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A SIGNAL FROM OTHER PLANETS: how many intelligent civilizations could there be in our galaxy

A SIGNAL FROM OTHER PLANETS: how many intelligent civilizations could there be in our galaxy
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Are we alone in the Universe? Previously, experts have given optimistic or pessimistic predictions ranging from «yes, we are alone in the entire space» to «there are millions of other civilizations». A new study by scientists from the University of Nottingham offers a fairly accurate (hypothetically) answer to the question that worries mankind.

Scientists have calculated the theoretically likely number of intelligent civilizations in our galaxy that possibly have means of communication. Professor of astrophysics from Nottingham Christopher Conselice, who conducted the study, explains the principle behind the search.

Relying on knowledge of Earth, scientists have suggested that it takes an average of 5 billion years for intelligent life forms to develop on planets of this type. Conselice says this is how the idea of applying the laws of Earth’s evolution at the galactic level came about. Such calculations have been called the Astrobiological Copernican Limit.

Tom Westby, co-author of the study, clarifies that classical theorizing usually takes into account the simple possibility of the presence of living forms. The advantage of the Astrobiological Copernican Limit is that it allows us to consider intelligent life forms that could have formed in 5 billion years — similar to what happened on Earth, where a contactable civilization emerged in 4.5 billion years.

This is why the possible active attempt by civilizations to send signals of their presence into space — such as satellites, television, etc. — is so important. Another factor, according to experts, is the concentration of metals (elements heavier than helium) in the substance of the parent star, which should be at the level of the Sun.

On the basis of these data, scientists have calculated that there may be 36 such active civilizations in our galaxy.

But there is one more important circumstance — these civilizations, if they exist, are at a distance of 17,000 light years from us, which, at the current level of technology for humanity, is an insurmountable distance. And no less significant detail: there remains the possibility that we are the only civilization in our galaxy if the duration of the existence of possible civilizations is the same as ours.

Scientists are convinced that such studies shed light not only on the possibility of the existence of intelligent civilizations but also suggest how long our civilization can exist. Professor Conselice believes that the search for extraterrestrial intelligent life forms, even if they are not successful, gives mankind a chance to explore their future and their destiny.


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Let us remind our readers that previous calculations were based on the hypotheses of astronomer Frank Drake, who in 1961 proposed the famous «Drake equation», which includes the factors required to assess the probability of the existence of intelligent life somewhere in the universe. The Drake equation allows us to assume the number of intelligent civilizations, even in the absence of clear evidence of their existence.

The primary function of the Drake equation is to reduce the big question of the number of intelligent civilizations to seven more minor problems:

1) The number of intelligent civilizations willing to make contact;

2) The fraction of stars that form in our galaxy in one year;

3) The number of Sun-like stars with planets;

4) The average number of planets (and satellites) with suitable conditions for the birth of civilization; the probability of life originating on a planet with suitable conditions;

5) The probability of intelligent life forms arising on a planet with life;

6) The ratio of the number of planets whose intelligent inhabitants are capable of seeking contact to the number of planets on which there is intelligent life;

7) The lifetime of such a civilization (i.e., the time during which the civilization exists and is capable and willing to make contact).

Drake’s equation gives a result of 10 hypothetical intelligent civilizations.

It is also important to remember the «Great Filter» hypothesis formulated by Robin Hanson in 1996. Hanson’s hypothesis is one of the attempts to explain why, with all the possibilities for contact with an intelligent civilization to occur, it does not.

Hanson argues that for interstellar civilization to occur, a series of evolutionary steps must take place: the emergence of a star system with planets on which life is possible; the emergence of self-replicating molecules (e.g., RNA) on one of the planets; the emergence of simple unicellular life (prokaryotes); the emergence of complex unicellular life (eukaryotes); the emergence of sexual reproduction; the emergence of multicellular organisms; the emergence of animals with advanced brains that use tools; and the achievement of the present state of the humanity, the spread of civilization through the process of space colonization.

The «Great Filter» hypothesis states that at some point in evolution from the first to the ninth step, there is some obstacle, a filter, which is unlikely or impossible to overcome. It may be a set of natural factors that prevent life from arising and developing at a sufficient level. Or it could be the death of intelligent life as a result of its own actions or the efforts of other civilizations.

Original study: Tom Westby, Christopher J. Conselice. The Astrobiological Copernican Weak and Strong Limits for Intelligent Life. The Astrophysical Journal, 2020


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