I often wonder if most of us choose to believe in life on other worlds because of scientific reasons, or sentimental ones. There are, of course, plenty of encouraging numbers about the many Earth-like exoplanets and the many radiating stars around which habitats might form. The odds seem to be in the favor of extraterrestrial life. But then there is the reality of things: the silence, the lack of any evidence that anyone out there is listening at all. How much of our belief in alien life stems from data, and how much of it stems from a desire not to be alone? Humans are social, bonded creatures. If we do not like to feel alone on a personal level, then perhaps we very much cherish the idea of alien life because we do not like to feel alone on a species-wide level.

Today I’ve decided to explore some of the most memorable explanations to the absence of alien life, or as it’s more formally known “The Fermi Paradox”. The Fermi paradox is the conflict between the lack of clear, obvious evidence for extraterrestrial life and various high estimates for their existence.

There have been many attempts to explain the Fermi paradox, primarily suggesting that intelligent extraterrestrial beings are extremely rare, that the lifetime of such civilizations is short, or that they exist but for various reasons humans see no evidence. This suggests that at universe time and space scales, two intelligent civilizations would be unlikely to ever meet, even if many developed during the life of the universe.

The list is not meant to be exhaustive, but it does aim to provide both familiar and refreshing new ideas. At the end I will leave you with my own personal belief.

1. Galactic colonization is not like humans exploring the globe and eventually colonizing all the continents. Instead think of galactic expansion as if an aquatic species is spreading out through Earth’s bodies of water. They are only interested in very specific environments for living. Similarly, alien life will not be interested in the sun or our Solar System. An advanced enough civilization may be interested in gathering energy directly from a host star, as with a Dyson sphere. Stars of spectral class O could be 800,000 times more luminous than the sun, allowing them to harvest 10¹⁸ times more energy than we could ever harvest here on Earth. Aliens in this scenario only colonize places with high energy potential such as black holes and powerful stars.
2. We live in a simulation. In this “Planetarium Scenario” the universe outside of Earth has been simulated and does not contain actual worlds capable of developing life. We are the only real beings in a universe that’s painfully false.
3. The “Zoo Scenario” is based on mankind’s approach to the natural world. Extraterrestrial civilizations have chosen to avoid contact with Earth because we represent nascent, primitive life in what is the universal equivalent of a wildlife preserve.
4. All technologically advanced civilizations eventually succumb to their own creations. This can happen either through something as explosive as a nuclear war or through something more intellectually sinister and conniving, such as artificial intelligence that gets out of control.
5. Aliens do not wish to communicate because they do not share our desire to explore the universe, nor do they care for the existence of other civilizations. Or they may not reach out because they have created virtual realities for themselves which are more interesting and fulfilling than anything in the real world.
6. Aliens are sending signals in a way which we do not recognize. Up until now we assumed their signals would come in the form of electromagnetic radiation because of its various advantages. Not only is it cheap and easy to produce, but it would also move at the speed of light and could be received by more primitive civilizations. Our focus on this kind of radiation could mean we’re missing signals elsewhere.
7. In the “Dark Forest Theory” of the universe civilizations choose to remain silent for fear of being attacked. When there is no way for you to know another person’s intentions, it’s best to play it safe.
8. Civilizations on other worlds may not be able to develop astronomy at all. Physical characteristics of their planet may make it impossible for them to study or explore the universe. Their skies may be perpetually dark, clouded, or otherwise perpetually bright so that the population does not even dream of other worlds.
9. In the “Rare Earth Hypothesis” we explore the idea that there were many fortunate events leading to the formation of life on Earth. Not only Earth’s placement in the Solar System and in the galaxy, but also its unique geothermal activity, its accompanying moon, our rare and stable star, and our proximity to a planet the size of Jupiter to protect us from accidental impacts. So many factors contributed to our existence, in fact, that it is unlikely life has developed on other worlds. And if life does exist on other worlds, it does not easily develop into complex and intelligent organisms. This may be because some step in our evolution is incredibly difficult to overcome.
10. More advanced civilizations have reached the technological singularity and are now beyond our understanding. They may even gravitate towards places of great power and mystery, such as the inside of a black hole.

And, finally, my own personal belief. This idea extends beyond mankind’s solitary status in the universe and applies to each one of us at the individual level. That we must learn, at least for some time, to be alone. What seems silent and empty now will someday be filled with the boisterous streams of radiation indicating busy civilizations at work.

It’s difficult to think that, for now, we’re still alone. But each human is born and dies alone; this is not so different. If we are the first civilization to arise then we can use this time to evolve into the best kind of beings: wise, welcoming, looking to guide and not destroy. Let us use these formidable years alone to become the very things we keep wishing to find.