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Physics (from the Greek, φυσικός (phusikos), "natural", and φύσις (phusis), "nature") is the Science of the natural world in the broadest sense, dealing with the fundamental constituents of the Universe, the forces they exert on one another, and the results produced by these forces. Physicists study a wide range of physical phenomena spanning all length scales: from the sub-atomic particles from which all ordinary (i.e., baryonic) Matter is made (Particle physics) to the behavior of the material Universe as a whole (cosmology).
Physics discoveries find applications throughout the other natural sciences, because it studies the basic constituents of the natural world. Some of the phenomena studied in physics, such as the Conservation of energy, are common to all material systems. These are often referred to as laws of physics. Physics is sometimes said to be the "fundamental science", because each of the other natural sciences (Biology, Chemistry, Geology, etc.) deals with particular types of material systems that obey the laws of physics. For example, chemistry is the science of molecules and the chemicals that they form in the bulk. The properties of a chemical 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 where physical laws can be precisely formulated and their predictions quantified. Physical theories are almost invariably expressed using mathematical relations, and the mathematics involved is generally more complicated than in the other sciences. The difference between physics and mathematics is that physics is ultimately concerned with descriptions of the material world, whereas mathematics is concerned with abstract patterns that need not have any bearing on it. The distinction, however, is not always clear-cut. There is a large area of research intermediate between physics and mathematics, known as Mathematical physics, devoted to developing the mathematical structure of physical theories.
While physics has a remarkably broad purview, it attempts only to describe those aspects of the world that can be dealt with by the Scientific method. It is not concerned with issues that cannot be verified even in principle, such as those studied in Metaphysics.
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.
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Roughly speaking, theorists seek to develop theories that can 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. 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.
The works of Ptolemy (Astronomy) and Aristotle were also found to not always match everyday observations.
The willingness to question previously held truths and search for new answers resulted in a period of major scientific advancements, now known as the Scientific revolution. Its origins can be found in the European re-discovery of Aristotle in the twelfth and thirteenth centuries. This period culminated with the publication of the Philosophiae Naturalis Principia Mathematica in 1687 by 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 the book De Revolutionibus was brought to the Polish astronomer Nicolaus Copernicus, who had written it about a dozen years earlier.
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, Joseph-Louis de Lagrange, 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, 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 field. These 20 equations were later reduced, using Vector calculus, to a set of four equations by Oliver Heaviside. [image] [image] 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 his theory of Special relativity. 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[:Physics}}#endnote_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 argued that astronomical observations demonstrate that the universe is expanding.
From the late 17th century onwards, Thermodynamics was developed by 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, 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 in nature, which proved to be the opening argument in the edifice that would become Quantum mechanics. Beginning in 1900, Planck, Einstein, Niels Bohr, and others developed Quantum theories to explain various anomalous experimental results by introducing discrete energy levels. In 1925, Heisenberg and 1926, Schrödinger and Paul Dirac formulated Quantum mechanics, which 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 Erwin Schrödinger, Werner 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. [image] 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[:Physics}}#endnote_parity]. In 1954, Yang and Robert Mills then developed a class of gauge theories[:Physics}}#endnote_gauge theories] which provided the framework for understanding the nuclear forces. 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 behavior of solids and liquids, including phenomena such as crystal structures, semiconductivity, and superconductivity. The pioneers of condensed matter physics include 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 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 have 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 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.
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 prophesized:
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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