The General Theory of Relativity created by Einstein in 1915 predicted an object with such a high mass that nothing can escape from its gravitional pull.
Many scientists didn’t take this prediction seriously and it was considered a side-effect of the calculations. This exotic phenomenon, commonly known as a “Black hole” was a term coined by astrophysicist John Wheeler.
Einstein actually had a very hard time grasping the mathematics around his theory, mostly due to the fact that it was very difficult mathematics (tensor calculus). When his theory of General Relativity was published, one of the first people to realize its implications was a German astrophysicist named Karl Schwarzchild.
Schwarzchild added the finishing touches to Einstein’s mathematical calculations. General Relativity suggests that gravity is not a force but a distortion of space-time. When you, an object with mass, stand on a trampoline the fabric gets stretched and warped downward and any objects near by would fall towards you. Conceptually, this is how General Relativity works. The escape velocity would be the velocity an object would have to travel at to get out of the pit created by your mass i.e. your gravitional pull.
Schwarzchild realized that the escape velocity of an object depends on its radius and its mass. For example, the escape velocity of Earth is 11.2km/sec while the escape velocity of the moon is only 2.4km/sec. He became aware of the fact that if you keep the mass the same but continue to make the radius smaller i.e. increase the density you would get an escape velocity greater than the speed of light. Since nothing can travel faster than the speed of light, nothing can escape from the gravitional pull of the object. This is how Schwarzchild, using Einstein’s equations came to the conclusion that black holes exist.
The radius required to create a black hole based on an objects mass is called the Schwarzchild radius. If an object reaches its Schwarzchild radius then its atomic and subatomic forces can’t hold its gravity so it collapses into a singularity. A singularity is an infinitesimally small point with zero volume but infinite density.
Schwarzchild had predicted a very strange phenomenon and it left many scientists curious. Many scientists didn’t take the idea of a black holes seriously. They were considered an interesting prediction of General Relativity but not an accurate represenation of reality. 5 decades later everyone would be surprised.
We can really see black holes because nothing can escape from one to give us information that it exists. However, black holes have effects on their surroundings. A black hole creates an accretion disk which is a collection of stellar gas, dust and other debris that is close to falling into the black hole but is far enough away. The debris in the accretion disk is accelerated to such high velocities that it emitts x-rays.
The first black hole was discovered in 1971 and it’s named Cygnus X-1. British astronomers, Louise Webster and Paul Murdin at the Royal Greenwich Observatory observed a massive but invisible source of x-ray radiation 6000 light years away which was later identified as a black hole.
Science predicted such an exotic and strange phenomenon that we couldn’t even observe for another 5 decades! Something our own eyes will never be able to see!