According to the currently accepted black hole mode, a black hole is a spacetime region (the 4-dimensional conceptual model combining the three dimensions of space and the dimension of time) surrounding a point of infinite density, the singularity, where both space and time break down, where the spacetime defining that precise location would cease to exist. Around that point, in a region bound by the event horizon, no particles nor any form of electromagnetic radiation can escape.
To our day, other than being fascinating objects of study, full of grandeur, whose impact, despite the astronomical distances separating us from black holes, can now be detected in laboratories, black holes provide for extreme test cases for theories such as quantum gravity. Equations and laws may seem flawless under laboratory conditions. However, black holes allow scientists to study what happens to these in the most extreme conditions that we know of in our universe with respect to temperature, mass, density, and time.
It is only not so long ago that scientists were able to observe black holes that had remained purely speculative and theoretical objects for centuries. As such, the following article will provide a brief journey through the development of our knowledge on black holes, outlining the most important discoveries and milestones leading up to the first image of a black hole obtained by the Event Horizon Telescope in 2019.
It all started in the year 1784 when a natural philosopher, John Michell, speculated about an astronomical body whose escape velocity, the velocity needed to escape its gravitational pull, would exceed the speed of light and came up with theoretical stars so dense not even a single light particle would be able to escape it and as such would not be seen, hence the name: “dark stars.” Shortly after, the mathematician Pierre-Simon Laplace independently calculated the radius of such objects. A process that yielded a radius as small as 6 kilometers for a body as massive as the Sun.
However, this initial concept of black holes was quickly dropped when the light was discovered to be a wave. For around 115 years, scientists could not comprehend how a wave could be affected by gravitational phenomena.
Einstein’s theory of relativity
The modern history of black holes began around the year 1915, when physicist Albert Einstein proposed his groundbreaking theory of General Relativity, offering an improved view on the universe and its phenomena, such as the motion and functioning of our universe’s massive objects. However, Einstein was not the one to come up with elegant solutions to his equations when studying black holes. Another physicist, Karl Schwarzschild, determined the radius necessary to which a planet of a certain mass had to be shrunk to collapse under its own gravitational pull.
Subsequently, any mass which falls within that radius would be unable to escape. This discovery contributed to a major step in understanding black holes. Today, the said radius is known as the Schwarzschild radius, giving rise to a boundary around the black hole, better known as the event horizon.
In 1930, physicist Subrahmanyan Chandrasekhar challenged a theory proposed by his own professor, Arthur Eddington, and contributed to a major step toward understanding the conditions necessary for black hole formation. While studying white dwarfs, extremely dense stars made up of mostly electron-degenerate matter, Eddington proposed that the reasons these stars did not eventually collapse into black holes were that this electron-degenerate matter or close packing of electrons exerted an increasing pressure, countering the pull of gravity towards the stellar core.
However, Chandrasekhar introduced some nuance to this assertion by limiting how massive white dwarf stars can get while still sustaining such pressure. A limit which is today known as the Chandrasekhar limit. Chandrasekhar’s work was a milestone in the history of black holes.
- The concept of Chandrasekhar limit in Astrophysics
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- The three types of black holes
WWII and Oppenheimer
WWII proved to be one of the turning points in the history of black holes. However, it was not recognized as such until about 20 years later due to the distraction brought about by Nazi Germany’s invasion of Poland in 1939. During that time, Robert Oppenheimer published a paper suggesting that even more massive stellar bodies than those suggested by Chandrasekhar were susceptible to succumbing to their own gravitational pull. However, in the midst of the war and due to the lack of concrete proofs or observations, Oppenheimer’s theories remained widely unconsidered and were discarded.
Scientists supporting relativity remained skeptical but grew keener on the idea of black holes. In 1963, Princeton University physicist John Wheeler officially popularised such black bodies under ‘black holes.’
In the same year, physicist Roy Kerr also independently developed another solution to Einstein’s equations of General Relativity, which would explain the existence of spinning black holes. To our day, we know that black holes may carry various properties, amongst which are mass, charge, and angular momentum.
- The concept of Hawking radiation from black holes
- The most important diagram in Astrophysics
- Nuclear reactions in stars
Black holes can emit radiation
Asserted in complete contradiction with a black hole’s fundamental concept, which states that nothing, no energy, may escape a black hole: this is what renowned physicist Stephen Hawking concluded in 1974. The universe accounts for continuous quantum fluctuations: the random and temporary change in energy at a set point. These fluctuations may give rise to pairs of particles with their anti-particle, which immediately annihilates.
However, if such an occurrence were to appear right on the event horizon of a black hole, and one of them would fall into the black hole while the other remained on the outside, annihilation would be unable to happen, and the black hole will have emitted a particle under the form a fundamental particle and black holes, over vast timescales would disappear by evaporating due to what is now known as Hawking radiation.
The culmination of our knowledge on black holes
After numerous theoretical phenomena and properties of black holes were concretized over the years, the scientific community and research on black holes reached a high point in 2015 when LIGO (Laser Interferometer Gravitational-Wave Observatory) detected the merger of two black holes while matching the predictions of the Theory of Relativity. This was the first proof of their existence, finally putting an end to growing suspicion in the scientific community.
The most recent milestone came to a climax in April 2019, when the first image of a black hole was finally released. It was developed through data of a radio-telescope, the Event Horizon Telescope. It depicted the supermassive black hole M87*, the center of the gargantuan elliptical galaxy M87, which contained 6.5 billion solar masses and was situated 53 million light-years away.
But, The Debate Continues…
Since the beginning, astronomers have divided opinions on actual mathematical black holes as predicted by General Relativity. Scientists like Einstein, Eddington, McCrea, Mitra, among others, did not believe in true mathematical black holes. On the other hand, scientists like Hawking, Chandrasekhar, Thorne, among others, had a different opinion. Over time, many flaws were found in the currently accepted black holes model, and many alternative models were proposed. Some of them include gravastars, naked singularities, and MECO.
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Of all these models, MECO is quite promising. MECO stands for Magnetospheric eternally collapsing object and was first proposed by Indian astrophysicist Abhas Mitra and later generalized by Darryl J. Leiter and Stanley L. Robertson. One of the major differences between the true black holes and MECOs is that the latter can produce their own magnetic fields, a fact backed by observation. Also, as stated in the currently accepted model, MECOs do not collapse to a singularity. MECOs are eternally collapsing objects. They will only hit singularity at infinity. The entire concept of MECOs is given in this newly published book.
If you want to know more about the MECOs, you can follow this link. In this blog, Prof Mirta has written about the flaws in the current model and how MECOs overcome those flaws and prove to be a better theoretical model. The blog also contains links to all the peer-reviewed publications for reference. You can also contact Prof. Mitra here and watch his TEDx talk on flaws on the current black hole model below.