general relativity

general relativity
Name	General relativity
Field	Theoretical physics
Year	1915
Creator	Albert Einstein
Related	Special relativity, Newton's law of universal gravitation

general relativity. It is a geometric theory of gravitation published by Albert Einstein in 1915, which revolutionized the understanding of gravity, space, and time. The theory superseded Isaac Newton's law of universal gravitation, describing gravity not as a force but as a consequence of the curvature of spacetime caused by the uneven distribution of mass and energy. Its predictions have been confirmed in numerous tests, from the orbit of Mercury to the detection of gravitational waves by the LIGO collaboration.

Overview and historical context

The development of general relativity was primarily the work of Albert Einstein between 1907 and 1915, building upon his earlier special relativity. Key influences included the equivalence principle, the mathematics of non-Euclidean geometry developed by Bernhard Riemann and others, and the need to reconcile gravity with the laws of special relativity. The theory was presented in its final form to the Prussian Academy of Sciences in November 1915. Early crucial support came from astronomers like Arthur Eddington, whose 1919 observations during a solar eclipse confirmed the predicted bending of starlight, catapulting Einstein to global fame. Subsequent work by figures like Karl Schwarzschild, who found the first exact solution to the Einstein field equations, and Alexander Friedmann, who developed cosmological solutions, rapidly expanded the theory's applications.

Fundamental principles

The theory rests on two core principles. The first is the equivalence principle, which states that the effects of gravity are locally indistinguishable from those of acceleration, famously illustrated by Einstein's thought experiment involving an elevator in free fall. The second is the concept that spacetime is a dynamic, four-dimensional manifold whose geometry is described by the metric tensor. The curvature of this spacetime, governed by the Einstein field equations, dictates the motion of inertial objects and light. This geometrization of gravity means that planets like Earth orbit the Sun not due to a force, but by following the straightest possible paths, or geodesics, in curved spacetime.

Einstein field equations

The core mathematical framework is encapsulated in the Einstein field equations, a set of ten interrelated, non-linear partial differential equations. They equate the geometry of spacetime (expressed by the Einstein tensor) to the distribution of matter and energy within it (expressed by the stress–energy tensor). The equations include the cosmological constant, a term Einstein initially introduced to allow a static universe, later calling it his "greatest blunder" after the discovery of the expansion of the universe by Edwin Hubble. Solving these complex equations for specific physical situations has yielded seminal solutions, such as the Schwarzschild metric describing non-rotating black holes and the Friedmann–Lemaître–Robertson–Walker metric underpinning Big Bang cosmology.

Tests and experimental evidence

General relativity has passed every stringent experimental test to date. The first major success was explaining the anomalous precession of the perihelion of Mercury, a puzzle that Newtonian mechanics could not fully resolve. The 1919 solar eclipse expedition led by Arthur Eddington confirmed the predicted gravitational deflection of starlight by the Sun. Later, the Shapiro time delay effect was confirmed by radar echo experiments with planets like Venus and spacecraft such as Viking 1. The Hafele–Keating experiment demonstrated gravitational time dilation using precise atomic clocks on airplanes. In 2015, the LIGO observatory directly detected gravitational waves from merging black holes, a monumental confirmation of a key prediction.

Astrophysical applications and predictions

The theory predicts several extreme astrophysical phenomena. It describes black holes, regions of spacetime from which not even light can escape, with properties like the event horizon and the central singularity. The theory also predicts that massive rotating objects, like the planet Jupiter or the black hole in the Messier 87 galaxy, drag spacetime around them, an effect known as frame-dragging. On cosmological scales, it forms the basis for models of the expansion of the universe, the existence of the cosmic microwave background, and the formation of structures like the Virgo Cluster. Observations of the orbit of the Hulse–Taylor binary pulsar provided indirect evidence for gravitational waves and earned Russell Alan Hulse and Joseph Hooton Taylor Jr. the Nobel Prize in Physics.

Relationship to other physical theories

General relativity stands as the modern description of gravity and macroscopic phenomena but remains famously difficult to reconcile with quantum mechanics, the theory governing the microscopic realm. The search for a theory of quantum gravity, such as string theory or loop quantum gravity, is a major goal of theoretical physics. General relativity reduces to Newton's law of universal gravitation in the limit of weak gravitational fields and low velocities, as demonstrated in the post-Newtonian expansion. It is also consistent with, and a generalization of, special relativity, which applies in the absence of significant gravity. The theory's implications continue to drive research at institutions like the Institute for Advanced Study and through missions like the Gravity Probe B satellite.

Category:Theories of gravitation Category:Albert Einstein Category:Relativity