If there are so many galaxies, stars and planets, where are all the aliens and why haven’t we heard of them? These are the simple questions at the heart of the Fermi Paradox. In a new paper, two researchers ask the next obvious question: How long will we have to survive to hear about another alien civilization?
Their response ? 400,000 years old.
400,000 years is a long time for a species that has only been around for a few hundred thousand years and only discovered agriculture around 12,000 years ago. But 400,000 years is the time we will need to continue this human experiment if we want to hear from extraterrestrial civilizations. That’s according to new research on Communicating Extraterrestrial Intelligent Civilizations (CETIs).
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The article is entitled “The number of possible CETIs in our galaxy and the probability of communication between these CETIs”. The authors are Wenjie Song and He Gao, both from the Astronomy Department of Beijing Normal University. The article is published in The Astrophysical Journal.
“As the only advanced intelligent civilization on Earth, one of the most puzzling questions for humans is whether our existence is unique,” the authors state. “There have been many studies of extraterrestrial civilization over the past few decades.” There certainly were, although it’s hard to study something you’re not even sure exists. But that doesn’t stop us.
Studying other civilizations in any way is baffling because we only have one data point: humans on Earth. Yet many researchers have approached the question as a sort of thought experiment, using rigorous scientific guidelines. A 2020 study, for example, concluded that there are likely 36 CETIs in the Milky Way.
The number of CETIs that can exist depends on how long we have to wait to hear one. “We have always wanted to know the answers to the following questions. First, how many CETIs are there in the Milky Way? This is a difficult question. We can only learn from a single known data point (ourselves)…” write the authors.
This is where the Drake equation comes in. Based on our growing knowledge of the Milky Way, the Drake equation attempts to estimate how many CETIs there may be in our galaxy. Drake’s equation has its flaws, as many reviewers have explained. For example, some of its variables are little more than guesswork, so the number of civilizations it calculates is unreliable. But the Drake equation is more of a thought experiment than an actual calculation. We have to start somewhere, and that gets us started.
It also got the authors of this new study started.
“Most studies of this problem are based on the Drake equation,” the researchers write. “The obvious difficulty with this method is that it is uncertain and unpredictable to quantify the likelihood that life could arise on a suitable planet and eventually develop into an advanced communicating civilization.”
If you’re skeptical about all of this, you’re not alone. We cannot scientifically know how many other civilizations exist, or even if any exist. We are not informed enough. Studies like this are part of an ongoing conversation we have with ourselves about our plight. Each helps us reflect on the context of our civilization.
So how did they find 400,000 years if we don’t even know how many CETIs there might be?
The research duo is not the first to tackle this question. Their paper describes some of the earlier scientific efforts to understand the impact of other civilizations in the Milky Way. For example, they refer to a 2020 study estimating that there are 36 CETIs in the Milky Way. This number came from calculations involving galactic star formation histories, metallicity distributions, and the likelihood of stars hosting Earth-like planets in their habitable zones. This article clarifies that “…the subject of intelligent and communicative extraterrestrial civilizations will remain entirely within the realm of hypothesis until a positive detection is made…” But they also point out that scientists can still produce valuable models based on logical assumptions “…that can produce at least plausible estimates of the rate of occurrence of such civilizations.
This study carries some of that same thinking forward. It deals with two parameters, both poorly understood. The first concerns the number of habitable terrestrial planets and the frequency with which life on these planets evolves into a CETI. The second is to know at what stage in the evolution of a host star a CETI would be born.
The researchers assigned each of these parameters a variable in their calculations. The probability of life arising and evolving into a CETI is (Fvs), and the required host star evolutionary stage is (F). Song and Gao ran a series of Monte Carlo simulations using different values for these variables. They arrived at two scenarios: an optimistic outlook and a pessimistic outlook.
The optimistic scenario uses the values F = 25% and Fvs = 0.1%. Thus, a star must have at least 25% of its lifetime before a CETI can emerge. And for each telluric planet, there is only a 0.1% chance that a CETI will appear. These bullish variables create over 42,000 CETIs, which sounds like a lot, but it’s not when they’re spread across the galaxy at different times. Moreover, we would need to survive another 2000 years to achieve two-way communication with us. It seems almost within reach.
But it’s the optimistic scenario that makes the Universe seem friendly and inhabited by other welcoming civilizations. Maybe some of them are already talking to each other, and we just need to join.
Now let’s move on to the pessimistic scenario.
In the pessimistic scenario, F = 75% and Fvs = 0.001%. Thus, a star cannot host a CETI until it is much older, and the probability of a single terrestrial planet hosting a CETI drops to a tiny percentage. Where does this lead us?
This pessimistic calculation produces only about 111 CETIs in the Milky Way. Worse still, we would need to survive another 400,000 years to have two-way communication with them. (For perspective, Star Trek starts in the middle of the 22nd century.)
This is where the Great Filter comes in. The Great Filter is whatever prevents matter from becoming life and then progressing to advanced civilization.
The authors address this subject when they write: “However, it has been proposed that the lifespan of civilizations is most likely limited, due to many potential disturbances, such as population problems, nuclear annihilation, change sudden climate, rogue comets, ecological problems. changes, etc. If the Doomsday argument is correct, in some pessimistic situations humans may receive no signals from other CETIs before extinction.
In their article, the scientists write that “…the values of Fvs and F are full of many unknowns. This is the case in all this type of work. This article, and others that address the same question, are more usefully considered thought experiments than solid results. We can’t know any of this for sure, but we can’t help but be compelled to explore it. It’s part of human nature. “It is quite uncertain what proportion of terrestrial planets can give rise to life, and the process of life evolving into a CETI and able to send detectable signals into space is highly unpredictable,” they write.
Will humanity ever encounter another civilization? It’s one of our most compelling questions, and almost certainly no one alive today will ever have an answer. First, there must be other CETIs, then you must exist simultaneously with them and communicate in one way or another. It is possible that another CETI had already detected life on Earth before being wiped out by the Great Filter or perhaps by a natural calamity like a supernova explosion. We’ll never know.
Perhaps humanity will survive for a long time. Perhaps Earth will become uninhabitable and humanity will flee to Mars or elsewhere. But would a Muskian outpost on a long-dead planet populated by the scruffy descendants of a ruined Earth be considered a CETI? We like to imagine other civilizations having successfully overcome problems we still struggle with. Will it be true? Or will the first CETI we discover be little more than the descendants of a once proud civilization that radiated confidence until the Great Filter struck?
Who knows? If humanity ever encounters another technological species, it could be so far in the future that our descendants will be almost unrecognizable from modern humans.
Or, perhaps, we will never have an answer, and the Great Filter will prevent us from finding one.
But if humanity needs a purpose, something to cling to that can keep hope alive, then the dream of communicating with another CETI could do that.