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General relativity

General relativity
General relativity, or the general theory of relativity, is the geometric theory of gravitation published by Albert Einstein in 1916[1] and the current description of gravitation in modern physics. General relativity generalizes special relativity and Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time, or spacetime. In particular, the curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present. The relation is specified by the Einstein field equations, a system of partial differential equations. Some predictions of general relativity differ significantly from those of classical physics, especially concerning the passage of time, the geometry of space, the motion of bodies in free fall, and the propagation of light. Einstein's theory has important astrophysical implications. History[edit] Albert Einstein developed the theories of special and general relativity. Related:  physics

Physics World reveals its top 10 breakthroughs for 2011 The two physics stories that dominated the news in 2011 were questions rather than solid scientific results, namely "Do neutrinos travel faster than light?" and "Has the Higgs boson been found?". However, there have also been some fantastic bona fide research discoveries over the last 12 months, which made it difficult to decide on the Physics World 2011 Breakthrough of the Year. But after much debate among the Physics World editorial team, this year's honour goes to Aephraim Steinberg and colleagues from the University of Toronto in Canada for their experimental work on the fundamentals of quantum mechanics. Using an emerging technique called "weak measurement", the team is the first to track the average paths of single photons passing through a Young's double-slit experiment – something that Steinberg says physicists had been "brainwashed" into thinking is impossible. We have also awarded nine runners-up (see below). 1st place: Shifting the morals of quantum measurement

Chaos theory A double rod pendulum animation showing chaotic behavior. Starting the pendulum from a slightly different initial condition would result in a completely different trajectory. The double rod pendulum is one of the simplest dynamical systems that has chaotic solutions. Chaos: When the present determines the future, but the approximate present does not approximately determine the future. Chaotic behavior can be observed in many natural systems, such as weather and climate.[6][7] This behavior can be studied through analysis of a chaotic mathematical model, or through analytical techniques such as recurrence plots and Poincaré maps. Introduction[edit] Chaos theory concerns deterministic systems whose behavior can in principle be predicted. Chaotic dynamics[edit] The map defined by x → 4 x (1 – x) and y → x + y mod 1 displays sensitivity to initial conditions. In common usage, "chaos" means "a state of disorder".[9] However, in chaos theory, the term is defined more precisely. where , and , is: .

Quantum mechanics Description of physical properties at the atomic and subatomic scale Quantum mechanics is a fundamental theory in physics that describes the behavior of nature at and below the scale of atoms.[2]: 1.1 It is the foundation of all quantum physics including quantum chemistry, quantum field theory, quantum technology, and quantum information science. Classical physics, the collection of theories that existed before the advent of quantum mechanics, describes many aspects of nature at an ordinary (macroscopic) scale, but is not sufficient for describing them at small (atomic and subatomic) scales. Most theories in classical physics can be derived from quantum mechanics as an approximation valid at large (macroscopic) scale.[3] Overview and fundamental concepts Quantum mechanics allows the calculation of properties and behaviour of physical systems. A fundamental feature of the theory is that it usually cannot predict with certainty what will happen, but only give probabilities. . and , where Here

Quantum Diaries (Thoughts on work and life from particle physicists from around the world.) Schrödinger's cat Schrödinger's cat: a cat, a flask of poison, and a radioactive source are placed in a sealed box. If an internal monitor detects radioactivity (i.e. a single atom decaying), the flask is shattered, releasing the poison that kills the cat. The Copenhagen interpretation of quantum mechanics implies that after a while, the cat is simultaneously alive and dead. Schrödinger's cat is a thought experiment, sometimes described as a paradox, devised by Austrian physicist Erwin Schrödinger in 1935.[1] It illustrates what he saw as the problem of the Copenhagen interpretation of quantum mechanics applied to everyday objects. Origin and motivation[edit] Real-size cat figure in the garden of Huttenstrasse 9, Zurich, where Erwin Schrödinger lived 1921 – 1926. The thought experiment[edit] Schrödinger wrote:[1][10] One can even set up quite ridiculous cases. You are the only contemporary physicist, besides Laue, who sees that one cannot get around the assumption of reality, if only one is honest.

Theory of everything A theory of everything (ToE) or final theory, ultimate theory, or master theory is a hypothetical single, all-encompassing, coherent theoretical framework of physics that fully explains and links together all physical aspects of the universe.[1]:6 Finding a ToE is one of the major unsolved problems in physics. Over the past few centuries, two theoretical frameworks have been developed that, as a whole, most closely resemble a ToE. The two theories upon which all modern physics rests are general relativity (GR) and quantum field theory (QFT). GR is a theoretical framework that only focuses on the force of gravity for understanding the universe in regions of both large-scale and high-mass: stars, galaxies, clusters of galaxies, etc. Through years of research, physicists have experimentally confirmed with tremendous accuracy virtually every prediction made by these two theories when in their appropriate domains of applicability. Historical antecedents[edit] Modern physics[edit] [edit]

Gravitational lens A light source passes behind a gravitational lens (point mass placed in the center of the image). The aqua circle is the light source as it would be seen if there was no lens, white spots are the multiple images (or Einstein ring) of the source. A gravitational lens is a distribution of matter (such as a cluster of galaxies) between a distant light source and an observer, that is capable of bending the light from the source as the light travels towards the observer. This effect is known as gravitational lensing, and the amount of bending is one of the predictions of Albert Einstein's general theory of relativity.[1][2] (Classical physics also predicts the bending of light, but only half that predicted by general relativity.[3]) Although Einstein made unpublished calculations on the subject in 1912,[4] Orest Khvolson (1924)[5] and Frantisek Link (1936)[citation needed] are generally credited with being the first to discuss the effect in print. Description[edit] 1. 2. 3. History[edit] Notes

Bohr–Einstein debates The Bohr–Einstein debates were a series of public disputes about quantum mechanics between Albert Einstein and Niels Bohr, who were two of its founders. Their debates are remembered because of their importance to the philosophy of science. An account of the debates has been written by Bohr in an article titled "Discussions with Einstein on Epistemological Problems in Atomic Physics".[1] Despite their differences of opinion regarding quantum mechanics, Bohr and Einstein had a mutual admiration that was to last the rest of their lives.[2] Pre-revolutionary debates[edit] Einstein was the first physicist to say that Planck's discovery of the quantum (h) would require a rewriting of physics. 1913 brought the Bohr model of the hydrogen atom, which made use of the quantum to explain the atomic spectrum. The quantum revolution[edit] Einstein rejected this interpretation. Post-revolution: First stage[edit] Figure A. Einstein's slit. Figure C. Figure D. . which satisfies the relation: . .

Particle physics Subatomic particles[edit] Modern particle physics research is focused on subatomic particles, including atomic constituents such as electrons, protons, and neutrons (protons and neutrons are composite particles called baryons, made of quarks), produced by radioactive and scattering processes, such as photons, neutrinos, and muons, as well as a wide range of exotic particles. Dynamics of particles is also governed by quantum mechanics; they exhibit wave–particle duality, displaying particle-like behavior under certain experimental conditions and wave-like behavior in others. In more technical terms, they are described by quantum state vectors in a Hilbert space, which is also treated in quantum field theory. Following the convention of particle physicists, the term elementary particles is applied to those particles that are, according to current understanding, presumed to be indivisible and not composed of other particles.[1] History[edit] Standard Model[edit] Theory[edit] Future[edit]

Gravitational microlensing Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects ranging from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers can only detect bright objects that emit lots of light (stars) or large objects that block background light (clouds of gas and dust). These objects make up only a tiny fraction of the mass of a galaxy. Microlensing allows the study of objects that emit little or no light. When a distant star or quasar gets sufficiently aligned with a massive compact foreground object, the bending of light due to its gravitational field, as discussed by Einstein in 1915, leads to two distorted unresolved images resulting in an observable magnification. Since microlensing observations do not rely on radiation received from the lens object, this effect therefore allows astronomers to study massive objects no matter how faint. How it works[edit] History[edit] . . . . .

Rapid eye movement sleep Rapid eye movement (REM) sleep is a stage of sleep characterized by the rapid and random movement of the eyes. Rapid eye movement sleep is classified into two categories: tonic and phasic.[1] It was identified and defined by Nathaniel Kleitman and his student Eugene Aserinsky in 1953. Criteria for REM sleep includes rapid eye movement, low muscle tone and a rapid, low-voltage EEG; these features are easily discernible in a polysomnogram,[2] the sleep study typically done for patients with suspected sleep disorders.[3] REM sleep typically occupies 20–25% of total sleep, about 90–120 minutes of a night's sleep. REM sleep is physiologically different from the other phases of sleep, which are collectively referred to as non-REM sleep (NREM sleep). Physiology[edit] Polysomnographic record of REM Sleep. Theories about the function(s) of REM sleep[edit] While the function of REM sleep is not well understood, several theories have been proposed. Memory-related theories[edit] Shift of gaze[edit]