Where are all the aliens?
Our universe is giant, and its expansion is only accelerating. There are an estimated 21.6 sextillion (that’s 2.16 x 1025!) planets in the universe, with an estimated 400 billion of them in the Milky Way. Out of that 400 million, astronomers believe that about 10 billion are habitable. And yet, with so many possible places for life to spring up, why haven’t we discovered or been contacted by any alien civilizations?
This question is the essence of the Fermi paradox, a simple question asked by Enrico Fermi over a lunchtime debate — “where is everyone?” Our sun is a relatively young star, and scientists have discovered far older stars with far older habitable planets. Shouldn’t life have sprouted up on those planets long before it happened on Earth, meaning those planets would have extremely advanced intelligent civilizations?
Let’s take a step back and talk about what exactly an “intelligent civilization” means. To categorize types of intelligent civilizations, Nicolai Kardashev proposed a model called the Kardashev Scale, which grouped civilizations into Type I, II, or III by how advanced they were. Type I civilizations are able to harness all the energy available on their planet — Earth has not yet reached this point, with Carl Sagan ranking our planet only at about a Type 0.7. Type IIs would be able to harness the energy of their star, for example through using a Dyson sphere, which would theoretically encapsulate the sun and extract its energy so humans would be able to use it. Type III civilizations, then, are the most advanced, being able to use the energy output of their entire galaxy. A civilization like this would most likely have mastered interstellar travel and maybe even have conquered the entire galaxy.
But why does this matter? With Earth only being roughly a Type 0.7 civilization and Type I status still centuries away, one possible answer to the Fermi paradox is simply that we haven’t been looking in the right places. Maybe other civilizations have been broadcasting their own messages, but our own technology isn’t advanced enough to pick them up yet. Another theory is that Earth doesn’t show up on other civilizations’ radars. While Earth has been transmitting messages through radio waves or attached to our satellites and probes for decades, it is exceedingly likely that other civilizations aren’t listening to the wavelengths our broadcasts have been sent out on; imagine trying to use something like cave paintings to communicate with a world of only text message users.
And yet, perhaps it’s a good thing that we haven’t been able to discover other sentient life out there. The Great Filter, a hypothesis introduced by Robin Hanson that stems from the Fermi Paradox, suggests that humanity hasn’t been able to find anyone out there because of some kind of inevitable roadblock in the timeline of a civilization’s development, “filtering” out civilizations past a certain point. There are two scenarios in the Great Filter — one where this roadblock is behind us, and one where it is ahead of us.
If the filter is still ahead of us, the radio silence from the rest of the universe means that there is something out there wiping out civilizations, preventing them from reaching out. This doesn’t bode well for humanity; if there’s some phenomenon or other issue that’s wiped out civilizations far more advanced than us, we likely don’t stand a chance. This filter could take the form of something like climate change, AIs taking over the world, nuclear war, or gamma-ray bursts/black holes.
But let’s look on the bright side — in the case that humanity has already passed this “filter,” Hanson suggests some possible steps in Earth’s evolution that wouldn’t normally have happened. Perhaps the emergence of RNA and other self-replicating molecules was the filter, or jump from prokaryotic to eukaryotic cells, or any other step that our ancestors took that would be near-impossible to recreate elsewhere. If this was the case, then we haven’t heard from other planets or galaxies because we are one of the rare few who have been able to overcome this obstacle.
For now, until we are able to learn more about our universe, our best bet is that we really are alone; the more common life is throughout the universe, and the more advanced it is, the higher the odds are that the Great Filter is ahead of us. Let’s hope that there are only empty planets out there, ready for humanity to discover and explore.
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Kanchwala, Hussain. “Kardashev Scale: Definition, Types and a Simple Explanation.” Science ABC, 27 Feb. 2019, www.scienceabc.com/nature/universe/what-is-kardashev-scale.html.
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