At the end of 2018, the observatory of gravitational waves LIGO announced that they had discovered the most distant and massive source of space-time pulsations ever observed: waves caused by pairs of black holes colliding in deep space. Only from 2015, we were able to observe these invisible astronomical bodies, which can be detected only by their gravitational attraction. The history of our hunt for these mysterious objects dates back to the 18th century, but the decisive stage occurred in a rather dark period of human history – World War II.
The concept of a body that could hold light and thus become invisible to the rest of the universe was first considered by natural philosophers John Michel and then by Pierre-Simon Laplace in the 18th century. They used Newton's gravitational laws to calculate the rate at which a particle of light emanates from a body, predicting the existence of stars so dense that light could not escape from them. Michell called them "dark stars."
But after it was discovered in 1801 that light took the form of a wave, it became unclear how light would affect the Newtonian gravitational field, so the idea of dark stars was discarded. It took about 115 years to understand how light in the form of a wave will behave under the influence of a gravitational field, with the help of Albert Einstein's General Theory of Relativity in 1915 and Karl Schwarzschild's solution to this problem a year later.
Schwarzschild also predicted the existence of a critical body circumference beyond which light can not cross: the Schwarzschild radius. This idea was similar to the idea of Michell, but now this critical circle was understood as an impenetrable barrier.
Only in 1933, Georges Lemaitre showed that this impenetrability was only an illusion that a distant observer would have. Using the now famous illustration of Alice and Bob, the physicist suggested that if Bob stops while Alice jumps into a black hole, Bob will see Alice’s image slow down to freezing just before reaching the Schwarzschild radius. Lemaitre also showed that Alice actually overcomes this barrier: Bob and Alice simply experience the event differently.
Despite this theory, at that time there was not a single known object of this size, and even close to a black hole. So no one believed that something like dark stars, as Michel suggested, would exist. In fact, no one even dared to take this opportunity seriously. Until World War II.
From dark stars to black holes
On September 1, 1939, the fascist German army invaded Poland, which led to the beginning of the war, which forever changed world history. It is noteworthy that on this very day the first scientific article on black holes was published. The recently acclaimed article by J. Robert Oppenheimer and Hartland Snyder, two American physicists, “On the Continuation of Gravitational Compression” was a decisive moment in the history of black holes. This time seems especially strange if we take into account the central place of the Second World War in the development of the theory of black holes.
This was Oppenheimer's third and final article in astrophysics. In it, he and Snyder predict the continuation of the contraction of a star under the influence of its own gravitational field, creating a body with an intense attractive force from which even light cannot escape. It was the first version of the modern concept of a black hole, an astronomical body, so massive that it can be detected only by gravitational attraction.
In 1939, this idea was too strange to believe in it. It will take two decades until the concept is sufficiently developed for physicists to begin to recognize the consequences of the continuing reduction described by Oppenheimer. And the Second World War itself played a decisive role in its development due to the investment of the US government in the study of atomic bombs.
Reborn from the ashes
Oppenheimer, of course, was not only an important character in the history of black holes. He will later become the head of the Manhattan Project, a research center that led to the development of atomic weapons.
Politicians understood the importance of investing in science in order to gain a military advantage. Consequently, in all areas, there were significant investments in the research of revolutionary physics related to the war, nuclear physics, and the development of new technologies. All sorts of physicists have devoted themselves to these kinds of research, and as a direct consequence, the fields of cosmology and astrophysics have been largely forgotten, including the Oppenheimer article.
Despite the decade lost to large-scale astronomical research, the discipline of physics generally flourished as a result of the war — in fact, military physics ended in an increase in astronomy. The USA left the war as the center of modern physics. The number of doctors of science has increased dramatically, and a new post-doctoral education tradition has been created.
By the end of the war, the study of the universe was resumed. A renaissance arose in the once underestimated theory of the general theory of relativity. The war changed the way we do physics: and as a result, this led to the fact that the fields of cosmology and general relativity gained the recognition they deserve. And it was fundamental for accepting and understanding black holes.
Princeton University has become the center of a new generation of relativists. It was there that nuclear physicist John A. Wheeler, who later popularized the name “black hole”, first became acquainted with the general theory of relativity and re-analyzed Oppenheimer's work. Initially skeptical, the influence of close relativists, new advances in computer modeling and radio engineering, developed during the war, turned him into the greatest enthusiast of Oppenheimer's prediction on the day the war began, September 1, 1939.
Since then, new properties and types of black holes have been theorized and discovered, but all this culminated only in 2015. Measuring gravitational waves created in a black hole binary system was the first concrete proof of the existence of black holes.
Scientists have discovered the largest known black hole collision.