Physics (from the Greek, φύσις (phúsis), "nature" and φυσικῆ (phusiké), "knowledge of nature") is the science concerned with the discovery and understanding of the fundamental laws which govern matter, energy, space, and time. Physics deals with the elementary constituents of the universe and their interactions, as well as the analysis of systems best understood in terms of these fundamental principles. ^{[1]} Because physics treats the core workings of the universe, including the quantum mechanical details which underpin all atomic interactions, it may be thought of as the foundational science, upon which stands the "central science" of chemistry, and the earth sciences, biological sciences, and social sciences. Discoveries in basic physics have important ramifications for all of science.
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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 nineteenth century, as the study of the changing world by philosophy. Mixed questions, of which solutions can be attempted through the applications of both disciplines (for example the divisibility of the atom) can involve natural philosophy in physics (the science) and vice versa.
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 often are 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.
As analytic solutions are not always possible, numerical analysis and simulations must be utilized. Thus, scientific computation is an integral part of physics, and the field of computational physics is an active area of research.
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.
Physicists study a wide range of physical phenomena, from quarks to black holes, from individual atoms to the manybody systems of superconductors.
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.
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, Galilean invariance, Mass, Momentum, Force, Energy, Angular momentum, Torque, Conservation law, Harmonic oscillator, Wave, Work, Power, Lagrangian, Hamiltonian, TaitBryan angles, Euler angles 
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, Maxwell relations, 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, Granular material 
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, Waveparticle duality, Zeropoint energy, Pauli Exclusion Principle, Heisenberg Uncertainty Principle 
Relativity  Special relativity, General relativity, Einstein field equations  Covariance, Einstein manifold, Equivalence principle, Fourmomentum, Fourvector, General principle of relativity, Geodesic motion, Gravity, Gravitoelectromagnetism, Inertial frame of reference, Invariance, Length contraction, Lorentzian manifold, Lorentz transformation, Massenergy 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, Stressenergy tensor, Time dilation, Twin paradox, World line 
Contemporary research in physics is divided into several distinct fields that study different aspects of the material world.
Since the twentieth 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" such as Albert Einstein (1879–1955) and Lev Landau (1908–1968), who were comfortable working in multiple fields of physics, now are very rare.
Many fields and subfields of physics are listed in the table below.
Field  Subfields  Major theories  Concepts 

Astrophysics  Cosmology, Gravitation physics, Highenergy astrophysics, Planetary astrophysics, Plasma physics, Space physics, Stellar astrophysics  Big Bang, LambdaCDM model, Cosmic inflation, General relativity, Newton's 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 electrodynamics, Quantum chromodynamics, Electroweak theory, Effective field theory, Lattice field theory, Lattice gauge theory, Gauge theory, Supersymmetry, Grand unification theory, Superstring theory, Mtheory  Fundamental force (gravitational, electromagnetic, weak, strong), Elementary particle, Spin, Antimatter, Spontaneous symmetry breaking, Neutrino oscillation, Seesaw mechanism, Brane, String, Quantum gravity, Theory of everything, Vacuum energy 
Condensed matter physics  Solid state physics, High pressure physics, Lowtemperature physics, Surface Physics,Nanoscale and Mesoscopic physics, Polymer physics  BCS theory, Bloch wave, Fermi gas, Fermi liquid, Manybody theory  Phases (gas, liquid, solid, BoseEinstein condensate, superconductor, superfluid), Electrical conduction, Magnetism, Selforganization, Spin, Spontaneous symmetry breaking 
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 still is 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.
Because relativity and quantum mechanics provide the most complete known description of fundamental interactions, however, 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 may 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).
The culture of physics research differs from the other 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 (e.g. American quantum chemist and biochemist Linus Pauling) have also been experimentalists, although 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 upon each other. Theoretical research in physics, however, may be considered further 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 Mtheory, a popular theory in highenergy physics for which no practical experimental test has ever been devised.
Scientific theories sometimes end up being discredited or superseded. In some of these cases the theory was announced prematurely and gained recognition before being discredited. Other times an established theory is overthrown and a new one erected in its place. Some famous examples are:
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 unclear and, to some extent, depend upon 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 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 use 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. [1]
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, Solidstate devices, Quantum chemistry, Quantum electronics, Quantum information science, Vehicle dynamics 
Since antiquity, people have tried to understand the behavior 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 behavior of celestial objects such as the sun and the moon. Several theories were proposed, most of which were incorrect. 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 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 seventeenth century. The precursors to the scientific revolution may 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 mathematicianastronomer 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 alHaitham (Alhazen); the Astrolabe invented by the Persian astronomer Muhammad alFazari; and the significant flaws in the Ptolemaic system pointed out by Persian scientist Nasir alDin 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 medieval Europe by the twelfth and thirteenth centuries.
This eventually led to the scientific revolution, held by most historians (e.g., Howard Margolis) to have begun in 1543, when the first printed copy of Nicolaus Copernicus's De Revolutionibus was brought to the influential astronomer from Nuremberg (Nürnberg), where it had been printed by Johannes Petreius. Most of its contents had been written years prior, but the publication had been delayed.
Further significant advances were made over the following century by Galileo Galilei, Christiaan Huygens, Johannes Kepler, and Blaise Pascal. During the early seventeenth 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.
The scientific revolution is considered to have culminated with the publication of the Philosophiae Naturalis Principia Mathematica in 1687 by the mathematician, physicist, alchemist and inventor Sir Isaac Newton (16431727).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 reformulated and extended by Leonhard Euler, French mathematician JosephLouis 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 seventeenth and eighteenth 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.
The existence of the atom was proposed in 1808 by John Dalton.
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 twenty equations that explained the interactions between electric and magnetic fields. These twenty 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 Xrays.
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 stressenergy 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. He tried, and failed, to fix his equation to allow for this. By 1929, however, Edwin Hubble's astronomical observations suggested that the universe is expanding at a possibly exponential rate.
From the late seventeenth century onward, 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 nineteenth century, is responsible for the modern form of statistical mechanics.
In 1895, Röntgen discovered Xrays, which turned out to be highfrequency electromagnetic radiation.
Radioactivity was discovered in 1896 by Henri Becquerel, and further studied by Maria SklodowskaCurie, 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. Its existence 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 manmade 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 behavior of matter at small distance scales. During the 1920s Schrödinger, Heisenberg, and Max Born were able to formulate a consistent picture of the chemical behavior 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, SinItiro 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 TsungDao 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 GellMann. 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 socalled 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 behavior 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 behavior 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 Laboratories.
The two themes of the twentieth 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 subatomic scales. This challenge is being attacked by string theory, which treats spacetime as composed, not of points, but of onedimensional objects, strings. Strings have properties similar to 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.
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 greatest unsolved theoretical problem is the explanation for hightemperature 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 nonzero mass. These experimental results appear to have solved the longstanding 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.
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. Currently, the leading candidates are Mtheory, superstring theory, and loop quantum gravity.
Many astronomical and cosmological phenomena have yet to be explained satisfactorily, including the existence of ultrahigh 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 highenergy, quantum, and astronomical physics, many everyday phenomena, involving complexity, chaos, or turbulence remain poorly understood. Complex problems that would appear to be soluble by a clever application of dynamics and mechanics, such as the formation of sand piles, nodes in trickling water, the shape of water droplets, mechanisms of surface tension catastrophes, or selfsorting 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 also has 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 nearstagnant 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.


Classical mechanics  Electromagnetism  Thermodynamics  General relativity  Quantum mechanics 

Particle physics  Condensed matter physics  Atomic, molecular, and optical physics 
The Four Fundamental "Forces" of Physics  

Gravitation  Electromagnetism  Strong Force  Weak Force 
General subfields within the Natural sciences 

Astronomy  Biology  Chemistry  Earth science  Ecology  Physics 