In 1950, a learned lunchtime conversation set the stage for decades of astronomical exploration. Physicist Enrico Fermi submitted to his colleagues around the table a couple contentions, summarized as 1) The galaxy is very old and very large, with hundreds of billions of stars and likely even more habitable planets. 2) That means there should be more than enough time for advanced civilizations to develop and flourish across the galaxy.
So where the heck are they?
This simple, yet powerful argument became known as the Fermi Paradox, and it still boggles many sage minds today. Aliens should be common, yet there is no convincing evidence that they exist.
Here are twelve possible reasons why this is so.
1. There aren't any aliens to find. As unlikely as it seems in a galaxy with hundreds of billions of stars and as many as 40 billion Earth-size planets in habitable zones, we could be alone.
2. There is no intelligent life besides us. (This assumes, of course, that humans count as intelligent.) Life may exist, but it could simply take the form of miniscule microbes or other cosmically "quiet" animals.
3. Intelligent species lack advanced technology. Currently, astronomers utilize radio telescopes to listen intently to the night sky. So if alien species aren't broadcasting any signals, we'd never know they existed.
4. Intelligent life self-destructs. Whether via weapons of mass destruction, planetary pollution, or manufactured virulent disease, it may be the nature of intelligent species to commit suicide, existing for only a short time before winking out of existence.
5. The universe is a deadly place. On cosmic timescales – think billions of years – life may be fleeting. All it takes is a single asteroid, supernova, gamma ray burst, or solar flare to render a life-harboring planet lifeless.
6. Space is big. The Milky Way alone is 100,000 light years across, so it's conceivable that the focused signals of intelligent aliens, which are limited to the speed of light, simply haven't reached us yet.
7. We haven't been looking long enough. Eighty years. That's the amount of time that radio telescopes, which allow us to detect alien signals, have been around. And we've been actively searching for aliens for maybe sixty years. That's not very long at all.
8. We're not looking in the correct place. As previously mentioned, space is big, so there are tons of regions to listen for alien signals. If we're not listening precisely in the direction from which a signal is originating, we'd never hear it. As Andrew Fain explained at Universe Today, it's like trying to speak with your friend on a 250,000,000,000-channel CB radio, without any knowledge of the frequency on which they are transmitting. You'll probably be channel flipping for a long time.
9. Alien technology may be too advanced. Radio technology may be commonplace here on Earth, but on far-flung worlds, alien societies may have graduated to more advanced communication technologies, like neutrino signals. We can't decipher those just yet.
10. Nobody is transmitting. Instead, everybody may be listening. That's basically how it is here on Earth. Apart from a few paltry efforts to broadcast strong signals over a narrow frequency band towards the stars above, we've barely made our presence known in the universe. In fact, if aliens have radio telescopes similar to what we have on Earth, our television and radio broadcasts would only be detectable up to 0.3 light-years away. That distance doesn't even transcend the farthest reaches of our solar system.
11. Earth is deliberately not being contacted. On Earth, we have policies about contacting indigenous peoples; it's possible that the same thing could be happening with us. Just like in Star Trek, advanced alien societies may enforce rules that limit contact only to species that attain a lofty degree of technological or cultural evolution.
12. Aliens are already here and we just don't realize it. Conspiracy theorists love this unlikely explanation. While the chances are remote, it's not impossible that government agencies are concealing the presence of aliens. Although it's more likely that aliens are already amongst us, observing humanity in the clever and ironic guise of lab mice.