General relativity

General relativity
Simulating eXtreme Spacetimes Lensing (SXS) · CC BY-SA 4.0 · source
Name	General relativity
Caption	Albert Einstein, 1921
Introduced	1915
Major contributors	Albert Einstein, David Hilbert, Marcel Grossmann, Karl Schwarzschild, Willem de Sitter, Arthur Eddington, Roy Kerr, Subrahmanyan Chandrasekhar, Roger Penrose, Stephen Hawking

General relativity is the modern classical theory of gravitation formulated in 1915 that replaces Newtonian gravity for strong fields and large scales. Conceived by Albert Einstein with mathematical input from David Hilbert and Marcel Grossmann, the theory describes gravitation as the geometry of spacetime rather than a force acting at a distance. It underlies predictions tested by experiments associated with Arthur Eddington, Roy Kerr, Vera Rubin, and contemporary observatories such as LIGO, Event Horizon Telescope, and Planck.

History and development

Einstein's work built on antecedents including Isaac Newton, Carl Friedrich Gauss, Bernhard Riemann, and Hermann Minkowski, and interacted with contemporaries such as David Hilbert and Marcel Grossmann. Early solutions like the Schwarzschild solution by Karl Schwarzschild and cosmological models by Willem de Sitter influenced debates involving Albert Einstein, Alexander Friedmann, and Georges Lemaître. Empirical attention followed Arthur Eddington's 1919 expedition during a solar eclipse, which involved institutions like the Royal Society and the Royal Astronomical Society and brought public figures such as Sir Frank Dyson into prominence. Mid-20th-century developments by Subrahmanyan Chandrasekhar, Lev Landau, and George Gamow intersected with work by John Wheeler, Felix Pirani, and Hermann Bondi; later breakthroughs by Roger Penrose and Stephen Hawking advanced singularity theorems and black hole thermodynamics, influencing research at institutions including Princeton University, Cambridge University, the Institute for Advanced Study, and California Institute of Technology. The discovery of pulsars by Jocelyn Bell Burnell and the development of radio astronomy at Jodrell Bank and Arecibo further solidified the empirical foundation, while contemporary international collaborations such as LIGO Scientific Collaboration, Virgo Collaboration, and Event Horizon Telescope have tested its strong-field predictions.

Mathematical framework

The theory employs differential geometry developed by Bernhard Riemann and refined by Élie Cartan, using the metric tensor, Levi-Civita connection, and Riemann curvature tensor. Field equations couple spacetime curvature to energy–momentum via the Einstein field equations, whose formulation echoes work by David Hilbert and Noether's theorem by Emmy Noether. Exact solutions include the Schwarzschild metric by Karl Schwarzschild, the Kerr metric by Roy Kerr, and the Friedmann–Lemaître–Robertson–Walker (FLRW) metrics associated with Alexander Friedmann and Georges Lemaître. Mathematical tools from tensor analysis and spinor methods introduced by Élie Cartan and Paul Dirac, and global techniques by Roger Penrose and Stephen Hawking, underpin singularity theorems and causal structure, while formulations by Richard Feynman, Julian Schwinger, and Gerard 't Hooft inform semiclassical approaches. Numerical relativity, advanced at institutions like Max Planck Institute for Gravitational Physics and Rochester, employs computational methods developed by Susan Hahn, Richard H. Price, and Frans Pretorius to simulate mergers observed by LIGO and Virgo.

Predictions and experimental tests

Predictions such as the perihelion precession of Mercury first highlighted by Urbain Le Verrier were explained by Einstein’s field equations, while light bending observed by Arthur Eddington, gravitational redshift measured by Pound and Rebka, and time dilation confirmed by atomic-clock experiments at National Institute of Standards and Technology (NIST) validate the theory. The existence of gravitational waves predicted by Einstein and searched for by Joseph Weber led to direct detection by LIGO and Virgo, spearheaded by Rainer Weiss, Kip Thorne, and Barry Barish. Black hole candidates identified in X-ray binaries and galactic centers, with observational work by Riccardo Giacconi, Maarten Schmidt, and Andrea Ghez, confirm properties of the Kerr solution; the Event Horizon Telescope imaged the shadow of the supermassive object in Messier 87 under the leadership of Sheperd Doeleman and the Event Horizon Telescope Collaboration. Cosmological tests via observations by Hubble Space Telescope, Planck, Wilkinson Microwave Anisotropy Probe (WMAP), and the Sloan Digital Sky Survey constrain FLRW models and parameters first considered by Alan Guth, Andrei Linde, and Paul Steinhardt. Precision timing of binary pulsars by Hulse and Taylor provided indirect evidence for gravitational radiation prior to LIGO detections.

Astrophysical and cosmological applications

Applications span black hole astrophysics studied by Subrahmanyan Chandrasekhar and Martin Rees, gravitational lensing used by Fritz Zwicky and Vera Rubin to probe dark matter, and cosmology tracing expansion histories in work by Edwin Hubble, Georges Lemaître, and Alexander Friedmann. Structure formation studies involve James Peebles, Simon White, and Carlos Frenk; inflationary scenarios incorporate Alan Guth, Andrei Linde, and Alexei Starobinsky. Observational programs at Keck Observatory, Very Large Telescope, Chandra X-ray Observatory, and the Square Kilometre Array inform models of galaxy evolution connected to Vera Rubin’s rotation-curve research. Dark energy, inferred from Type Ia supernovae surveys by Saul Perlmutter, Adam Riess, and Brian Schmidt, sits alongside alternatives such as modified gravity theories explored by theorists at CERN, Kavli Institute, and Perimeter Institute. Gravitational-wave astronomy opens multimessenger studies linking LIGO/Virgo detections with electromagnetic follow-up by Swift, Fermi, and IceCube.

Extensions and alternatives

Efforts to quantize the theory include canonical quantization by Bryce DeWitt, loop quantum gravity developed by Carlo Rovelli and Lee Smolin, and string theory advanced by Michael Green, John Schwarz, and Edward Witten. Semiclassical approaches by Stephen Hawking and Jacob Bekenstein led to black hole thermodynamics and Hawking radiation. Alternatives and modifications such as f(R) gravity, Brans–Dicke theory by Carl Brans and Robert Dicke, massive gravity, and MOND proposed by Mordehai Milgrom have been investigated alongside constraints from experiments at CERN, LHC experiments, and cosmological surveys like DES and Euclid. Effective field theory methods by Donoghue and others provide low-energy expansions, while asymptotic safety programs trace back to Steven Weinberg. Mathematical generalizations by Élie Cartan and Tullio Regge inform discrete approaches used in Regge calculus.

Philosophical and conceptual implications

The geometric reinterpretation influenced philosophy of science through figures such as Hans Reichenbach, Moritz Schlick, and Thomas Kuhn, intersecting debates in epistemology, realism, and the nature of spacetime discussed at institutions like the University of Vienna and London School of Economics. Issues concerning determinism, causality, and the problem of time appear in work by John Bell, David Albert, and Julian Barbour. Conceptual puzzles such as the information paradox involve Stephen Hawking, Don Page, and Gerard 't Hooft, while dialogue with quantum theory engages Niels Bohr, Werner Heisenberg, and Hugh Everett. Ongoing philosophical inquiry considers the implications for ontology and scientific methodology in light of empirical programs led by the Royal Society, National Academy of Sciences, and international collaborations.

Category:Physics