Scientists have some ideas as to why we haven't found aliens yet. They think there are two possibilities: either there's a lot of life in the universe, or almost none at all. A study published in International Journal of Astrobiology gives some reasons for this secrecy.
Scientists searching for extraterrestrials believe that there should be many advanced civilizations beyond our own. They have used mathematics and logic, based on the work of John Haldane, to explain why we have not yet found any extraterrestrial civilizations.
Imagine a group of planets like Earth. You would expect life to appear on all of them, or on none of them. There is no clear reason why only half of these planets would have life.
This idea can be represented by a U-shaped graph: on one side there is a high probability that no planets have life, and on the other side a high probability that many do. In the middle is a low probability that only some planets have life.
The Drake Equation: What Is It?
Scientists used Haldane's logic to change the renowned Drake equation. Astronomer Frank Drake created this equation to guess how many alien civilizations might be in the Milky Way.
The Drake equation looks like this: N = R* x fp x ne x fl x fi x fc x L
Astronomers know a lot about star formation and how many stars have planets, but they're not sure how many planets could support life. The rest of the equation is still a mystery, so scientists have to guess.
The study’s authors say the first six parts of the Drake equation tell us how alien civilizations might arise. The final part, L, tells us when those civilizations might disappear.
New formula for counting alien civilizations
By separating the parts of the Drake equation, scientists have found a simpler way to look at it: the average number of alien civilizations in a galaxy equals the rate at which they were born times the rate at which they died. This means scientists don't have to worry too much about the unknown parts of the Drake equation.
They also added a novel idea called the population fraction, F. A high F, close to 1, would mean that almost every planet has life. A low F, close to 0, would mean that very little or no planets have life.
The problem is that, given what scientists have observed, F is probably not close to 1. If it were, we would have discovered aliens building things or sending radio signals into space long ago.
This means that if we are not alone in the Universe, F could be closer to 0.5, which places us in the unlikely part of the U-shaped graph. This suggests that knowledgeable life is occasional in the Universe.
Scientists believe that if alien civilizations ever existed in the Milky Way, they could have spread throughout the galaxy within 10 to 100 million years. Even if they disappeared, we should find evidence of their existence. Since we haven't found anything, it seems that alien civilizations are occasional and don't last long.
If aliens exist, why haven't we found them?
The authors also suggest that if we ignore the lack of evidence, the Drake equation shows that the universe could be full of life or nearly empty.
If the Universe were full of life, F would be close to 1, which would still be possible under certain conditions.
Maybe alien civilizations stay in their part of space, and our solar system is not a place they visit. Maybe traveling between stars is too challenging, or they simply don't need to.
It is also possible that although there is much sophisticated life in the Universe, there are very few technologically advanced civilizations or they take a long time to develop.
There's even a chance that the birth and death rates of alien civilizations haven't balanced out yet, so perhaps we still have time to find them. However, given how senior the universe is, that seems unlikely.
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