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Quantum Graphity
Induced gravity (or emergent gravity) is an idea in quantum gravity that spacetime curvature and its dynamics emerge as a mean field approximation of underlying microscopic degrees of freedom, similar to the fluid mechanics approximation of Bose–Einstein condensates. The concept was originally proposed by Andrei Sakharov in 1967. Overview Sakharov observed that many condensed matter systems give rise to emergent phenomena that are analogous to general relativity. For example, crystal defects can look like curvature and torsion in an Einstein–Cartan spacetime. This allows one to create a theory of gravity with torsion from a world crystal model of spacetime in which the lattice spacing is of the order of a Planck length. Sakharov's idea was to start with an arbitrary background pseudo-Riemannian manifold (in modern treatments, possibly with torsion) and introduce quantum fields (matter) on it but not introduce any gravitational dynamics explicitly. This gives rise to an e ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Quantum Gravity
Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics; it deals with environments in which neither gravitational nor quantum effects can be ignored, such as in the vicinity of black holes or similar compact astrophysical objects, such as neutron stars. Three of the four fundamental forces of physics are described within the framework of quantum mechanics and quantum field theory. The current understanding of the fourth force, gravity, is based on Albert Einstein's general theory of relativity, which is formulated within the entirely different framework of classical physics. However, that description is incomplete: describing the gravitational field of a black hole in the general theory of relativity leads physical quantities, such as the spacetime curvature, to diverge at the center of the black hole. This signals the breakdown of the general theory of relativity and the need for a theory that goes b ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Effective Action
In quantum field theory, the quantum effective action is a modified expression for the classical action taking into account quantum corrections while ensuring that the principle of least action applies, meaning that extremizing the effective action yields the equations of motion for the vacuum expectation values of the quantum fields. The effective action also acts as a generating functional for one-particle irreducible correlation functions. The potential component of the effective action is called the effective potential, with the expectation value of the true vacuum being the minimum of this potential rather than the classical potential, making it important for studying spontaneous symmetry breaking. It was first defined perturbatively by Jeffrey Goldstone and Steven Weinberg in 1962, while the non-perturbative definition was introduced by Bryce DeWitt in 1963 and independently by Giovanni Jona-Lasinio in 1964. The article describes the effective action for a single scalar ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Entropic Gravity
Entropic gravity, also known as emergent gravity, is a theory in modern physics that describes gravity as an ''entropic force''—a force with macro-scale homogeneity but which is subject to quantum-level disorder—and not a fundamental interaction. The theory, based on string theory, black hole physics, and quantum information theory, describes gravity as an ''emergent'' phenomenon that springs from the quantum entanglement of small bits of spacetime information. As such, entropic gravity is said to abide by the second law of thermodynamics under which the entropy of a physical system tends to increase over time. The theory has been controversial within the physics community but has sparked research and experiments to test its validity. Significance At its simplest, the theory holds that when gravity becomes vanishingly weak—levels seen only at interstellar distances—it diverges from its classically understood nature and its strength begins to decay ''linearly with dist ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Entropy
Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynamics, where it was first recognized, to the microscopic description of nature in statistical physics, and to the principles of information theory. It has found far-ranging applications in chemistry and physics, in biological systems and their relation to life, in cosmology, economics, sociology, weather science, climate change, and information systems including the transmission of information in telecommunication. The thermodynamic concept was referred to by Scottish scientist and engineer William Rankine in 1850 with the names ''thermodynamic function'' and ''heat-potential''. In 1865, German physicist Rudolf Clausius, one of the leading founders of the field of thermodynamics, defined it as the quotient of an infinitesimal amount of hea ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Erik Verlinde
Erik Peter Verlinde (; born 21 January 1962) is a Dutch theoretical physicist and string theorist. He is the identical twin brother of physicist Herman Verlinde. The Verlinde formula, which is important in conformal field theory and topological field theory, is named after him. His research deals with string theory, gravity, black holes and cosmology. Currently, he works at the Institute for Theoretical Physics at the University of Amsterdam. At a symposium at the Dutch Spinoza-institute on December 8, 2009 he introduced a theory of entropic gravity. In this theory, gravity exists because of a difference in concentration of information in the empty space between two masses and its surroundings; he also extrapolates this to general relativity and quantum mechanics. He said in an interview with the newspaper '' de Volkskrant'', "On the smallest level Newton's laws don't apply, but they do for apples and planets. You can compare this to the pressure of a gas. Molecules themse ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Thanu Padmanabhan
Thanu Padmanabhan (10 March 1957 – 17 September 2021) was an Indian theoretical physicist and cosmologist whose research spanned a wide variety of topics in gravitation, structure formation in the universe and quantum gravity. He published nearly 300 papers and reviews in international journals and ten books in these areas. He made several contributions related to the analysis and modelling of dark energy in the universe and the interpretation of gravity as an emergent phenomenon. He was a Distinguished Professor at the Inter-University Centre for Astronomy and Astrophysics (IUCAA) at Pune, India. Life and career Born to Thanu Iyer and Lakshmi on 10 March 1957 in Thiruvananthapuram (then Trivandrum), Padmanabhan attended school there. He earned his B.Sc. (1977) and M.Sc. (1979) in Physics from the University College Thiruvananthapuram, part of Kerala University. He published his first research paper (on general relativity) when he was still a B.Sc. student, at the age of 20 ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Rindler Coordinates
In relativistic physics, the coordinates of a ''hyperbolically accelerated reference frame'' constitute an important and useful coordinate chart representing part of flat Minkowski spacetime. In special relativity, a uniformly accelerating particle undergoes hyperbolic motion, for which a uniformly accelerating frame of reference in which it is at rest can be chosen as its proper reference frame. The phenomena in this hyperbolically accelerated frame can be compared to effects arising in a homogeneous gravitational field. For general overview of accelerations in flat spacetime, see Acceleration (special relativity) and Proper reference frame (flat spacetime). In this article, the speed of light is defined by , the inertial coordinates are , and the hyperbolic coordinates are . These hyperbolic coordinates can be separated into two main variants depending on the accelerated observer's position: If the observer is located at time at position (with as the constant proper acceler ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Einstein Field Equations
In the general theory of relativity, the Einstein field equations (EFE; also known as Einstein's equations) relate the geometry of spacetime to the distribution of matter within it. The equations were published by Einstein in 1915 in the form of a tensor equation which related the local ' (expressed by the Einstein tensor) with the local energy, momentum and stress within that spacetime (expressed by the stress–energy tensor). Analogously to the way that electromagnetic fields are related to the distribution of charges and currents via Maxwell's equations, the EFE relate the spacetime geometry to the distribution of mass–energy, momentum and stress, that is, they determine the metric tensor of spacetime for a given arrangement of stress–energy–momentum in the spacetime. The relationship between the metric tensor and the Einstein tensor allows the EFE to be written as a set of nonlinear partial differential equations when used in this way. The solutions of the EFE are t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Theodore Jacobson
Theodore A. "Ted" Jacobson (born November 27, 1954) is an American theoretical physicist. He is known for his work on the connection between gravity and thermodynamics. In particular, in 1995 Jacobson proved that the Einstein field equations describing relativistic gravity can be derived from thermodynamic considerations.Ted Jacobson, "Thermodynamics of Spacetime: The Einstein Equation of State", ''Physical Review Letters'', Vol. 75, Issue 7 (August 14, 1995), pp. 1260-1263, , . Also at , April 4, 1995. Also availablhereanhere Additionally available aan entryin the Gravity Research Foundation's 1995 essay competitionMirror linkLee Smolin, '' Three Roads to Quantum Gravity'' (New York, N.Y.: Basic Books, 2002), pp. 173 and 175, , . Jacobson is professor of physics at the University of Maryland's Center for Fundamental Physics. His current research focuses on the dark energy problem and cosmic expansion.Bob Swarup"Much Ado About Nothing: Does the vacuum regenerate itself to fill t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mark Van Raamsdonk
Mark Van Raamsdonk is a professor at the Department of Physics and Astronomy at the University of British Columbia since 2002. Before that, he was a postdoc at Stanford University from 2000 until 2002 and studied as a graduate student at Princeton University from 1995 until 2000 when he received his PhD under the supervision of Washington Taylor. Before that, he did a combined mathematics/physics undergraduate degree at University of British Columbia where he graduated with what is believed to be the highest GPA in the University's prior history. In 2009 Mark Van Raamsdonk started to work on the relationship between quantum mechanics and gravity during his first sabbatical year. He published his results "Building up spacetime with quantum entanglement" as an essay in 2010, which won the first prize of the annual essay contest run by the Gravity Research Foundation. Van Raamsdonk is a member of the "It from Qubit" collaboration, which was formed in 2015. Mark Van Raamsdonk plays t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
AdS/CFT Correspondence
In theoretical physics, the anti-de Sitter/conformal field theory correspondence, sometimes called Maldacena duality or gauge/gravity duality, is a conjectured relationship between two kinds of physical theories. On one side are anti-de Sitter spaces (AdS) which are used in theories of quantum gravity, formulated in terms of string theory or M-theory. On the other side of the correspondence are conformal field theories (CFT) which are quantum field theories, including theories similar to the Yang–Mills theories that describe elementary particles. The duality represents a major advance in the understanding of string theory and quantum gravity.de Haro et al. 2013, p. 2 This is because it provides a non-perturbative formulation of string theory with certain boundary conditions and because it is the most successful realization of the holographic principle, an idea in quantum gravity originally proposed by Gerard 't Hooft and promoted by Leonard Susskind. It also provides a powerf ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Weinberg–Witten Theorem
In theoretical physics, the Weinberg–Witten (WW) theorem, proved by Steven Weinberg and Edward Witten, states that massless particles (either composite or elementary) with spin ''j'' > 1/2 cannot carry a Lorentz-covariant current, while massless particles with spin ''j'' > 1 cannot carry a Lorentz-covariant stress-energy. The theorem is usually interpreted to mean that the graviton (''j'' = 2) cannot be a composite particle in a relativistic quantum field theory. Background During the 1980s, preon theories, technicolor and the like were very popular and some people speculated that gravity might be an emergent phenomenon or that gluons might be composite. Weinberg and Witten, on the other hand, developed a no-go theorem that excludes, under very general assumptions, the hypothetical composite and emergent theories. Decades later new theories of emergent gravity are proposed and some high-energy physicists are still using this theorem to try and refute such theories. Becaus ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
