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Why would someone who has never won a Nobel Prize lead such an important project? What on earth is he capable of, Oppenheimer?

I wonder if everyone has seen the movie Oppenheimer? Whether you have seen it or not, the Manhattan Project is no stranger to everyone. Coupled with the online film interpretation of "Oppenheimer" and the popularity of the characters can be seen everywhere, so today we will not talk about the plot of the film, nor about politics and humanities, but try to talk about Oppenheimer as a physicist from the scientific point of view. What contribution did he make in physics?

Born in 1904, Oppenheimer is the same wave as Pauli, Heisenberg, Dirac and Fermi. As a post-00 generation, they are all "aborigines" of the quantum age.

From an early age, Oppenheimer was a man with a wide range of hobbies, from literature to chemistry, from history to philosophy, to graduate students. Later he went to the University of Gottingen to worship under Marx born, where Oppenheimer met Heisenberg and Pauli.

As a physicist, Oppenheimer is a bit like Bohr, not good at doing experiments, but good at theoretical research. In his early years, Oppenheimer focused on the spectrum, often discussing the continuity of the spectrum with Pauli. He also invented a method to calculate the photoelectric effect of hydrogen and X-rays. Through the calculation, he found that the sunlight accords with the absorption of hydrogen but not helium. It was not until many years later that it was discovered that hydrogen was the main element that made up the sun.

As a student of born, Oppenheimer and his tutor also proposed the famous born-Oppenheimer approximation (BO approximation). Born-Oppenheimer approximation says that, for example, when calculating the energy level and wave function of a molecule, we need to calculate each electron and every nucleus according to the Schrodinger equation. This leads to not only a frightening number of variables in the wave function, but also an exaggeration of the whole calculation step.

However, Oppenheimer thought: the mass of the nucleus and the electron is very different, and compared with the electron, the nucleus is almost stationary, so we can simplify the model accordingly, so the calculation process will be greatly simplified. Since then, born-Oppenheimer approximation has become one of the most famous mathematical methods in molecular physics, and has been widely used in the field of quantum chemistry.

In the same period, Dirac unified special relativity and quantum mechanics for the first time and put forward the famous Dirac equation. As we said in the vulgar Quantum, there is a very strange solution to the Dirac equation, that is, there is an unknown particle with a mass equal to that of an electron but with a positive charge. Dirac himself prefers it as a proton, but Oppenheimer doesn't think so. Because all experiments show that the mass of protons is much larger than that of electrons, he asserts that such positively charged particles should be positrons. Sure enough, Carl Anderson found this kind of positron in cosmic rays two years later.

Boy, there are so many secrets hidden in cosmic rays! So after that, Oppenheimer's interest turned to cosmic rays. Since then, he has made an important contribution in this regard, and the study of field electron emission has led to the emergence of the concept of "quantum tunneling".

At that time, Oppenheimer was already an associate professor in Berkeley, and he still had a wide range of interests, and he often discussed a variety of issues in group meetings with students. He even studied Sanskrit in order to read the original Bhagavad Gita.

Oppenheimer and his students, however, did not prevent him from continuing to produce in his old field of quantum physics. First, as a theoretical physicist, he provided theoretical support for Lawrence's cyclotron experimental data, and then with his student Phillips, he put forward the nuclear reaction theory, which was later called the "Oppenheimer-Phillips process". The theory successfully explained some of the abnormal results in Lawrence's experiment. This is also one of Oppenheimer's early contributions to nuclear physics. After all, at that time, people were not so clear about what happened in the nucleus compared to the world outside the nucleus.

In 1936, under the influence of his good friend Tolman, Oppenheimer's interest again turned to astrophysics, or, to be exact, theoretical astronomy. Theoretical astronomy is based on physical and chemical models to describe and explain astronomical phenomena in principle. To put it bluntly, based on known astronomical phenomena, theorists first build some theoretical models, and then astronomers will see which of these models can better predict astronomical phenomena.

Astronomical research often deals with massive celestial bodies such as planets and stars. On this scale, Newton's classical mechanics has been somewhat inadequate, and some minor problems have to be corrected by Einstein's general theory of relativity. But even so, until the middle of the 20th century, general relativity was considered too weird, and people treated it with a "play" attitude to see if anything new could be found with it.

Soon Oppenheimer made some research on the stability of stellar cores based on general relativity and nuclear physics, which he is good at.

Before 1939, Oppenheimer and his students published three blockbuster papers in succession. In the second paper on massive Neutron Nuclei, Oppenheimer and his student Volkoff proposed that there is a maximum value for the mass of a neutron star, beyond which the neutron star will no longer remain stable and will continue to collapse. This mass limit of the neutron star was later called the Tolman-Oppenheimer-Volkoff limit (TOV limit), also known as the Oppenheimer limit.

How long will the collapse last? there has to be an end, right? The third paper gives the answer-there is no end! Once the mass of the neutron star breaks through the Oppenheimer limit, gravity will become absolutely dominant, and there will be no force inside the celestial body to compete with it, so it will collapse until the Schwarzschild radius.

The co-author of this paper is Hartland Snyder, another student of Oppenheimer, who jointly proposed the Oppenheimer-Snyder model. This model describes how extreme celestial bodies collapse into black holes step by step, and this paper has become an important paper to predict the existence of black holes. Hey, on the importance of being a mentor, is there?

To say more, the Oppenheimer limit is not as accurate as the Chandra Seka limit. The Chandra Seka limit describes the upper mass limit of a white dwarf, which is about 1.4 times the mass of the sun. Beyond this mass, a white dwarf will collapse into a neutron star. The internal structure of a neutron star is very complex, but it is not such a large pile of neutrons as people think. In addition, most of the real neutron stars are rotating pulsars, so that to this day, scientists still have different opinions on the material composition of neutron stars.

So Oppenheimer's second paper focused on neutron stars in an ideal state of non-rotation, with a limit of about 0.7 times the solar mass. This value is even smaller than the mass limit of a white dwarf, which is obviously wrong. But there was nothing wrong with Oppenheimer's thinking, and this limit was later revised to 2.2 times the solar mass, corresponding to the star's initial mass of about 15 to 20 times the solar mass.

Just about to do a big job in astrophysics, it wasn't long before the United States began the Manhattan Project. Strongly recommended by Groves, the "number one" of the project, Oppenheimer was appointed director of the secret weapons laboratory, responsible for the research and development of atomic bomb-related technology, similar to CTO in today's company.

Everyone wondered why the then unknown Oppenheimer was allowed to lead such an important project. In terms of technology, Oppenheimer has never won a Nobel Prize, and there are more people than his major; in terms of management, he has never had any experience in managing large projects, not to mention that he is not good at doing experiments himself, and can only be regarded as a theoretical physicist. Perhaps because of his wide range of interests, Oppenheimer is very knowledgeable, and nuclear physics is his field of study, so Oppenheimer's "compound talent" is suitable for unprecedented things such as designing and building atomic bombs. Of course, there are some other reasons besides technology, not to mention.

There is an interlude: just as Oppenheimer was called "the father of the atomic bomb", Edward Taylor was given the title of "father of the hydrogen bomb". But unlike Oppenheimer's appointment, Taylor's appointment can be said to be dramatic.

Edward Taylor was in the early stages of the atomic bomb project, and Taylor was a member of the theoretical team. One day the team leader Hans Bette and Oppenheimer complained, "can you get that Taylor kid out of here?" day by day he has so many ideas that everyone can't get down to business. "

It turns out that Taylor is also the kind of unrestrained person, new ideas emerge every day, but also always like to discuss with others, so seriously affected the progress of the project. But Oppenheimer and Taylor have a good relationship, so it is not appropriate to drive people away directly, so Oppenheimer came up with an idea. He said to Taylor, "Ty, I have an important secret task for you now. I think you are absolutely one in a million wizards, so now you are appointed as the leader of the secret task force, and the task of studying the hydrogen bomb is up to you!" As soon as Taylor was overjoyed, he turned around and formed a team to study how to make a hydrogen bomb.

Edward Taylor, because the study of the atomic bomb had just begun, and the hydrogen bomb had nothing to do with it, so Oppenheimer wanted to push the boat along the river, no problem. But who would have thought that just five years after the first atomic bomb exploded, the first hydrogen bomb would be successfully tested.

From this we can see that the more difficult the research content is (especially the new unknown territory), the more suitable it is for people like Oppenheimer and Taylor who are flexible and dare to try.

Since then, although Oppenheimer has been the director of the Princeton Institute for Advanced Studies, his focus has shifted from the front line of scientific research to the management of researchers. At the same time, Oppenheimer has been making active efforts to prevent nuclear proliferation, including strong opposition to the development of hydrogen bombs. Perhaps as he quoted the Bhagavad Gita as saying: "at this moment, I have become death, the destroyer of the world."

references

[1] https://en.wikipedia.org/wiki/J._Robert_Oppenheimer

[2] https://en.wikipedia.org/wiki/Born%E2%80%93Oppenheimer_approximation

[3] https://en.wikipedia.org/wiki/Oppenheimer%E2%80%93Phillips_process

[4] https://en.wikipedia.org/wiki/Melba_Phillips

[5] https://en.wikipedia.org/wiki/Tolman%E2%80%93Oppenheimer%E2%80%93Volkoff_limit

[6] https://en.wikipedia.org/wiki/Oppenheimer%E2%80%93Snyder_model

[7] https://en.wikipedia.org/wiki/Chandrasekhar_limit

[8] https://journals.aps.org/pr/abstract/10.1103/PhysRev.54.540

[9] https://journals.aps.org/pr/abstract/10.1103/PhysRev.55.374

[10] https://journals.aps.org/pr/abstract/10.1103/PhysRev.56.455

[11] https://wuli.iphy.ac.cn/cn/article/doi/10.7693/wl20210905

[12] https://mp.weixin.qq.com/s/DX6SB2EBRNnsYouAn8HRIA

This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo

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