When Jules Verne wrote his novel From the Earth to the Moon , he knew that his projectile had to go out fast enough to overcome the gravitational pull of our planet. The cannon was supposed to launch it at a speed of about 40,000 km/h. This velocity is called the escape velocity. Compared to what our cars reach, it is immense, but quite small when compared to the 620 km/s (more than 2 million kilometers per hour!) necessary to escape from the surface of the Sun.
It is evident that the denser, more compact, a body is, the greater the speed necessary to overcome its gravitational field. Can it happen that there is one with a mass sufficient for its escape velocity to be equal to that of light? This same question was asked by the astronomer John Mitchell, the parish priest of Thornhill, in Yorkshire, England. In an article read on November 27, 1783 in the Royal Society of London -and published a year later in his Philosophical Transactions- , Mitchell wrote: “… light could not escape from a body that had the same density as the Sun but with a radius 500 times greater ”. Following the old and very healthy tradition of giving endless titles to writings, John Mitchell titled his article On the means of discovering the distance, magnitude, etc., of the fixed stars, in consequence of the diminution of their light, in case such a diminution should be found to take place in any of them, and such other data should be procured from observations, as would be further necessary for that purpose .
A few years later, in 1796, the mathematician and astronomer Pierre Simon Laplace studied the existence of these ‘dark bodies’. “It is possible that the largest luminous stars in the universe may be invisible,” he wrote in his Exposition of the World System . Both works were based on the corpuscular theory of light formulated by Newton and on his law of universal gravitation. However, different experiments carried out at the beginning of the 19th century revealed that light behaved like a wave and not as if it were made up of small particles. Seeing how one of the foundations of his prediction crumbled, Laplace retracted and those studies became a simple curiosity. Until Einstein and his general theory of relativity arrived.
It may be surprising that the first solution to the equations of general relativity was not found by Einstein himself . Who obtained it was the German astronomer Karl Schwarzschild, director of the Potsdam observatory . At the age of forty, he left the tranquility of his position to enlist as a volunteer after the outbreak of the Great War. In those conditions that were so inauspicious for developing scientific work, such as the trenches of the Russian front, he studied the articles that Einstein had presented at the Prussian Academy of Sciences in November 1915. A month later he found an analytical solution to the problem of a mass point located in empty space.
Unfortunately he was unable to defend his work at the Academy. During his stay on the eastern front he contracted a skin disease, pemphigus , an autoimmune disease whose basic characteristic is the appearance of extensive, very fragile blisters all over his body. It is accompanied by anorexia, tiredness, fever, joint pain… This situation worsens gradually and at that time -before the appearance of corticosteroids- it led to death. Urgently repatriated, he died on May 11, 1916 in a Potsdam hospital.
The manuscript excited Einstein , who read it at the Academy when Schwarzschild lay on his deathbed. In it he not only gave a correct description of the gravitational field of the Solar System, but also introduced the existence of black holes . Schwarzschild showed that if a mass is sufficiently concentrated, the curvature of space in nearby regions will reach such a magnitude that it will be separated, isolated, from the rest of the universe. We are facing a cosmic funnel : anything that rushes into it will be irretrievably lost and trapped there, with no possible connection to the rest of the universe.
Why? As we get closer to a body, the speed necessary to escape from its gravitational field is increasing. If that body is sufficiently massive and compact, at a certain distance the speed to escape from its gravitational embrace is exactly that of light. And if from that point we continue to get closer to the object, the escape velocity will become greater than the speed of light. As the speed of light marks the physical limit to all the possible speeds of the existing particles in our universe, nothing can get out: the light and everything that is in that region of space is trapped, with no possible connection with the rest of the universe. universe. This distance that marks the no-return limit is known as the Schwarzschild radius or the event horizon . Nothing that could happen inside it will be seen, heard or known by any external observer.
The idea that such an unspeakably strange body could exist revolted a large number of physicists, including Einstein himself. Many physicists devoted great efforts to find some mechanism that would prevent their existence in nature. Unfortunately for them, in 1939 Robert Oppenheimer -the father of the atomic bomb- and Hartland Snyder showed that such objects were not mere mathematical fireworks and could exist in the real world.