The small library of Tongji University is located in a Spanish style house with a history of several decades, which is specially used for some small academic seminars, where some special books on space exploration are treasured, and it is a gathering place for astronomy enthusiasts.

Usually there are not many people in this library, but now it is past 7 o'clock in the evening, and the lights in the entire library hall are dim and there are few people.

Tang Wenzhe took Li Ting and Wang Mingxia to the second floor of the library, where there were not many people in the spacious reading room, some were flipping through the materials, and some were typing softly in front of the computer.

On one wall, bookcases from top to bottom are filled with books, and on a row of counters, two young library student volunteers are talking to people standing in front of the counter to borrow books.

Directly opposite the service desk is a dark corridor, there are only a few gray wall lamps in the corridor, stepping on the 'creaking' old wooden floor, Tang Wenzhe and his party walked to the end of the dark corridor, leaning on the door of a small conference room in the innermost corner, pushing open an old wooden door whose paint is peeling off and almost a little scattered, the lights in the room are quite bright, a chandelier in the house hangs down from the high roof beam, and the chandelier seems to be a little swaying from side to side under the agitation of the wind blowing in from the window cracks.

Tang Wenzhe's doctoral supervisor at Tongji University, Ding Wenjun, was sitting at the table with his back to the door, operating the computer with reading glasses, as if he knew that Tang Wenzhe was coming behind, and said: "Student Tang, you came on time, sit down, I am preparing some materials." ”

"Thank you teacher for taking the time to teach us. Tang Wenzhe said, by the way, put a few plastic bags containing things in his hand on an old-fashioned oval western dining table and conference table, Wang Mingxia took out a large pizza packaging box from the bag, and put a few bottles of canned drinks of various colors on the table.

After Tang Wenzhe sat down, he folded the floral silk scarf around his neck into a square and put it in his pocket, tonight Tang Wenzhe was wearing a suit but no tie, Li Ting was wearing a navy blue suit, and the collar of the white shirt was turned over to the outside of the collar of the suit, which looked very spiritual, which is also a common clothing style for Li Ting at work. Wang Mingxia wore a pink long-sleeved dress, which made her body even more delicate and delicate.

Instructor Ding Wenjun is sitting on the side of the oval table, using a laptop computer to connect with a projector in front of the table, and directly hitting the content displayed on the computer screen to a white screen hanging on the wall in front of him, and now he is showing a picture of several planets in the solar system surrounding the sun, and the image quality is very clear.

Ding Wenjun said: "Ladies and gentlemen, at the invitation of Tang Wenzhe today, I would like to share with you some of my recent research experiences in deep space exploration, you should all have some understanding of the solar system, simply put, the solar system is a celestial system centered on the sun and maintained by its gravity, and the sun is located about 27,000 light-years away from the center of the Milky Way and 23,000 light-years away from the edge. We know that the Milky Way is about 100,000 light-years in diameter and contains 150 billion stars, and the sun is just one of them.

"Yes, what the teacher said is that the solar system we are familiar with is simply too small in the whole galaxy. Tang Wenzhe immediately replied respectfully.

Wang Mingxia was standing at the end of the table cutting the oversized pizza, and Li Ting just stopped laughing, thinking that you Tang Wenzhe still has the confidence to speak in the unit, how can you speak in front of the teacher and nod your head, and feel a little funny, but it is a little impolite to laugh on this academic exchange occasion.

"Look at how big the solar system is, there are several theories in the scientific community at present: the first point of view: according to the orbits of the planets in the solar system that have been discovered, that is, an average of 82 astronomical units, and one astronomical unit is about 150 million kilometers. When Professor Ding Wenjun refers to Tang Wenzhe, he will change with the scene of his speech, and when he treats Tang Wenzhe as a student who listens to the class, he will call him Tang, and when he treats Tang Wenzhe as a friend, he will call him Wenzhe.

"Oh, let me explain, the astronomical unit that Professor Ding is talking about here is the distance between the sun and the earth as mentioned by insiders, which is about 150 million kilometers. ”

"I only know some common sense about the solar system, and I don't have much specific in-depth research, but today is a long knowledge. Li Ting said curiously.

Ding Wenjun pointed to a solar system pattern on the projection screen with a laser pointer and continued: "The second point of view is determined by the distance that the solar wind can reach: about 700 astronomical units. The third view is determined by the extent of the sun's gravitational influence, which is about 2,000 astronomical units. Generally speaking, scientists around the world agree that the diameter of the solar system is about 12 billion kilometers. But no matter which definition is made, there are many things in astronomy that are uncertain, and human understanding of the universe is still at a very low level. ”

Li Ting asked Tang Wenzhe beside her softly: "Isn't there a Milky Way outside the solar system? The solar system is so big, how big is the Milky Way?"

"This female student asked a very good question, which shows that you are thinking about the problem, the superior galaxy of the solar system is the Milky Way, which is a giant barred spiral galaxy, the diameter of the Milky Way is about 120,000 light-years, it has a huge disk structure, and the entire universe is currently observed to be 96 billion light-years and may be larger. According to the latest research by scientists, the Milky Way has four well-defined and symmetrical spiral arms, which are 4,500 light-years apart. Our solar system is not on the four main arms, but very close to a branch that is independent of the four main arms, and the distance to the core of the Milky Way is about 27,000 light-years, and the sun orbits the galactic core for about 250 million years. The Milky Way includes 100 billion ~ 400 billion stars and a large number of star clusters, nebulae, and various types of interstellar gas and interstellar dust. In the center of the Milky Way lies a supermassive black hole with a mass of about 4.3 million suns."

"Light-year, I know, is the distance traveled by light particles in a straight line in a vacuum, which is generally used to measure the space-time distance between celestial bodies, calculated at the speed of light at 300,000 kilometers per second in a vacuum, about one light-year is 9.46 trillion kilometers. Li Ting said after taking a pen and calculating on a piece of paper.

"I think this female student has a lot of potential for deep space exploration, what major are you studying? Ding Wenjun said.

"Professor Ding, I'm Li Ting, graduated from the Department of Computer Science of Beijing University of Posts and Telecommunications, and now I am a colleague with Mr. Tang Wenzhe, so I'm sorry to interrupt your speech. ”

"Hey, you can't say that, we are discussing scientific issues, discussing each other's strengths and complementing each other's weaknesses, there is no problem of interrupting the speech, Mr. Li, your question is very good, although you graduated from computer science, you still have a foundation in astrophysics, and the questions raised are representative. Ding Wenjun said.

"Thank you Professor Ding for your encouragement, I originally graduated from Nanxiang Middle School in this city, I used to be a member of the school's astronomy enthusiast training camp, and I went to Sheshan Astronomical Station to do starry sky observation practice activities, so I can only say that I have a little hobby and interest in astronomy. Li Ting said.

"No wonder there is a basis for the questions asked, and many great scientists start their focus on a certain field of science with interest. Ding Wenjun said encouragingly.

At this time, Wang Mingxia put the cut pizza on several plates, and put the first one in front of Professor Ding who was speaking, and then pushed the drink on the table slightly in front of Professor Ding, and said, "Professor Ding, please use snacks and drinks." ”

Professor Ding picked up a can of soda from the table and said, "Wen Zhe still remembers that I like to drink some soda." ”

"Yes, Professor Ding occasionally has a bit of high uric acid, and the doctor advised him to drink some soda. Tang Wenzhe said.

Wang Mingxia saw Professor Ding take the plate of pizza and soda, and then put two sliced pizzas in front of Tang Wenzhe and Li Ting.

Li Ting took the pizza handed over by Wang Mingxia and said 'thank you', and took a few napkins and a bottle of orange juice from the table.

"When Tang was studying with us, sometimes we discussed problems late and relied on pizza, this model is very good, it can not only fill the stomach but also save time, and it is also delicious to eat occasionally, Tang student, are you right?" Ding Wenjun said.

"Yes, we were all poor students at that time, we didn't have much money in our pockets, and the pizzas were all bought for us by Professor Ding, and now we can be regarded as repaying the teacher. ”

"Then the price of your return to the teacher is too small, just a pizza. Li Ting said while eating pizza.

"Hey, it's okay to get your mind, you have a successful career now, my teacher has face, eating pizza is secondary, and the most important thing is for everyone to exchange their opinions. Ding Wenjun hurried up to eat a few bites of pizza and drank some soda and said.

"Professor Ding, let me ask you a question, since the Milky Way is so big, how big is the entire universe?" asked Wang Mingxia.

This student asked a very good question, and now it seems that the friends brought by Tang Wenzhe today are very in-depth, and this question is quite profound, and it is exactly the topic we are analyzing and researching now. Ding Wenjun was very satisfied with the in-depth questions.

"How big is the universe is is a question of general concern, this is not only a scientific question, but the most important thing is directly related to the evolution of our earth, and whether there is only a lonely existence of our earthlings in the universe. That's why our deep space exploration is so important. When Ding Wenjun said this, he had already guided the direction of the discussion towards the topic of deep space exploration that he was concerned about.

Of course, Tang Wenzhe knew what Professor Ding wanted to express, but now he tried not to interrupt Ding Wenjun's speech.

Ding Wenjun continued: " Human beings have launched a number of probes into space since the 70s, through the observation and calculation of scientists, according to the current detectable size of the universe is 93 billion light years, in the observable universe, there are at least 2 trillion galaxies similar to the size of the Milky Way, according to the average existence of 35 billion stars in a galaxy cluster, the entire cosmic system exists as many as 700 trillion star systems, and the number of stars is as high as 5600 trillion, and our earth is just one of the 5600 trillion planets, which shows that compared to human beings, the inhabited earth is not worth mentioning in the universeOur earth is a speck of dust in the universe, not even dust, only a particle. ”

"Professor Ding, let me ask you another question, according to your analysis and research, how much time will it take for us human beings to fly out of the solar system according to the current level of science and technology?" asked Wang Mingxia.

"That's a good question, a good question, is it possible to fly out of the solar system with the current level of science and technology of our human beings? My research answer is not yet. Voyager 1 is said to be the greatest probe ever built by mankind, was sent into space on September 5, 1977, with a speed of up to 17 kilometers per second, it carried the most advanced instruments and energy devices at that time, as well as friendly information brought by human beings to extraterrestrial civilizations, and began its long space travel, human beings have been eager to fly out of the solar system, maybe one day we human beings immigrate to the outer galaxy may no longer be a dream, but the ideal is very rich, and the reality is very skinny. ”

"Your Professor Ding is a very fashionable word, and the word backbone is very popular now. Li Ting whispered in Tang Wenzhe's ear.

Tang Wenzhe gave a thumbs up and said to Li Ting: "Yes, Professor Ding's important views are still behind?"

Professor Ding was as passionate as he was lecturing in a lecture hall, pointing to a picture of the solar system on the screen and continuing: "To understand this question, let's first understand where the edge of the solar system is, in fact, until now, the edge of the solar system has not been completely defined. In a narrow sense, the solar system consists only of the sun itself and the eight planets that surround it, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Nephra, and then the five dwarf planets, Pluto, Ceres, Eris, Orion, and Haumea, as well as nearly 200 confirmed moons, millions of asteroids, and countless comets and interstellar dust. The entire solar system is moving in an orderly manner under the strong gravitational pull of the sun. The narrow definition of the solar system is the orbit of Pluto and has a diameter of about 79 astronomical units. Voyager 1 is now more than 140 AU away from the Sun, so by narrow standards, it has indeed left the solar system.

But in fact, the scope of the solar system is much larger, in the 20th century discovered by the sun's gravity controlled by the "Oort cloud", so that the boundary of the solar system greatly extended, scientists now believe that this boundary can be extended to 5 to 150,000 astronomical units from the sun, scientists now believe that the solar system belongs to the "Oort cloud" boundary is the real edge of the solar system.

Therefore, Voyager 1 is still under the control of the sun's gravity, and it has only flown out of the narrow edge of the solar system, far from the true edge of the solar system. At its current rate, it will take at least 10,000 years to truly break free from the shackles of the sun's gravity, and it will take a long time for human beings to truly get out of the solar system. After speaking, Professor Ding reluctantly put down the pizza in his hand, picked up a can of soda and took a sip.

"I completely agree with Professor Ding's point of view, which is a cruel fact that scientists have reluctantly accepted based on the current level of science and technology of mankind. Tang Wenzhe interjected, and at the same time, he could let Professor Ding pause for a short rest.

"More trouble is still to come, 'Voyager 1' has been working in space for more than 40 years, its energy is not based on solar energy, but nuclear power batteries, it is expected that the battery will reach its design life by 2025, and in a few years, Voyager 1 will be completely cut off from human beings, it will fly into space endlessly, but like a kite with a broken string, we have no trace of its whereabouts. ”

"It's a shame that we've lost an eye to observe the universe in the distant starry sky, and humanity will have a farther way to understand the universe beyond the solar system. Li Ting said regretfully.

But don't be disappointed, in addition to using Voyager 1 as a space probe, humans have also used optical telescopes such as the Hubble telescope, which can see luminous objects as far away as 27 billion light-years away from Earth. There is also the use of space particle detectors to collect cosmic rays in space, to explore dark matter and antimatter in space, and to explore the mysteries of the origin of the universe. The most noteworthy is the world's largest 500-meter giant radio telescope (FAST) built by China, also known as China's 'Sky Eye', which is composed of 4,450 reflection units with a total area of 250,000 square meters, equivalent to the size of 30 standard football fields. It will detect distant signals and materials in the universe, such as electromagnetic waves, microwaves, lasers, various organic matter in the universe, interstellar matter, stars, etc.

"Yes, this is a great contribution, although some scientists in the world oppose the contact between earthlings and the extraterrestrial world, but the 'Sky Eye' has epoch-making significance in the history of the development of science in the world, especially in the deep space exploration of the universe. Tang Wenzhe said.

"There are two basic points in my research that Tang Wenzhe knows, the first is to look at the universe from a philosophical point of view, and the earth we live on will never exist alone in the universe. Second: we earthlings will never be the only intelligent beings in the universe. Professor Ding is very confident when he talks about his theoretical views and insights.

Professor Ding's first point has been confirmed, and scientists are now using astronomical telescopes to discover that the Earth may be the only planet in the solar system where life exists. So suppose that there are only planets in the Milky Way with life on Earth, and that there are 200 billion galaxies in the universe. Earth is only one billionth of the planets in the Milky Way. That means there are still 200 billion planets like Earth in the entire universe. As for the second question...," Tang Wenzhe said, pausing here.

"Student Tang, you told me in an email a year ago that you found out what happened to the password to enter the 'parallel world', and whether it is related to intelligent life?" asked Professor Ding.

"This... I'm afraid that's not appropriate tonight. Tang Wenzhe hesitated.

"What does it matter, we are all materialists, academic discussions, don't worry about being doubted and criticized, our purpose is to discover our unknown world in deep space exploration, you might as well say it. ”

Tang Wenzhe didn't answer, took out a small wooden comb from the pocket of his suit underwear and looked at it. Li Ting lowered her head and cut a piece of pizza on her plate, while Wang Mingxia looked up and drank a bottle of orange juice drink.

At this time, there were several slight knocks on the door of the small conference room, and as the door was slowly opened from the outside, a trace of horror was felt in the room, and Professor Ding stood up and said, "Who?" Professor Ding stepped out of the door and saw dimly lit along the corridor to the reading hall outside, and only saw the gray-haired old library attendant on the shelf sorting out books at the service desk in the distance.

"It's strange, I think I heard a knock on the door, why is there no one outside. After Professor Ding finished speaking, he closed the door and returned to the conference table, only to find that Tang Wenzhe, who was sitting across from his table just now, was gone...