What are black holes, anyway? - Part 1

Gobbling monsters, evaporating pin pricks and tiny balls of string

Aug. 5, 2020 |  Categories:  Space  

By Marquis Lee and Mark Davis, Theoretical Physicists

"Black holes" conjure up a lot of curiosity. The concept just sprang out of the mathematics of Einstein's General Theory of Relativity. Einstein himself did not believe in the concept; he thought the concept was far too outlandish to even bother to investigate.

Black Hole Dynamic Behavior and Surroundings Mapped by XMM-Newton ...

Black holes are said to be one of the weirdest things known to exist. The theoretical possibility of black holes has been around for almost a hundred years now. Yet, after almost 30 years of strong observational evidence, we still do not understand it fully.

A black hole 20 miles across would weigh about 5.4 times the mass of the Sun.

History of black holes

The first inklings of the possibility of a black hole date back as much as the 18th Century when a geologist, John Mitchell, wrote to the Royal Society of London suggesting that a star 500 times larger than the Sun would generate so much of gravitational force, that even light would not be able to escape from its clutches. John was inspired by the renowned natural philosophers of his time, who were succeeding in measuring the finite and constant speed of light.

The third Astronomer Royal, James Bradley was the first to successfully calculate the currently accepted speed of light. Working in Greenwich, London, in 1728, he discovered a strange movement in the position of stars, amounting to just 1/200 of a degree. At first, he thought he had detected stellar parallax. Stellar parallax is the apparent shift of position of any nearby star (or another object) against the background of distant objects. But he soon realized that all the stars he measured had the same amount of angular displacement and so, the effect could not be stellar parallax, which would have varied with the star's distance from the Earth. He proposed that the movement was caused by the finite speed of light. In the same way that you tilt your umbrella slightly against the rain because it looks like the rain is approaching you diagonally (that is caused by your motion), James Bradley was having to angle his telescope to compensate for the Earth's motion through space. The angle of the tilt allowed Bradley to calculate the speed of light in relation to the Earth. He computed a figure of 186,000 miles per second (or around 300,000 kilometers per second).

John Mitchell took Bradley's figure and used Newtonian gravity to estimate the size of the body needed to have an escape velocity (the minimum force required to escape the gravity of a body equal to the speed of light, and came to his final estimate of around 500 times the mass of the Sun. This idea sparked a debate as astronomers mulled the possibility of such "dark stars". Eventually, the natural philosophers of the time decided that Newton's laws precluded light from being affected by a gravitational field, and so the light would always leave a celestial body, no matter how strong its gravity. 

The matter rested 3for a couple of centuries until Einstein came along and published his General Theory of Relativity and proved that gravity did indeed affect light. Less than 2 months after Einstein's publication, German mathematician Karl Schwarzchild found that Einstein's equations allowed celestial bodies to become so dense that they create gravitational traps. The size of each trap dubbed its "Schwarzschild Radius", is determined by the amount of mass inside the celestial body. For example, a black hole the mass of the Earth would have a Schwarzschild radius of a small coin.

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On Aug. 20, 2020, 8:31 a.m.  Adith wrote:

Could you please explain the Lorentz Transformation in a future series?