Black hole

• 1. Introduction

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  A black hole is so compact that its gravity stops everything, including light, from escaping. That boundary of no escape has a name: the event horizon. Cross it, and you are trapped inside, yet you feel no local change at the moment of crossing. Einstein's theory of general relativity, which treats gravitation as the curvature of spacetime, predicts that any sufficiently compact mass will form one. For a long time these objects were dismissed as a mathematical curiosity. So how did a strange solution to a set of equations become an object that astronomers photograph and weigh? Why did serious physicists, including Einstein himself, spend decades trying to prove black holes could not exist? And what happens at the central singularity, where the curvature of spacetime becomes infinite?

• 2. Curiosity To Consensus

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  In the late 18th century, English astronomer and clergyman John Michell imagined a body so massive that even light could not escape it. In a short part of a letter published in 1784, he calculated that a star of the sun's density but 500 times its radius would have an escape velocity exceeding the speed of light. Michell suggested such non-radiating bodies might be detected through their gravitational pull on nearby visible objects.

  Pierre-Simon Laplace reached a similar idea independently. In 1796, in his book Exposition du Systeme du Monde, the French scientist suggested a star could be invisible if it were large enough. When Franz Xaver von Zach asked for a mathematical analysis, Laplace provided one and published it in von Zach's journal. Both men pictured very large stars, not the extremely dense objects we now describe.

  The first solution of general relativity that would characterise a black hole arrived in 1916. Only by the late 1950s did physicists begin to read it as a region of space from which nothing can escape. The shift from curiosity to consensus came in the 1960s, when theoretical work showed black holes were a generic prediction of general relativity. The first widely accepted black hole, Cygnus X-1, was identified by several researchers independently in 1971.

• 3. The Reluctant Theorists

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  Karl Schwarzschild solved Einstein's field equations only a few months after they were published, assuming spherical symmetry with no spin. Johannes Droste, a student of Hendrik Lorentz, independently found the same solution a few months later. At a certain radius the solution became singular, with terms in the equations turning infinite. That radius later became the Schwarzschild radius, and at the time no one understood what it meant.

  Many early 20th-century physicists doubted black holes could be real. In a 1926 popular science book, Arthur Eddington criticised the idea of a star compressed to its Schwarzschild radius as a flaw in a poorly understood theory. In 1939, Einstein used general relativity to argue that black holes were impossible, leaning on pressure or centrifugal force balancing gravity. He missed the possibility that implosion would drive a system below the critical value.

  Subrahmanyan Chandrasekhar opened a different door. In 1931 he studied electron-degenerate matter and found it stable only below a limiting mass. When he announced his results, Eddington called the consequence absurd, insisting some unknown mechanism would halt the collapse. Lev Landau later took a similar view. The next breakthrough would not come from balancing forces but from following the collapse all the way down.

• 4. Watching A Star Freeze

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  Robert Oppenheimer and his student Hartland Snyder formulated the modern concept of black holes in 1939. Working in general relativity, J. Robert Oppenheimer and George Volkoff had already predicted that neutron stars below a certain mass, the Tolman-Oppenheimer-Volkoff limit, would be stable under neutron degeneracy pressure. Above that limit, either their model failed or gravitational contraction would not stop.

  In the Oppenheimer-Snyder model, Oppenheimer and Snyder solved Einstein's equations for an idealised imploding star and described the view from far outside. The star collapses inward as expected. As its density rises, gravitational time dilation grows, and from afar the collapse seems to slow until the star reaches its Schwarzschild radius and appears frozen in time.

  David Finkelstein clarified the picture in 1958, identifying the Schwarzschild surface as an event horizon. He called it "a perfect unidirectional membrane: causal influences can cross it in only one direction." Events inside cannot affect events outside. Finkelstein built a new reference frame to include the view of infalling observers, linking events at the surface of an imploding star to events far away. By 1962 the two viewpoints were reconciled, persuading many sceptics that implosion into a black hole made physical sense.

• 5. The Golden Age

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  The era from the mid-1960s to the mid-1970s was called the golden age of black hole research, when general relativity and black holes became mainstream subjects. In 1963, Roy Kerr found the exact solution for a rotating black hole. Two years later, Ezra Newman found the solution for a black hole both rotating and electrically charged.

  Werner Israel showed in 1967 that any non-spinning, uncharged collapsing star yields a spherically symmetric black hole, with any asymmetry somehow vanishing. In 1972, Richard H. Price found that the asymmetry was converted into gravitational waves. Wheeler captured this stripping away of detail with the phrase "a black hole has no hair." The full no-hair theorem took another 15 years and many physicists, and it states that a stationary black hole is described by just three parameters of the Kerr-Newman metric: mass, angular momentum, and electric charge.

  Roger Penrose proved in 1965 that general relativity predicts singularities appear in all black holes, though this may change once quantum mechanics is included. Vladimir Belinski, Isaak Khalatnikov, and Evgeny Lifshitz had tried to show singularities would not appear in generic, asymmetric cases, and they later reversed their positions. Observation kept pace with theory. In 1967, Antony Hewish and Jocelyn Bell Burnell discovered pulsars, shown by 1969 to be rapidly rotating neutron stars, proving that compact objects from gravitational collapse were physically real.

• 6. From X-Rays To Thermodynamics

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  In 1971, X-ray telescope observations by Riccardo Giacconi's team showed Cygnus X-1 emitting X-rays in a rapid, sporadic way consistent with a compact source. Optical spectroscopy and modelling matched a binary system: a massive ordinary star feeding gas into an invisible compact companion that generated the X-rays. By 1974 the object was widely considered a black hole, though 100% confidence for Cygnus X-1 may not be possible. In 2011, the masses were estimated at 14.1 for the black hole and 19.2 for its stellar companion.

  Work by James Bardeen, Carter, and Hawking in the early 1970s produced the laws of black hole mechanics, which mirror the laws of thermodynamics. Jacob Bekenstein strengthened the analogy, linking mass, surface area, and surface gravity to energy, entropy, and temperature. The link held only as an analogy until quantum mechanics entered the story.

  Hawking completed it in 1974, showing that quantum field theory implies black holes radiate like a black body, with a temperature proportional to surface gravity. This effect is now called Hawking radiation, and its rate of emission is inversely proportional to mass. The catch is stark: even stellar black holes gain mass from the cosmic microwave background faster than they lose it this way. A stellar black hole's Hawking temperature of 62 nanokelvins sits far below the 2.7 K of the cosmic background, so such holes grow rather than shrink.

• 7. Weighing The Invisible

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  Sagittarius A*, the radio source at the core of the Milky Way, contains a supermassive black hole of about 4.3 million solar masses. Astronomers reached that conclusion by tracking the motions of over 100 stars orbiting an invisible object at that position. One star, called S2, completed a full orbit. Fitting these motions to Keplerian orbits pinned the hidden mass into a radius of less than 0.002 light-years, with no other plausible way to confine so much unseen mass in so small a space.

  Supermassive black holes earned recognition more slowly than stellar ones. When the Hubble Space Telescope launched in the 1990s, optical studies of galaxy Messier 87 revealed a large concentration of mass. In 1995, microwave spectra from the Very Long Baseline Array tracked masers orbiting the centre of NGC 4258, and the orbits ruled out dense stellar clusters, leaving supermassive black holes as the only explanation. In 1999, David Merritt proposed the M-sigma relation, tying a galaxy bulge's velocity dispersion to the mass of its central black hole.

  Gravitational waves opened a direct channel. In late 2015, the LIGO Scientific Collaboration and Virgo Collaboration made the first direct detection, GW150914, the first observation of a black hole merger. The two holes were roughly 30 and 35 solar masses, about 1.4 billion light-years away. In 2017, Rainer Weiss, Kip Thorne, and Barry Barish received the Nobel Prize in Physics. On the 10th of April 2019, the Event Horizon Telescope published the first direct image of a black hole, the supermassive object at the centre of Messier 87.

• 8. Inside The Horizon

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  For a non-rotating black hole, the singularity is a single point; for a rotating one it smears into a ring lying in the plane of rotation. In both cases the singular region has zero volume, yet holds all of the black hole's mass, giving it effectively infinite density. An observer falling into a Schwarzschild black hole cannot avoid being carried inward once past the event horizon.

  Tidal forces tear an infalling observer apart in a process sometimes called spaghettification or the noodle effect, stretching and compressing them as they approach the centre. Rotating or charged black holes add an inner horizon, the Cauchy horizon, split into ingoing and outgoing sections. At the ingoing section, infalling matter and radiation build up and drive the spacetime curvature toward infinity, a phenomenon called mass inflation.

  Some physicists argue that in realistic black holes, accretion and Hawking radiation would stop mass inflation from occurring. Alternative theories soften the picture further. The fuzzball model, based on string theory, holds that black holes are made of quantum microstates and need not have a singularity or an event horizon at all. Loop quantum gravity proposes that the curvature and density at the centre are large but not infinite, which keeps the question of what truly lies inside an open one, bound up with the information loss paradox.