A Black Hole is an area of spacetime where gravity is so intense that no electromagnetic wave, not even light, has the energy to escape. According to general relativity theory, a compact enough mass can bend spacetime into a black hole. The event horizon is the line beyond which there is no escape. Despite having a significant impact on the outcome and circumstances of an object traversing it, general relativity states that it lacks any locally observable characteristics. A black hole behaves in many ways like a perfect black body since it does not reflect light.
Additionally, event horizons are predicted to emit Hawking radiation, which has the same spectrum as a black body and a temperature that is inversely proportional to mass, according to quantum field theory in curved spacetime. For stellar black holes, this temperature is on the order of billionths of a kelvin, making direct observation virtually difficult.
John Michell and Pierre-Simon Laplace were the first to think about objects whose gravitational fields are so powerful that light cannot escape. Karl Schwarzschild devised the first general relativity model that would adequately describe a black hole in 1916. In 1958, David Finkelstein published the first article defining a “black hole” as an area of space from which nothing can egress.
Black holes were once thought to be a mathematical oddity, but theoretical research in the 1960s revealed that they were actually a general prediction of general relativity. Jocelyn Bell Burnell’s 1967 discovery of neutron stars stimulated research into gravitationally compacted compact objects as a potential astrophysical reality. Cygnus X-1 was the first black hole to be discovered, and it was done so independently by many researchers.
When large stars end their lives and collapse, black holes of stellar mass are created. A black hole can expand by absorbing mass from its surroundings after it has created. It is possible for supermassive black holes to absorb additional stars and merge with other black holes to produce masses of millions of solar masses (M). Most galaxies’ centers are believed to contain supermassive black holes.
Through its interactions with other stuff and electromagnetic radiation like visible light, black holes can be detected. Any material that falls into a black hole has the potential to create an exterior accretion disk that is heated by friction and gives rise to quasars, some of the brightest objects in the universe. Supermassive black holes can shred stars into streamers that shine brilliantly before being “swallowed” if they are approached too closely.
If there are stars orbiting a black hole, the mass and location of the black hole can be inferred from the stars’ orbits. By making such findings, one can rule out potential alternatives like neutron stars. The Sagittarius A* radio source, at the center of the Milky Way galaxy, has a supermassive black hole with an estimated mass of 4.3 million solar masses, according to astronomers, who have also detected a number of star black hole candidates in binary systems.
Albert Einstein created his theory of general relativity in 1915 after demonstrating earlier that gravity does affect the velocity of light. Karl Schwarzschild solved the Einstein field equations that explain the gravitational field of a point mass and a spherical mass only a few months later. Johannes Droste, a Hendrik Lorentz student, independently provided the identical answer for the point mass a few months after Schwarzschild and went into greater detail regarding its characteristics. At what is now known as the Schwarzschild radius, this solution exhibited odd behavior when it become singular, which meant that some of the terms in the Einstein equations turned infinite.
At the time, it was unclear exactly what kind of surface this was. Arthur Eddington demonstrated in 1924 that the singularity vanished with a change in coordinates, but it wasn’t until 1933 that Georges Lemaître understood this meant the Schwarzschild radius singularity was a non-physical coordinate singularity.
In a book published in 1926, Arthur Eddington did address the possibility of a star with mass compressed to the Schwarzschild radius, noting that Einstein’s theory allows us to rule out excessively high densities for visible stars like Betelgeuse because “a star of 250 million km radius could not possibly have so high a density as the Sun.” First, light couldn’t escape because the gravitational pull would be so strong.
Second, the spectral lines’ red shift would be so extreme that the spectrum would be completely shifted. Thirdly, the mass would cause the spacetime metric to curve so strongly that space would contract around the star, leaving us outside (or nowhere).
The singularity at the Schwarzschild radius’s edge was regarded by Oppenheimer and his co-authors as proof that this was the edge of a bubble where time stood still. For independent observers, however, this is not a valid point of view. Because of this characteristic, collapsed stars were given the nickname “frozen stars” because, from a distance, the star’s surface appeared to be frozen in place at the precise moment that its collapse brought it to the Schwarzschild radius.
The Schwarzschild surface is an event horizon, according to David Finkelstein, who described it as “a perfect unidirectional membrane: causal influences can cross it in only one direction” in 1958. This broadened Oppenheimer’s findings to take into account the viewpoint of infalling observers rather than directly contradicting them. The Schwarzschild solution was expanded by Finkelstein to account for observers falling into black holes in the future. Martin Kruskal had previously discovered a complete extension, and he was persuaded to disclose it.
These findings arrived at the start of general relativity’s “golden age,” which was characterized by general relativity and black holes becoming popular research topics. This procedure was aided by Jocelyn Bell Burnell’s 1967 discovery of pulsars, which by 1969 had been identified as rapidly rotating neutron stars. Until that point, neutron stars and black holes were thought of as purely theoretical wonders, but the discovery of pulsars demonstrated their physical significance and sparked an increased interest in all kinds of compact objects that might be created by gravitational collapse.[Reference needed]
More general black hole solutions were discovered during this time. Roy Kerr discovered the precise answer to a revolving black hole in 1963. Ezra Newman discovered the axisymmetric answer for a revolving, electrically charged black hole two years later.The no-hair theorem, which states that the three components of the Kerr-Newman metric—mass, angular momentum, and electric charge—completely characterize a stationary black hole solution, was developed through the efforts of Werner Israel, Brandon Carter, and David Robinson.
The first direct gravitational wave detection—the first discovery of a black hole merger—was made public on February 11, 2016, by the Virgo and LIGO Scientific Collaborations. Following studies of the supermassive black hole in the galactic center of Messier 87 by the Event Horizon Telescope (EHT) in 2017, the first direct image of a black hole and its surroundings was released on April 10, 2019. The closest known object that is believed to be a black hole as of 2021 is located roughly 1,500 light-years (460 parsecs) distant.
There are believed to be hundreds of millions of black holes in the Milky Way, the majority of which are solitary and do not emit radiation, despite the fact that only a few dozen have been discovered so far. Consequently.
In a letter from November 1783 to Henry Cavendish, John Michell used the term “dark star”, and physicists started using the term “gravitationally collapsed object” in the early 20th century. The term “black hole” is credited to physicist Robert H. Dicke, who is said to have made the comparison in the early 1960s. The Black Hole of Calcutta is infamous for being a prison where inmates enter but never leave alive.
Life and Science News magazines first used the term “black hole” in print in 1963, and science writer Ann Ewing also used it in her article “‘Black Holes’ in Space” from 18 January 1964, which was a report on an American Association for the Advancement of Science meeting that took place in Cleveland, Ohio.
Properties and structure
The no-hair theory states that a black hole only has three independent physical properties once it reaches a stable state after formation: mass, electric charge, and angular momentum; the black hole is otherwise featureless. If the hypothesis is correct, any two black holes with the same values for these parameters are identical and cannot be distinguished from one another. It is currently unknown to what extent the conjecture holds true for actual black holes under the laws of contemporary physics.
These characteristics stand out because they can be seen from outside of a black hole. A charged black hole, for instance, repels other charged objects just like any other charged object. The gravitational equivalent of Gauss’s law (through the ADM mass), applied distant from the black hole, can also be used to determine the total mass inside a sphere containing a black hole. The gravitomagnetic field can also be used to detect angular momentum (or spin) from a distance, for example, through the Lense-Thirring effect.
The most basic static black holes are massless; they lack both angular momentum and electric charge. Schwarzschild black holes are the name given to these black holes in honor of Karl Schwarzschild, who discovered this answer in 1916. It is the only spherically symmetric vacuum solution, according to Birkhoff’s theorem. This indicates that there is no discernible difference between the gravitational field of a black hole like this one and that of any other spherical object with the same mass from a distance.
Therefore, the common perception that a black hole “sucks in everything” in its surroundings is only true close to the black hole’s horizon; distant from the black hole, the external gravitational field is the same as that of any other body of the same mass.
There are also solutions that describe black holes in a broader sense. The Reissner-Nordström metric describes non-rotating charged black holes, whereas the Kerr metric describes non-rotating non-charged black holes. The Kerr-Newman metric, which characterizes a black hole with both charge and angular momentum, is the most widely accepted stationary black hole solution.
One of the universe’s most intriguing and mysterious objects are black holes. These celestial behemoths, which were created when enormous stars died, have an unstoppable gravitational pull that snares anyone unlucky enough to enter their event horizon. They may appear to be cosmic vacuum cleaners, but they also have a huge impact on how we perceive the cosmos.