Hua Feng heard the teacher continue: In addition to satisfying curiosity, exploring alien civilizations will also provide many conveniences for the development of human beings:

1. If a more advanced civilization can be discovered, then it can obviously provide a good reference model for the development of human civilization, and perhaps the development process will take many detours;

2. The rapid growth of population has made it more and more difficult for the earth to support the huge energy consumption, and the ecological environment and other problems have affected people's healthy life to some extent. Maybe one day, Earth will become uninhabitable, and immigrants like Mars (or a planet farther away but better?) will also benefit from the assistance of extraterrestrial civilizations.

Although we speculate that there are many extraterrestrial civilizations in the universe and in the galaxy, why don't they establish direct contact with humanity and let humanity make a potentially futile effort? Can we find these cosmic brothers? Maybe they are looking for us, but are blocked from the distance for the same reason. These reasons can be:

The sheer size of the universe makes it extremely difficult to connect with each other. Presumably, in the Milky Way, the maximum possible outcome is that 500 stars produce an extraterrestrial civilization, so that we can only find an extraterrestrial civilization if we find 501 stars, and according to the stellar density, 500 stars occupy a radius of 35 light-years, which means that the nearest extraterrestrial civilization may be 35 light-years away, and we send a signal there, and it will take 70 years to hear back at the earliest.

We use electromagnetic waves to connect with extraterrestrials, but because the electromagnetic spectrum is so wide, we don't know what spectrum they use.

Since there are different types of civilizations, if the other side is a highly developed civilization and reaches the level of type II or III civilization, then they may dismiss us and avoid us. If the other party is less civilized than we are, they will not be able to communicate with us.

We think about civilizations that are similar to Earth's civilizations, but what if there are other life forms, such as silicon people, such as spider-men and little green men in science fiction, they can't communicate with us. And if they exist in a form unknown to us, we have to remain unknown to them forever.

In ancient times, not too far ago, due to the underdevelopment of science and technology, when people sailed in the vast sea, in order to preserve and transmit information, they often put the note with the information in a bottle, and then sealed it tightly and threw it into the sea. These bottles may be washed ashore by ocean currents, where they can be spotted and informed of the condition of the person delivering the message. But this method of "drifting bottle" is extremely unreliable, because it is possible that it does not drift to the shore, but continues to "wander" in the sea. In this way, hundreds, thousands, or even tens of thousands of years later, the message of the transmitter will be lost to the sea, and no one will know it again.

Just as the ancients traveled in the vast sea, the "travel" of modern people in the vast universe has to adopt a "modern drifting bottle" method to make contact. This method is to use some things that may be common to extraterrestrial civilizations (mainly radio waves and numbers) as the carrier of information, and then communicate with aliens through these cosmic languages.

Radio is a reasonable way to deal with extraterrestrial civilizations. This is because the level of technology that uses radio waves is possible in a civilized society. It's as fast as light, and our fastest spacecraft can only reach a few thousandths of the speed of light. The nearest star is 4.2 light-years away, and if we were to get there, even the fastest spacecraft would fly for 25,000 years, and most stars are farther than that. At the same time, with the current level of technology, we cannot communicate with alien civilizations in spaceships.

In fact, as early as the 19th century, people were trying to find ways to get in touch with extraterrestrial civilizations. For a quarter of the 20th century, we were bombarding space with radio waves, which have now traveled 70 light-years in all directions, covering thousands of star systems. We can imagine that intelligent life on a certain planet may have turned on the radio by now and is listening to some popular songs on Earth.

In 1960 and 1972, the United States implemented the "Ozma" program, which listened to the radio signals of two nearby stars, and in 1974 sent a short telegram to the constellation Hercules.

We know that the farther and weaker the radio signal travels, and coupled with the interference of other radiation and noise, the possibility of the alien civilization receiving the signal is very small. Scientists have discovered that in order for an extraterrestrial civilization in the universe to receive our radio signals, a transmitter must be built, the emitting power of which is comparable to the radiation power of a medium star.

Transmitting a single-frequency signal with a 21-centimeter wave that only intelligent beings can receive requires not only a transmitter of incredible power, but also a huge spherical antenna system. If the emitted power is 10 to the 26th power, the range is required to be 30,000 light years, and its spherical radius should be 0.1 astronomical units (about 15 million kilometers). Although the Earth's atmosphere absorbs only 1% of 21 centimeter waves, the system dissipates up to 10 24 watts of energy in the atmosphere, which is a million times the amount of solar energy absorbed by the Earth. Therefore, once this radio transmitter is activated, the end of humanity will also come.

Therefore, if there must be such a launch pad, we can only build it in space. However, building it in space is expensive. For example, if we were to build a radio transmitter with a power of 200 kilowatts, a radius of 5,000 kilometers and a distance of 10,000 light-years at a distance of 100 kilowatts from the Earth, using materials with an average density of 100 kg/(m∧3), the total mass of the spherical antenna would be 5× (10∧19) tons, which is about 1 500% of the mass of the Earth.

If a nuclear-powered spacecraft with a thousandth of the speed of light is used to transport these metallic materials, the energy that will be released during flight should generally be limited to 0.1% of the star's radiated energy. In the solar system, it is 3 × (10∧23) watts. It would take at least 3 million years to transport antenna materials to a spacecraft that meets this condition. In addition, it takes 30 million years to excavate materials, raw materials, and fuel, and transport laborers and food. In addition, when the launch pad is completed, 100 million tons of nuclear fuel will be consumed every year, and the power required to establish a line to connect with the launch pad will greatly exceed the power of the launch pad. It's too much! Humans don't have that ability yet. Even if it can be carried out, it will be more than worth the loss.

If the method of receiving signals from extraterrestrial civilizations is adopted, the cost is much cheaper and the feasibility is greater. In this way, we must build a multi-channel narrowband omnidirectional receiving antenna system to receive signals from extraterrestrials. But signals from extraterrestrial civilizations, even in very close planetary systems, are difficult for us to receive due to various disturbances.

Suppose that when communicating, the other party emits extremely strong radio waves, and the equipment on this side of the earth can also receive them. So first of all, the radio telescope on the earth must be aimed at hundreds of billions of planets in the Milky Way, and the other party must also send signals to hundreds of billions of planets because they don't know the earth, so there are too few opportunities for the two to meet. Even if we meet and we receive a signal from an extraterrestrial civilization, what about it? Because the universe is unusually far away, tens of thousands of light-years from Earth. During this period, it is difficult to know whether the earth civilization and the alien civilization that talks about it still exists.

Even if we receive signals from extraterrestrial civilizations, whether we can distinguish and identify them is a question. In 1899, the electrical technician Nicola Tesla, who had just invented the radio, received strange radio wave marks, and later, when wireless communication between the United States and Europe had just begun, a radio amateur received a much lower frequency mark than the one he used, which was thought to be the radio wave sent by Mars, but no one knows where these radio wave marks came from.

Today, a century later, astronomers receive countless radio waves every day, but these waves may come from the Earth itself, or from probes or satellites launched by the Earth, so it is difficult to distinguish them.

It seems that "cherries are delicious and difficult to plant". Although the use of radio waves is the most appropriate method, it is undoubtedly difficult to implement. Despite this, scientists have not given up on their efforts to communicate with extraterrestrial civilizations with radio waves. In the summer of 1982, the International Astronomical Union (IAL) launched a great search for extraterrestrial life, and in October 1992, the United States launched two powerful radio telescopes in Puerto Rico and California, called the "Microwave Exploration Program".

In the past, scientists liked to use microwaves ranging from 1420 MHz to 1720 MHz for reception, and this area is called a water hole, which is considered to be the most likely emission frequency for aliens. Because a water molecule (H2O) is composed of a hydrogen atom and a hydroxide (OH), when the hydrogen atoms collide in the air, the energy frequency radiated is 1420 MHz, and when the hydroxide (OH) collides in space, the energy frequency of radiation is 1720 MHz, so it is called a water hole.

Since hydrogen is the most basic element in the universe, intelligent life may emit radio waves in this way, and the water hole is a relatively quiet area of the spectrum. The radiation of hydrogen atoms is very weak, and in some other bands there is a lot of strong radiation, which can interfere with the signals of possible intelligent life. The Ozma plan of 1960 was too distracting to come to fruition.

The Microwave Discovery Program is different. It can receive more than 10 million channels simultaneously, and it gets more information in 30 seconds than it has in the last 30 years.

It currently has two plans: one is target search, which will search for 1,000 sun-like stars in 10 years, or all stars within 75 light years above Earth, using frequencies in water holes, and the other is space surrounding, which will search every place in the 1,000 MHz to 10,000 MHz band in 10 years. The program commemorates the 500th anniversary of Christopher Columbus's discovery of the New World, demonstrating the belief of scientists on Earth in their search for extraterrestrial civilizations.