Black hole
In 1784, John Michell published a letter suggesting that some stars could be so massive and dense that light would never escape their gravity. He calculated that a star with the same density as the Sun but five hundred times its radius would trap all emitted light. This idea remained obscure until Pierre-Simon Laplace independently proposed similar thoughts in his 1796 book on the origin of the Solar System. Laplace later removed these comments from subsequent editions after Thomas Young's wave theory of light cast doubt on the corpuscular nature of light used in his calculations. For over a century, the concept existed only as a mathematical curiosity without physical evidence or mainstream acceptance.
Albert Einstein published his general theory of relativity in 1917, explaining how matter affects spacetime and how this curvature influences motion. Just months later, Karl Schwarzschild found the first exact solution to Einstein's equations for a non-rotating, uncharged mass. His work revealed a critical radius where terms in the equations became infinite, though physicists did not understand its meaning at the time. Arthur Eddington discussed this compressed state in a 1926 popular science book but dismissed it as absurd rather than real. Einstein himself tried to prove black holes impossible in 1939 by assuming pressure forces would always prevent collapse beyond this critical value.
From the mid-1960s through the mid-1970s, theoretical physics experienced what historians call the golden age of black hole research. Roy Kerr discovered an exact solution for rotating black holes in 1963, while Ezra Newman found solutions for objects that were both rotating and electrically charged two years later. Werner Israel proved in 1967 that non-spinning black holes could only be defined by their mass alone. Roger Penrose demonstrated in 1965 that singularities must appear in all black holes according to general relativity without quantum mechanics. The discovery of pulsars by Antony Hewish and Jocelyn Bell Burnell in 1967 provided physical evidence for compact objects formed by gravitational collapse.
In 1971, researchers independently identified Cygnus X-1 as the first astronomical object commonly accepted to be a black hole. Louise Webster and Paul Murdin, along with Charles Thomas Bolton, analyzed data from Greenwich and Toronto observatories showing the source was part of a binary system with the supergiant star HDE 226868. Their calculations revealed the invisible companion had too much mass to be either a white dwarf or neutron star. By the end of 1973, the scientific community generally accepted this stellar-mass black hole candidate. This confirmation came decades before similar recognition would be granted to supermassive black holes at galactic centers.
On the 11th of February 2016, the LIGO Scientific Collaboration announced the first direct detection of gravitational waves named GW150914. Two black holes approximately 1.4 billion light-years away merged, creating ripples in spacetime that traveled across the universe. The resulting black hole weighed about 62 solar masses while three solar masses radiated away as gravitational energy. Rainer Weiss, Kip Thorne, and Barry Barish received the Nobel Prize in Physics in 2017 for their leadership on the project. On the 10th of April 2019, the Event Horizon Telescope published the first direct image of a black hole's shadow surrounding Messier 87's galactic center.
The event horizon marks the boundary where nothing can escape inward toward the singularity at the center. For non-rotating black holes, this surface forms a perfect sphere with radius proportional to mass through the Schwarzschild equation. Rotating black holes create an ergosphere outside the event horizon where frame dragging forces matter into rotation faster than light speed allows stillness. A photon sphere exists at 1.5 times the Schwarzschild radius where light rays orbit completely around the object before falling in or escaping. Accretion disks form when gas falls toward the black hole, heating to millions of degrees and emitting X-rays visible from Earth.
Black holes typically form when massive stars exhaust their hydrogen fuel and undergo gravitational collapse during supernova explosions. Stars below about eight solar masses become white dwarfs held by electron degeneracy pressure while more massive ones may form neutron stars via neutron degeneracy pressure. If stellar cores exceed these limits, even neutron degeneracy fails and collapse continues until a black hole forms. Primordial fluctuations in the early universe might have created micro black holes as small as Planck mass that evaporate quickly through Hawking radiation. Supermassive black holes grow by absorbing surrounding matter or merging with other black holes over billions of years.
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Common questions
When did John Michell first suggest that some stars could be so massive and dense that light would never escape their gravity?
John Michell published a letter suggesting this phenomenon in 1784. He calculated that a star with the same density as the Sun but five hundred times its radius would trap all emitted light.
Who discovered the first exact solution to Einstein's equations for a non-rotating uncharged mass in 1916?
Karl Schwarzschild found the first exact solution to Einstein's equations just months after Albert Einstein published his general theory of relativity in 1917. His work revealed a critical radius where terms in the equations became infinite though physicists did not understand its meaning at the time.
What year was Cygnus X-1 identified as the first astronomical object commonly accepted to be a black hole?
Researchers independently identified Cygnus X-1 as the first astronomical object commonly accepted to be a black hole in 1971. Louise Webster and Paul Murdin along with Charles Thomas Bolton analyzed data from Greenwich and Toronto observatories showing the source was part of a binary system with the supergiant star HDE 226868.
On what date did the LIGO Scientific Collaboration announce the first direct detection of gravitational waves named GW150914?
The LIGO Scientific Collaboration announced the first direct detection of gravitational waves named GW150914 on the 11th of February 2016. Two black holes approximately 1.4 billion light-years away merged creating ripples in spacetime that traveled across the universe.
When did the Event Horizon Telescope publish the first direct image of a black hole's shadow surrounding Messier 87's galactic center?
The Event Horizon Telescope published the first direct image of a black hole's shadow surrounding Messier 87's galactic center on the 10th of April 2019. This event marked a major milestone following the discovery of gravitational waves by Rainer Weiss Kip Thorne and Barry Barish who received the Nobel Prize in Physics in 2017 for their leadership on the project.