Chapter 314 Green’s Function

"Come, have a cup of coffee and continue practicing tonight."

In the laboratory, Wu Bin put the coffee he just bought on Lin Ziqi's table and said.

Facing this sudden surprise, Lin Ziqi happily held up the coffee cup and said, "Thank you~ Where did you buy it?"

"The coffee at Winter Story at the door tastes quite good." Wu Bin replied casually.

"Why did you suddenly think of going to the coffee shop?" Lin Ziqi asked curiously.

"Talk about patents."

"Oh, how did the conversation go?"

"All the preliminary preparations have been discussed, and all that's left is to wait for the results."

"That's great~" Lin Ziqi had an envious expression on her face, then picked up her phone and looked at it and said hurriedly: "Oh, I have to go to class, thank you for the coffee."

"Remember to come back and write your paper in the evening." Wu Bin warned Lin Ziqi's back.

"I got it, Professor Wu!" Lin Ziqi made a face at Wu Bin and ran out of the laboratory.

"Well... I almost have to go to the classroom." Wu Bin looked at the time, then walked back to his seat and picked up his schoolbag.

The courses assigned to him by Academician Chen Xianhui are comprehensive, ranging from advanced quantum mechanics, quantum field theory, electrodynamics, and introduction to condensed matter physics in superconductivity theory to engineering, electromagnetism, heat transfer, fluid mechanics, etc. that apply superconductivity. Wait.

A while ago, because Wu Bin was basically busy in the laboratory, he had not attended a few classes yet. Now, taking advantage of the gap of patent application, Wu Bin happened to make up for all these classes.

However, for Wu Bin, it is not so much that he is going to class, but that he wants to hear more insights and ideas about superconductivity. After all, with his self-study ability, he has learned these related courses as early as his freshman year. I had a great time learning it, and since then I have read hundreds of documents from various countries.

Based on his current understanding of superconductivity, he can clearly feel that applied superconductivity is much friendlier than theoretical superconductivity research, because the content of applied superconductivity can still be seen, and everyone can get together to discuss the results during the experiment. discuss.

But theoretical superconductivity is completely different. If you study this thing a little deeper, you will see a complete darkness. Physics masters from all over the world are also walking in the dark, and whenever this group of people gather to discuss, they slam the door. It's easy to go there, but it's common for two friends to suddenly escalate into a Facebook quarrel while chatting.

As for why this is the case, to put it simply, it is like blind men touching an elephant. No one knows where the future of superconductivity lies, so some say it is long, some say it is thick, and some say it can spray water.

In short, everyone firmly believes that the path they take is the path to the truth, otherwise how can they persist.

After arriving in the classroom, Wu Bin sat in the relatively empty third row. As soon as he put down his schoolbag, he noticed an acquaintance of his walking in from outside the classroom.

‘It turns out that Professor Cheng is teaching this class. ’

Professor Cheng's full name is Cheng Guangji. He is a doctoral student in the Department of Physics and Astronomy at the University of Pittsburgh. He has been working on quantum nanoelectronics since returning to China. For a period of time, Wu Bin often discussed with him the issues of condensed matter physics in low-latitude experiments.

When Wu Bin saw Cheng Guangji, Cheng Guangji also saw him. He smiled and raised his hand to say hello. Wu Bin immediately waved and smiled at him.

After about half of the people in the classroom were seated, Professor Cheng stood on the podium and picked up a piece of chalk.

【Green's function】

After writing four big words on the blackboard, Cheng Guangji turned around and asked with a smile: "Which student can explain the physical meaning of Green's function?"

"..."

The students below all looked confused and wondered what happened to Professor Cheng today. Why did he ask questions as soon as the class started? We haven’t even talked about it yet!

However, when everyone was confused, Wu Bin frowned and realized that things were not simple.

Sure enough, after a brief silence, Cheng Guangji smiled and said, "Come on, the male student in blue who is sitting in the middle of the third row, please get up and talk."

"..."

Wu Bin was speechless for a while and stood up slowly. He didn't know how he had offended Professor Cheng. He had just said hello with a smile, but why did he suddenly get angry? ?

When Wu Bin stood up, the students around him couldn't help but focus their attention on him, wanting to see who was so unlucky, but most people's first reaction was.

"It seems like I've never seen this guy before..."

But many people exclaimed in low voices: "Isn't this Wu Shen!?"

When he said this, everyone around him also remembered it.

‘This is the big guy who graduated from the second year of undergraduate studies. ’

Whether it was the Guo Award or his graduation ceremony, Wu Bin has made a strong presence in HKUST this year, making it difficult for others to recognize him.

"Okay, quiet, quiet!" Cheng Guangji patted the podium and shouted.

In an instant, after the classroom became quiet, Cheng Guangji looked at Wu Bin again and said with a smile, "Classmate, please answer."

After showing a dumbfounded expression, Wu Bin said: "In physics, there are a large number of non-homogeneous differential equations with boundary conditions or initial conditions."

"These differential equations are essentially describing the relationship between the source and the field. Among them, there is a good method called Green's function method. The function obtained by using it is called Green's function. In fact, Green's function is the corresponding A field.”

"Well, that's right." Cheng Guangji, who was standing on the podium, clapped his hands twice and continued: "The rest... I guess it's more troublesome to explain it in words. You'd better go on stage and explain it."

"???"

Wu Bin, who was confused again, had no choice but to slowly walk up to the podium under the watchful eyes of the whole class, and accepted the chalk handed over by Cheng Guangji with a smile.

Taking a deep breath, Wu Bin wrote on the blackboard and said: "Generally speaking, the definite solution is equal to the general solution plus boundary conditions, and finding the general solution is equal to the integral. But if you use Green's function to solve it, a brand new idea will appear."

As he spoke, Wu Bin drew a line on the blackboard, and then drew an irregular shape on the line.

"In unbounded space, if there is an irregular charged body, the charge density is rho(r'). It should be noted that this is the field source of any branch."

"Then it will excite an electric field in space, and the TV generated in space r will be wirelessly divided and then superimposed."

Saying this, Wu Bin began to list the formulas on the blackboard.

[(r→)=4πε/1∫∫∫r→-r→’/p(r→’)dr→’]

"Where dr'→ represents not only a vector, but also a volume element. The electric potential at a certain point in space is uniquely determined by the charge density distribution."

Glancing at the students taking notes below, Wu Bin added: "We know that the problem without boundary conditions has a solution but it is not unique. Given the boundary conditions, the problem can have a unique solution. So how can we find the solution through the superposition of point charge potentials? Can we obtain any distribution of charge density and potential under any boundary conditions?"

"The question then arises, what kind of boundary conditions do we need?"

And just when everyone was looking at Wu Bin curiously, Wu Bin suddenly turned around and said to Cheng Guangji: "Professor Cheng, I don't understand the next part. I'm asking for your advice."

Cheng Guangji was stunned for a moment after hearing this, and then laughed loudly and said: "Okay, you are so slippery." After laughing, he took the chalk from Wu Bin's hand and said: "Okay, go down, go down. If you don't understand, I will tell you more later." Come to class, you know?”

"knew."

Wu Bin breathed a sigh of relief and returned to his seat. He almost knew why Professor Cheng suddenly targeted him. It was probably just a small expression of dissatisfaction with him for taking so long to come to class, and there was no malicious intention.

After Wu Bin sat back in his seat, Cheng Guangji picked up the chalk and wrote under Wu Bin's formula: "This student has already explained the first half very well. So for the definite solution problem, we can give a general Poisson equation... …”

…

As a piece of soft music played, Cheng Guangji, who had written on several blackboards, announced that "get out of class is over."

Just as Wu Bin was packing his schoolbag, two girls sitting in the front row suddenly walked over to him.

"Um...hello." The taller girl took a step forward and waved to Wu Bin.

"Hello." Wu Bin nodded towards her.

"We think your explanation of Green's function was very good in the first class." After speaking, he lowered his voice and said, "Actually, you know how to continue the explanation."

After hearing this, Wu Bin quickly waved his hands and said, "No, no, no, I really don't know. Otherwise, why would I come to Professor Cheng's class?"

The two girls chuckled softly after hearing this, with obvious disbelief on their faces.

After laughing, the tall girl said again: "Actually... we came here just to learn from the experience and ask the master for some learning experience."

"It's okay, God. If I can help, I will definitely answer."

The tall girl clapped her hands and said happily: "The master is really friendly to the people. In fact, the two of us have been abused by the graduate courses just a few weeks after the school started! I don't know whether it is because of the large span of undergraduate majors or the large span of professional levels. Anyway, I really listened to the class. don't know."

“Some powerful tutors often tell us about the cutting-edge of their respective fields or the research progress of their own laboratories in a mixture of Chinese and English. It sounds very high-level... but after class, I feel that I have learned nothing, and my IQ is not good enough. I have been crushed, and recently I have started to doubt whether I was the right choice to go to graduate school... Now I feel like a fake antique that has slipped into a museum, and I don’t fit in no matter how I look at it.”

Another girl on the side also nodded fiercely, looking very troubled.

"This..." Wu Bin thought for a while, "Which classes do you find difficult to understand?"

"Well...high quantities, field theory, group theory, quantum physics..."

As she spoke, the tall girl's face turned red, because she seemed to have holes all over her body...

In order to break the embarrassment, Wu Bin first replied: "Well...you two are taking these classes to learn about superconductivity, right? In this case, I actually suggest that you read more literature. The scope of this field is too large, and everyone in it None of the courses are easy, and if you really feel you can’t keep up, you can start the semester with applied superconductivity, which will actually make it a little easier to get started.”

PS: Just got home, coded out a chapter, and will continue later.