Why did Oppenheimer become the leader of the Manhattan Project?

Popular Science Times reporter Chen Jie

Nolan's new film "Oppenheimer" has been released in mainland China for more than two weeks. While arousing the public's enthusiasm for watching movies, it has also stimulated people's interest in hardcore physics knowledge.

On September 8, Zhang Chaoyang, a doctor of physics, chairman and CEO of Sohu, gave an offline class called "Zhang Chaoyang's Physics Class", which can be regarded as a hardcore viewing guide for the film.

Made important contributions to the field of quantum mechanics

In the classroom, Zhang Chaoyang, dressed in a retro suit, a top hat and holding a cigar, not only restored Oppenheimer's classic image, but also used the same hard-core formula deductions to tell the legendary figure's contribution to quantum mechanics.

The famous "Born-Oppenheimer approximation" was proposed by Oppenheimer and his mentor Born in a paper co-authored by him. It is still regarded as a major breakthrough in understanding molecular behavior using quantum mechanics and constitutes the cornerstone of modern quantum chemistry. Zhang Chaoyang used this method to process hydrogen molecular ions and then used the variational method to solve its wave function, explaining that the physical essence of molecular bonds is the attractive potential brought by electrons, and gave a hard-core interpretation of Oppenheimer's early contributions to quantum mechanics.

Why did Oppenheimer become the leader of the Manhattan Project? Zhang Chaoyang believes that first of all, Oppenheimer not only understood physics and metallurgy, but also had top-notch management skills. The US military believed that Oppenheimer was the smartest scientist who could best grasp the essence of the problem. "Later, it was proved that this was an extremely correct decision," Zhang Chaoyang said.

Nuclear fission was discovered by a female scientist

The most shocking scene in the film is the moment when the world's first atomic bomb was detonated. When asked by students on the scene what the difference is between the physical principles of atomic bombs and hydrogen bombs, Zhang Chaoyang explained, "One is nuclear fission, and the other is nuclear fusion, but both are applications of Einstein's E=mc^2."

In the early stages of atomic bomb development, Oppenheimer encountered the problem of "a fission chain reaction that could explode the Earth", which was restored in the film. Zhang Chaoyang gave a supplementary explanation from a physical perspective: nuclear fission produces extremely high temperatures, triggering a chain reaction of hydrogen fusion or nitrogen fusion in the air, which in turn explodes the Earth. "However, it was later confirmed that the concentration of hydrogen in the air was not that high, and the energy of nuclear fission was not enough to convert the helium in the air into higher elements, so a chain reaction could not occur."

In addition, Zhang Chaoyang added a detail that was not mentioned in the movie, "The discoverer of nuclear fission was a female scientist. She was far away in Denmark and guided her colleagues in Berlin through letters to use neutrons to hit heavy metal nuclei and realize the occurrence of fission." Oppenheimer used this to create the atomic bomb. As the movie said, the turning point of human destiny was the first when humans invented fire, and the second when they found energy from the atomic nucleus.

Neutron deceleration: Betting on heavy water led Germany's nuclear program astray

The atomic bomb is an uncontrollable fission chain reaction, and the nuclear materials must be purified before making the atomic bomb. When analyzing the principle of the atomic bomb, Zhang Chaoyang said that nuclear fission is actually the fission produced by bombarding the nucleus with neutrons. The higher the probability of the two colliding, the more neutrons are released, and these neutrons will continue to bombard the next nucleus. "In order to increase the collision rate, the best way is to slow down the speed of the neutrons."

How to slow down neutrons is not only a scientific problem, but also directly led to the collapse of Germany's atomic bomb program during World War II.

"Historically, the German atomic bomb project went astray because of the use of heavy water to slow down the neutrons." Zhang Chaoyang said that German scientists Heisenberg and Fermi from Oppenheimer's team both thought of using graphite to slow down neutrons. The experiment found that the neutrons were slowed down, but the carbon in the graphite also absorbed the neutrons. Heisenberg had no choice but to abandon graphite and use heavy water to slow down. Fermi believed that there were too many impurities in graphite, and the problem would be solved by using higher purity graphite. After the heavy water plant built by Hitler in Norway was bombed by the Allies, Germany did not even have the most basic neutron deceleration.

In the movie, after arriving in Chicago, scientist Bohr excitedly told Oppenheimer that Heisenberg used heavy water to slow down, "They have gone astray, they have gone to a fork in the road."