In other words, Ding Mu took the four Tianying under his command and asked them to find Guo Pingfan. In the war-torn Diamond Country, he has already organized an army that has withstood the test of blood and fire, and this army is the reserve force for future wars.

Ding Mu thanked the Dahua army for its support, and took his family to a large villa on the coast of California.

His son, Ding Yan, is here.

Ding Mu came to the United States this time, mainly to see his son. Ding Yan is about to go to elementary school, and Ding Mu is going to get him back to Dahua from the United States. The reason is very simple, today's basic education in Dahua is obviously better than that of the United States.

"Daddy, Daddy!" a handsome young man jumped up to Ding Mu's side, opened his hand, and pounced.

Although the father and son have never really met, Ding Mu often communicates with him through video. Ding Yan already knew a lot of things, and he knew that his father was a great scientist. The people around him adored him.

Cheng Qianzhan, these old friends of Yunliu, who are also the so-called loyal ministers of Tuogu, have always told Ding Yan about Ding Mu's glorious deeds, and established Ding Mu as a man who stands up to the sky, under their influence, men should be like this.

In addition, the United States is also a country with strong personal heroism, and Ding Yan has also been influenced by this since he was a child, and feels that his father is a great scientist.

Therefore, although Ding Yan had no contact with Ding Mu, he admired him very much, and in his small heart, he felt that his father was an extremely great person.

This time, when he heard that his father was coming to see him, Ding Yan was so excited that he didn't sleep well last night.

Ding Mu picked up Ding Yan and said with a smile: "Yan'er, dad hugs, it's so big before you know it." Dad likes Yan'er the most. ”

Ding Yan pouted on Ding Mu's cheek, kissed it, and then said, "Yan'er also likes Dad." Dad, did your research succeed?"

Ding Mu nodded with a smile and said, "Well, the stage has been successful." ”

"yes, that's great. Ding Yan's face showed a heartfelt smile, "Dad, can you tell me the story of the earth and aliens?"

Ding Mu nodded and said, "Okay." Daddy can tell you stories. ”

Just when Ding Mu wanted to open the conversation, Cheng Qianzhan walked in and said, "Young master, young master has basketball lessons in the afternoon, and we are about to set off." ”

"Grandpa Cheng, I don't want to go to basketball class, I want my dad to tell me stories. Dad, it's okay for me not to go to basketball class. Ding Yan looked at Ding Mu with a blank look in his eyes, his eyes full of pleading.

How could Ding Mu refuse this kind of look, he said: "Uncle Cheng, our father and son finally met, forget it today." ”

Of course, Cheng Qianzhan would not disobey Ding Mu's words, and nodded and retreated.

Ding Yan felt very happy when he saw that Cheng Qianzhan was gone, and he could spend an afternoon with his father, and his beautiful Danfeng eyes raised upward, and asked like a cannon: "Dad, are there any aliens? What do aliens look like? Will aliens come to attack the earth? If aliens come, can we win?"

Ding Mu smiled, it was not strange for him to ask Ding Yan this kind of question, children, they always like to explore the unknown, especially children who grew up in the United States, they have a strong atmosphere, and they are always very interested in alien civilizations and aliens.

Ding Mu said: "Yan'er, let's sit down and talk slowly, you ask your father so many questions at once, and your father has to talk slowly." ”

"Yes, yes. Ding Yan smiled and moved the small bench and took two bottles of mineral water, and handed one to Ding Mu: "Dad sits on the sofa and drinks water, and Yan'er sits on the small bench and listens to the story." ”

Ding Mu looked at Ding Yan's cute appearance and narrowed his eyes.

After taking a sip of water, Ding Mu said affirmatively: "Whether there are aliens or not, this answer has been relatively clear." Aliens are there. ”

"Yay. I knew there must be aliens. Ding Yan was encouraged.

Of course, Ding Mu knew that there were aliens, and his own real-life beautification system was left by aliens. He said: "The universe is so big, and it is impossible for human beings to be alone in such a large area. ”

"Dad, how big is the universe?" Ding Yan asked curiously.

Ding Mu picked up a pen, and then wrote and drew on the notebook that Ding Yan usually scribbled, while saying: "The research results of contemporary astronomy show that the universe is a hierarchical structure, constantly expanding, diverse in material forms, and constantly moving and developing celestial systems.

The Earth where we live is in the solar system. Planets, asteroids, comets, and meteoroids all orbit around the central celestial body, the Sun, which together make up the solar system.

There are other planetary systems outside the solar system as well. About 250 billion sun-like stars and interstellar matter make up the Milky Way, a much larger celestial system.

The Milky Way is about 100,000 light-years in diameter, and the Solar System is located in one of the spiral arms of the Milky Way, about 30,000 light-years from the galactic center. ”

"Dad, what do you mean by light years?" Ding Yan asked curiously, "Is this a distance unit?"

"Well, that's right. "A light-year is a unit of distance, a year's distance that light travels in a straight line in a cosmic vacuum. The speed of light is about 300,000 kilometers per second, specifically 299,792,458 meters per second. ”

Ding Yan did the math and said, "1 light year is 9460730472580800 meters. ”

Ding Mu shrugged his shoulders and said, "Son, your mental arithmetic skills are good." ”

Ding Yan smiled and said, "I learned this from Grandpa Sun." Dad, come on, what's out of the galaxy?"

"There are many similar celestial systems outside the Milky Way, called extragalactic galaxies, often referred to simply as galaxies," Ding said. 100 billion galaxies are currently observed.

Galaxies are clustered into large and small groups, called galaxy clusters. On average, each galaxy cluster has about 100 galaxies with a diameter of tens of millions of light-years. Tens of thousands of galaxy clusters have been discovered. A small cluster of about 40 galaxies, including the Milky Way, is called the Local Group.

I personally study the latest statistics, there are at least 2 trillion galaxies in the universe. On such a large scale, logically speaking, the probability of having aliens is very high. ”

"But where are they?" asked Ding Yan.

"Hahaha. Ding Mu smiled and said, "Good son, this question of yours was asked seventy years ago, that person is a famous physicist, or a Nobel laureate, his Fermi, we call this question the Fermi paradox." ”

"Oh......" Ding Yan seemed a little disappointed that Ding Mu said this, and continued to ask, "What does the Fermi paradox mean?" ”

Dingmu explained: "The Fermi paradox may be a bit complicated for you. He deals with the contradiction between arguments about scale and probability and the scarcity of evidence, and the remarkable scale and age of the universe implying that a higher extraterrestrial civilization should exist. But this hypothesis is not supported by sufficient evidence.

The first point of Fermi's paradox, the problem of scale, is an order of magnitude estimate: there are about 250 billion (2.5 x 10^11) stars in the Milky Way, and 700 (7 x 10^22) stars in the observable universe. Even if intelligent life appears in the planets around these stars with a small probability, then there should be a sizable number of civilizations within the galaxy alone. This is also in line with the idea of the banal principle, that the Earth is not special, but just a typical planet with the same laws and phenomena as other stars.

The second point of Fermi's paradox is a response to the scale view: given the ability of intelligent life to overcome resource scarcity and its propensity for external expansion, it is likely that any advanced civilization will seek out new resources and pioneer the star systems in which they are located, and then set foot in neighboring galaxies. Because 13.7 billion years after the birth of the universe, we have not found reliable evidence of the existence of other intelligent life on Earth or elsewhere in the observable universe, and it can be said that intelligent life is very rare, or that our understanding of the general behavior of intelligent life is wrong. ”

Unexpectedly, Ding Yan understood these at once, and said: "Then I know, this is a theoretical and logical discussion about whether there are aliens." I think it can be divided into three situations. 1. There is no other civilization in the universe. Second, extraterrestrial civilizations exist, but they have not been able to contact us so far. 3. Extraterrestrial civilizations have come to Earth, but we don't know it. ”

Ding Mu said: "It's great, you summed it up perfectly. Son, however, more and more facts tend to prove that extraterrestrial civilizations exist and have come to Earth, but the vast majority of people are not yet capable of contacting them. Or, in other words, I have come into contact with them, and I don't know their alien identity. Maybe there's an alien behind you. ”

"Dad. I'm afraid. Ding Yan suddenly felt a little scared, got up from the small bench, and snuggled into Ding Mu's arms.

Ding Mu smiled, rubbed Ding Yan's head, and said, "Manly man, don't be afraid." Dad told you that interstellar travel across galaxies in the true sense of the word requires a huge amount of energy. It takes at least a second-level civilization to be relatively easy to achieve. ”

Ding Yan asked, "Dad, what is a second-level civilization, and how many levels are we?"

Ding Mu was extremely patient, and slowly replied: "The civilization level I am talking about is called the Kardashev level

Based on the amount of energy that a civilization can use to communicate with the outer planets. In other words, planets that can communicate with the outside world with a lot of energy can be counted in the Kardashev class.

It is based on a level 1 civilization: this level of civilization has complete control over all available resources and everything on their planet. It is the absolute ruler of the planet, and can control the change, rise and fall of the weather, rivers, land, other organisms, oceans and even the matter in the earth's crust at will. It can be said that on their planet they can do anything.

Level 2 Civilizations: Can take full advantage of the energy of their stars and fully control their solar system. You can live and travel freely on the planets in the solar system, and you don't have to worry about energy problemsBecause you know how to efficiently use the endless energy emitted from the stars, the society has become highly developed and the development tends to be highly accelerated.

It should be mentioned that if a civilization reaches the second type, then it is almost indestructible. At least all the natural and man-made means known to mankind cannot touch a single hair of them. Even a supernova explosion near them would not allow them to perish. They can move their galaxy away from the erupting star, or simply harness the energy of the eruption.

Level 3 civilizations: are no longer limited to their solar system, but the entire galaxy in which they are located, such as our Milky Way. Harnesses the energy of all stars in the galaxy. This kind of civilization is beyond our imagination, and we can only guess how they lived. According to Einstein's theory of relativity, exceeding the speed of light is theoretically untenable. A galaxy is very large, and it takes millions of years to fly from one end to the other, even if it is close to the speed of light, so the third type of civilization is not like in science fiction movies, driving huge spaceships and shuttling through the stars. They should have the ability to artificially develop wormholes. ”

"Dad, what level of civilization are we?"

"Five years ago, the human index was 0.73 (2× 1013 watts per second), and the growth of the 0.1 index meant a 10-fold increase in energy, and the growth of the 0.01 index meant the 10th root of 10, which is a 1.259-fold increase, and humans had to increase their existing energy production thousands of times to reach the first level of civilization. However, this is not counting the few technologies I have developed recently. Your dad's greatest contribution is to raise this human index to close to 8. ”