model rests on two theoretical pillars: Albert Einstein's general relativity and the
cosmological principle. Cosmologists have recently established a
precise model of the evolution of the universe, which include
cosmic inflation,
dark energy and
dark matter.
The discovery by
Karl Jansky in 1931 that radio signals were emitted by celestial bodies initiated the science of
radio astronomy. Most recently, the frontiers of astronomy have been expanded by space exploration. Perturbations and interference from the earth’s atmosphere make space-based observations necessary for
infrared,
ultraviolet,
gamma-ray, and
X-ray astronomy. The
Hubble Space Telescope, launched in 1990, has made possible visual observations of a quality far exceeding those of earthbound instruments; earth-bound observatories using telescopes with
adaptive optics will now be able to compensate for the turbulence of
Earth's
atmosphere.
Applied physics
Applied physics is a general term for physics which is intended for a particular
use.
Applied is distinguished from
pure by a subtle combination of factors such as the motivation and attitude of researchers and the nature of the relationship to the technology or science that may be affected by the work. It usually differs from
engineering in that an applied physicist may not be designing something in particular, but rather is using physics or conducting physics research with the aim of developing new technologies or solving a problem. The approach is similar to that of
applied mathematics. Applied physicists can also be interested in the use of physics for scientific research. For instance, people working on
accelerator physics might seek to build better particle detectors for research in theoretical physics.
Physics is used heavily in
engineering. For example,
statics, a subfield of
mechanics, is used in the building of
bridges or other structures, while
acoustics is used to design better concert halls. An understanding of physics is important to the design of realistic
flight simulators, video games, and movies.
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