"Let's look at this diagram, assuming that the abscissa X axis in the coordinate system represents the physical coordinates of three-dimensional space, and the Y axis represents the time axis, then at the beginning, the elder brother and the younger brother were both on the earth, and they were both located at a point on the X axis, assuming that it was this P point!"

"When the elder brother left the earth in a spaceship, the younger brother and the earth did not move, they only moved on the time axis, so you can draw a straight line parallel to the Y axis through the P point, and take the P' point is the point where the two brothers meet, then the timeline (world line) that the younger brother experienced is the straight line of PP'. Pen "Fun" Pavilion www.biquge.info"

Seeing that everyone in the audience was listening carefully, Zhou Chen continued: "Since my brother first stayed away from the earth and then returned to the earth, his timeline is an arc that starts from point P, bends in the positive direction of the X axis (spatial coordinates), and finally returns to point P'!" ”

"Then you can see that the timeline experienced by the younger brother is the straight line of PP, and the timeline experienced by the elder brother is the arc of PP, many people should think that the straight line must be shorter than the arc, but it is not!"

"This is because all four-dimensional space-time involving time is pseudo-Euclidean geometry, and the symbol of time is the opposite of space, so a plus sign should be used in coordinates where no time is all space, and once time is involved, a negative sign must be added in front, so in pseudo-Euclidean space-time, the seemingly longer arc of PP is actually a short-range line, which is shorter than the straight line of PP!"

"As for the student's question about why we use pseudo-Euclidean space-time, it's because we're talking about four-dimensional space-time, which is composed of three-dimensional space and a timeline, which is very similar to a simple four-dimensional space, but one of the dimensions (time dimension) uses imaginary numbers. Then some people will ask, is it okay for me to just take time as the fourth dimension and not introduce imaginary units? ”

"Absolutely! However, in this case, the Lorentz transform matrix is just an ordinary fourth-order square matrix, and in such a system, the velocity, force, and momentum defined by classical mechanics do not have the covariance under the Lorentz transform, and it will become impossible to calculate them. ”

"So we introduced the imaginary dimension, so that the four-dimensional velocity, four-dimensional momentum, and so on defined by this new system have covariance with respect to the Lorentz transform, and in terms of people's aesthetic habits, they must also prefer symmetrical things, such as the orthogonal matrix here in Min's space (pseudo-Euclidean space-time), which is very perfect."

The fat boy nodded in realization, but then he asked again: "I have another question, since my brother left the earth in a spaceship, he is moving at a high speed relative to the earth, so he looks young; But on the other hand, if you take the elder brother as a reference, it is the earth that is moving away at high speed, and the younger brother is moving at high speed, so why is the final result still that the elder brother is younger? ”

"It depends on whether the target person feels the real acceleration."

Zhou Chen replied: "If you feel the acceleration and inertial force, then it shows that he is in absolute motion, which is an important dividing line for judging the time trend!" ”

"Thank you, Mr. Zhou, I understand!" The fat boy said with a grateful face.

Then the classroom was quiet for a moment, and then a young associate professor in the physics department stood up and prepared to ask the next question.

I saw him think a little and said humbly: "Hello Mr. Zhou, we all know that you have an article "On the Law of Thermodynamics of Black Holes" that has caused a big sensation in the theoretical community, which is the honor of our Beijing Normal University and the Department of Physics, and the question I want to ask is related to this aspect. ”

"Please." Zhou Chen said with a smile.

"That's right, Hawking thinks that black holes have temperature, and proves that black holes have thermal radiation, so I want to ask, what is the main way to detect black holes at present, is it by observing the radiation emitted by black holes?"

"First of all, thank you very much for asking questions that are in my professional direction."

Zhou Chen smiled and said without much thought: "Although the theoretical community has now confirmed that black holes do have temperatures, the temperature of black holes is actually very low, only about 10 to the minus 6th power, and this temperature is even many orders of magnitude smaller than the 2.7K of cosmic microwave background radiation, so the radiation it produces is also very weak, and it will definitely not work by directly observing the Hawking radiation produced by black holes." ”

"At present, the observation of black holes is generally through the study of the intense effects produced by the absorption of some matter by black holes, but unfortunately neutron stars will also have similar effects, so there are still problems in direct observation of black holes at present."

The young associate professor nodded, and he continued: "I have a second question, because black holes have always had black-body radiation, and if we assume that the law of conservation of energy can always exist, will there be an end point for black holes, and eventually it will disappear from the universe?" ”

Zhou Chen replied: "Yes, the black hole will become smaller after experiencing radiation, and the heat capacity of the black hole is negative, the more it emits thermal radiation, the higher its temperature will be, and the temperature will increase, the stronger its radiation will be, this is a vicious circle, so for a small black hole, once its thermal balance is broken, the radiation process will be very intense, and it will blow up." In the case of a large black hole, it is still a relatively stable structure because the radiation is negligible to its whole. ”

"Thank you for your answer!" The young professor nodded.

"You're welcome!" Zhou Chen smiled faintly.

After class, walking on the campus of Beijing Normal University, for some reason there are always crows hovering in the sky, and sometimes they land on the poplar trees on both sides.

This is a common scene in Beijing Normal University, and after a long time, you will get used to this kind of scene.

Just as Zhou Chen was strolling around the campus, a phone call came, it was Cen Xian'an, the head of the physics department.

"Hey, hello."

"Xiao Zhou, where are you now? Just finished taking an elective? All right...... Come to my office! Director Cen's tone seemed very excited, and urged Zhou Chen to hurry to his office.

Zhou Chen shook his head and hung up the phone, then turned around and walked towards Director Cen's office.

When he came to the administrative office of the department, Zhou Chen first met Director Gao, who had received him, and Director Gao was very polite when he saw Zhou Chen, and kept asking for warmth.

Now Zhou Chen is a rising star of Beijing Normal University, a model figure among young teachers, even if he is the deputy director of the department, when he sees him, he will put down his body, give face, and try his best to maintain a good relationship.

After greeting Director Gao for a while, Zhou Chen finally walked into Director Cen's office. Director Cen had been waiting for Zhou Chen for a long time, and when he saw him coming, he hurriedly invited him to take a seat, and told Zhou Chen the good news he had gotten.

"Congratulations, Xiao Zhou, we have just received news that in view of your outstanding contributions to the study of black hole theory, the Chinese Physical Society has nominated you as the winner of next year's 'Zhou Peiyuan Physics Prize'!" Director Cen said with a smile on his face.

Zhou Chen was a little surprised, but he quickly recovered, in fact, with the popularity of "On the Laws of Thermodynamics of Black Holes" at home and abroad, he was not surprised by this result. It is impossible to ignore his achievements at home.

Seeing that he was indifferent, Director Cen nodded slightly. He couldn't help but feel emotional, this young man is really unusual, he didn't have such calmness when he won the award.

The "Zhou Peiyuan Physics Prize" is one of the "eight major awards" of the Chinese Physical Society, along with the "Five Physics Prizes", the Xie Xide Physics Prize, and the Huang Kun Physics Prize, and is the highest award in the domestic physics community.

Each of the eight awards has a focus, such as the "Xie Xide Physics Prize" for female physicists, the "Huang Kun Physics Prize" for solid-state and semiconductor researchers, the "Li Gangfu Physics Prize" for experimental technology, the "Rao Yutai Physics Prize" for optics, acoustics, atomic and molecular physics, the "Ye Qisun Physics Prize" for condensed matter physics, the "Wu Youxun Physics Prize" for nuclear physics, and the "Wang Ganchang Physics Prize" for particle physics and inertial confinement nuclear fusion.

The "Zhou Peiyuan Physics Award" won by Zhou Chen this time focuses on young and middle-aged physicists, and it is a very high gold award!

……

Just as Zhou Chen was talking with the head of the department in Director Cen's office, the joint meeting of the party and government of Beijing Normal University was also being held lively, and one of the topics discussed at the meeting was directly related to Zhou Chen.

In fact, as Zhou Chen's "On the Laws of Thermodynamics of Black Holes" caused a sensation at home and abroad, the voice of Zhou Chen's exceptional promotion to associate professor of Beijing Normal University began to circulate in the circle of teachers of Beijing Normal University.

Coupled with Zhou Chen's excellent performance in the elective course "Astrophysics", some people asked: such an excellent teacher is just a lecturer, how does the outside world view the construction of the teaching team of Beijing Normal University?

This problem is very realistic, so that the party and government leaders have to look at it carefully, and with the news that Zhou Chen was nominated as the winner of the "Zhou Peiyuan Physics Award", this has become a problem that cannot be avoided.

Strictly speaking, Zhou Chen's position as a lecturer is already an exception, but now he has to make another exception to discuss whether he should be awarded the title of associate professor.

Of course, there are many people in the school who oppose it, they think that the rules and regulations are hard and rigid, how can they casually do something out of the ordinary? In the long run, who else will look at the rules and regulations? Everyone is out of the ordinary, and there are no rules. This is a very bad precedent!

But there are also many people who believe that the rules are dead, people are alive, trees move to die, people move to live, obviously have the corresponding strength, why do you still stick to the rules? That's not called sticking to principles, but not knowing how to be flexible! It's a sign of conformism.

The two sides were at a stalemate, and Secretary Liu and Principal Zhang had a headache about how to clean up the scene.

At this time, a staff member from the principal's office suddenly ran in, with surprise on his face, and said breathlessly, "Good news...... Good news......"

"What's the good news?" Everyone stopped arguing and turned their eyes to it.

The staff member was stunned, adjusted his tone, and said as calmly as possible: "Teacher Zhou Chen from the physics department of our school, he ...... He received a ...... from the International Center for Theoretical Physics Dirac Medal!! ”