I think it’s safe to say that we are all fascinated by the possibility of extraterrestrial life. Regardless of whether you believe it actually exists or not, the mere chance of someone else being out there is enough to light something inside us.
After all, the implications of alien life being real would be life-altering! What would happen to our perceived place in the universe? How would the existence of extraterrestrial life affect our religious sense of self? Does each sentient civilization have its own deity? Would those deities have some connection to ours? Is there an alien Jesus, alien Buddha, or alien Muhammed? I mean, the idea of humanity being god’s chosen vanishes into thin air if there are other civilizations out there.
There are also, of course, issues that are Hollywood’s bread and butter. Would aliens be friendly or aggressive? Would an intergalactic encounter spark a war between worlds?
While finding alien life would certainly be as troubling as it would be fascinating, the alternative isn’t much more comforting, to be honest. What if there is absolutely no one else out there? What if the universe, in its vast infinity, is nothing but a desolate wasteland devoid of any semblance of life, intelligent or otherwise?
I don’t know about you, but that also seems like quite a chilling prospect.
Anyway, while I’d love to dive deep into each of those questions and give you all my opinions on them, they are not the purpose of today’s blog. (Maybe some other time?) But the preamble was not in vain, I promise. Let me now ask the question that inspired me to write this:
The “Paradox” Explained
Where are all the aliens?
I mean it as a serious question. Let me walk you through the logic here.
The best of our current scientific understanding suggests that the universe is about 13.7 billion years old.
Our galaxy, the Milky Way, is estimated to have around 20 billion sun-like stars (that is, stars that are similar in size to ours). Of those, between 1 and 3% have at least one planet within its habitable zone. The habitable zone is where water can exist in its liquid form (i.e, further away it would freeze, closer it would evaporate). So, taking the least likely scenario, we can expect at least 200 million earthlike planets in our galaxy alone.
Now, we don’t know how rare life is, and that is where different people will give you different guesses. And that is what they are, guesses. Do not let anyone’s fancy degree fool you. We have a sample size of planets that can harbor life of exactly one: Earth. We can’t really extrapolate from a sample size of one! If you meet one person from a country, it would be illogical to assume that everyone from that country behaves or thinks like that one individual.
Similarly, we don’t know how life might form somewhere other than Earth. How rare or frequent is it? We truly do not know. Some people might be more optimistic or more pessimistic, but there is little we can do to narrow our uncertainty here. Still, let us do our best to work through it.
With 200 million earthlike planets, even if 0.00001% of them harbor intelligent life, we can expect 20 planets with intelligent life in our galaxy.
Well, a planet harboring life isn’t the only prerequisite for us to spot it. After all, we can’t just focus our telescopes on any random planet and see if we spot a little green alien walking around. No, we need aliens to become an interplanetary species before we can hope to see them. But that is not necessarily as much of a tough ask as it might seem at first. Although we might still be a couple of hundred or even a thousand years away from becoming truly interplanetary, it might be reasonable to use our species for this assessment. It has taken humans about two million years to get to where we are now (the extra time we still need barely dents this number). And while it may sound like an extremely long time for us puny humans, it is merely a blink of an eye in the cosmic timescale. To put this into perspective, since the beginning of the universe, there have been six thousand, eight hundred and fifty intervals of two million years. In other words, a species like ours could have gone from becoming sentient to interplanetary six thousand eight hundred and fifty times, one after the other. That is plenty of opportunities!
(For my fellow nerds out there who may want to argue that the early universe was not inhabitable and that I should shrink the time window: Yes, you are correct. However, it is a relatively small timeframe that I am willing to neglect for the sake of simplicity)
And so far I have been just talking about the Milky Way. There are around 20 billion galaxies in the observable universe. If each one has 20 planets that could harbor life, that translates to 400 billion earthlike planets.
But it doesn’t stop there. Once a species has expanded to another planet, the growth becomes exponential. After a lag time, colonization parties will depart not only from the home planet, but from each newly colonized one! Even if it takes each colony ten thousand years to generate its own colony, within a matter of a few million years, we could expect the entire galaxy to be filled with life, or at least remnants of it.
The mystery deepens. Keep in mind that our planet is relatively young; it was formed “only” about 4.6 billion years ago. This means there were almost nine billion years for life to develop and expand around us. From the moment our species was born, we should have been able to see signs of life across the galaxy.
Yet this is not the case. There is no one sending us signals, no one answering our calls. There are no ruins of ancient civilizations nor hints of current ones.
So I ask once again, where are all the aliens?
This is the famous Fermi paradox: the apparent discrepancy between the absence of alien life and the (relatively) high likelihood of its occurrence.
I encourage you to think about this and let me know your thoughts in the comments. In the next blog, we will discuss one of the possible answers, The Great Filter
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