A week later, Hua Feng and the others began to systematically study the types of civilizations. They know that in the vast universe, the civilization established by humans on Earth is insignificant. Because the earth's civilization is so short, it has only been tens of thousands of years since human beings began to create civilization, only a few hundred years to develop science and technology, and only a few decades to explore space technology, which is no different from 4.5 billion years of the earth's age and 10 billion to 15 billion years of the galaxy's age.

In 1964, astrophysicist Kadashev proposed the Kadashev scale as a measure of the technological level of a civilization. This scale is only theoretical, and the object of its measurement, that is, highly evolved civilizations, is only in human speculation, and no one has seen it firsthand, but the significance of this scale is that it considers the energy consumption of the entire civilization from the perspective of the universe:

Type I civilization: Able to mobilize energy equivalent to the entire output power of the earth (at that time, he estimated that the power output of the earth was about 10 to the 15th power ~ 16th power watts) for communication. Now we are able to circumnavigate our Earth in one day, learn about events happening on Earth, and also leave Earth to do a lightning visit to the Moon.

Type II civilizations: Able to use the equivalent of a typical star (10 to the 26th power) for communication.

Type III civilizations: 10 to the 36th power for communication, which is approximately equal to the power output of an entire typical galaxy.

As of 2011, the current civilization on Earth can only be roughly defined as Type 0.72 - not even Type I. However, considering that in 1900 we were still a type 0.58 civilization, it is expected that in the next one or two hundred years, mankind will enter a type 1 civilization.

In addition, the American astronomer Carl Sagan suggested that the three-type civilization can be divided into 10 subtypes according to the energy scale, that is, 10 to the 16th power of the tile is 1.0 type, 10 of the 17th power tile is 1.1 type, 10 of the 18th power tile is 1.2 type, and so on. These three types of civilization are not insurmountable, but because of their qualitative differences, it is conceivable that the process will be very long and difficult.

Nikolai Semenovich Kadashev (Николай Семёнович Кардашёв), born on April 25, 1932 in Moscow, is a Russian (former Soviet Union) astrophysicist, deputy director of the Institute of Cosmonautics of the Russian Space Research Institute, and an academician of the Russian Academy of Sciences.

Kadashev graduated from Moscow State University in 1955 and received his doctorate in 1962 under the tutelage of Schrowski at the Berg Institute of Astronomy.

Kadashev became an Associate Academician of the Department of General Physics and Astronomy of the USSR Academy of Sciences in 1976 and an Academician of the Russian Academy of Sciences in 1994.

In 1963, Kadashev began to work on quasars such as CTA-102 and was the first to propose a classification of extraterrestrial intelligence (search for extraterrestrial civilizations). In this work, he came up with ideas that might make it possible to recognise millions or billions of extraterrestrial or even extragalactic civilizations, and he created the Kadashev Classification of Cosmic Civilizations, as well as the types of civilizations. In the search for extraterrestrial civilizations, Russia proposed some similar programs earlier than the United States. Other well-known experts in the USSR were Vsevolod and Skrowsky (Kadashev's mentor).

In 1960, Frank Drake, an astronomer at the University of California, proposed a possible way to find extraterrestrial civilizations, which was the following formula: N=R*×Fp×Ne×Fl×Fi×Fc×L

This formula seems a bit complicated, and it uses the product of a series of possibilities to calculate how many civilizations (N) could exist in our galaxy.

R* represents the number of new stars born in our galaxy in a year. Space is made up of fragments scattered by supernova explosions (which occur every 30 years on average) and hydrogen gas, a byproduct of the Big Bang that formed the universe. Gradually, under the influence of gravity and the impact of new supernova explosions, this material concentrates in one place, slowly accumulates, and finally becomes a star. This process is repeated over and over again.

Fp refers to the number of planets on average of a new star formed in this way. Among the stars, there are called double stars and triple stars, which have two or three suns of the same size, alternately surrounding each other. It is argued that it is impossible to form a planet in this case. But in our solar system, aren't there huge planets like Jupiter and Saturn? These can be considered triples, and there may be planets in doubles, triples. However, stars usually have several planets, which cannot be determined.

Ne indicates that there is a chance that life will occur and evolve on these planets. For life to arise, there must be a lot of liquid water. However, if the planet is too far away from the star, the water will freeze into ice, and if it is too close, it will turn into water vapor. In order for life to evolve, it is necessary to have a land made of rocks. If the planet is too large, it cannot have these conditions. In addition, there is an atmosphere, and it is possible for asteroids to drift away due to lack of gravity and lose their atmosphere. Moreover, if the rotation period is too long, not only will the temperature difference between day and night be too large, but also the continuous blowing of strong winds will make it difficult for life to occur.

Fl indicates the proportion of planets that meet these conditions that actually have life and evolved evolution. Evolution must have DNA, which is the genetic method of producing extremely complex and huge compounds, and the possibility of this formation is very small.

Fi denotes the probability of forming life to evolve to intelligence. Bacteria, trees, and grasses cannot evolve into sapient organisms. They do not have a cranial nervous system, nor do they have the spine necessary to grow to a certain size. The most primitive animal with a nervous system and spine is a fish.

And if the fish is always in the water, it will not be able to evolve into intelligent creatures. First the fish had to become four-legged and able to walk on land, then climb trees, learn to grasp things with their fingers, and evolve to walk upright. In this way, the feet and hands are divided, and after a long period of time, they will have sufficient wisdom. However, the process doesn't have to be that smooth. There are quite a lot of planets where there is life, but if there are only bacteria, plants, and shellfish, then it is difficult to produce intelligent life.

Fc said the proportion of intelligent beings who are able to communicate with the outside world. At present, the earth mainly uses radio waves as the means of interstellar communication, but deep in the Amazon basin of South America, deep in the mountains of New Guinea, there are uncivilized hominids.

They did not know about farming and animal husbandry, and lacked the skills to make metal items such as iron tools, so they could only hunt with simple bows, arrows, and clubs. They didn't evolve from ape-man to human recently, they evolved hundreds of thousands of years ago, just like us. It is necessary to have advanced science and technology, and also to have certain conditions, which are enough to accommodate countless changes and experience accumulation. Therefore, the planets with intelligent life may not all reach a high level of civilization, and therefore may not all be connected to the outside world, so that we can perceive their existence.

L stands for the average lifespan of a civilization. The earth's civilization is not yet highly developed, but it is already facing the threat of nuclear war and public pollution. The higher the level of civilization, the greater the likelihood of a devastating blow, because the forces that push it forward and destroy it are so great that an unexpected event can bring civilization to its death. If all developed civilizations had a short lifespan, then if there were developed civilizations on other planets, they would have perished long ago.

We can't extrapolate L and Fc, so scientists disagree on the value of N. Carl Sagan calculated that for every 1 million stars in the Milky Way, there is a highly developed extraterrestrial civilization, and he calculated that there is a civilized planet for every 100,000 stars. So Sagan artificially said that life could have existed on stars that were very close to each other, such as Twassage and Jetret Curie I and II.

Based on this formula, Drake predicted that there would be 4,000 communicative civilizations. Asimov counted 530,000 in the book "Extraterrestrial Civilizations".

There are also people who question this extraterrestrial civilization equation, thus denying that there are developed civilizations in the galaxy besides Earth. For example, Shukroski of the former Soviet Union believed that other planets have nuclear weapons and will perish due to nuclear war, so the lifespan of developed civilizations is very short, and there may be no other civilization in the galaxy except Earth.

Another opinion is that equations predict too many and are unreliable. For example, the American astronomer McKale Hart believes that if there were developed civilizations in the Milky Way, then it would have colonized the Milky Way and so on, and it is impossible that we have not discovered it so far. This suggests that there are no other civilizations within the galaxy other than Earth, while there may be outside the galaxy.

The exploration of extraterrestrial civilizations is first of all a kind of curiosity for human beings, and for the unknown, human beings are always trying to uncover the mystery of their appearance. But humanity is so small in relation to the vastness of the universe that until now we have not even been able to figure out where we came from. To some extent, the exploration of extraterrestrial civilizations is actually a process of exploration of human civilization itself.

It is likely that the chemical evolution of prelife was not confined to Earth for reasons including:

1. The time lag between the formation of the earth's crust and the emergence of the earliest cellular life is relatively short;

2. Astrochemical studies have shown that interstellar matter and other celestial bodies in the universe contain a large number of organic molecules;

3. There is evidence that organic molecules outside the Earth have ever reached the Earth's surface through certain pathways.

If the birth of life on Earth is inseparable from extraterrestrials, then the exploration of extraterrestrial life is actually an exploration of its own origins.