A Man of Science and Engineering in Troubled Times Wandering in the Late Qing Dynasty

Chapter 695 Di Shen Starts
"Is it raining outside?" asked Blackett in the lab.

Dirac stood up, walked to the window, returned to his seat and said, "It's not raining now."

Kapitsa just happened to come in with a few cups of milk tea, put them in front of their table, and said with a smile: "Answer accurately."

These three later winners of the Nobel Prize in Physics were friends in Cambridge.

However, the personalities of the three were a little different. Dirac was taciturn throughout his life and generally did not speak. If he spoke, it would definitely be useful; and he did not like literature, art, or drama.

Kapitsa likes to chat and loves literary and artistic drama.

Kapitsa was skeptical of abstract theoretical physics, while Dirac regarded abstract theories as his life.

Blackett is relatively moderate.

Earlier, Kapitsa defeated Blackett and won the Cambridge Experimental Scholarship for the Best Student, which was one of Kapitsa's narrow victories over Blackett.

Blackett asked, "Kapitsa, are you still learning Greek and Latin?"

"There's no way I can do it. I have to learn it. It's because you British gentlemen like these useless things." Kapitsa said jokingly.

A considerable number of students at British universities such as Cambridge and Oxford come from private aristocratic schools such as Eton College and Harrow School, where students learn Latin, Greek and the skills to participate in current trendy topics.

Blackett said, “Then I suppose you can easily discuss Eliot’s modernist poetry or review Shaw’s latest play.”

Kapitsa said proudly: "I can even talk to you about the relationship between Star Wars, aliens and mechanics, and I can also write a few Chinese characters."

"Then you can definitely join those small groups," Blackett praised.

Kapitsa scoffed: "I learn these things not to fit in with them, but just to prove that I understand."

"I'm too lazy to even go into the proof," said Dirac, who never participated in those topics.

"I am a Soviet," Kapitsa said with a hint of helplessness, then he cheered up. "But I am increasingly in favor of Lenin's views, and I like the pleasure of throwing out the pampered land-owning aristocrats during the Russian Revolution."

Blackett asked: "You have not joined the Bolsheviks, have you?"

“No,” Kapitsa said, “but I support them and their insistence on rebuilding society led by the working class.”

Blackett said, "You'd better be careful. The college prohibits excessive talk of politics."

Kapitsa said: "That's because the British government is afraid of this trend of thought. I really don't understand what they are afraid of. MI5 and the London Police Department have conducted several investigations on me for this. It's very annoying!"

Blackett said: "Didn't you just return to the Soviet Union a few months ago? I heard that you gave advice to your successor Stalin on his industrial plan."

Kapitsa said: "Isn't this normal? When I return to the Soviet Union this year, I will give Trotsky suggestions on how to carry out the country's electrification reform."

"As expected," Blackett reminded, "Professor Keynes said that you'd better keep a low profile when doing these things, otherwise MI5 will really arrest you, because you are wild and have no selfish desires, so it's easy for you to be regarded as a radical, or at least have the characteristics of a radical."

Dirac said, "I don't think so. It should be replaced with words like confidence and courage."

Kapitsa smiled and said, "Although you don't talk much, every word you say makes sense."

Blackett looked at his pocket watch and said, "Class is about to start. Mr. Li Yu will be teaching today."

Kapitsa said: "It was great not to have to listen to Mr. Eddington's long and incoherent speeches. He always tried to turn astronomy and physics into dazzling prose. But I couldn't stand his habitual jumping rhythm. He would suddenly switch to another question without finishing it, as if he had forgotten the previous question."

Dirac agreed: "His prose is not as good as yours."

Kapitsa gave a thumbs up: "Thank you for the compliment!"

Cavendish's lecture hall was very small and not crowded, maintaining the discussion mode of a hundred years ago. It was not until many years later that a new building was built in Cambridge.

"Hello, Professor."

Dirac, Blackett, Kapitsa, Oppenheimer and several other students greeted together.

With so many big guys in front of him, Li Yu must have felt it difficult to cope with it before, but fortunately he has adapted to it now, but he has never attended classes in a British university.

He had no choice but to ask, "What do you usually do?"

Several people were stunned. Kapitsa said, "Nothing much. I just finish the homework left by Professor Fowler and Rutherford, read the recommended books and the latest journals, and review the notes in class. This is the case every week except Sunday."

It's so casual, then it's easy.

"What do you want to hear?" Li Yu asked.

"Shouldn't it be you who's saying something?" Blackett said.

"Anyway, it was a substitute teaching assignment arranged by Fowler," Li Yu said, "It's like asking someone to continue a novel. He didn't give me a topic, so it's not his responsibility to decide what the next part of the story will be like."

Kapitsa said happily: "You are really irresponsible."

Li Yu pointed to the loose-leaf notebook in his hand and said, "Don't be too happy too soon. Mr. Fowler has already given you two weeks' homework."

Kapitsa's smile froze instantly: "Why is Professor Fowler still so responsible? I would rather he forget it."

These assignments are completely different from those in elementary and middle school. They are all challenging and unknown questions, and each person is different.

Dirac said: "In fact, we really want to know about the amazing paper that de Broglie published not long ago. I heard that it was praised by both Mr. Li Yu and Mr. Einstein."

"Wave-particle duality?" Li Yu said.

Dirac nodded: "That's right."

"It's very confusing and weird," Blackett said. "How can it be both a wave and a particle?"

Li Yu said: "I have a different idea, which may be different from yours."

Blackett said, "Please sir, tell me about it."

Li Yu said: "Rather than thinking that it is both a wave and a particle, it is better to say that it is neither a wave nor a particle, but just happens to behave in a stereotypical way at certain times."

Li Yu then drew a picture on the blackboard: "Cambridge is best at geometry and projective geometry. You must be familiar with this example. The projection of an object in different directions can be completely different. When projected from left to right, it is a circle; but when viewed from right to left, it is a rectangle. But it is actually neither a rectangle nor a circle, but a new thing that a two-dimensional life form can never understand: a cylinder!"

The people listening below were completely shocked. Dirac felt his heart tremble and got goose bumps all over his body: "Neither a wave nor a particle? A two-dimensional life form will never be able to understand it?"

Li Yu said: "To put it another way, if you look at it from the perspective of a wave, it is a wave; if you look at it from the perspective of a particle, it is a particle."

"This...!!" Dirac couldn't help asking, "Is there any mathematical explanation?"

Li Yu said leisurely: "Don't worry, it will appear soon."

Kapitsa is a good experimenter. He suddenly thought of the single-photon interference experiment that Li Yu did many years ago: "You firmly believed in the wave-particle duality at that time?!"

Li Yu said: "Yes, if you are interested, you can repeat this experiment."

The principle of the experiment is not complicated, but it is difficult to design the experimental equipment, especially how to make a single-photon light source. Li Yu wanted to keep it simple at the time and used the method of reducing the light intensity.

If you want to be more rigorous, you can actually use an atomic-level radiation source, and then control the number of atoms and the frequency of radiation to produce single photons. Although there is a high probability, as long as you can ensure that the time interval between two emissions is greater than the experimental requirements, it can be considered a single-photon light source.

Kapitsa was eager to try: "I will prepare to replicate this experiment right away!"

It is not surprising to say that de Broglie opened a new era. His statement "particles can be waves and waves can be particles" was indeed too explosive. After all, wave-particle duality is a concept that touches the core of quantum mechanics and is about to lead to two other god-level theories.

Li Yu was chatting with them enthusiastically, but at this time the bell rang. Flipping through the loose-leaf book left by Fowler, Li Yu said to Dirac: "The homework Fowler left for you is to investigate the changes on the surface of stars (such as the sun)."

"Astronomy?" Dirac was confused. "How come I still can't escape Eddington's astronomy clutches? It must have been his suggestion!"

Astronomy was a required course in the Department of Physics at that time, and one of Heisenberg's five doctoral dissertation supervisors taught astronomy.

After handing out the assignments one by one, Kapitsa asked again, "Mr. Academician, would you like to participate in our discussion?"

"What discussion?" Li Yu asked.

Kapitsa said: "It's called the Kapitsa Club, which specializes in discussing some new and popular physics topics once a week, and there are also rich dinners."

"Why haven't I heard Professor Rutherford mention it?" Li Yu said.

"He doesn't like seminars like this."

"why?"

"Professor Rutherford said that those of us who work on theories are just playing symbol games, and the real Cavendish people are the ones who verify the truth of nature," said Kapitsa.

Li Yu smiled and said, "If I have time, I will join in the fun."

These seminars were actually similar to lectures, and Li Yu’s lectures were all “nonsense” anyway, with no syllabus at all - and it was impossible for Cambridge to assign him a syllabus task.

In short, Li Yu got along very well with this group of people.

A few days later, Rutherford found Li Yu with a package: "There is a letter from Göttingen."

"Who sent it?" Li Yu asked.

"It's signed by a doctoral student named Heisenberg," said Rutherford, who had never heard of Heisenberg's name at that time.

Li Yu opened it and found a 15-page draft of Heisenberg's paper, which was the extremely difficult-to-understand matrix mechanics paper that went on for a long time but did not use matrix methods.

It contains several corrections made by Heisenberg in his characteristic italic handwriting, as well as an emphatically marked aside: "The order of multiplication becomes important."

Heisenberg specifically wrote this in the hope that reviewers would not be offended by this point of view and feel that his theory was too far-fetched to be worth publishing.

It can be seen that Heisenberg himself was still quite confused and puzzled about this.

For Li Yu, this paper is actually a mystery. If you don’t know what he wants to say in advance, it is really difficult to understand.

So he decided not to continue reading and looked out the window, and happened to see Dirac.

Dirac was walking, and if the weather was good, he would sometimes walk for hours. Dirac was wearing the suit he wore to school, with his hands behind his back, and walked with a slightly outward-turned foot, taking long steps at a rhythm that never changed like a metronome. No wonder many of his Cambridge classmates said that Dirac walked "like a groom in an Italian wedding photo."

"Dirac, please come here." Li Yu shouted.

"Okay, Mr. Li Yu!"

Arriving at the office, Dirac looked at this very strange paper and asked in confusion: "German?"

Li Yu said: "Dr. Heisenberg is a German who graduated from Heisenberg University."

Dirac usually had the habit of reading the latest journals. He said: "I have seen his graduation thesis, which is about turbulence, but this paper does not seem to be about fluid mechanics."

Dirac's interests were indeed wide-ranging.

When Heisenberg first entered the University of Munich, his mentor Sommerfeld did not want him to study atomic physics at the beginning because it was difficult, so he asked Heisenberg to study the simpler turbulence first.

Well, it’s quite simple!
Later Heisenberg discovered that this thing was not as simple as you said!
Let’s study atomic physics!

Li Yu said: "Now Heisenberg has left the University of Munich and is studying quantum theory with Bohr in Copenhagen and Born in Göttingen."

"Quantum theory again," said Dirac.

Li Yu said, “This paper is more mathematical, but it has some weird aspects, so I want you to take a look and hear your thoughts.”

Dirac put Heisenberg's paper in his bag: "Is it an assignment?"

Li Yu smiled and said, "Not really, just read it in your spare time."

The paper must be given to Dirac, because the biggest turning point in his academic life came from this strange paper by Heisenberg.

After returning to his residence, Dirac first briefly flipped through the obscure German paper. Although he could intuitively tell that it was not an ordinary quantum theory mathematical exercise that repeated previous ideas, after a cursory glance, Dirac still felt that Heisenberg's method was too complicated and arbitrary and did not arouse any interest, so he decided to put the paper aside for the time being.

Dirac made himself a meal, then poured himself a glass of lemonade - he was a teetotaler in the Methodist tradition, then turned on the newly purchased radio and listened to the announcer telling the day's news.

After listening for a while, he found that he was even less interested because the announcer's stiff tone sounded like a funeral speech. Dirac turned off the radio, sat down at his desk again, and opened Heisenberg's paper.

This time he read it more carefully. Halfway through, Dirac's nerves began to tense up.

Dirac had an excellent foundation in mathematics, and he quickly discovered the most critical part of the paper: Heisenberg wrote that some quantities in the theory have a strange property, that is, if one quantity is multiplied by another, the result is sometimes different from the result obtained after the order of multiplication is reversed.

Unlike Heisenberg, who had no experience with commutativity, Dirac was very familiar with it.

Dirac was certain that noncommutative quantities held the key to the mystery.

The advantage of having a wide range of knowledge immediately became apparent. Dirac vaguely remembered having seen something similar when he was taking an elective course called Analytical Mechanics.

So he ran to the library and searched for a long time, and finally pulled out a big book called "Analytical Dynamics of Particles and Rigid Bodies", written by Edmund Whittaker, a professor of mathematics at the University of Edinburgh.

Following the table of contents, he turned to page 299 and confirmed that, as Dirac had conjectured, the form of the Poisson bracket proposed by French mathematician Denis Poisson in his work more than a century ago was the product of two mathematical quantities minus the product of two related quantities. The product and the minus sign looked very much like the expression PQ-QP.

Through this discovery, Dirac found the key clue, and was able to see the whole picture from a small part.

The process is naturally not as simple as it sounds. It will take a long time to figure this out, that is, to clarify the mathematical basis of the analogy between quantum theory and classical theory.

It is said to be a long time, but in fact it was only three or four years. Dirac first published several papers and then completed the epic masterpiece "Principles of Quantum Mechanics", which established his lofty position in the field of quantum mechanics.

It seems that it was because of Dirac that analytical mechanics became a compulsory course instead of an elective.

In addition, after matrix mechanics was confirmed to be correct, physics researchers, whether students or professors, had to start cramming for matrices that they had never heard of...

It was a nightmare for physicists at that time.

Li Yu was not in a hurry at all. He was ready to wait a little longer and let the bosses' bullets continue flying for a while.