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Physics

2007 Schools Wikipedia Selection. Related subjects: General Physics

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   Physics (from the Greek, φύσις (phúsis), "nature" and φυσική (phusiké),
   "knowledge of nature") is the science concerned with the discovery and
   understanding of the fundamental laws which govern matter, energy,
   space and time. That is, physics deals with the elementary constituents
   of the Universe and their interactions, as well as the analysis of
   systems which are best understood in terms of these fundamental
   principles. Physics is a study of the inorganic, physical world, as
   opposed to the organic world of biology, physiology, etc. Chemistry
   concerning the electro-chemical interactions of substances overlaps
   with physics.

Introduction

   Physics attempts to describe the natural world by the application of
   the scientific method, including modelling by theoreticians. Formerly,
   physics included the study of natural philosophy, its counterpart which
   had been called "physics" (earlier physike) from classical times up to
   the separation of physics from philosophy as a positive science in the
   19th century, as the study of the changing world by philosophy. Mixed
   questions, of which solutions can be attempted through the applications
   of both disciplines (e.g. the divisibility of the atom) can involve
   natural philosophy in physics (the science) and vice versa .

Connected Studies

   Many other sciences and fields of thought are related to physics.

   Discoveries in physics find connections throughout the other natural
   sciences as they regard the basic constituents of the Universe. Some of
   the phenomena studied in physics, such as the phenomenon of
   conservation of energy, are common to all material systems. These are
   often referred to as laws of physics. Other phenomena, such as
   superconductivity, stem from these laws, but are not laws themselves
   because they only appear in some systems. Physics is often said to be
   the "fundamental science", because each of the other sciences (biology,
   chemistry, geology, physiology, archaeology, anthropology, etc.) deals
   with particular types of material systems that obey the laws of
   physics. For example, chemistry is the science of matter (such as atoms
   and molecules) and the chemical substances that they form in the bulk.
   The structure, reactivity, and properties of a chemical compound are
   determined by the properties of the underlying molecules, which can be
   described by areas of physics such as quantum mechanics (called in this
   case quantum chemistry), thermodynamics, and electromagnetism. (Refer
   to Branches of physics)

   Physics relies on mathematics, which provides the logical framework in
   which physical laws can be precisely formulated and their predictions
   quantified. Physical definitions, models and theories are invariably
   expressed using mathematical relations. There is a large area of
   research intermediate between physics and mathematics, known as
   mathematical physics.

   Physics is also closely related to engineering and technology. For
   instance, electrical engineering is the study of the practical
   application of electromagnetism. Statics, a subfield of mechanics, is
   responsible for the building of bridges. Further, physicists, or
   practitioners of physics, invent and design processes and devices, such
   as the transistor, whether in basic or applied research. Experimental
   physicists design and perform experiments with particle accelerators,
   nuclear reactors, telescopes, barometers, synchrotrons, cyclotrons,
   spectrometers, lasers, and other equipment.

   Beyond the known Universe, the field of theoretical physics also deals
   with hypothetical issues, such as parallel universes, a multiverse, or
   whether the universe could have expanded as predominantly antimatter
   rather than matter.

Branches of physics

   Physicists study a wide range of physical phenomena, from quarks to
   black holes, from individual atoms to the many-body systems of
   superconductors.

Central theories

   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. Electromagnetism encompasses various real-world
       electromagnetic 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 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, is:

     *
          + A physical theory which is based on two postulates (1) that
            the speed of light in a vacuum is constant and independent of
            the source or observer and (2) that it is impossible to
            determine ones absolute velocity in any inertial systems and
            which leads to the deduction of the equivalence of mass and
            energy and of change in mass, dimension, and time with
            increased velocity — called also special relativity, special
            theory of relativity;
          + An extension of the theory to include gravitation and related
            acceleration phenomena — called also general relativity,
            general theory of relativity.

   Theory Major subtopics Concepts
   Classical mechanics Newton's laws of motion, Lagrangian mechanics,
   Hamiltonian mechanics, Kinematics, Statics, Dynamics, Chaos theory,
   Acoustics, Fluid dynamics, Continuum mechanics Density, Dimension,
   Gravity, Space, Time, Motion, Length, Position, Velocity, Acceleration,
   Mass, Momentum, Force, Energy, Angular momentum, Torque, Conservation
   law, Harmonic oscillator, Wave, Work, Power
   Electromagnetism Electrostatics, Electrodynamics, Electricity,
   Magnetism, Maxwell's equations, Optics Capacitance, Electric charge,
   Current, Electrical conductivity, Electric field, Electric
   permittivity, Electric potential, Electrical resistance,
   Electromagnetic field, Electromagnetic induction, Electromagnetic
   radiation, Gaussian surface, Magnetic field, Magnetic flux, Magnetic
   monopole, Magnetic permeability
   Thermodynamics and Statistical mechanics Heat engine, Kinetic theory
   Boltzmann's constant, Conjugate variables, Enthalpy, Entropy, Equation
   of state, Equipartition theorem, Free energy, Heat, Ideal gas law,
   Internal energy, Laws of thermodynamics, Irreversible process, Ising
   model, Mechanical action, Partition function, Pressure, Reversible
   process, Spontaneous process, State function, Statistical ensemble,
   Temperature, Thermodynamic equilibrium, Thermodynamic potential,
   Thermodynamic processes, Thermodynamic state, Thermodynamic system,
   Viscosity, Volume, Work
   Quantum mechanics Path integral formulation, Scattering theory,
   Schrödinger equation, Quantum field theory, Quantum statistical
   mechanics Adiabatic approximation, Blackbody radiation, Correspondence
   principle, Free particle, Hamiltonian, Hilbert space, Identical
   particles, Matrix Mechanics, Planck's constant, Observer effect,
   Operators, Quanta, Quantization, Quantum entanglement, Quantum harmonic
   oscillator, Quantum number, Quantum tunneling, Schrödinger's cat, Dirac
   equation, Spin, Wavefunction, Wave mechanics, Wave-particle duality,
   Zero-point energy, Pauli Exclusion Principle, Heisenberg Uncertainty
   Principle
   Theory of relativity Special relativity, General relativity, Einstein
   field equations Covariance, Einstein manifold, Equivalence principle,
   Four-momentum, Four-vector, General principle of relativity, Geodesic
   motion, Gravity, Gravitoelectromagnetism, Inertial frame of reference,
   Invariance, Length contraction, Lorentzian manifold, Lorentz
   transformation, Mass-energy equivalence, Metric, Minkowski diagram,
   Minkowski space, Principle of Relativity, Proper length, Proper time,
   Reference frame, Rest energy, Rest mass, Relativity of simultaneity,
   Spacetime, Special principle of relativity, Speed of light,
   Stress-energy tensor, Time dilation, Twin paradox, World line

Major fields of physics

   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. A magnet levitating above a high-temperature
       superconductor (with boiling liquid nitrogen underneath),
       demonstrating the Meissner effect, is a phenomenon of importance to
       the field of condensed matter physics.

     * The field of atomic, molecular, and optical physics deals with the
       behaviour 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.

   Many fields and subfields of physics are listed in the table below.
   Field Subfields Major theories Concepts
   Astrophysics Cosmology, Gravitation physics, High-energy astrophysics,
   Planetary astrophysics, Plasma physics, Space physics, Stellar
   astrophysics Big Bang, Lambda-CDM model, Cosmic inflation, General
   relativity, Law of universal gravitation Black hole, Cosmic background
   radiation, Cosmic string, Cosmos, Dark energy, Dark matter, Galaxy,
   Gravity, Gravitational radiation, Gravitational singularity, Planet,
   Solar system, Star, Supernova, Universe
   Atomic, molecular, and optical physics Atomic physics, Molecular
   physics, Atomic and Molecular astrophysics, Chemical physics, Optics,
   Photonics Quantum optics, Quantum chemistry, Quantum information
   science Atom, Molecule, Diffraction, Electromagnetic radiation, Laser,
   Polarization, Spectral line, Casimir effect
   Particle physics Nuclear physics, Nuclear astrophysics, Particle
   astrophysics, Particle physics phenomenology Standard Model, Quantum
   field theory, Quantum chromodynamics, Electroweak theory, Effective
   field theory, Lattice field theory, Lattice gauge theory, Gauge theory,
   Supersymmetry, Grand unification theory, Superstring theory, M-theory
   Fundamental force ( gravitational, electromagnetic, weak, strong),
   Elementary particle, Spin, Antimatter, Spontaneous symmetry breaking,
   Brane, String, Quantum gravity, Theory of everything, Vacuum energy
   Condensed matter physics Solid state physics, High pressure physics,
   Low-temperature physics, Nanoscale and Mesoscopic physics, Polymer
   physics BCS theory, Bloch wave, Fermi gas, Fermi liquid, Many-body
   theory Phases (gas, liquid, solid, Bose-Einstein condensate,
   superconductor, superfluid), Electrical conduction, Magnetism,
   Self-organization, Spin, Spontaneous symmetry breaking

Classical, quantum and modern physics

   Since the construction of quantum mechanics in the early twentieth
   century, it generally became evident to the physical community that it
   would be preferable for many known descriptions of nature to be
   quantized, that is, to follow the postulates of quantum mechanics. To
   this effect, all results that were not quantized are called classical:
   this includes the Special Theory and General Theory of Relativity.
   Simply because a result is classical does not mean that it was
   discovered before the advent of quantum mechanics. Classical theories
   are, generally, much easier to work with and much research is still
   being conducted on them without the express aim of quantization.
   However, there exist problems in physics in which classical and quantum
   aspects must be combined to attain some approximation or limit that may
   acquire several forms as the passage from classical to quantum
   mechanics is often difficult — such problems are termed semiclassical.

   However, because relativity and quantum mechanics provide the most
   complete known description of fundamental interactions, and because the
   changes brought by these two frameworks to the physicist's world view
   were revolutionary, the term modern physics is used to describe physics
   which relies on these two theories. Colloquially, modern physics can be
   described as the physics of extremes: from systems at the extremely
   small (atoms, nuclei, fundamental particles) to the extremely large
   (the Universe) and of the extremely fast (relativity).

Theoretical and experimental physics

   The culture of physics research differs from the other sciences in the
   separation of theory and experiment. Since the 20th 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
   (e.g. American quantum chemist and biochemist Linus Pauling) have also
   been experimentalists, though this is changing as of late.

   Roughly speaking, theorists seek to develop through abstractions and
   mathematical models theories that can both describe and interpret
   existing experimental results and successfully predict future results,
   while experimentalists devise and perform experiments to explore new
   phenomena and test theoretical predictions. Although theory and
   experiment are developed separately, they are strongly dependent on
   each other. However, theoretical research in physics may further be
   considered to draw from mathematical physics and computational physics
   in addition to experimentation. Progress in physics frequently comes
   about when experimentalists make a discovery that existing theories
   cannot account for, necessitating the formulation of new theories.
   Likewise, ideas arising from theory often inspire new experiments. In
   the absence of experiment, theoretical research can go in the wrong
   direction; this is one of the criticisms that has been leveled against
   M-theory, a popular theory in high-energy physics for which no
   practical experimental test has ever been devised.

Discredited theories

   Scientific theories sometimes end up being discredited or superseded.
   In some of these cases the theory was announced prematurely and gained
   press attention before being discredited. Other times an established
   theory is overthrown and a new one erected in its place. Some famous
   examples are:
     * Dynamic theory of gravity — Announced in a press release by Nikola
       Tesla in 1937 but never published.
     * Steady state theory — An established theory of cosmology in the
       early and middle 20th century, made obsolete by the success of Big
       Bang theory.
     * Luminiferous aether — An established theory in the late 19th
       century, which was contradicted by observations and made
       "superfluous" by relativity.
     * Cold fusion — Announced in a press conference in 1989 but never
       confirmed. Still controversial.
     * Phlogiston theory — An established theory of the 18th century that
       attributed combustion to the liberation of phlogiston from a
       material.

Phenomenology

   Phenomenology is intermediate between experiment and theory. It is more
   abstract and includes more logical steps than experiment, but is more
   directly tied to experiment than theory. The boundaries between theory
   and phenomenology, and between phenomenology and experiment, are
   somewhat fuzzy and to some extent depend on the understanding and
   intuition of the scientist describing these. An example is Einstein's
   1905 paper on the photoelectric effect, " On a Heuristic Viewpoint
   Concerning the Production and Transformation of Light".

Applied physics

   Applied physics is physics that is intended for a particular
   technological or practical use, as for example in engineering, as
   opposed to basic research. This approach is similar to that of applied
   mathematics. Applied physics is rooted in the fundamental truths and
   basic concepts of the physical sciences but is concerned with the
   utilization of scientific principles in practical devices and systems,
   and in the application of physics in other areas of science. "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.
                         Branches of Applied Physics
   Accelerator physics, Acoustics, Agrophysics, Biophysics, Chemical
   Physics, Communication Physics, Econophysics, Engineering physics,
   Fluid dynamics, Geophysics, Materials physics, Medical physics,
   Nanotechnology, Optics, Optoelectronics, Photovoltaics, Physical
   chemistry, Physics of computation, Plasma physics, Solid-state devices,
   Quantum chemistry, Quantum electronics, Quantum information science,
   Vehicle dynamics

History

   Since antiquity, people have tried to understand the behaviour of
   matter: why unsupported objects drop to the ground, why different
   materials have different properties, and so forth. The character of the
   Universe was also a mystery, for instance the Earth and the behaviour
   of celestial objects such as the Sun and the Moon. Several theories
   were proposed, most of which were wrong. These first theories were
   largely couched in philosophical terms, and never verified by
   systematic experimental testing as is popular today. The works of
   Ptolemy and Aristotle, however, were also not always found to match
   everyday observations. There were exceptions and there are anachronisms
   - for example, Indian philosophers and astronomers gave many correct
   descriptions in atomism and astronomy, and the Greek thinker Archimedes
   derived many correct quantitative descriptions of mechanics and
   hydrostatics.

   The willingness to question previously held truths and search for new
   answers eventually resulted in a period of major scientific
   advancements, now known as the Scientific Revolution of the late 17th
   century. The precursors to the scientific revolution can be traced back
   to the important developments made in India and Persia, including the
   elliptical model of the planets based on the heliocentric solar system
   of gravitation developed by Indian mathematician-astronomer Aryabhata;
   the basic ideas of atomic theory developed by Hindu and Jaina
   philosophers; the theory of light being equivalent to energy particles
   developed by the Indian Buddhist scholars Dignāga and Dharmakirti; the
   optical theory of light developed by Muslim scientist Ibn al-Haitham
   (Alhazen); the Astrolabe invented by the Persian astronomer Muhammad
   al-Fazari; and the significant flaws in the Ptolemaic system pointed
   out by Persian scientist Nasir al-Din Tusi.

   As the influence of the Arab Empire expanded to Europe, the works of
   Aristotle preserved by the Arabs, and the works of the Indians and
   Persians, became known in Europe by the 12th and 13th centuries. This
   eventually led to the scientific revolution which culminated with the
   publication of the Philosophiae Naturalis Principia Mathematica in 1687
   by the mathematician, physicist, alchemist and inventor Sir Isaac
   Newton ( 1643- 1727).

   The Scientific Revolution is held by most historians (e.g., Howard
   Margolis) to have begun in 1543, when the first printed copy of
   Nicolaus Copernicus's De Revolutionibus (most of which had been written
   years prior but whose publication had been delayed) was brought to the
   influential Polish astronomer from Nuremberg.

   Further significant advances were made over the following century by
   Galileo Galilei, Christiaan Huygens, Johannes Kepler, and Blaise
   Pascal. During the early 17th century, Galileo pioneered the use of
   experimentation to validate physical theories, which is the key idea in
   modern scientific method. Galileo formulated and successfully tested
   several results in dynamics, in particular the Law of Inertia. In 1687,
   Newton published the Principia, detailing two comprehensive and
   successful physical theories: Newton's laws of motion, from which arise
   classical mechanics; and Newton's Law of Gravitation, which describes
   the fundamental force of gravity. Both theories agreed well with
   experiment. The Principia also included several theories in fluid
   dynamics. Classical mechanics was re-formulated and extended by
   Leonhard Euler, French mathematician Joseph-Louis Comte de Lagrange,
   Irish mathematical physicist William Rowan Hamilton, and others, who
   produced new results in mathematical physics. The law of universal
   gravitation initiated the field of astrophysics, which describes
   astronomical phenomena using physical theories.

   After Newton defined classical mechanics, the next great field of
   inquiry within physics was the nature of electricity. Observations in
   the 17th and 18th century by scientists such as Robert Boyle, Stephen
   Gray, and Benjamin Franklin created a foundation for later work. These
   observations also established our basic understanding of electrical
   charge and current.

   In 1821, the English physicist and chemist Michael Faraday integrated
   the study of magnetism with the study of electricity. This was done by
   demonstrating that a moving magnet induced an electric current in a
   conductor. Faraday also formulated a physical conception of
   electromagnetic fields. James Clerk Maxwell built upon this conception,
   in 1864, with an interlinked set of 20 equations that explained the
   interactions between electric and magnetic fields. These 20 equations
   were later reduced, using vector calculus, to a set of four equations
   by Oliver Heaviside.

   In addition to other electromagnetic phenomena, Maxwell's equations
   also can be used to describe light. Confirmation of this observation
   was made with the 1888 discovery of radio by Heinrich Hertz and in 1895
   when Wilhelm Roentgen detected X rays. The ability to describe light in
   electromagnetic terms helped serve as a springboard for Albert
   Einstein's publication of the theory of special relativity in 1905.
   This theory combined classical mechanics with Maxwell's equations. The
   theory of special relativity unifies space and time into a single
   entity, spacetime. Relativity prescribes a different transformation
   between reference frames than classical mechanics; this necessitated
   the development of relativistic mechanics as a replacement for
   classical mechanics. In the regime of low (relative) velocities, the
   two theories agree. Einstein built further on the special theory by
   including gravity into his calculations, and published his theory of
   general relativity in 1915.

   One part of the theory of general relativity is Einstein's field
   equation. This describes how the stress-energy tensor creates curvature
   of spacetime and forms the basis of general relativity. Further work on
   Einstein's field equation produced results which predicted the Big
   Bang, black holes, and the expanding universe. Einstein believed in a
   static universe and tried (and failed) to fix his equation to allow for
   this. However, by 1929 Edwin Hubble's astronomical observations
   suggested that the universe is expanding.

   From the late 17th century onwards, thermodynamics was developed by
   physicist and chemist Boyle, Young, and many others. In 1733, Bernoulli
   used statistical arguments with classical mechanics to derive
   thermodynamic results, initiating the field of statistical mechanics.
   In 1798, Thompson demonstrated the conversion of mechanical work into
   heat, and in 1847 Joule stated the law of conservation of energy, in
   the form of heat as well as mechanical energy. Ludwig Boltzmann, in the
   19th century, is responsible for the modern form of statistical
   mechanics.

   In 1895, Röntgen discovered X-rays, which turned out to be
   high-frequency electromagnetic radiation. Radioactivity was discovered
   in 1896 by Henri Becquerel, and further studied by Marie Curie, Pierre
   Curie, and others. This initiated the field of nuclear physics.

   In 1897, Joseph J. Thomson discovered the electron, the elementary
   particle which carries electrical current in circuits. In 1904, he
   proposed the first model of the atom, known as the plum pudding model.
   (The existence of the atom had been proposed in 1808 by John Dalton.)

   These discoveries revealed that the assumption of many physicists that
   atoms were the basic unit of matter was flawed, and prompted further
   study into the structure of atoms.

   In 1911, Ernest Rutherford deduced from scattering experiments the
   existence of a compact atomic nucleus, with positively charged
   constituents dubbed protons. Neutrons, the neutral nuclear
   constituents, were discovered in 1932 by Chadwick. The equivalence of
   mass and energy (Einstein, 1905) was spectacularly demonstrated during
   World War II, as research was conducted by each side into nuclear
   physics, for the purpose of creating a nuclear bomb. The German effort,
   led by Heisenberg, did not succeed, but the Allied Manhattan Project
   reached its goal. In America, a team led by Fermi achieved the first
   man-made nuclear chain reaction in 1942, and in 1945 the world's first
   nuclear explosive was detonated at Trinity site, near Alamogordo, New
   Mexico.

   In 1900, Max Planck published his explanation of blackbody radiation.
   This equation assumed that radiators are quantized, which proved to be
   the opening argument in the edifice that would become quantum
   mechanics. By introducing discrete energy levels, Planck, Einstein,
   Niels Bohr, and others developed quantum theories to explain various
   anomalous experimental results. Quantum mechanics was formulated in
   1925 by Heisenberg and in 1926 by Schrödinger and Paul Dirac, in two
   different ways that both explained the preceding heuristic quantum
   theories. In quantum mechanics, the outcomes of physical measurements
   are inherently probabilistic; the theory describes the calculation of
   these probabilities. It successfully describes the behaviour of matter
   at small distance scales. During the 1920s Schrödinger, Heisenberg, and
   Max Born were able to formulate a consistent picture of the chemical
   behaviour of matter, a complete theory of the electronic structure of
   the atom, as a byproduct of the quantum theory.

   Quantum field theory was formulated in order to extend quantum
   mechanics to be consistent with special relativity. It was devised in
   the late 1940s with work by Richard Feynman, Julian Schwinger,
   Sin-Itiro Tomonaga, and Freeman Dyson. They formulated the theory of
   quantum electrodynamics, which describes the electromagnetic
   interaction, and successfully explained the Lamb shift. Quantum field
   theory provided the framework for modern particle physics, which
   studies fundamental forces and elementary particles.

   Chen Ning Yang and Tsung-Dao Lee, in the 1950s, discovered an
   unexpected asymmetry in the decay of a subatomic particle. In 1954,
   Yang and Robert Mills then developed a class of gauge theories which
   provided the framework for understanding the nuclear forces (Yang,
   Mills 1954). The theory for the strong nuclear force was first proposed
   by Murray Gell-Mann. The electroweak force, the unification of the weak
   nuclear force with electromagnetism, was proposed by Sheldon Lee
   Glashow, Abdus Salam and Steven Weinberg and confirmed in 1964 by James
   Watson Cronin and Val Fitch. This led to the so-called Standard Model
   of particle physics in the 1970s, which successfully describes all the
   elementary particles observed to date.

   Quantum mechanics also provided the theoretical tools for condensed
   matter physics, whose largest branch is solid state physics. It studies
   the physical behaviour of solids and liquids, including phenomena such
   as crystal structures, semiconductivity, and superconductivity. The
   pioneers of condensed matter physics include Felix Bloch, who created a
   quantum mechanical description of the behaviour of electrons in crystal
   structures in 1928. The transistor was developed by physicists John
   Bardeen, Walter Houser Brattain and William Bradford Shockley in 1947
   at Bell Telephone Laboratories.

   The two themes of the 20th century, general relativity and quantum
   mechanics, appear inconsistent with each other. General relativity
   describes the universe on the scale of planets and solar systems while
   quantum mechanics operates on sub-atomic scales. This challenge is
   being attacked by string theory, which treats spacetime as composed,
   not of points, but of one-dimensional objects, strings. Strings have
   properties like a common string (e.g., tension and vibration). The
   theories yield promising, but not yet testable results. The search for
   experimental verification of string theory is in progress.

   The United Nations declared the year 2005, the centenary of Einstein's
   annus mirabilis, as the World Year of Physics.

Future directions

   Research in physics is progressing constantly on a large number of
   fronts, and is likely to do so for the foreseeable future.

   In condensed matter physics, the biggest unsolved theoretical problem
   is the explanation for high-temperature superconductivity. Strong
   efforts, largely experimental, are being put into making workable
   spintronics and quantum computers.

   In particle physics, the first pieces of experimental evidence for
   physics beyond the Standard Model have begun to appear. Foremost
   amongst these are indications that neutrinos have non-zero mass. These
   experimental results appear to have solved the long-standing solar
   neutrino problem in solar physics. The physics of massive neutrinos is
   currently an area of active theoretical and experimental research. In
   the next several years, particle accelerators will begin probing energy
   scales in the TeV range, in which experimentalists are hoping to find
   evidence for the Higgs boson and supersymmetric particles.
   Thousands of particles explode from the collision point of two
   relativistic (100 GeV per nucleon) gold ions in the STAR detector of
   the Relativistic Heavy Ion Collider; an experiment done in order to
   investigate the properties of a quark gluon plasma such as the one
   thought to exist in the ultrahot first few microseconds after the big
   bang.
   Enlarge
   Thousands of particles explode from the collision point of two
   relativistic (100 GeV per nucleon) gold ions in the STAR detector of
   the Relativistic Heavy Ion Collider; an experiment done in order to
   investigate the properties of a quark gluon plasma such as the one
   thought to exist in the ultrahot first few microseconds after the big
   bang.

   Theoretical attempts to unify quantum mechanics and general relativity
   into a single theory of quantum gravity, a program ongoing for over
   half a century, have not yet borne fruit. The current leading
   candidates are M-theory, superstring theory and loop quantum gravity.

   Many astronomical and cosmological phenomena have yet to be
   satisfactorily explained, including the existence of ultra-high energy
   cosmic rays, the baryon asymmetry, the acceleration of the universe and
   the anomalous rotation rates of galaxies.

   Although much progress has been made in high-energy, quantum, and
   astronomical physics, many everyday phenomena, involving complexity,
   chaos, or turbulence are still poorly understood. Complex problems that
   seem like they could be solved by a clever application of dynamics and
   mechanics, such as the formation of sandpiles, nodes in trickling
   water, the shape of water droplets, mechanisms of surface tension
   catastrophes, or self-sorting in shaken heterogeneous collections are
   unsolved. These complex phenomena have received growing attention since
   the 1970s for several reasons, not least of which has been the
   availability of modern mathematical methods and computers which enabled
   complex systems to be modeled in new ways. The interdisciplinary
   relevance of complex physics has also increased, as exemplified by the
   study of turbulence in aerodynamics or the observation of pattern
   formation in biological systems. In 1932, Horace Lamb correctly
   prophesied the success of the theory of quantum electrodynamics and the
   near-stagnant progress in the study of turbulence:

     I am an old man now, and when I die and go to heaven there are two
     matters on which I hope for enlightenment. One is quantum
     electrodynamics, and the other is the turbulent motion of fluids.
     And about the former I am rather optimistic.

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