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Should humans cry out in the dark forest of the universe?

Although there is still no conclusion about the existence of extraterrestrial civilizations, the related discussion has become an important research topic in the history of science.

In 1957, the US Natural Science Foundation funded Ohio State University to build a radio antenna dedicated to the search for extraterrestrial civilizations. This may be the earliest extraterrestrial civilization funding project. In 1971, NASA also joined the funding ranks for monitoring extraterrestrial civilizations.

Since 1980, related researchers have received relatively stable funding. In 1971, some researchers even proposed the "Cyclops" plan to build a vast radio telescope array consisting of 1,000 surfaces and 100-meter apertures, dedicated to searching radio signals from extraterrestrial civilizations, with a total budget of up to 10 billion US dollars. However, obtaining financial support for such a grand vision is difficult, and the search for extraterrestrial civilizations has yet to make breakthrough progress.

In the 1970s, as the Search for Extraterrestrial Civilization Project (abbreviated as SETI) found nothing and its opposite, another practical means of trying to contact extraterrestrial civilizations—Message to the ExtraTerrestrial Intelligence (abbreviated as METI) was also put on the agenda.

METI is based on such a conjecture: We have not found traces of extraterrestrial civilizations because they probably do not know the existence of human beings; therefore, they can actively inform extraterrestrial civilizations of the existence of human beings by sending positioning radio signals into outer space. So far, the influential METI projects have been implemented four times.


Arecibo Message

Cosmic call 1999

Teen Age Message

Cosmic call 2003











Target Star

  • Globular cluster M13

  • Hercules

  • HD190363Cygnus

  • HD190464Sagitta

  • HD178428Sagitta

  • HD186408Cygnus

  • HD9512Ursa Major

  • HD76151Hydra

  • HD50692Gemini

  • HD126053Virgo

  • HD193664Draco

  • Hip4872Cassiopeia

  • HD245409Orion

  • HD75732Cancer

  • HD10307Andromeda

  • HD95128Ursa Major

Amount of Information

1679 bits

370967 bits

648220 bits

​500472 bits







3 minutes

960 minutes

366 minutes

900 minutes

Tansmit Power

83 kJ

8640 kJ

2200 kJ

8100 kJ

Serious Controversy Over METI

METI has also caused much controversy in the scientific community since its implementation. November 6, 1974, after the first interstellar radio message was sent to the globular cluster M13 via the Arecibo radar. This year's annual Nobel laureate, radio astronomer Martin Ryle sent a statement of disapproval warning that "...any beings in outer space are likely to be malevolent and hungry..." and calling for an international ban on any attempt to contact and send signals to it.

Ryle's statement was supported by some scientific people who believed that METI might be a risky behavior that would bring disaster to the entire human race because of a few people's reckless curiosity and paranoia. Human beings don't know yet whether extraterrestrial civilizations are benevolent—or in other words, for human beings on earth, even if they come into contact with an extraterrestrial civilization, they may not be seriously treated.

In this case, human beings at the lowest level of civilization in the universe rashly send signals to outer space that will reveal their position in space and thus invite attacks from aggressive civilizations. And the earth's history has repeatedly proved that when a relatively backward civilization encounters another advanced civilization, almost without exception, the result is a disaster.

The scientist David Brin, also known for writing science fiction, is also on the stand against METI. He speculates that the reason why human beings have not found any traces of extraterrestrial civilization——Brin called it the "Great Silence". It may be because of a danger unknown to humans that keeps all other cosmic civilizations silent. The METI project implemented by humans is tantamount to a suicide cry in the cosmic jungle.

And as a justification for the necessity of METI, Satseff calls on Brin and others to say that humans only need to implement SETI and METI should be banned, questioning the human beings who claim that they should keep silent in the cosmic jungle. If neither is happy to make a sound, how can he expect a response from his cosmic companions with a clear conscience?

This is Satseff's famous SETI Paradox.

In other words, considering the universe's scale, if no cosmic civilization thinks it is necessary to send signals to other civilizations, then the one-way search implemented by SETI is meaningless. It is destined never to find anything.

However, in contrast to this, another point of view is that: radio waves radiating from the earth into space have had a continuous operation 24 hours a day since the 1970s. They are responsible for some national security defense tasks and as an interstellar collision early warning military radar system.

Their radio signals have made the earth's civilization reveal its existence in the universe. Extraterrestrial intelligent life -- if they exist, they will find these signals sooner or later. So, it is too late for humans to keep silent now.

However, some scientists point out that this view is less convincing than it appears on the surface, although it is widely circulated as an indirect argument supporting METI.

Because generally speaking, military radar signals have dissipated below the interstellar noise level within a few light-years, making them difficult to detect.

In contrast, positioning transmission signals emitted by large radio astronomy telescopes are different. Their transmission power is much more substantial than the former, and they are much easier to capture.

In this regard, Sutseff countered that the radar signal leaked into outer space had been weakened to be undetectable by Type I civilization. But that doesn't mean it won't be captured by super civilizations like Type II or Type III.

Therefore, to avoid these radiation signals gradually passing across the sky area, those "interstellar invaders" detection, it would be necessary to ban all activities related to radar detection—and such an approach is undoubtedly impractical. Therefore, in this sense, to be able to be detected by some young type I civilizations living near the parent star, it is necessary to carry out positioning signal transmission.

"Fermi's paradox" and its solution

Accompanied by the progress of METI, on the other hand, to theoretically explore the question of "whether extraterrestrial civilization exists" has also begun to be widely spread in the scientific community. The related discussion was later often referred to as "Fermi's paradox" and its resolution.

The lack of solid evidence to prove the existence or non-existence of extraterrestrial civilizations makes the "Fermi Paradox" an extremely open problem, leading to various solutions.

An important reason given by scientific people who reject METI is that alien civilizations may be malicious. Regarding this point, many sci-fi works have already been described in various ways, and works such as World War, Mars Attack, Alien, and Independent Day are all representative works of this type.

What's more worth mentioning is that some science fiction writers have gone one step further. In addition to creating beautiful stories, they also put forward different explanations for Fermi's paradox.

In his sci-fi classic, The Berserker series, sci-fi author Fred Saberhagen envisions an intelligent doomsday weapon, the Berserker, that would survive in 500,000 years. It was left over from the previous interstellar war. It was equipped with intelligent machines by the killer fleet, and an asteroid base uniformly controlled it. In addition to being able to reproduce itself autonomously, the only instruction given was to destroy all organic life in the universe.

Inspired by the story of "The Berserker", a strict interpretation of the "Fermi Paradox" holds that the universe may be full of aggressive doomsday weapons similar to the Berserker, hindering or eliminating other extraterrestrial civilizations. The remaining extraterrestrial civilizations are afraid of attracting their attention, so they dare not send out signals, which makes it impossible for humans to search for related information.

In addition, Chinese science fiction writer Liu Cixin's recent science fiction novel "Three-Body Problem" series 441-449 also proposed a more refined interpretation of Fermi's paradox—the law of the dark forest.

This rule is a sound enrichment and expansion of the previous Bollinger conjecture based on two fundamental assumptions and two basic concepts.

Two fundamental assumptions are: (1) survival is the first need of civilization; (2) civilization continues to grow and expand, but the total amount of matter in the universe remains unchanged.

The two basic concepts are the "chain of suspicion" and "technology explosion". The "chain of suspicion" is caused by the inability to conduct instant and effective communication between civilizations in the universe. And no civilization can trust other civilizations (in the daily instant and effective communication we are familiar with, even if one party is deceived also means the truncation of the "chain of suspicion").

"Technological explosion" means that the technology in civilization may break through and develop explosively at any time, making it impossible to accurately estimate the technological level of any distant civilization.

Due to the above two basic assumptions, we can only draw the following inference: all civilizations in the universe are bound to be in contention for resources, and the "chain of suspicion" and "technical explosion" make it impossible for any civilization to believe in the goodwill of other civilizations, nor can not guarantee their technological leadership. So the universe is a dark forest where the weak prey on the strong.

At the end of "Three-Body II", the author expresses his interpretation of the "Fermi Paradox" through the mouth of the protagonist Luo Ji: "The universe is a dark forest, and every civilization is a hunter with a gun, like a ghost." He must be careful because hunters sneak like him everywhere in the forest. If he finds another life... there is only one thing he can do: shoot and destroy it.

In this forest, other people are hell, an eternal threat, and any life that exposes its existence will be wiped out soon. This picture of cosmic civilization explains Fermi's paradox: " Humans take the initiative to send themselves into outer space. You will become a silly child in the dark forest who lights a bonfire and shouts, "here I am".

However, it should be mentioned that whether it is the theory of "Rage Warrior" derived from Saberhagen's novels or Liu Cixin's "Law of the Dark Forest", as a solution to the "Fermi Paradox", there are limitations. Because whether the way of thinking, such as good and evil, defined by human beings from their behavior patterns, can be applied to all extraterrestrial civilizations is a very controversial issue.

For example, the Polish science-fiction writer Stanislaw Lem dismissed similar ideas—in a sense, his strange and puzzling novel Solaris is a result of confronting this view.

In comparison, in the later debates by Sutseff and others, another explanation is more reasonable. They believe that SETI has not searched for radio signals from other cosmic civilizations for many years because the distances between stars in the universe are very far away. The sky area is where it is located, but because the signal becomes very weak, it cannot be detected at all.

The possibility that extraterrestrial civilizations may discover the signals sent by human beings actively positioning themselves in outer space is minimal. Still, once the results are produced, the impact will be huge, and this impact will inevitably affect all aspects of human society, such as science, culture, religion, and philosophy.

Therefore, in searching for alien civilizations, the more important question should be: What should we do if we find alien civilizations?


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