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Physics (from the Greek, φ?σις (phúsis), "nature" and φυσικός (phusikós), "natural"), the most fundamental physical science, is concerned with the underlying principles of the natural world. Consequently, physics deals with the elementary constituents of the Universe — that is, all classes of matter and energy — and their interactions, as well as the analysis of systems which are best understood in terms of these fundamental principles.
Contents 
Introduction
Physics attempts to describe the natural world by the application of the scientific method. In contrast, natural philosophy, its counterpart which had also been called "physics" from classical times to at least up to the separation of physics from philosophy as a positive science in the 19th century, is 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.
Discoveries in physics find applications 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. Others, 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" (chemistry is sometimes included), because each of the other sciences (biology, chemistry, geology, material science, engineering, medicine 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.
Physics is closely related to 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. A key difference between physics and mathematics is that because physics is ultimately concerned with descriptions of the material world, it tests its theories by observations (called experiments), whereas mathematics is concerned with abstract logical patterns not limited by those observed in the real world (because the real world is limited in the number of dimensions and in many other ways it does not have to correspond to richer mathematical structures). The distinction, however, is not always clearcut. 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.
Branches of physics
Physicists study a wide range of physical phenomena, from quarks to black holes, from individual atoms to the manybody 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 realworld 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 the mathematical forms of the laws of physics are invariant in all inertial systems and which leads to the assertion 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.
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.
 The field of atomic, molecular, and optical physics deals with the behavior of individual atoms and molecules, and in particular the ways in which they absorb and emit light.
 The field of particle physics, also known as "highenergy 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.
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.
 The field of atomic, molecular, and optical physics deals with the behavior of individual atoms and molecules, and in particular the ways in which they absorb and emit light.
 The field of particle physics, also known as "highenergy 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.
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 every known description 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 and general theories 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.
Classical physics is physics based on principles developed before the rise of quantum theory, usually including the special theory of relativity and general theory of relativity. Mathematically, classical physics equations are ones in which Planck's constant does not appear. (In contrast, modern physics is a slightly looser term which may refer to just quantum physics or to 20th and 21st century physics in general and so always includes quantum theory and may include relativity.) There are no restrictions on the application of classical principles, but, practically, the scale of classical physics is the level of isolated atoms and molecules on upwards, including the macroscopic and astronomical realm. Inside the atom and among atoms in a molecule, the laws of classical physics break down and generally do not provide a correct description. Moreover, the classical theory of electromagnetic radiation is somewhat limited in its ability to provide correct descriptions, since light is inherently a quantum phenomenon. Unlike quantum physics, classical physics is generally characterized by the principle of complete determinism.
In physics, the adjective semiclassical has different precise meanings depending on the context. All these meanings usually refer to some approximation, limit or situation that combines quantum and classical aspects in a given problem. The plurality of meanings comes from the fact that the passage from quantum to classical mechanics is generally a very difficult task. Some of the possible significations are the following:
First, semiclassical approximation may refer to quantummechanical calculations that are obtained by considering a small perturbation of a classical calculation, for example the WKB approximation in nonrelativistic quantum mechanics or the loop expansion or the instanton methods in quantum field theory. In quantum field theory, a semiclassical correction arises from oneloop Feynman diagrams. The semiclassical effective action is
Second, in the context of open quantum systems and measurement theory, where one considers the dynamics of a given quantum system in interaction with an environment, the semiclassical regime may refer to the situation in which the wavefunction of the system is approximately peaked around the solution of the corresponding classical equations of motion. Corrections to the classical trajectory and the dispersion of the solution around the mean value are usually considered. Third, semiclassical gravity is the approximation to the yet unknown theory of quantum gravity in which one treats matter fields as being quantum and the gravitational field as being classical. The classical Einstein equations are computed with the expectation value of the quantum matter fields in the classical background. Semiclassical gravity has applications in black hole physics and physical cosmology. A semiclassical approximation is any high frequency approximation (or "high energy approximation"), less extreme than classical mechanics, that is used to approximate quantum mechanics. Fourthly, certain early theories in quantum mechanics, notably the Bohr model of the hydrogen atom, are sometimes called semiclassical, as they incorporate the newer quantum ideas into an essentially classical framework.
A physical system on the classical level is a physical system in which the laws of classical physics are valid. Among the branches of theory included in classical physics are:
 Classical mechanics
 Newton's laws of motion
 Classical Lagrangian and Hamiltonian formalisms
 Classical electrodynamics (Maxwell's Equations)
 Classical thermodynamics
 Special theory of relativity and General theory of relativity
 Classical chaos theory and nonlinear dynamics
According to the correspondence principle and Ehrenfest's theorem as a system becomes larger or more massive (action >> Planck's constant) the classical dynamics tends to emerge, with some exceptions, such as superfluidity. This is why we can usually ignore quantum mechanics when dealing with everyday objects; instead the classical description will suffice. However, one of the most vigorous ongoing fields of research in physics is classicalquantum correspondence. This field of research is concerned with the discovery of how the laws of quantum physics give rise to classical physics in the limit of the large scales of the classical level.
Quantum theory is a theory of physics that uses Planck's constant. In contrast to classical physics, many of the variables in a quantum theory take on discrete values. The quantum was a concept that grew out of the realisation that electromagnetic radiation came in discrete packets, called quanta. The process of converting a classical theory into a quantum theory is called quantisation and is divided into stages: first quantisation, second quantisation, etc depending on the extent to which the theory is quantised. Quantum physics is the set of quantum theories. There are several of them:
 quantum mechanics  a first quantised or semiclassical theory in which particle properties are quantised, but not particle numbers, fields and fundamental interactions.
 quantum field theory or QFT  a second or canonically quantised theory in which all aspects of particles, fields and interactions are quantised, with the exception of gravitation. Quantum electrodynamics, quantum chromodynamics and electroweak theory are examples of relativistic fundamental QFTs which taken together form the Standard Model. Solid state physics is a nonfundamental QFT.
 quantum gravity  a third quantised theory in which general relativity (i.e. gravity) is also quantised. This theory is incomplete, and is hoped to be finalised within the framework of a theory of everything, such as string theory or Mtheory.
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).
The beginning of the 20th century brought the start of a revolution in physics. In 1904, Thomson proposed the first model of the atom, known as the plum pudding model. The existence of atoms of different weights had been proposed in 1808 by John Dalton to explain the law of multiple proportions. The convergence of various estimates of Avogadro's number lent decisive evidence for atomic theory. In 1911, Ernest Rutherford deduced from scattering experiments the existence of a compact atomic nucleus, with positively charged constituents dubbed protons. The first quantum mechanical model of the atom, the Bohr model, was published in 1913 by Niels Bohr. Sir W. H. Bragg and his son Sir William Lawrence Bragg, also in 1913, began to unravel the arrangement of atoms in crystalline matter by the use of xray diffraction. Neutrons, the neutral nuclear constituents, were discovered in 1932 by James Chadwick.
The Lorentz transformations, the fundamental equations of special relativity, were published in 1897 and 1900 and also by Joseph Larmor and by Hendrik Lorentz in 1899 and 1904. They both showed that Maxwell's equations were invariant under the transformations. In 1905, Einstein formulated the theory of special relativity. In 1915, Einstein extended special relativity to describe gravity with the general theory of relativity. One principal result of general relativity is the gravitational collapse into black holes, which was anticipated two centuries earlier, but elucidated by Robert Oppenheimer. Important exact solutions of Einstein's field equation were found by Karl Schwarzschild in 1915 and Roy Kerr only in 1963.
According to Cornelius Lanczos, any physical law which can be expressed as a variational principle describes an expression which is selfadjoint[1] or Hermitian. Thus such an expression describes an invariant under a Hermitian transformation. Felix Klein's Erlangen program attempted to identify such invariants under a group of transformations. Noether's theorem identified the conditions under which the Poincaré group of transformations (what is now called a gauge group) for general relativity define conservation laws. The relationship of these invariants (the symmetries under a group of transformations) and what are now called conserved currents, depends on a variational principle, or action principle. Noether's papers made the requirements for the conservation laws precise. Noether's theorem remains right in line with current developments in physics to this day.
Beginning in 1900, Max Planck, Albert Einstein, Niels Bohr, and others developed quantum theories to explain various anomalous experimental results,e.g. the photoelectric effect and the black body spectrum, by introducing discrete energy levels and in 1925 Wolfgang Pauli elucidated the Pauli exclusion principle and introduced the notion of quantized spin and fermions. In that year Erwin Schrödinger formulated wave mechanics, which provided a consistent mathematical method for describing a wide variety of physical situations such as the particle in a box and the quantum harmonic oscillator which he solved for the first time. Werner Heisenberg described, also in 1925, an alternative mathematical method, called matrix mechanics, which proved to be equivalent to wave mechanics. In 1928 Paul Dirac produced a relativistic formulation built on Heinsberg's matrix mechanics, and predicted the existence of the positron and founded quantum electrodynamics.
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. Quantum mechanics also provided the theoretical tools for understanding condensed matter physics, which studies the physical behavior of solids and liquids, including phenomena such as electrical conductivity in crystal structures. 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. Much of the behavior of solids was elucidated within a few years with the discovery of the Fermi surface which was based on the idea of the Pauli exclusion principle applied to systems having many electrons. The understanding of the transport properties in semiconductors as described in William Shockley's Electrons and holes in semiconductors, with applications to transistor electronics enabled the electronic revolution of the twentieth century through the development of the ubiquitous, ultracheap transistor.
In 1929, Edwin Hubble published his discovery that the speed at which galaxies recede positively correlates with their distance. This is the basis for understanding that the universe is expanding. Thus, the universe must have been smaller and therefore hotter in the past. In 1933 Karl Jansky at Bell Labs discovered the radio emission from the Milky Way, and thereby starting the science of radio astronomy.By the 1940s, researchers like George Gamow proposed the Big Bang theory[2], evidence for which was discovered in 1964[3]; Enrico Fermi and Fred Hoyle were among the doubters in the 1940s and 1950s. Hoyle had dubbed Gamow's theory the Big Bang in order to debunk it. Today, it is one of the principal results of cosmology.
In 1934, the Italian physicist Enrico Fermi had discovered strange results when bombarding uranium with neutrons, which he believed at first to have created transuranic elements. In 1939, it was discovered by the chemist Otto Hahn and the physicist Lise Meitner that what was actually happening was the process of nuclear fission, whereby the nucleus of uranium was actually being split into two pieces, releasing a considerable amount of energy in the process. At this point it became clear to a number of scientists independently that this process could potentially be harnessed to produce massive amount of energy, either as a civilian power source or as a weapon. Leó Szilárd actually filed a patent on the idea of a nuclear chain reaction in 1934. In America, a team led by Fermi and Szilárd achieved the first manmade nuclear chain reaction in 1942 in the world's first nuclear reactor, and in 1945 the world's first nuclear explosive was detonated at Trinity Site, north of Alamogordo, New Mexico. After the war, central governments would become the primary sponsors of physics. The scientific leader of the Allied project, theoretical physicist Robert Oppenheimer, noted the change of the imagined role of the physicist when he noted in a speech that:
 "In some sort of crude sense, which no vulgarity, no humor, no overstatement can quite extinguish, the physicists have known sin, and this is a knowledge which they cannot lose."
Though the process had begun with the invention of the cyclotron by Ernest O. Lawrence in the 1930s, nuclear physics in the postwar period entered into a phase of what historians have called "Big Science", requiring costly huge accelerators and particle detectors, and large collaborative laboratories to test open new frontiers. The primary patron of physics became central governments, who recognized that the support of "basic" research could sometimes lead to technologies useful to both military and industrial applications. Toward the end of the twentieth century, a European collaboration of 20 nations, CERN, became the largest particle physics laboratory in the world. Another "big science" was the science of ionized gases, plasma, which had begun with Crookes tubes late in the 19th century. Large international collaborations over the last half of the twentieth century embarked on a long range effort to produce commercial amounts of electricity through fusion power, which remains a distant goal.
Further understanding of the physics of metals, semiconductors and insulators led a team of three men at Bell labs, William Shockley, Walter Brattain and John Bardeen in 1947 to the first transistor and then to many important variations, especially the bipolar junction transistor. Further developments in the fabrication and miniaturization of integrated circuits in the years to come produced fast, compact computers that came to revolutionize the way physics was donesimulations and complex mathematical calculations became possible that were undreamed of even a few decades previous. The discovery of nuclear magnetic resonance in 1946 led to many new methods for examining the structures of molecules and became a very widely used tool in analytical chemistry, and it gave rise to an important medical imaging technique, magnetic resonance imaging.
Starting in 1960 the military establishment of the United States began using atomic clocks to construct the global positioning system which in 1984 achieved its full configuration of 24 satellites in low earth orbits. This came to have many important civilian and scientific uses as well. Superconductivity, discovered in 1911 by Kamerlingh Onnes, was shown to be a quantum effect and was satisfactorily explained in 1957 by Bardeen, Cooper, and Schrieffer. A family of high temperature superconductors, based on cuprate perovskite, were discovered in 1986, and their understanding remains one of the major outstanding challenges for condensed matter theorists. Quantum field theory was formulated in order to extend quantum mechanics to be consistent with special relativity. It achieved its modern form in the late 1940s with work by Richard Feynman, Julian Schwinger, SinItiro Tomonaga, and Freeman Dyson. This provided the framework for modern particle physics, which studies fundamental forces and elementary particles. In 1954, Yang Chen Ning and Robert Mills developed a class of gauge theories, which provided the framework for the Standard Model. This was largely completed in the 1970s and successfully describes almost all elementary particles observed to date.
In 1974 Stephen Hawking discovered the spectrum of radiation emanating during the collapse of matter into black holes. These mysterious objects became objects of intense interest to astrophysicists and even the general public in the latter part of the twentieth century. Attempts to unify quantum mechanics and general relativity made significant progress during the 1990s. At the close of the century, a Theory of everything was still not in hand, but some of its characteristics were taking shape. String theory, loop quantum gravity and black hole thermodynamics all predicted quantized spacetime on the Planck scale. A number of new efforts to understand the physical world arose in the last half of the twentiety century that generated widespread interest: fractals and scaling, selforganized criticality, complexity and chaos, power laws and noise, networks, nonequilibrium thermodynamics, sandpiles, nanotechnology, cellular automata and the anthropic principle were only a few of these important topics.
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 Mtheory, a popular theory in highenergy physics for which no practical experimental test has ever been devised.
Theories lifecycles
Scientific theories sometimes end up being discredited. 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.
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".
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 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. The current leading candidates are Mtheory, superstring theory and loop quantum gravity. Many astronomical and cosmological phenomena have yet to be satisfactorily explained, 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 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 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 has also increased, as exemplified by the study of turbulence in aerodynamics or the observation of pattern formation in biological systems.
External articles and references
 arXiv Cornell University Archive
 Rama Corporation Engineering Guide  A free 35page PDF displaying power and wattage requirements, energy calculations and material properties is made available by this 50year old electrical heater manufacturer. (Thanks RLP)
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