“Dark Stars” – A Brief History of Black Holes

Artist’s concept illustration of a supermassive black hole emitting an X-ray jet. Credit: NASA/JPL-Caltech At the end of 2018, the gravitational wave observatory, LIGOannounced that they had detected the most distant and massive source of

Artist’s concept illustration of a supermassive black hole emitting an X-ray jet. Credit: NASA/JPL-Caltech

At the end of 2018, the gravitational wave observatory, LIGOannounced that they had detected the most distant and massive source of space-time ripples never monitored: gravitational waves triggered by pairs of black holes colliding in deep space. It is only since 2015 that we have been able to observe these invisible astronomical bodies, which at the time could only be detected by their gravitational pull. Then, in a breakthrough in 2019, the Event Horizon Telescope captured a picture of a black hole and his shadow for the first time.

The history of our hunt for these enigmatic objects goes back to the 18th century, but the crucial phase took place in a sufficiently dark period of human history – the Second World War.

The concept of a body that would trap light, and thus become invisible to the rest of the universe, was first envisioned by the natural philosophers John Michell and later Pierre-Simon Laplace in the 18th century. They calculated the escape velocity of a luminous particle from a body using Newton’s laws of gravity, predicting the existence of stars so dense that light could not escape. Michell called them “black 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, so the idea of ​​dark stars was dropped. It took about 115 years to understand how light in wave form would behave under the influence of a gravitational field, with Albert Einstein’s discovery General relativity theory in 1915, and Karl Schwarzschild solution to this problem one year later.

Schwarzschild also predicted the existence of a critical circumference of a body, beyond which light could not pass: the Schwarzschild radius. This idea was similar to Michell’s, but now this critical circumference was understood as an impenetrable barrier.

Schwarzchild radius

The Schwarzchild radius. Credit: Tetra Quark/Wikimedia Commons, CC BY-SA

It was not until 1933 that George Lemaître watch that this impenetrability was only an illusion that a distant observer would have. Using the now famous illustration of Alice and Bob, the physicist theorized that if Bob stood still while Alice jumped into the black hole, Bob would see Alice’s image slow to freeze just before reaching the Schwarzschild radius. Lemaître also showed that in reality, Alice crosses this barrier: Bob and Alice simply experience the event differently.

Despite this theory, at the time there was no known object of such size, nothing even close to a black hole. As a result, no one believed that anything resembling the dark stars as Michell had hypothesized would exist. In fact, no one even dared to treat this possibility seriously. Not before World War II.

From black stars to black holes

On September 1, 1939, the Nazi German army invaded Poland, triggering the start of the war that forever changed world history. Remarkably, it was on this same day that the first academic paper on black holes was published. The now acclaimed article, On continuous gravitational contraction, by J Robert Oppenheimer and Hartland Snyder, two American physicists, was a crucial point in the history of black holes. This moment seems particularly odd considering the centrality of the rest of World War II to the development of black hole theory.

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

In 1939, it was still an idea too strange to be believed. It would take two decades for the concept to be sufficiently developed for physicists to begin to accept the consequences of the continuous contraction described by Oppenheimer. And World War II itself played a crucial role in its development, due to the US government’s investment in research atomic bombs.

Einstein Oppenheimer

Einstein and Oppenheimer, around 1950. Credit: Wikimedia Commons

Reborn from the ashes

Oppenheimer, of course, was not just an important figure in black hole history. He would later become the head of the Manhattan Projectthe research center that led to the development of atomic weapons.

Politicians understood the importance of investing in science in order to provide military advantage. Therefore, across the board, there has been a large investment in groundbreaking war-related physics research, nuclear physics, and the development of new technologies. All sorts of physicists devoted themselves to this type of research, and as an immediate result the fields of cosmology and astrophysics were mostly forgotten, including Oppenheimer’s paper.

Despite the lost decade for large-scale astronomical research, the discipline of physics as a whole flourished in the aftermath of the war – in fact, military physics eventually augmented astronomy. The United States left the war as the center of modern physics. The number of doctors skyrocketsand a new tradition of postgraduate training has been established.

When the war ended, the study of the universe was revived. There has been a revival of the once underrated theory of general relativity. The war changed the way we do physics: and ultimately it led to the fields of cosmology and general relativity getting the recognition they deserve. And that was fundamental to the acceptance and understanding of black holes.

princeton university then became the center of a new generation of relativists. It was here that 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 near relativists, new advances in computer simulation, and radio technology – developed during the war – made him the biggest enthusiast for Oppenheimer’s prediction on the day the war broke out, September 1, 1939.

Since then, new properties and new types of black holes have been theorized and discovered, but all this came to fruition only in 2015. The measurement of the gravitational waves created in a binary black hole system was the first concrete proof that black holes exist.

Written by Carla Rodrigues Almeida, Visiting Postdoctoral Fellow, Max Planck Institute for the History of Science.

This article was first published in The conversation.The conversation