Uncover the story behind "Oppenheimer" that Nolan didn't film

Recently, the biographical film "Oppenheimer" written and directed by Nolan was released worldwide, bringing this legendary scientist known as the "father of the atomic bomb" back into people's attention. However, Oppenheimer's complicated life experience has caused most people to ignore his academic contributions beyond the development of the atomic bomb, especially his achievements in the fields of astrophysics. Today, let's talk about the story behind Oppenheimer.

A life of suffering for the atomic bomb

Oppenheimer was called "America's Prometheus". In 1942, he was appointed as the chief scientist of the Manhattan Project, responsible for developing the atomic bomb. In the movie "Oppenheimer", there is such a detail: Groves, the military director of the Manhattan Project, once asked Oppenheimer what the probability was that the atomic bomb would destroy all life on Earth? Oppenheimer replied calmly: "The probability is close to zero." Groves asked back: "Close to zero? Zero is good."

Stills from the movie Oppenheimer

This idea seems almost bizarre today, but in fact, concerns that atomic bombs could set the earth on fire ran throughout the Manhattan Project.

This is because Oppenheimer discovered in the Manhattan Project that under the extremely high temperature conditions provided by nuclear fission, light atoms such as hydrogen can be combined into heavier atomic nuclei, thereby releasing more enormous energy. This discovery later led to the development of hydrogen bombs. Although there is very little hydrogen in the atmosphere, the high temperature after detonating an atomic bomb may cause hydrogen atoms in water vapor to separate, thereby triggering large-scale chain nuclear fusion. In other words, an atomic bomb may turn the earth itself into a huge, expanding fireball.

In 1945, Oppenheimer led a team to develop the world's first atomic bomb and successfully completed the test explosion. When a huge mushroom cloud rose into the sky of New Mexico, Oppenheimer was not happy that the earth did not catch fire. He later described that he remembered a sentence from the Indian epic "Bhagavad Gita": "I am now the god of death, the destroyer of worlds." Although later generations quoted this sentence to prove Oppenheimer's anti-nuclear stance, according to historians citing some onlookers at the scene, Oppenheimer actually blurted out a more clichéd sentence at the time - "This thing works."

One month later, the U.S. military dropped atomic bombs on Hiroshima and Nagasaki, Japan, ending World War II. After that, Oppenheimer never engaged in weapons research again.

The forgotten achievements of astrophysics

The period before leading the Manhattan Project was the highlight of Oppenheimer's academic career, especially in the fields of astrophysics. To understand Oppenheimer's contribution to astrophysics, let's first review some physics knowledge.

A star is mainly composed of hydrogen and helium. The huge gravity of the star will cause these substances to undergo a series of nuclear fusion reactions in the core of the star and release energy. This energy provides a huge outward radiation pressure inside the star, thus preventing the star from collapsing inward under the huge gravitational conditions. The equilibrium state at this stage is what astronomy calls the main sequence star stage.

When the nuclear reaction inside a star reaches its limit, what force will the star rely on to maintain after running out of fuel? According to the Pauli Exclusion Principle, in the microscopic world composed of fermions, there cannot be two or more particles in exactly the same state, and they must repel each other when they are together. Electrons, protons, and neutrons are all fermions, so they all obey the Pauli Exclusion Principle. Under this principle, the force of fermions repelling each other shapes the fate of stars after they run out of nuclear reaction fuel.

Stills from the movie Oppenheimer

Indian-American physicist Chandrasekhar proposed that a conventional white dwarf would use the degeneracy pressure of electrons to resist its own huge gravity, but if the mass of a white dwarf exceeds a certain upper limit, the electrons will be squeezed and combined with the protons in the nucleus, making the white dwarf's efforts to resist its own gravity ineffective, causing a catastrophic collapse. Chandrasekhar calculated the upper limit of the mass of a white dwarf in 1930, which is the "Chandrasekhar limit". For a typical white dwarf, the "Chandrasekhar limit" is about 1.4 times the mass of the sun.

In a paper published in 1939 in collaboration with Volkoff, Oppenheimer proposed a stable solution denser than a white dwarf - a non-rotating neutron star model. This model uses general relativity and the ideal Fermi gas equation to prove that there is an upper limit on the mass of a stable non-rotating neutron star. Later, this upper limit was called the "Tolman-Oppenheimer-Volkoff limit."

However, this paper only considered the combined pressure between neutrons, while ignoring the strong force between thermal pressure and neutrons, so the upper limit of the mass obtained was only 0.7 times the mass of the sun. Modern nuclear physics can basically determine that the value of the "Tolman-Oppenheimer-Volkoff limit" should be more than 2 times the mass of the sun. In 2017, the Laser Interferometer Gravitational-Wave Observatory detected the merger of two neutron stars for the first time in the NGC4993 galaxy 130 million light-years away. The observation results showed that the "Tolman-Oppenheimer-Volkoff limit" of neutron stars can exceed 2.17 times the mass of the sun.

Predicting the existence of black holes

In 1939, Oppenheimer and his student Snyder published another groundbreaking paper, "On the Continuous Gravitational Contraction", which deduced the fate of stars to the extreme. In this paper, they proved that after a star with a large enough mass has exhausted its nuclear fuel, it will continue to shrink under its own gravity and shrink into the Schwarzschild radius. At and within the Schwarzschild radius, even light cannot escape, so the entire celestial body becomes black. Such celestial bodies were later called "black holes".

Einstein and Oppenheimer meeting in 1950

The paper used Einstein's general theory of relativity to not only clearly propose the formation process of black holes, but also specifically pointed out a series of black hole characteristics that have not become obsolete to this day. Interestingly, when Oppenheimer was completing this black hole paper within the framework of general relativity, Einstein, who proposed general relativity, was seeking to prove that black holes could not exist.

Three years later, Oppenheimer went to Los Alamos, where he consolidated his image as the "father of the atomic bomb" to the public. In the eyes of physicists, Oppenheimer's most important contribution in his life was his research that proved the inevitable formation of black holes.

Yang Zhenning once commented with regret: "When he died of cancer in 1967, the existence of black holes was not yet widely recognized in astrophysics. If he had lived another 5 years, black holes would have been recognized by everyone." This is the story that the director of this film, Nolan, did not tell.

(Author: Wang Zhongshan Photo source: Stills Review expert: Jiang Fan, deputy director of the Science and Technology Committee of China Aerospace Science and Technology Corporation)