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# General relativity

General relativity (GR, also known as the general theory of relativity or GTR) is the geometric theory of gravitation published by Albert Einstein in 1915 and the current description of gravitation in modern physics. [1]

## Abstract index notation

Abstract index notation is a mathematical notation for tensors and spinors that uses indices to indicate their types, rather than their components in a particular basis.

## Acceleration

In physics, acceleration is the rate of change of velocity of an object with respect to time.

## Accretion (astrophysics)

In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter, typically gaseous matter, in an accretion disk.

## Active galactic nucleus

An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much higher than normal luminosity over at least some portion—and possibly all—of the electromagnetic spectrum, with characteristics indicating that the excess luminosity is not produced by stars.

The ADM formalism (named for its authors Richard Arnowitt, Stanley Deser and Charles W. Misner) is a Hamiltonian formulation of general relativity that plays an important role in canonical quantum gravity and numerical relativity.

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.

## Albert Einstein

Albert Einstein (14 March 1879 – 18 April 1955) was a German-born theoretical physicist who developed the theory of relativity, one of the two pillars of modern physics (alongside quantum mechanics).

## Alcubierre drive

The Alcubierre drive or Alcubierre warp drive (or Alcubierre metric, referring to metric tensor) is a speculative idea based on a solution of Einstein's field equations in general relativity as proposed by Mexican theoretical physicist Miguel Alcubierre, by which a spacecraft could achieve apparent faster-than-light travel if a configurable energy-density field lower than that of vacuum (that is, negative mass) could be created.

## Alexander Friedmann

Alexander Alexandrovich Friedmann (also spelled Friedman or Fridman; Алекса́ндр Алекса́ндрович Фри́дман) (June 16, 1888 – September 16, 1925) was a Russian and Soviet physicist and mathematician.

## Alternatives to general relativity

Alternatives to general relativity are physical theories that attempt to describe the phenomenon of gravitation in competition to Einstein's theory of general relativity.

## Angular momentum

In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational equivalent of linear momentum.

## Anisotropy

Anisotropy, is the property of being directionally dependent, which implies different properties in different directions, as opposed to isotropy.

## Anti-de Sitter space

In mathematics and physics, n-dimensional anti-de Sitter space (AdSn) is a maximally symmetric Lorentzian manifold with constant negative scalar curvature.

## Apparent horizon

In general relativity, an apparent horizon is a surface that is the boundary between light rays that are directed outwards and moving outwards, and those directed outward but moving inward.

## Apsidal precession

In celestial mechanics, apsidal precession or orbital precession is the precession (rotation) of the orbit of a celestial body.

## Apsis

An apsis (ἁψίς; plural apsides, Greek: ἁψῖδες) is an extreme point in the orbit of an object.

## Arthur Eddington

Sir Arthur Stanley Eddington (28 December 1882 – 22 November 1944) was an English astronomer, physicist, and mathematician of the early 20th century who did his greatest work in astrophysics.

## Ashtekar variables

In the ADM formulation of general relativity, spacetime is split into spatial slices and a time axis.

## Astrophysical jet

An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as an extended beam along the axis of rotation.

## Astrophysics

Astrophysics is the branch of astronomy that employs the principles of physics and chemistry "to ascertain the nature of the astronomical objects, rather than their positions or motions in space".

## Atomic clock

An atomic clock is a clock device that uses an electron transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element.

## Background independence

Background independence is a condition in theoretical physics, that requires the defining equations of a theory to be independent of the actual shape of the spacetime and the value of various fields within the spacetime.

## Baryon

A baryon is a composite subatomic particle made up of three quarks (a triquark, as distinct from mesons, which are composed of one quark and one antiquark).

## Big Bang

The Big Bang theory is the prevailing cosmological model for the universe from the earliest known periods through its subsequent large-scale evolution.

## Big Bang (book)

Big Bang: The most important scientific discovery of all time and why you need to know about it is a book written by Simon Singh and published in 2004 by Fourth Estate.

## Big Bang nucleosynthesis

In physical cosmology, Big Bang nucleosynthesis (abbreviated BBN, also known as primordial nucleosynthesis, arch(a)eonucleosynthesis, archonucleosynthesis, protonucleosynthesis and pal(a)eonucleosynthesis) refers to the production of nuclei other than those of the lightest isotope of hydrogen (hydrogen-1, 1H, having a single proton as a nucleus) during the early phases of the Universe.

## Big Crunch

The Big Crunch is one possible scenario for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately causing the cosmic scale factor to reach zero or causing a reformation of the universe starting with another Big Bang.

## Binary black hole

A binary black hole (BBH) is a system consisting of two black holes in close orbit around each other.

## Binary pulsar

A binary pulsar is a pulsar with a binary companion, often a white dwarf or neutron star.

## Binary system

A binary system is a system of two astronomical bodies which are close enough that their gravitational attraction causes them to orbit each other around a barycenter (also see animated examples).

## BKL singularity

A Belinsky-Khalatnikov-Lifshitz (BKL) singularity is a model of the dynamic evolution of the Universe near the initial singularity, described by an anisotropic, homogeneous, chaotic solution to Einstein's field equations of gravitation.

## Black hole thermodynamics

In physics, black hole thermodynamics is the area of study that seeks to reconcile the laws of thermodynamics with the existence of black-hole event horizons.

## Blueshift

A blueshift is any decrease in wavelength, with a corresponding increase in frequency, of an electromagnetic wave; the opposite effect is referred to as redshift.

## Brans–Dicke theory

In theoretical physics, the Brans–Dicke theory of gravitation (sometimes called the Jordan–Brans–Dicke theory) is a theoretical framework to explain gravitation.

## Canonical quantization

In physics, canonical quantization is a procedure for quantizing a classical theory, while attempting to preserve the formal structure, such as symmetries, of the classical theory, to the greatest extent possible.

## Causal dynamical triangulation

Causal dynamical triangulation (abbreviated as CDT) theorized by Renate Loll, Jan Ambjørn and Jerzy Jurkiewicz, and popularized by Fotini Markopoulou and Lee Smolin, is an approach to quantum gravity that like loop quantum gravity is background independent.

## Causal sets

The causal sets program is an approach to quantum gravity.

## Caustic (mathematics)

In differential geometry and geometric optics, a caustic is the envelope of rays either reflected or refracted by a manifold.

## Center of mass

In physics, the center of mass of a distribution of mass in space is the unique point where the weighted relative position of the distributed mass sums to zero, or the point where if a force is applied it moves in the direction of the force without rotating.

## Center of mass (relativistic)

In physics, relativistic center of mass refers to the mathematical and physical concepts that define the center of mass of a system of particles in relativistic mechanics and relativistic quantum mechanics.

## Charge (physics)

In physics, a charge may refer to one of many different quantities, such as the electric charge in electromagnetism or the color charge in quantum chromodynamics.

## Charged black hole

A charged black hole is a black hole that possesses electric charge.

## Classical limit

The classical limit or correspondence limit is the ability of a physical theory to approximate or "recover" classical mechanics when considered over special values of its parameters.

## Classical mechanics

Classical mechanics describes the motion of macroscopic objects, from projectiles to parts of machinery, and astronomical objects, such as spacecraft, planets, stars and galaxies.

## Classical physics

Classical physics refers to theories of physics that predate modern, more complete, or more widely applicable theories.

## Closed timelike curve

In mathematical physics, a closed timelike curve (CTC) is a world line in a Lorentzian manifold, of a material particle in spacetime that is "closed", returning to its starting point.

## Compactification (mathematics)

In mathematics, in general topology, compactification is the process or result of making a topological space into a compact space.

## Compactification (physics)

In physics, compactification means changing a theory with respect to one of its space-time dimensions.

## Conformal geometry

In mathematics, conformal geometry is the study of the set of angle-preserving (conformal) transformations on a space.

## Connection (mathematics)

In geometry, the notion of a connection makes precise the idea of transporting data along a curve or family of curves in a parallel and consistent manner.

## Conservation of energy

In physics, the law of conservation of energy states that the total energy of an isolated system remains constant, it is said to be ''conserved'' over time.

## Coordinate conditions

In general relativity, the laws of physics can be expressed in a generally covariant form.

## Coordinate system

In geometry, a coordinate system is a system which uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space.

## Cosmic Background Explorer

The Cosmic Background Explorer (COBE), also referred to as Explorer 66, was a satellite dedicated to cosmology, which operated from 1989 to 1993.

## Cosmic background radiation

Cosmic background radiation is electromagnetic radiation from the big bang.

## Cosmic censorship hypothesis

The weak and the strong cosmic censorship hypotheses are two mathematical conjectures about the structure of singularities arising in general relativity.

## Cosmic distance ladder

The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects.

## Cosmic string

Cosmic strings are hypothetical 1-dimensional topological defects which may have formed during a symmetry breaking phase transition in the early universe when the topology of the vacuum manifold associated to this symmetry breaking was not simply connected.

## Cosmological constant

In cosmology, the cosmological constant (usually denoted by the Greek capital letter lambda: Λ) is the value of the energy density of the vacuum of space.

## Covariant derivative

In mathematics, the covariant derivative is a way of specifying a derivative along tangent vectors of a manifold.

## Curvature

In mathematics, curvature is any of a number of loosely related concepts in different areas of geometry.

## Curvature form

In differential geometry, the curvature form describes the curvature of a connection on a principal bundle.

## Dark energy

In physical cosmology and astronomy, dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe.

## Dark matter

Dark matter is a theorized form of matter that is thought to account for approximately 80% of the matter in the universe, and about a quarter of its total energy density.

## De Sitter universe

A de Sitter universe is a cosmological solution to the Einstein field equations of general relativity, named after Willem de Sitter.

## Density

The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume.

## Derivations of the Lorentz transformations

There are many ways to derive the Lorentz transformations utilizing a variety of physical principles, ranging from Maxwell's equations to Einstein's postulates of special relativity, and mathematical tools, spanning from elementary algebra and hyperbolic functions, to linear algebra and group theory.

## Differential geometry

Differential geometry is a mathematical discipline that uses the techniques of differential calculus, integral calculus, linear algebra and multilinear algebra to study problems in geometry.

## Divergence

In vector calculus, divergence is a vector operator that produces a scalar field, giving the quantity of a vector field's source at each point.

## Double star

In observational astronomy, a double star or visual double is a pair of stars that appear close to each other in the sky as seen from Earth when viewed through an optical telescope.

## Eötvös experiment

The Eötvös experiment was a famous physics experiment that measured the correlation between inertial mass and gravitational mass, demonstrating that the two were one and the same, something that had long been suspected but never demonstrated with the same accuracy.

## Eddington luminosity

The Eddington luminosity, also referred to as the Eddington limit, is the maximum luminosity a body (such as a star) can achieve when there is balance between the force of radiation acting outward and the gravitational force acting inward.

## Edwin Hubble

Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer.

## Effective field theory

In physics, an effective field theory is a type of approximation, or effective theory, for an underlying physical theory, such as a quantum field theory or a statistical mechanics model.

The Ehrenfest paradox concerns the rotation of a "rigid" disc in the theory of relativity.

## Eikonal approximation

In theoretical physics, the eikonal approximation (Greek εἰκών for likeness, icon or image) is an approximative method useful in wave scattering equations which occur in optics, seismology, quantum mechanics, quantum electrodynamics, and partial wave expansion.

## Einstein field equations

The Einstein field equations (EFE; also known as Einstein's equations) comprise the set of 10 equations in Albert Einstein's general theory of relativity that describe the fundamental interaction of gravitation as a result of spacetime being curved by mass and energy.

## Einstein ring

In observational astronomy an Einstein ring, also known as an Einstein–Chwolson ring or Chwolson ring, is the deformation of the light from a source (such as a galaxy or star) into a ring through gravitational lensing of the source's light by an object with an extremely large mass (such as another galaxy or a black hole).

## Einstein tensor

In differential geometry, the Einstein tensor (named after Albert Einstein; also known as the trace-reversed Ricci tensor) is used to express the curvature of a pseudo-Riemannian manifold.

## Einstein's thought experiments

A hallmark of Albert Einstein's career was his use of visualized thought experiments (Gedankenexperiment) as a fundamental tool for understanding physical issues and for elucidating his concepts to others.

## Einstein–Cartan theory

In theoretical physics, the Einstein–Cartan theory, also known as the Einstein–Cartan–Sciama–Kibble theory, is a classical theory of gravitation similar to general relativity.

## Einstein–Hilbert action

The Einstein–Hilbert action (also referred to as Hilbert action) in general relativity is the action that yields the Einstein field equations through the principle of least action.

## Electric charge

Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field.

## Electric field

An electric field is a vector field surrounding an electric charge that exerts force on other charges, attracting or repelling them.

## Electricity

Electricity is the set of physical phenomena associated with the presence and motion of electric charge.

In physics, electromagnetic radiation (EM radiation or EMR) refers to the waves (or their quanta, photons) of the electromagnetic field, propagating (radiating) through space-time, carrying electromagnetic radiant energy.

## Electromagnetic spectrum

The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.

## Electromagnetism

Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electrically charged particles.

## Elementary particle

In particle physics, an elementary particle or fundamental particle is a particle with no substructure, thus not composed of other particles.

## Ellipse

In mathematics, an ellipse is a curve in a plane surrounding two focal points such that the sum of the distances to the two focal points is constant for every point on the curve.

## Energy

In physics, energy is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object.

## Energy condition

In relativistic classical field theories of gravitation, particularly general relativity, an energy condition is one of various alternative conditions which can be applied to the matter content of the theory, when it is either not possible or desirable to specify this content explicitly.

## Energy density

Energy density is the amount of energy stored in a given system or region of space per unit volume.

## Entropy

In statistical mechanics, entropy is an extensive property of a thermodynamic system.

## Equation of state

In physics and thermodynamics, an equation of state is a thermodynamic equation relating state variables which describe the state of matter under a given set of physical conditions, such as pressure, volume, temperature (PVT), or internal energy.

## Equivalence principle

In the theory of general relativity, the equivalence principle is any of several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.

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## Euclidean geometry

Euclidean geometry is a mathematical system attributed to Alexandrian Greek mathematician Euclid, which he described in his textbook on geometry: the Elements.

## Euclidean space

In geometry, Euclidean space encompasses the two-dimensional Euclidean plane, the three-dimensional space of Euclidean geometry, and certain other spaces.

## Event (relativity)

In physics, and in particular relativity, an event is the instantaneous physical situation or occurrence associated with a point in spacetime (that is, a specific place and time).

## Event horizon

In general relativity, an event horizon is a region in spacetime beyond which events cannot affect an outside observer.

## Exact solutions in general relativity

In general relativity, an exact solution is a Lorentzian manifold equipped with tensor fields modeling states of ordinary matter, such as a fluid, or classical nongravitational fields such as the electromagnetic field.

## Existence theorem

In mathematics, an existence theorem is a theorem with a statement beginning 'there exist(s)..', or more generally 'for all,,...

## Expansion of the universe

The expansion of the universe is the increase of the distance between two distant parts of the universe with time.

## Experimental data

Experimental data in science are data produced by a measurement, test method, experimental design or quasi-experimental design.

## F(R) gravity

f(R) gravity is a type of modified gravity theory which generalizes Einstein's general relativity.

## Field equation

In theoretical physics and applied mathematics, a field equation is a partial differential equation which determines the dynamics of a physical field, specifically the time evolution and spatial distribution of the field.

## Flatness problem

The flatness problem (also known as the oldness problem) is a cosmological fine-tuning problem within the Big Bang model of the universe.

## Force

In physics, a force is any interaction that, when unopposed, will change the motion of an object.

## Force carrier

In particle physics, force carriers or messenger particles or intermediate particles are particles that give rise to forces between other particles.

## Four-momentum

In special relativity, four-momentum is the generalization of the classical three-dimensional momentum to four-dimensional spacetime.

## Fourier series

In mathematics, a Fourier series is a way to represent a function as the sum of simple sine waves.

## Frame of reference

In physics, a frame of reference (or reference frame) consists of an abstract coordinate system and the set of physical reference points that uniquely fix (locate and orient) the coordinate system and standardize measurements.

## Frame-dragging

Frame-dragging is an effect on spacetime, predicted by Einstein's general theory of relativity, that is due to non-static stationary distributions of mass–energy.

## Francis Bacon

Francis Bacon, 1st Viscount St Alban, (22 January 15619 April 1626) was an English philosopher, statesman, scientist, jurist, orator, and author.

## Free fall

In Newtonian physics, free fall is any motion of a body where gravity is the only force acting upon it.

## Friction

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.

## Friedmann–Lemaître–Robertson–Walker metric

The Friedmann–Lemaître–Robertson–Walker (FLRW) metric is an exact solution of Einstein's field equations of general relativity; it describes a homogeneous, isotropic, expanding or contracting universe that is path connected, but not necessarily simply connected.

## Galaxy

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.

## Galilean invariance

Galilean invariance or Galilean relativity states that the laws of motion are the same in all inertial frames.

## Gauge fixing

In the physics of gauge theories, gauge fixing (also called choosing a gauge) denotes a mathematical procedure for coping with redundant degrees of freedom in field variables.

## Gödel metric

The Gödel metric is an exact solution of the Einstein field equations in which the stress–energy tensor contains two terms, the first representing the matter density of a homogeneous distribution of swirling dust particles (dust solution), and the second associated with a nonzero cosmological constant (see lambdavacuum solution).

## General covariance

In theoretical physics, general covariance, also known as diffeomorphism covariance or general invariance, consists of the invariance of the form of physical laws under arbitrary differentiable coordinate transformations.

## General relativity

General relativity (GR, also known as the general theory of relativity or GTR) is the geometric theory of gravitation published by Albert Einstein in 1915 and the current description of gravitation in modern physics.

## General Relativity (book)

General Relativity is a popular textbook on Einstein's theory of general relativity written by Robert Wald.

## GEO600

GEO600 is a gravitational wave detector located near Sarstedt in the South of Hanover, Germany.

## Geodesic

In differential geometry, a geodesic is a generalization of the notion of a "straight line" to "curved spaces".

## Geodesics in general relativity

In general relativity, a geodesic generalizes the notion of a "straight line" to curved spacetime.

## Geodetic effect

The geodetic effect (also known as geodetic precession, de Sitter precession or de Sitter effect) represents the effect of the curvature of spacetime, predicted by general relativity, on a vector carried along with an orbiting body.

## Geometry

Geometry (from the γεωμετρία; geo- "earth", -metron "measurement") is a branch of mathematics concerned with questions of shape, size, relative position of figures, and the properties of space.

## Georges Lemaître

Georges Henri Joseph Édouard Lemaître, RAS Associate (17 July 1894 &ndash; 20 June 1966) was a Belgian Catholic Priest, astronomer and professor of physics at the Catholic University of Leuven.

## Geroch energy

The Geroch energy or Geroch mass is one of the possible definitions of mass in general relativity.

## Global Positioning System

The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Air Force.

## Gowdy solution

Gowdy universes or, alternatively, Gowdy solutions of Einstein's equations are simple model spacetimes in general relativity which represent an expanding universe filled with a regular pattern of gravitational waves.

In mathematics, the gradient is a multi-variable generalization of the derivative.

## Gravitation (book)

Gravitation is a physics book on Einstein's theory of gravity, written by Charles W. Misner, Kip S. Thorne, and John Archibald Wheeler and originally published by W. H. Freeman and Company in 1973.

## Gravitational constant

The gravitational constant (also known as the "universal gravitational constant", the "Newtonian constant of gravitation", or the "Cavendish gravitational constant"), denoted by the letter, is an empirical physical constant involved in the calculation of gravitational effects in Sir Isaac Newton's law of universal gravitation and in Albert Einstein's general theory of relativity.

## Gravitational lens

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.

## Gravitational microlensing

Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect.

## Gravitational potential

In classical mechanics, the gravitational potential at a location is equal to the work (energy transferred) per unit mass that would be needed to move the object from a fixed reference location to the location of the object.

## Gravitational redshift

In astrophysics, gravitational redshift or Einstein shift is the process by which electromagnetic radiation originating from a source that is in a gravitational field is reduced in frequency, or redshifted, when observed in a region at a higher gravitational potential.

## Gravitational singularity

A gravitational singularity or spacetime singularity is a location in spacetime where the gravitational field of a celestial body becomes infinite in a way that does not depend on the coordinate system.

## Gravitational time dilation

Gravitational time dilation is a form of time dilation, an actual difference of elapsed time between two events as measured by observers situated at varying distances from a gravitating mass.

## Gravitational wave

Gravitational waves are the disturbance in the fabric ("curvature") of spacetime generated by accelerated masses and propagate as waves outward from their source at the speed of light.

## Gravitational-wave astronomy

Gravitational-wave astronomy is an emerging branch of observational astronomy which aims to use gravitational waves (minute distortions of spacetime predicted by Einstein's theory of general relativity) to collect observational data about objects such as neutron stars and black holes, events such as supernovae, and processes including those of the early universe shortly after the Big Bang.

## Gravitational-wave observatory

A gravitational-wave observatory (or gravitational-wave detector) is any device designed to measure gravitational waves, tiny distortions of spacetime that were first predicted by Einstein in 1916.

## Graviton

In theories of quantum gravity, the graviton is the hypothetical elementary particle that mediates the force of gravity.

## Gravity

Gravity, or gravitation, is a natural phenomenon by which all things with mass or energy—including planets, stars, galaxies, and even light—are brought toward (or gravitate toward) one another.

## Gravity Probe A

Gravity Probe A (GP-A) was a space-based experiment to test the equivalence principle, a feature of Einstein's theory of relativity.

## Gravity Probe B

Gravity Probe B (GP-B) was a satellite-based mission which launched on 20 April 2004 on a Delta II rocket.

## Gravity well

A gravity well or gravitational well is a conceptual model of the gravitational field surrounding a body in space – the more massive the body, the deeper and more extensive the gravity well associated with it.

## Gyroscope

A gyroscope (from Ancient Greek γῦρος gûros, "circle" and σκοπέω skopéō, "to look") is a device used for measuring or maintaining orientation and angular velocity.

## Hafele–Keating experiment

The Hafele–Keating experiment was a test of the theory of relativity.

## Hawking energy

The Hawking energy or Hawking mass is one of the possible definitions of mass in general relativity.

Hawking radiation is blackbody radiation that is predicted to be released by black holes, due to quantum effects near the event horizon.

## Hertz

The hertz (symbol: Hz) is the derived unit of frequency in the International System of Units (SI) and is defined as one cycle per second.

## History of string theory

The history of string theory spans several decades of intense research including two superstring revolutions.

## Homogeneity (physics)

In physics, a homogeneous material or system has the same properties at every point; it is uniform without irregularities.

## Hoop Conjecture

The hoop conjecture, proposed by Kip Thorne in 1972, states that an imploding object forms a black hole when, and only when, a circular hoop with a specific critical circumference could be placed around the object and rotated.

## Horizon problem

The horizon problem (also known as the homogeneity problem) is a cosmological fine-tuning problem within the Big Bang model of the universe.

## Hubble's law

Hubble's law is the name for the observation in physical cosmology that.

## Hulse–Taylor binary

PSR B1913+16 (also known as PSR J1915+1606, PSR 1913+16, and the Hulse–Taylor binary after its discoverers) is a pulsar (a radiating neutron star) which together with another neutron star is in orbit around a common center of mass, thus forming a binary star system.

## Inertia

Inertia is the resistance of any physical object to any change in its position and state of motion.

## Inertial frame of reference

An inertial frame of reference in classical physics and special relativity is a frame of reference in which a body with zero net force acting upon it is not accelerating; that is, such a body is at rest or it is moving at a constant speed in a straight line.

## Inflation (cosmology)

In physical cosmology, cosmic inflation, cosmological inflation, or just inflation, is a theory of exponential expansion of space in the early universe.

## Inflaton

The inflaton field is a hypothetical scalar field that is theorized to drive cosmic inflation in the very early universe.

## Institut Henri Poincaré

The Henri Poincaré Institute (or IHP for Institut Henri Poincaré) is a mathematics research institute part of Sorbonne University, in association with the Centre national de la recherche scientifique (CNRS).

## Integrable system

In the context of differential equations to integrate an equation means to solve it from initial conditions.

## Interferometric gravitational wave detector

An interferometric gravitational wave detector is a gravitational wave detector that uses laser interferometry to detect the influence of gravitational waves on light that is moving back and forth between test masses.

## Introduction to the mathematics of general relativity

The mathematics of general relativity is complex.

## Invariant (mathematics)

In mathematics, an invariant is a property, held by a class of mathematical objects, which remains unchanged when transformations of a certain type are applied to the objects.

## Isolated system

In physical science, an isolated system is either of the following.

## Isotropy

Isotropy is uniformity in all orientations; it is derived from the Greek isos (ἴσος, "equal") and tropos (τρόπος, "way").

## John Archibald Wheeler

John Archibald Wheeler (July 9, 1911 – April 13, 2008) was an American theoretical physicist.

## John C. Baez

John Carlos Baez (born June 12, 1961) is an American mathematical physicist and a professor of mathematics at the University of California, Riverside (UCR) in Riverside, California.

## Joseph Hooton Taylor Jr.

Joseph Hooton Taylor Jr. (born March 29, 1941) is an American astrophysicist and Nobel Prize in Physics laureate for his discovery with Russell Alan Hulse of a "new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation.".

## Karl Schwarzschild

Karl Schwarzschild (October 9, 1873 – May 11, 1916) was a German physicist and astronomer.

## Kerr metric

The Kerr metric or Kerr geometry describes the geometry of empty spacetime around a rotating uncharged axially-symmetric black hole with a spherical event horizon.

## Komar mass

The Komar mass (named after Arthur Komar) of a system is one of several formal concepts of mass that are used in general relativity.

## Kurt Gödel

Kurt Friedrich Gödel (April 28, 1906 – January 14, 1978) was an Austrian, and later American, logician, mathematician, and philosopher.

## LAGEOS

LAGEOS, Laser Geodynamics Satellite or Laser Geometric Environmental Observation Survey, are a series of two scientific research satellites designed to provide an orbiting laser ranging benchmark for geodynamical studies of the Earth.

## Lagrangian (field theory)

Lagrangian field theory is a formalism in classical field theory.

## Lane P. Hughston

Lane P. Hughston (born 24 December 1951) is an American mathematician.

## Laser Interferometer Space Antenna

The Laser Interferometer Space Antenna (LISA) is a European Space Agency mission designed to detect and accurately measure gravitational waves—tiny ripples in the fabric of space-time—from astronomical sources.

## Laws of thermodynamics

The four laws of thermodynamics define fundamental physical quantities (temperature, energy, and entropy) that characterize thermodynamic systems at thermal equilibrium.

## Leonard Susskind

Leonard Susskind (born 1940)his 60th birthday was celebrated with a special symposium at Stanford University.

## Levi-Civita connection

In Riemannian geometry, the Levi-Civita connection is a specific connection on the tangent bundle of a manifold.

## Light-year

The light-year is a unit of length used to express astronomical distances and measures about 9.5 trillion kilometres or 5.9 trillion miles.

## LIGO

The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool.

## Limiting case (philosophy of science)

In the philosophy of science, under the correspondence principle, a limiting case theory is an earlier theory which becomes incorporated into a later, usually broader theory; that is to say, the earlier (limiting case) theory proves to be a special or limited case of the later theory.

## Linearized gravity

Linearized gravity is an approximation scheme in general relativity in which the nonlinear contributions from the spacetime metric are ignored, simplifying the study of many problems while still producing useful approximate results.

## LISA Pathfinder

LISA Pathfinder, formerly Small Missions for Advanced Research in Technology-2 (SMART-2), was an ESA spacecraft that was launched on 3 December 2015.

## List of contributors to general relativity

This is a partial list of persons who have made major contributions to the development of standard mainstream general relativity.

## List of gravitational wave observations

This is a list of observed gravitational wave events.

## Living Reviews in Relativity

Living Reviews in Relativity is a peer-reviewed open-access scientific journal publishing reviews on relativity in the areas of physics and astrophysics.

## Local reference frame

In theoretical physics, a local reference frame (local frame) refers to a coordinate system or frame of reference that is only expected to function over a small region or a restricted region of space or spacetime.

## Local spacetime structure

Local spacetime structure refers to the structure of spacetime on a local level, i.e. only considering those points in an open region of a point.

## Loop quantum gravity

Loop quantum gravity (LQG) is a theory of quantum gravity, merging quantum mechanics and general relativity.

## Loránd Eötvös

Baron Loránd Eötvös de Vásárosnamény (vásárosnaményi báró Eötvös Loránd Ágoston or Loránd Eötvös,; 27 July 1848 – 8 April 1919), more commonly called Baron Roland von Eötvös in English literature, was an Austro-Hungarian physicist of ethnic Hungarian origin.

## Lorentz covariance

In relativistic physics, Lorentz symmetry, named for Hendrik Lorentz, is an equivalence of observation or observational symmetry due to special relativity implying that the laws of physics stay the same for all observers that are moving with respect to one another within an inertial frame.

## Lunar Laser Ranging experiment

The ongoing Lunar Laser Ranging experiment measures the distance between Earth and the Moon using laser ranging.

## M-theory

M-theory is a theory in physics that unifies all consistent versions of superstring theory.

## Magnetic field

A magnetic field is a vector field that describes the magnetic influence of electrical currents and magnetized materials.

## Manifold

In mathematics, a manifold is a topological space that locally resembles Euclidean space near each point.

## Mars Global Surveyor

Mars Global Surveyor (MGS) was an American robotic spacecraft developed by NASA's Jet Propulsion Laboratory and launched November 1996.

## Mass

Mass is both a property of a physical body and a measure of its resistance to acceleration (a change in its state of motion) when a net force is applied.

## Matter

In the classical physics observed in everyday life, matter is any substance that has mass and takes up space by having volume.

## Max Planck Institute for Gravitational Physics

The Max Planck Institute for Gravitational Physics (Albert Einstein Institute) is a Max Planck Institute whose research is aimed at investigating Einstein’s theory of relativity and beyond: Mathematics, quantum gravity, astrophysical relativity, and gravitational wave astronomy.

## Mercury (planet)

Mercury is the smallest and innermost planet in the Solar System.

## Metric tensor (general relativity)

In general relativity, the metric tensor (in this context often abbreviated to simply the metric) is the fundamental object of study.

## Microquasar

A microquasar, the smaller version of a quasar, is a compact region surrounding a black hole with a mass several times that of our sun, and its companion star.

## Milky Way

The Milky Way is the galaxy that contains our Solar System.

## Millisecond pulsar

A millisecond pulsar (MSP) is a pulsar with a rotational period in the range of about 1–10 milliseconds.

## Minkowski diagram

The Minkowski diagram, also known as a spacetime diagram, was developed in 1908 by Hermann Minkowski and provides an illustration of the properties of space and time in the special theory of relativity.

## Minkowski space

In mathematical physics, Minkowski space (or Minkowski spacetime) is a combining of three-dimensional Euclidean space and time into a four-dimensional manifold where the spacetime interval between any two events is independent of the inertial frame of reference in which they are recorded.

## Modern physics

Modern physics is the post-Newtonian conception of physics.

## Momentum

In Newtonian mechanics, linear momentum, translational momentum, or simply momentum (pl. momenta) is the product of the mass and velocity of an object.

## Naked singularity

In general relativity, a naked singularity is a gravitational singularity without an event horizon.

## National Center for Supercomputing Applications

The National Center for Supercomputing Applications (NCSA) is a state-federal partnership to develop and deploy national-scale cyberinfrastructure that advances research, science and engineering based in the United States of America.

## Negative mass

In theoretical physics, negative mass is matter whose mass is of opposite sign to the mass of normal matter, e.g. −1 kg.

## Neutron star

A neutron star is the collapsed core of a large star which before collapse had a total of between 10 and 29 solar masses.

## Newton's law of universal gravitation

Newton's law of universal gravitation states that a particle attracts every other particle in the universe with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.

## Newton's laws of motion

Newton's laws of motion are three physical laws that, together, laid the foundation for classical mechanics.

## Newton–Cartan theory

Newton–Cartan theory (or geometrized Newtonian gravitation) is a geometrical re-formulation, as well as a generalization, of Newtonian gravity first introduced by Élie Cartan and Kurt Friedrichs and later developed by Dautcourt, Dixon, Dombrowski and Horneffer, Ehlers, Havas, Künzle, Lottermoser, Trautman, and others.

## No-hair theorem

The no-hair theorem postulates that all black hole solutions of the Einstein-Maxwell equations of gravitation and electromagnetism in general relativity can be completely characterized by only three externally observable classical parameters: mass, electric charge, and angular momentum.

## Nobel Prize

The Nobel Prize (Swedish definite form, singular: Nobelpriset; Nobelprisen) is a set of six annual international awards bestowed in several categories by Swedish and Norwegian institutions in recognition of academic, cultural, or scientific advances.

## Nonlinear system

In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input.

## Normal mode

A normal mode of an oscillating system is a pattern of motion in which all parts of the system move sinusoidally with the same frequency and with a fixed phase relation.

## Numerical integration

In numerical analysis, numerical integration constitutes a broad family of algorithms for calculating the numerical value of a definite integral, and by extension, the term is also sometimes used to describe the numerical solution of differential equations.

## Numerical relativity

Numerical relativity is one of the branches of general relativity that uses numerical methods and algorithms to solve and analyze problems.

## Observational astronomy

Observational astronomy is a division of astronomy that is concerned with recording data about the observable universe, in contrast with theoretical astronomy, which is mainly concerned with calculating the measurable implications of physical models.

## Occam's razor

Occam's razor (also Ockham's razor or Ocham's razor; Latin: lex parsimoniae "law of parsimony") is the problem-solving principle that, the simplest explanation tends to be the right one.

## Orbit

In physics, an orbit is the gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet.

## Orbital eccentricity

The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle.

## Orbital period

The orbital period is the time a given astronomical object takes to complete one orbit around another object, and applies in astronomy usually to planets or asteroids orbiting the Sun, moons orbiting planets, exoplanets orbiting other stars, or binary stars.

## Order of magnitude

An order of magnitude is an approximate measure of the number of digits that a number has in the commonly-used base-ten number system.

## Parallel transport

In geometry, parallel transport is a way of transporting geometrical data along smooth curves in a manifold.

## Parameterized post-Newtonian formalism

Post-Newtonian formalism is a calculational tool that expresses Einstein's (nonlinear) equations of gravity in terms of the lowest-order deviations from Newton's law of universal gravitation.

## Partial differential equation

In mathematics, a partial differential equation (PDE) is a differential equation that contains unknown multivariable functions and their partial derivatives.

## Particle horizon

The particle horizon (also called the cosmological horizon, the comoving horizon (in Dodelson's text), or the cosmic light horizon) is the maximum distance from which particles could have traveled to the observer in the age of the universe.

## Particle physics

Particle physics (also high energy physics) is the branch of physics that studies the nature of the particles that constitute matter and radiation.

## Path integral formulation

The path integral formulation of quantum mechanics is a description of quantum theory that generalizes the action principle of classical mechanics.

## Penrose diagram

In theoretical physics, a Penrose diagram (named after mathematical physicist Roger Penrose) is a two-dimensional diagram capturing the causal relations between different points in spacetime.

## Penrose process

The Penrose process (also called Penrose mechanism) is a process theorised by Roger Penrose wherein energy can be extracted from a rotating black hole.

## Penrose–Hawking singularity theorems

The Penrose–Hawking singularity theorems are a set of results in general relativity that attempt to answer the question of when gravitation produces singularities.

## Perturbation theory

Perturbation theory comprises mathematical methods for finding an approximate solution to a problem, by starting from the exact solution of a related, simpler problem.

## Physical body

In physics, a physical body or physical object (or simply a body or object) is an identifiable collection of matter, which may be constrained by an identifiable boundary, and may move as a unit by translation or rotation, in 3-dimensional space.

## Physical cosmology

Physical cosmology is the study of the largest-scale structures and dynamics of the Universe and is concerned with fundamental questions about its origin, structure, evolution, and ultimate fate.

## Physical law

A physical law or scientific law is a theoretical statement "inferred from particular facts, applicable to a defined group or class of phenomena, and expressible by the statement that a particular phenomenon always occurs if certain conditions be present." Physical laws are typically conclusions based on repeated scientific experiments and observations over many years and which have become accepted universally within the scientific community.

## Planck's law

Planck's law describes the spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature T. The law is named after Max Planck, who proposed it in 1900.

## Planet

A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.

## Poincaré group

The Poincaré group, named after Henri Poincaré (1906), was first defined by Minkowski (1908) as the group of Minkowski spacetime isometries.

## Point particle

A point particle (ideal particle or point-like particle, often spelled pointlike particle) is an idealization of particles heavily used in physics.

## Polarization (waves)

Polarization (also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations.

## Post-Newtonian expansion

Post-Newtonian expansions in general relativity are used for finding an approximate solution of the Einstein field equations for the metric tensor.

## Potential

Potential generally refers to a currently unrealized ability.

## Pound–Rebka experiment

The Pound–Rebka experiment is a well known experiment to test Albert Einstein's theory of general relativity.

## Precession

Precession is a change in the orientation of the rotational axis of a rotating body.

## Pressure

Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed.

## Principle of relativity

In physics, the principle of relativity is the requirement that the equations describing the laws of physics have the same form in all admissible frames of reference.

## Proper time

In relativity, proper time along a timelike world line is defined as the time as measured by a clock following that line.

## Prussian Academy of Sciences

The Royal Prussian Academy of Sciences (Königlich-Preußische Akademie der Wissenschaften) was an academy established in Berlin, Germany on 11 July 1700, four years after the Akademie der Künste, or "Arts Academy," to which "Berlin Academy" may also refer.

## Pseudo-Riemannian manifold

In differential geometry, a pseudo-Riemannian manifold (also called a semi-Riemannian manifold) is a generalization of a Riemannian manifold in which the metric tensor need not be positive-definite, but need only be a non-degenerate bilinear form, which is a weaker condition.

## PSR J0737-3039

PSR J0737−3039 is the only known double pulsar.

## Pulsar

A pulsar (from pulse and -ar as in quasar) is a highly magnetized rotating neutron star or white dwarf that emits a beam of electromagnetic radiation.

## Pulsar timing array

A pulsar timing array (PTA) is a set of pulsars which is analyzed to search for correlated signatures in the pulse arrival times.

## Quantum cosmology

Quantum cosmology is the attempt in theoretical physics to develop a quantum theory of the Universe.

## Quantum field theory

In theoretical physics, quantum field theory (QFT) is the theoretical framework for constructing quantum mechanical models of subatomic particles in particle physics and quasiparticles in condensed matter physics.

## Quantum gravity

Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics, and where quantum effects cannot be ignored, such as near compact astrophysical objects where the effects of gravity are strong.

## Quantum mechanics

Quantum mechanics (QM; also known as quantum physics, quantum theory, the wave mechanical model, or matrix mechanics), including quantum field theory, is a fundamental theory in physics which describes nature at the smallest scales of energy levels of atoms and subatomic particles.

## Quasar

A quasar (also known as a QSO or quasi-stellar object) is an extremely luminous active galactic nucleus (AGN).

In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium.

## Raychaudhuri equation

In general relativity, the Raychaudhuri equation, or Landau–Raychaudhuri equation, is a fundamental result describing the motion of nearby bits of matter.

## Redshift

In physics, redshift happens when light or other electromagnetic radiation from an object is increased in wavelength, or shifted to the red end of the spectrum.

## Regge calculus

In general relativity, Regge calculus is a formalism for producing simplicial approximations of spacetimes that are solutions to the Einstein field equation.

## Reissner–Nordström metric

In physics and astronomy, the Reissner&ndash;Nordström metric is a static solution to the Einstein-Maxwell field equations, which corresponds to the gravitational field of a charged, non-rotating, spherically symmetric body of mass M. The metric was discovered by Hans Reissner, Hermann Weyl, Gunnar Nordström and G. B. Jeffery.

## Relativity priority dispute

Albert Einstein presented the theories of special relativity and general relativity in publications that either contained no formal references to previous literature, or referred only to a small number of his predecessors for fundamental results on which he based his theories, most notably to the work of Hendrik Lorentz for special relativity, and to the work of Carl F. Gauss, Bernhard Riemann, and Ernst Mach for general relativity.

## Renormalization

Renormalization is a collection of techniques in quantum field theory, the statistical mechanics of fields, and the theory of self-similar geometric structures, that are used to treat infinities arising in calculated quantities by altering values of quantities to compensate for effects of their self-interactions.

## Ricci calculus

In mathematics, Ricci calculus constitutes the rules of index notation and manipulation for tensors and tensor fields.

## Ricci curvature

In differential geometry, the Ricci curvature tensor, named after Gregorio Ricci-Curbastro, represents the amount by which the volume of a small wedge of a geodesic ball in a curved Riemannian manifold deviates from that of the standard ball in Euclidean space.

## Riemann curvature tensor

In the mathematical field of differential geometry, the Riemann curvature tensor or Riemann–Christoffel tensor (after Bernhard Riemann and Elwin Bruno Christoffel) is the most common method used to express the curvature of Riemannian manifolds.

## 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.

## Roger Penrose

Sir Roger Penrose (born 8 August 1931) is an English mathematical physicist, mathematician and philosopher of science.

## Rose (mathematics)

In mathematics, a rose or rhodonea curve is a sinusoid plotted in polar coordinates.

## Russell Alan Hulse

Russell Alan Hulse (born November 28, 1950) is an American physicist and winner of the Nobel Prize in Physics, shared with his thesis advisor Joseph Hooton Taylor Jr., "for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation".

## Scalar curvature

In Riemannian geometry, the scalar curvature (or the Ricci scalar) is the simplest curvature invariant of a Riemannian manifold.

## Scalar field

In mathematics and physics, a scalar field associates a scalar value to every point in a space – possibly physical space.

## Schrödinger equation

In quantum mechanics, the Schrödinger equation is a mathematical equation that describes the changes over time of a physical system in which quantum effects, such as wave–particle duality, are significant.

## Schwarzschild metric

In Einstein's theory of general relativity, the Schwarzschild metric (also known as the Schwarzschild vacuum or Schwarzschild solution) is the solution to the Einstein field equations that describes the gravitational field outside a spherical mass, on the assumption that the electric charge of the mass, angular momentum of the mass, and universal cosmological constant are all zero.

The Schwarzschild radius (sometimes historically referred to as the gravitational radius) is a physical parameter that shows up in the Schwarzschild solution to Einstein's field equations, corresponding to the radius defining the event horizon of a Schwarzschild black hole.

## Science (journal)

Science, also widely referred to as Science Magazine, is the peer-reviewed academic journal of the American Association for the Advancement of Science (AAAS) and one of the world's top academic journals.

## Scientific theory

A scientific theory is an explanation of an aspect of the natural world that can be repeatedly tested, in accordance with the scientific method, using a predefined protocol of observation and experiment.

## Self-energy

In most theoretical physics such as quantum field theory, the energy that a particle has as a result of changes that it itself causes in its environment defines self-energy \Sigma, and represents the contribution to the particle's energy, or effective mass, due to interactions between the particle and its system.

## Semi-major and semi-minor axes

In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the widest points of the perimeter.

## Shapiro time delay

The Shapiro time delay effect, or gravitational time delay effect, is one of the four classic solar-system tests of general relativity.

## Solar eclipse of May 29, 1919

A total solar eclipse occurred on May 29, 1919.

## Solar mass

The solar mass is a standard unit of mass in astronomy, equal to approximately.

## Solar System

The Solar SystemCapitalization of the name varies.

## Solid-state physics

Solid-state physics is the study of rigid matter, or solids, through methods such as quantum mechanics, crystallography, electromagnetism, and metallurgy.

## Solutions of the Einstein field equations

Solutions of the Einstein field equations are spacetimes that result from solving the Einstein field equations (EFE) of general relativity.

## Space

Space is the boundless three-dimensional extent in which objects and events have relative position and direction.

## Spacecraft

A spacecraft is a vehicle or machine designed to fly in outer space.

## Spacetime

In physics, spacetime is any mathematical model that fuses the three dimensions of space and the one dimension of time into a single four-dimensional continuum.

## Spacetime topology

Spacetime topology is the topological structure of spacetime, a topic studied primarily in general relativity.

## Special relativity

In physics, special relativity (SR, also known as the special theory of relativity or STR) is the generally accepted and experimentally well-confirmed physical theory regarding the relationship between space and time.

## Speed of light

The speed of light in vacuum, commonly denoted, is a universal physical constant important in many areas of physics.

## Spin network

In physics, a spin network is a type of diagram which can be used to represent states and interactions between particles and fields in quantum mechanics.

## Stanford University

Stanford University (officially Leland Stanford Junior University, colloquially the Farm) is a private research university in Stanford, California.

## Star

A star is type of astronomical object consisting of a luminous spheroid of plasma held together by its own gravity.

## Static spacetime

In general relativity, a spacetime is said to be static if it does not change over time and is also irrotational.

## Stationary spacetime

In general relativity, specifically in the Einstein field equations, a spacetime is said to be stationary if it admits a Killing vector that is asymptotically timelike.

## Stellar black hole

A stellar black hole (or stellar-mass black hole) is a black hole formed by the gravitational collapse of a massive star.

## Stellar collision

A stellar collision is the coming together of two stars caused by gravity, gravitational radiation, or other mechanisms not well understood.

## Stellar evolution

Stellar evolution is the process by which a star changes over the course of time.

## Stress (mechanics)

In continuum mechanics, stress is a physical quantity that expresses the internal forces that neighboring particles of a continuous material exert on each other, while strain is the measure of the deformation of the material.

## Stress–energy tensor

The stress–energy tensor (sometimes stress–energy–momentum tensor or energy–momentum tensor) is a tensor quantity in physics that describes the density and flux of energy and momentum in spacetime, generalizing the stress tensor of Newtonian physics.

## String theory

In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings.

## Subrahmanyan Chandrasekhar

Subrahmanyan Chandrasekhar FRS (19 October 1910 – 21 August 1995) was an Indian American astrophysicist who spent his professional life in the United States.

## Sun

The Sun is the star at the center of the Solar System.

## Supergravity

In theoretical physics, supergravity (supergravity theory; SUGRA for short) is a modern field theory that combines the principles of supersymmetry and general relativity where supersymmetry obeys locality; in contrast to non-gravitational supersymmetric theories such as the Minimal Supersymmetric Standard Model.

## Superluminal motion

In astronomy, superluminal motion is the apparently faster-than-light motion seen in some radio galaxies, BL Lac objects, quasars and recently also in some galactic sources called microquasars.

## Supermassive black hole

A supermassive black hole (SMBH or SBH) is the largest type of black hole, on the order of hundreds of thousands to billions of solar masses, and is found in the centre of almost all currently known massive galaxies.

## Supernova

A supernova (plural: supernovae or supernovas, abbreviations: SN and SNe) is a transient astronomical event that occurs during the last stellar evolutionary stages of a star's life, either a massive star or a white dwarf, whose destruction is marked by one final, titanic explosion.

## Superposition principle

In physics and systems theory, the superposition principle, also known as superposition property, states that, for all linear systems, the net response caused by two or more stimuli is the sum of the responses that would have been caused by each stimulus individually.

## Supersymmetry

In particle physics, supersymmetry (SUSY) is a theory that proposes a relationship between two basic classes of elementary particles: bosons, which have an integer-valued spin, and fermions, which have a half-integer spin.

## Symmetry

Symmetry (from Greek συμμετρία symmetria "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance.

## TAMA 300

TAMA 300 was a gravitational wave detector located at the Mitaka campus of the National Astronomical Observatory of Japan.

## Taub–NUT space

The Taub–NUT metric is an exact solution to Einstein's equations.

## Teleparallelism

Teleparallelism (also called teleparallel gravity), was an attempt by Albert Einstein to base a unified theory of electromagnetism and gravity on the mathematical structure of distant parallelism, also referred to as absolute or teleparallelism.

## Tensor

In mathematics, tensors are geometric objects that describe linear relations between geometric vectors, scalars, and other tensors.

## Test particle

In physical theories, a test particle is an idealized model of an object whose physical properties (usually mass, charge, or size) are assumed to be negligible except for the property being studied, which is considered to be insufficient to alter the behavior of the rest of the system.

## Tests of general relativity

Tests of general relativity serve to establish observational evidence for the theory of general relativity.

## The Large Scale Structure of Space-Time

The Large Scale Structure of Space-Time is 1973 book by Stephen Hawking and George Ellis on the theoretical physics of spacetime.

## Theoretical physics

Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain and predict natural phenomena.

## Theory of everything

A theory of everything (ToE), 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.

## Theory of relativity

The theory of relativity usually encompasses two interrelated theories by Albert Einstein: special relativity and general relativity.

Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter.

## Thought experiment

A thought experiment (Gedankenexperiment, Gedanken-Experiment or Gedankenerfahrung) considers some hypothesis, theory, or principle for the purpose of thinking through its consequences.

## Tidal force

The tidal force is an apparent force that stretches a body towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for the diverse phenomena, including tides, tidal locking, breaking apart of celestial bodies and formation of ring systems within Roche limit, and in extreme cases, spaghettification of objects.

## Time evolution

Time evolution is the change of state brought about by the passage of time, applicable to systems with internal state (also called stateful systems).

## Time in physics

Time in physics is defined by its measurement: time is what a clock reads.

## Time travel

Time travel is the concept of movement between certain points in time, analogous to movement between different points in space by an object or a person, typically using a hypothetical device known as a time machine.

## Timeline of gravitational physics and relativity

Timeline of gravitational physics and general relativity.

## Tipler cylinder

A Tipler cylinder, also called a Tipler time machine, is a hypothetical object theorized to be a potential mode of time travel&mdash;although results have shown that a Tipler cylinder could only allow time travel if its length were infinite or with the existence of negative energy.

## Torsion tensor

In differential geometry, the notion of torsion is a manner of characterizing a twist or screw of a moving frame around a curve.

## Trajectory

A trajectory or flight path is the path that a massive object in motion follows through space as a function of time.

## Transponder

In telecommunication, a transponder can be one of two types of devices.

## Trapped surface

Closed trapped surfaces are a concept used in black hole solutions of general relativity which describe the inner region of an event horizon.

## Twin Quasar

The Twin Quasar (also known as Twin QSO, Double Quasar, SBS 0957+561, TXS 0957+561, Q0957+561 or QSO 0957+561 A/B), was discovered in 1979 and was the first identified gravitationally lensed object.

## Twistor theory

Twistor theory was proposed by Roger Penrose in 1967 as a possible path to quantum gravity and has evolved into a branch of theoretical and mathematical physics.

## Two-body problem in general relativity

The two-body problem (or Kepler problem) in general relativity is the determination of the motion and gravitational field of two bodies as described by the field equations of general relativity.

## Universe

The Universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy.

## Unruh effect

The Unruh effect (or sometimes Fulling–Davies–Unruh effect) is the prediction that an accelerating observer will observe blackbody radiation where an inertial observer would observe none.

## Urbain Le Verrier

Urbain Jean Joseph Le Verrier (11 March 1811 – 23 September 1877) was a French mathematician who specialized in celestial mechanics and is best known for predicting the existence and position of Neptune using only mathematics.

## Venus

Venus is the second planet from the Sun, orbiting it every 224.7 Earth days.

## Very-long-baseline interferometry

Very-long-baseline interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy.

## Virgo interferometer

The Virgo interferometer is a large interferometer designed to detect gravitational waves predicted by the general theory of relativity.

## Weak gravity conjecture

The weak gravity conjecture (WGC) is a conjecture regarding the strength gravity can have in a theory of quantum gravity relative to the gauge forces in that theory.

## Wheeler–DeWitt equation

The Wheeler–DeWitt equation is a field equation.

## Whitehead's theory of gravitation

In theoretical physics, Whitehead's theory of gravitation was introduced by the mathematician and philosopher Alfred North Whitehead in 1922.

## Wilkinson Microwave Anisotropy Probe

The Wilkinson Microwave Anisotropy Probe (WMAP), originally known as the Microwave Anisotropy Probe (MAP), was a spacecraft operating from 2001 to 2010 which measured temperature differences across the sky in the cosmic microwave background (CMB) – the radiant heat remaining from the Big Bang.

## WKB approximation

In mathematical physics, the WKB approximation or WKB method is a method for finding approximate solutions to linear differential equations with spatially varying coefficients.

## World line

The world line (or worldline) of an object is the path that object traces in -dimensional spacetime.

## X-ray burster

X-ray bursters are one class of X-ray binary stars exhibiting periodic and rapid increases in luminosity (typically a factor of 10 or greater) that peak in the X-ray regime of the electromagnetic spectrum.

## X-ray pulsar

X-ray pulsars or accretion-powered pulsars are a class of astronomical objects that are X-ray sources displaying strict periodic variations in X-ray intensity.

## 1,000,000,000

1,000,000,000 (one billion, short scale; one thousand million or milliard, yard, long scale) is the natural number following 999,999,999 and preceding 1,000,000,001.

## References

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