Black Holes, Dark Bodies
A black hole is a region of spacetime that exhibits such a strong gravitational effect that nothing can escape from its interior, not even particles or electromagnetic radiation such as light. Today, it is generally accepted that there are supermassive black holes in the centers of most galaxies. This concept is usually thought to have originated in the 20th century with Albert Einstein’s theory of general relativity. However, John Michell was the true father of such a concept. In a paper sent to Cavendish in November 1783, which was later published in the Royal Society’s journal, he discussed how he discovered this exotic object.
Michell was devising a method to measure the mass of a star. He reasoned that when light particles were emitted from a star, the gravitational pull of the star would reduce their speed and produce a shift in the starlight. The speed of light was known, and Michell understood the concept of escape velocity and its dependence on the mass and size of the star. When he pondered what would happen if the escape velocity of a star were the same as the speed of light, he came up with the idea of black holes. He hypothesized that if the size and mass of a star were large enough, the proper gravity of the star would be able to attract light and make it return towards the star, thus making it invisible to astronomers. He thought that astronomers could not see many objects in the universe because they emitted no light. Nonetheless, he had an idea on how to identify them that is similar to how astronomers infer the existence of black holes today. According to him, these dark stars could be detected if they had a luminous twin circling them.
The contemporary understanding of black holes is based on the theory of general relativity. A black hole can be identified due to its interaction with matter and electromagnetic radiation. Black holes have only three visible physical properties: mass, charge and angular momentum. The defining feature of a black hole is the event horizon, a boundary where the deformation of spacetime is so strong that all possible paths for a particle take it closer to the center of the black hole. When matter or light falls into a black hole, all of its information is dispersed along the event horizon, is lost to outside observers and cannot escape the interior. According to the gravitational time dilation effect, clocks near a black hole appear to tick more slowly to distant observers, compared to those farther away from the black hole. Correspondingly, an object falling into a black hole appears to slow down as it reaches the event horizon. Likewise, the gravitational redshift effect makes the light emitted by objects redder and dimmer to observers, until it is no longer visible.
Gravity, stars and black holes were some of the topics Michell researched in his last years. He was a scientist with an incredibly broad understanding of the Earth, the universe and the laws governing them. John Michell was interested by the natural processes involved in stratigraphy, underground processes associated with earthquakes and volcanoes, and natural phenomena such as gravity. His research has been in obscurity because he never promoted his groundbreaking ideas. He was interested in many different fields, but his contributions to the field of geology are so significant that we can say that John Michell was a true geologist.
Michell was devising a method to measure the mass of a star. He reasoned that when light particles were emitted from a star, the gravitational pull of the star would reduce their speed and produce a shift in the starlight. The speed of light was known, and Michell understood the concept of escape velocity and its dependence on the mass and size of the star. When he pondered what would happen if the escape velocity of a star were the same as the speed of light, he came up with the idea of black holes. He hypothesized that if the size and mass of a star were large enough, the proper gravity of the star would be able to attract light and make it return towards the star, thus making it invisible to astronomers. He thought that astronomers could not see many objects in the universe because they emitted no light. Nonetheless, he had an idea on how to identify them that is similar to how astronomers infer the existence of black holes today. According to him, these dark stars could be detected if they had a luminous twin circling them.
The contemporary understanding of black holes is based on the theory of general relativity. A black hole can be identified due to its interaction with matter and electromagnetic radiation. Black holes have only three visible physical properties: mass, charge and angular momentum. The defining feature of a black hole is the event horizon, a boundary where the deformation of spacetime is so strong that all possible paths for a particle take it closer to the center of the black hole. When matter or light falls into a black hole, all of its information is dispersed along the event horizon, is lost to outside observers and cannot escape the interior. According to the gravitational time dilation effect, clocks near a black hole appear to tick more slowly to distant observers, compared to those farther away from the black hole. Correspondingly, an object falling into a black hole appears to slow down as it reaches the event horizon. Likewise, the gravitational redshift effect makes the light emitted by objects redder and dimmer to observers, until it is no longer visible.
Gravity, stars and black holes were some of the topics Michell researched in his last years. He was a scientist with an incredibly broad understanding of the Earth, the universe and the laws governing them. John Michell was interested by the natural processes involved in stratigraphy, underground processes associated with earthquakes and volcanoes, and natural phenomena such as gravity. His research has been in obscurity because he never promoted his groundbreaking ideas. He was interested in many different fields, but his contributions to the field of geology are so significant that we can say that John Michell was a true geologist.