Article by News Staff June 15, 2020 (sci-news.com)
• A study paper published (June 15th) in the Astrophysical Journal (by scientists at the University of Nottingham) attempts to calculate the ‘Astrobiological Copernican Limit’ on the number of alien civilizations in the galaxy.
• Estimating the number of intelligent civilizations relies on making guesses of values relating to life. But “opinions about such matters vary quite substantially,” noted Dr. Tom Westby, first author of the study. “Our new study simplifies these assumptions using new data, giving us a solid estimate of the number of civilizations in our Galaxy.”
• Since our own civilization on Earth formed after 4.5 billion years, the study’s criteria focused on other planets that formed at least 5 billion years ago. Assuming that other technological civilizations last 100 years, as we have so far, then there will be about 36 ongoing intelligent technical civilizations throughout our galaxy.
• “Our new research suggests that searches for extraterrestrial intelligent civilizations not only reveal the existence of how life forms, but also give us clues for how long our own civilization will last,” senior author Christopher Conselice said. It is possible that we are the only civilization within our galaxy, depending on the length of time that a technological civilization survives. “If we find that intelligent life is common then this would reveal that our civilization could exist for much longer than a few hundred years. Alternatively if we find that there are no active civilizations in our Galaxy it is a bad sign for our own long-term existence.”
• With our present technology the average distance to these civilizations would be 17,000 light-years away, making detection and communication very difficult.
• [Editor’s Note] Senior SETI astronomer, Seth Shostak, argues in his article, “How Many Alien Societies Are There?” that the flaw in the Nottingham study is that the author’s ‘Astrobiological Copernican Principle’ basis for their estimate assumes that “whatever we on Earth have done, the rest of the universe also does, or has done. For instance, the Nottingham scientists assume that all technological cultures will start by using radio waves to search for other civilizations for one hundred years, but no longer. That’s like saying because we’ve had airplanes for a century, everyone will have airplanes for a century, and no longer. This arbitrary assumption by the authors is largely responsible for their strikingly low estimate of the number of alien societies. Given the usefulness of radio, you could easily claim that the technological lifetime of societies is 10,000 years, not 100. If you argue for the larger number, the tally of inhabited worlds increases by a factor of 100.
The Nottingham paper also assumes that every Earth-size planet in the habitable zone of its solar system will spawn life, and after about 4 to 5 billion years, intelligent life. That’s like saying that every kid who takes piano lessons will inevitably win the Van Cliburn Prize. Venus and Mars are in the ‘habitable zone’ for life in this solar system. But they both appear to contain no life whatsoever.
By making one’s own assumptions, you can derive just about any estimate you wish for the number of intelligent cosmic species.
“There should be at least a few dozen active CETI civilizations in our Galaxy under the assumption that it takes 5 billion years for intelligent life to form on other planets, as on Earth,” said Professor Christopher Conselice, senior author of the study.
“The idea is looking at evolution, but on a cosmic scale. We call this calculation the Astrobiological Copernican Limit.”
“The classic method for estimating the number of intelligent civilizations relies on making guesses of values relating to life, whereby opinions about such matters vary quite substantially,” added Dr. Tom Westby, first author of the study.
“Our new study simplifies these assumptions using new data, giving us a solid estimate of the number of civilizations in our Galaxy.”
The two Astrobiological Copernican limits are that intelligent life forms in less than 5 billion years, or after about 5 billion years — similar to on Earth where a communicating civilization formed after 4.5 billion years.
In the strong criteria, whereby a metal content equal to that of the Sun is needed, the authors calculate that there should be around 36 active CETI civilizations in the Milky Way.
They show that the number of civilizations depends strongly on how long they are actively sending out signals of their existence into space, such as radio transmissions from satellites, television, etc.
If other technological civilizations last as long as ours which is currently 100 years old, then there will be about 36 ongoing intelligent technical civilizations throughout our Galaxy.
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