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A letter history of black holes



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Late in 2018, the gravitational wave observatory, LIGO, has announced that it has become the most distant and massive source of ripples of spacetime ever watched: Only since 201

5 have we been able to observe these invisible astronomical bodies, which can only be detected by their gravitational attraction. The history of our hunt for these enigmatic objects traces back to the 18th century, but the crucial phase took place in a suitably dark period of human history – World War II.
                                

John Michell and later Pierre-Simon Laplace in the 18th century.

The concept of a body that would trap light, has now become invisible to the rest of the universe. They use Newton's gravitational laws to calculate the escape velocity of a light particle from a body, predicting the existence of stars so that they could not escape from them. Michell called them "dark stars".

But after the discovery that light took the form of a wave in 1801, it became unclear how light would be affected by the Newtonian gravitational field. It took roughly 1913 to understand Einstein's General Relativity Theory in 1915, and Karl Schwarzschild's solution to this problem a year later.

predicted the existence of a critical circumference of a body, beyond which would be light to cross: the Schwarzschild radius. This idea was similar to that of Michell, but now this critical circumference was understood as an impenetrable barrier.

It was only in 1933 that George Lemaître showed that this impenetrability was only an illusion that a distant observer would have. Alice and Bob illustration, the physicist hypothesised that if Bob stood still while Alice jumped into the black hole, Bob would see Alice's image slowing down just before reaching the Schwarzschild radius. Lemaître thus showed that in reality, Alice crosses that barrier: Bob and Alice just experience the event differently.

Despite this theory, at the time there is no known object of such a size, nothing even close to a black hole. So nobody believed that something similar to the dark stars as hypothesised by Michell would exist. In fact, no one dared to treat the possibility with seriousness. Not until World War II.

From dark stars to black holes

On September 1 1939, the Nazi German army invaded Poland, the world's history forever went down. Remarkably, it's on this very same day. The now acclaimed article, On Continued Gravitational Contraction, by Robert Oppenheimer and Hartland Snyder, two American physicists, what a crucial point in the history of black holes. This is the timing of the theory of black holes.

Tetra Quark / Wikimedia Commons, CC BY-SA ">
                     A letter history of black holes
The Schwarzschild radius. Credit: Tetra Quark / Wikimedia Commons, CC BY-SA
    

This was Oppenheimer's third and final papers in astrophysics. In it, he and Snyder predict the continued contraction of a star under the influence of its own gravitational field, creating a body with an intense force that could not even escape from it. This is the first version of the modern concept of a black hole, so astronomical body that it can only be detected by its gravitational attraction.

In 1939, this was still an idea that was too strange to be believed. It would take two decades until the concept of physicists would start to accept the consequences of contraction. And World War II itself was a crucial role in its development because of the US government's investment in researching atomic bombs.

Reborn from the ashes

Oppenheimer, of course, which is not only an important character in the history of black holes.

Politicians understood the importance of investing in science in order to bring military advantage. Consequently, across the board, there was a lot of investment in war-related revolutionary physics research, nuclear physics and the development of new technologies. All sorts of physicists dedicated themselves to this kind of research, as well as to immediate consequences, the fields of cosmology and astrophysics were mostly forgotten, including Oppenheimer's paper.

astronomical research, the discipline of physics thrived as a whole as a result of the war – in fact, military physics ended up augmenting astronomy. The US left was as the center of modern physics. The number of Ph.D.'s skyrocketed, and a new tradition of postdoctoral education was set up.

By the end of the war, the study of the universe was rekindled. There is a renaissance in the once underestimated theory of general relativity. The theory has been changed.

Princeton University has become the center of a new generation of relativists. It was there that the nuclear physicist, John A Wheeler, who later popularized the name "black hole", had his first contact with general relativity, and reanalyzed Oppenheimer's work. Skeptical at first, the influence of close relativists, new advances in computational simulation and radio technology – developed during the war – turned it into the greatest enthusiast for Oppenheimer's prediction on the day that was broke out, September 1 1939.

Since then The measurement of the gravitational waves created in a black hole binary system. What the first concrete proof that black holes exist.
                                                                


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