If there are so many galaxies, stars, and planets, where are all the aliens, and why haven’t we heard from them? Those are the simple questions at the heart of the Fermi Paradox. In a new paper, a pair of researchers ask the next obvious question: how long will we have to survive to hear from another alien civilization?

Their answer? 400,000 years.

400,000 years is a long time for a species that’s only been around for a couple hundred thousand years and only discovered farming about 12,000 years ago. But 400,000 years is how long we’ll need to keep this human experiment going if we want to hear from any alien civilizations. That’s according to some new research into Communicating Extraterrestrial Intelligent Civilizations (CETIs.)

The paper is “The Number of Possible CETIs within Our Galaxy and the Communication Probability among These CETIs.” The authors are Wenjie Song and He Gao, both from the Department of Astronomy at Beijing Normal University. The paper is published in The Astrophysical Journal.

“As the only advanced intelligent civilization on the Earth, one of the most puzzling questions for humans is whether our existence is unique,” the authors state. “There have been many studies on extraterrestrial civilization in the past few decades.” There certainly have been, even though it’s difficult to study something we’re not even sure exists. But that doesn’t stop us.

Studying other civilizations in any way is confounding because we only have one data point: humans on Earth. Still, many researchers have tackled the question as a kind of thought experiment, using rigorous scientific guidelines. One study from 2020, for example, concluded that there are likely 36 CETIs in the Milky Way.

How many CETIs might exist is tied up with how long we might have to wait to hear from one. “We have always wanted to know the answers to the following questions. First, how many CETIs exist in the Milky Way? This is a challenging question. We can only learn from a single known data point (ourselves) …” the authors write.

This figure from the study shows the results of some of the simulations. The percentage F across the top is the stage of the host star's evolution required for a CETI to develop. The fc percentage is the percentage of terrestrial planets that can host a CETI. The number of CETIs that exist or did exist in the Milky Way ranges from an optimistic 42,000 + to a pessimistic 111. image Credit: Song and Gao 2022.
This figure from the study shows the results of some of the simulations. The percentage F across the top is the stage of the host star’s evolution required for a CETI to develop. The fc percentage is the percentage of terrestrial planets that can host a CETI. The number of CETIs that exist or did exist in the Milky Way ranges from an optimistic 42,000 + to a pessimistic 111. image Credit: Song and Gao 2022.

This is where the Drake Equation comes in. Based on our growing knowledge of the Milky Way, the Drake Equation tries to estimate how many CETIs there may be in our galaxy. The Drake Equation has its flaws, as many critics have explained. For example, some of its variables are little more than conjecture, so the number of civilizations it calculates isn’t reliable. But the Drake Equation is more of a thought experiment than an actual calculation. We have to start somewhere, and it gets us started.

It got the authors of this new study started, too.

“Most studies on this problem are based on the Drake equation,” the researchers write. “The obvious difficulty of this method is that it is uncertain and unpredictable to quantify the probability that life may appear on a suitable planet and eventually develop into an advanced communicating civilization.”

If you’re skeptical about any of this, you’re not alone. We can’t know scientifically how many other civilizations there are, or even if any exist. We’re not knowledgeable enough. Studies like this are part of an ongoing conversation we have with ourselves about our predicament. Each one helps us think about the context of our civilization.

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