Ilustração artística de um buraco negro supermassivo emitindo um jato de raios-x. Crédito: NASA/JPL-Caltech
No final de 2018, o observatório de ondas gravitacionais, LIGO, anunciou que havia detectado a fonte mais distante e massiva de ondulações do espaço-tempo já monitorada: ondas gravitacionais desencadeadas por pares de buracos negros colidindo no espaço profundo. Somente desde 2015 conseguimos observar esses corpos astronômicos invisíveis, que naquela época só podiam ser detectados por sua atração gravitacional. Então, em um avanço em 2019, o Event Horizon Telescope capturou uma imagem de um buraco negro e sua sombra pela primeira vez.
A história de nossa busca por esses objetos enigmáticos remonta ao século 18, mas a fase crucial ocorreu em um período adequadamente sombrio da história humana – a Segunda Guerra Mundial.
O conceito de um corpo que prenderia a luz e, assim, se tornaria invisível para o resto do universo, havia sido considerado pela primeira vez pelos filósofos naturais John Michell e depois Pierre-Simon Laplace no século XVIII. Eles calcularam a velocidade de escape de uma partícula de luz de um corpo usando as leis gravitacionais de Newton, prevendo a existência de estrelas tão densas que a luz não poderia escapar delas. Michell as chamou de “estrelas escuras”.
Mas após a descoberta de que a luz assumiu a forma de uma onda em 1801, ficou claro como a luz seria afetada pelo campo gravitacional newtoniano, então a ideia de estrelas escuras foi abandonada. Demorou cerca de 115 anos para entender como a luz em forma de onda se comportaria sob a influência de um campo gravitacional, com Albert Einstein Teoria da Relatividade Geral em 1915, e Karl Schwarzschild solução para este problema um ano depois.
Schwarzschild também previu a existência de uma circunferência crítica de um corpo, além da qual a luz seria incapaz de cruzar: o raio de Schwarzschild. Essa ideia era semelhante à de Michell, mas agora essa circunferência crítica era entendida como uma barreira impenetrável.
O raio de Schwarzchild. Crédito: Tetra Quark/Wikimedia Commons, CC BY-SA
Foi somente em 1933 que George Lemaître mostrou que essa impenetrabilidade era apenas uma ilusão que um observador distante teria. Usando a agora famosa ilustração de Alice e Bob, o físico levantou a hipótese de que se Bob ficasse parado enquanto Alice pulava no[{” attribute=””>black hole, Bob would see Alice’s image slowing down until freezing just before reaching the Schwarzschild radius. Lemaître also showed that in reality, Alice crosses that barrier: Bob and Alice just experience the event differently.
Despite this theory, at the time there was no known object of such a size, nothing even close to a black hole. As a result, no one believed that something resembling the dark stars as hypothesized by Michell would exist. In fact, no one even dared to treat the possibility with seriousness. Not until the Second World War.
From dark stars to black holes
On September 1, 1939, the Nazi German army invaded Poland, triggering the beginning of the war that changed the world’s history forever. Remarkably, it was on this very same day that the first academic paper on black holes was published. The now acclaimed article, On Continued Gravitational Contraction, by J Robert Oppenheimer and Hartland Snyder, two American physicists, was a crucial point in the history of black holes. This timing seems particularly odd when you consider the centrality of the rest of World War II in the development of the theory of black holes.
This was Oppenheimer’s third and final paper 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 attraction force that not even light could escape from it. This 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 attraction.
In 1939, this was still an idea that was too strange to be believed. It would take two decades until the concept was developed enough that physicists would start to accept the consequences of the continued contraction described by Oppenheimer. And World War II itself had a crucial role in its development, because of the US government’s investment in researching atomic bombs.
Einstein and Oppenheimer, around 1950. Credit: Wikimedia Commons
Reborn from the ashes
Oppenheimer, of course, was not only an important character in the history of black holes. He would later become the head of the Manhattan Project, the research center that led to the development of atomic weapons.
Politicians understood the importance of investing in science in order to bring military advantage. Consequently, across the board, there was a wide 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, and as an immediate consequence, the fields of cosmology and astrophysics were mostly forgotten, including Oppenheimer’s paper.
In spite of the decade lost to large-scale 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 the war as the center of modern physics. The number of PhDs 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 was a renaissance in the once underestimated theory of general relativity. The war changed the way we do physics: and eventually, this led to the fields of cosmology and general relativity getting the recognition they deserve. And this was fundamental to the acceptance and understanding of the black holes.
Princeton University then became 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 him into the greatest enthusiast for Oppenheimer’s prediction on the day that war broke out, September 1, 1939.
Since then, new properties and types of black holes have been theorized and discovered, but all this only culminated in 2015. The measurement of the gravitational waves created in a black hole binary 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.
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