The vast and unknowable cosmos could be home to millions upon millions of unique species of intelligent life forms. Or, it could house exactly one.
From our vantage point on Earth looking out into the universe, we see no evidence of life on other planets. How could this be, given how large and varied the observable universe is?
What Is the Fermi Paradox?
Named for physicist Enrico Fermi, the Fermi Paradox attempts to explain the presence of humans in a seemingly lifeless universe. In short, despite the millions of Sun-like stars in the galaxy, and those stars’ high likelihood of having Earth-like planets, we seem to have detected no signs of intelligent life among the stars.
Assuming life on Earth is not unique, Earth-like planets should have yielded life forms of similar intelligence to that of humans at some point. Given that many of the sun-like stars in the galaxy are billions of years old, and traversing the galaxy would have taken a space-faring civilization only millions of years, it stands to reason that, by now, every system in the galaxy should have been visited by intelligent life.
At the very least, we should be able to detect megastructures or the evidence that such intelligent life explored the galaxy at some point in the distant past. However, there is no such evidence. As far as humanity can tell, we are the only intelligent life forms in our galaxy. We may even be the only intelligent life forms in the observable universe.
How Could This Be?
There are a few proposed solutions to the Fermi Paradox. Many of them are far from comforting. The most common solution that is often brought forward is that intelligent life could be more rare than we have previously assumed. Another solution proposes that there could be abundant life among the stars, even life like ours, but none that has progressed to a space-faring stage.
A far darker view of life holds that, while there may have been intelligent life among the stars, it is the very nature of life to destroy itself. This theory, known as the Great Filter hypothesis, asserts that intelligent life has reached the capacity of space flight in the past, but that wars, famines, and lack of resources inevitably mean the species is wiped out before it travels through the galaxy.
There are numerous other proposed answers to this problem. However, none of them have any more weight than others simply because we have only our own Earth to look to for examples of the evolution of intelligent life.
Maybe in time, we will learn the answer to the Fermi Paradox. Or, maybe we are simply all alone in the universe.