We’ve been looking to the stars for alien life ever since we developed the technology. It is presumed that we are attempting to locate other life in the universe, but what if we are looking to ensure that there is none?
Here’s an equation, and it’s a fairly frightening one: N = R*× fP × ne × f1 × fi × fc × L. The Drake equation describes the number of alien civilizations with whom we might be able to communicate in our galaxy. Its terms correspond to numbers such as the fraction of stars that have planets, the fraction of planets that could support life, the fraction of planets that can support intelligent life, and so on. The minimal outcome of this equation, using cautious estimates, is 20. There should be at least 20 intelligent alien civilizations in the Milky Way that we can communicate with and who can communicate with us. But there aren’t any.
The Drake equation is an example of a broader issue in the scientific community—considering the sheer size of the universe and our knowledge that intelligence life has evolved at least once, there should be evidence for alien life. This is known as the Fermi paradox, after scientist Enrico Fermi, who was the first to investigate the contradiction between the high probability of alien civilizations and their apparent absence. Fermi summed it up nicely when he asked, “Where is everybody?”
But perhaps I asked the wrong question. A better question, albeit a more troubling one, might be “What happened to everybody?” Unlike asking where life exists in the universe, this question has a clearer potential answer: the Great Filter.
Why the universe is empty
There is likely alien life, but none that we can see. Therefore, it could be the case that somewhere along the trajectory of life’s development, there is a massive and common challenge that ends alien life before it becomes intelligent enough and widespread enough for us to see—a great filter.
This filter could take many different shapes. It could be that having a planet in the Goldilocks’ zone—the narrow band around a star where it is neither too hot nor too cold for life to exist—and having that planet contain organic molecules capable of accumulating into life is extremely unlikely. We’ve seen plenty of planets in the Goldilocks zone of various stars (there are thought to be 40 billion in the Milky Way), but maybe the conditions are still not right there for life to exist.
The Great Filter could happen at the very beginning of life. When you were in high school bio, you might have had the refrain drilled into your head “mitochondria are the powerhouse of the cell.” I most certainly did. However, mitochondria were once distinct bacteria with their own existence. A single-celled organism tried to consume one of these bacteria at some point on Earth, but instead of being digested, the bacterium teamed up with the cell, creating extra energy that enabled the cell to evolve in ways that led to higher forms of life. An event like this may be so rare that it has only happened once in the Milky Way.
Or, the filter could be the development of large brains, as we have. After all, we live on a planet with many organisms, and human intelligence has only occurred once. It may be overwhelmingly likely that living creatures on other planets simply don’t need to evolve the energy-demanding neural structures necessary for intelligence.
What if the filter is ahead of us?
These options assume that the Great Filter has passed us by—that humanity is a lucky species that has overcome a hurdle that practically all other life has been unable to overcome. This might not be the case, however; life might evolve to our level all the time but get wiped out by some unknowable catastrophe. The development of nuclear power is a certain consequence for any advanced society, but it also has the capability to destroy such a country. The current process of climate change is an example of how to use a planet’s resources to construct an advanced civilisation damages the world. Or it may be something completely unknown, a major threat that we cannot see and will not see until it is too late.
The Great Filter’s bleak, counterintuitive conclusion is that discovering alien life, particularly alien life with a degree of technological advancement comparable to our own, would be a bad sign for humanity. If our galaxy is actually empty and dead, we’ve most likely already passed through the Great Filter. The galaxy could be empty because all other life failed some challenge that humanity passed.
If we discover another alien civilization but not a universe teeming with alien civilizations, the implication is that the Great Filter awaits us. The galaxy should be full of life, but it isn’t; one more instance of life would imply that the many other civilizations that should exist were wiped out by some catastrophe that we and our alien counterparts have yet to face.
Fortunately, we haven’t found any life. Although it might be lonely, it means humanity’s chances at long-term survival are a bit higher than otherwise.