The Fermi Paradox
Are we alone in the universe?
The Fermi Paradox is relatively simple.
The Universe is 13.8 billions years old. There are around 100 billion planets in our galaxy alone. Even if only one out of a thousand could support life, that would leave 100 million planets — and many studies report that the number of habitable planets could be even higher. That means hundreds of millions of potential species.
Then where are all the aliens?
That’s it. That’s the Fermi Paradox. We haven’t discovered one smidgen of identifiable evidence of alien life, despite the fact that the galaxy (according to our premises) should be full of them. There was one signal received in 1977 that bore some of the hallmarks of what we might expect to come from a message from intelligent life, but it was far from definite that it came from extraterrestrial life, and has not been repeated.
So what explains this discrepancy? Are our premises wrong? There are dozens of theories. I’ll go over some of the major explanations one by one.
1. Rare Earth Hypothesis
This hypothesis posits that we have not seen any aliens because the conditions necessary to produce life or complex life are far more specific and therefore more improbable than we think. The Earth, and the features of it and the Solar System that combine to support life, are extraordinarily rare. According to Rare Earth proponents, this includes being just the right distance from the sun, not being part of a “dead zone,” a terrestrial planet of the right size, having a moon, having an atmosphere, orbiting the right kind of star, and several possessing other features. The chance of all of these characteristics occurring for one planet are extremely low.
Therefore, the proportion of planets in the universe that are habitable is extremely low. Instead of 1/1000 planets being habitable, it is closer from 1/1,000,000 to 1/1,000,000,000. Therefore, life as a whole is extremely rare, and, combined with the extraordinary distances between planets in the galaxy, it is completely logical that we have not seen any evidence of alien life. It exists, just not very often, and not necessarily anywhere close — or in the small span of time homo sapiens has existed.
Of course, like many of the hypotheses I’ll cover after this one, it is hard to evaluate, as we lack much of the necessary evidence. We don’t have much data on the physical conditions of other planets. We can estimate a few things (including a planet’s distance from their sun, and it’s potential temperature range) — but lack the ability to do detailed analysis.
Personally, I like this theory because it is simple, and using Occam’s Razor, it is probably one of the most probable hypotheses.
2. There Are Aliens, But They’re Not Intelligent
Life does exist on other planets, and it may even be common. But intelligent life is rare to non-existent. The evolution of an intelligent species on Earth, homo sapiens, was an extremely improbable event.
This has a certain plausibility to it. Life has existed on Earth for around 3.5 billions years, and it is only within the last few hundred thousand years that homo sapiens emerged. How many thousands, millions of species have emerged on the Earth that were not “intelligent?” Clearly, the emergence of an intelligent species, at least on Earth, is a rare event.
And the jump from mere intelligence to advanced technology may also be another barrier. Homo sapiens have only started to use technology that is detectable across space in the past several centuries or so. Who’s to say other intelligent species ever develop advanced technology?
3. Advanced Intelligence Life is Self-Destructive
This hypothesis is a bit more depressing. Intelligent life does emerge on other planets… but doesn’t usually last for very long, and wipes itself out before it can make any lasting marks that we would detect. Whether it’s from nuclear warfare or some other technology that we haven’t discovered yet, intelligent life has a tendency to wipe itself out.
If we’re looking in the mirror based on our own history, this theory seems sort of plausible… but I’m less of a fan. If life has evolved elsewhere, well… it would be alien. This hypothesis assumes that in most if not all of the alien life that has emerged, it self-destructs due to its own primitive/warlike tendencies… which seems like a very large logical leap. Alien life would be different. We can’t assume that the tendencies and flaws we see in humanity would be present in alien life, especially potentially hundreds if not millions of different species. The odds of that would be, in my mind, extremely low to nonexistent.
All in all, I think this hypothesis is pretty weak. Sure, one or two would wipe themselves out, but all of them? Even most? This theory would have to be combined with a rare Earth hypothesis or something else that already is limiting the amount of alien life for it to have the kind of magnitude of difference that I think would be necessary.
4. The Dark Forest Hypothesis
Alien life is out there, but the universe is a hostile place. You don’t want to be found. The only way to mathematically guarantee your safety from other species is to hide, and if another lifeform is detected, to wipe them out immediately. After all, you don’t know what’s out there. Only humans are stupid enough to broadcast their own existence.
This hypothesis came into increased prominence following the release of the Remembrance of Earth’s Past science-fiction trilogy.
This theory is extremely depressing, and unfortunately, I find it more plausible than the “self-destructive” hypothesis, if certain game theory frameworks are in operation. Here’s a simple thought experiment to explain why I find it plausible.
Assume there is Country A and Country B. These two countries know nothing about one another — there is no cross-travel, no cultural contact, no anything. They don’t even know that the other country exists.
Each country has a weapon. This weapon will completely destroy any target. The technology behind it is not too difficult or complex — but protecting oneself from said weapon is extremely difficult, if not impossible.
All it needs is the location of the target.
Country A discovers the existence of Country B. It might have the weapon too. The only way Country A can completely ensure its own safety is to fire first, before Country B discovers Country A’s existence. There is a first strike advantage. Country A knows that if Country B discovers its location, then it will run the same calculation.
Country A even knows of other countries that have been wiped out by the weapon. They, like species subject to evolutionary pressures, have evolved with a sense of self-preservation.
Country A uses the weapon.
Now imagine those countries were planets, or alien civilizations. That’s what the Dark Forest Hypothesis is.
Now, as you might have noticed, the plausibility of this theory rests on several assumptions. The first is that there is a relatively stable axis of technological development (girded by universal physical laws?) that results in powerful offensive weaponry with the capacity to destroy distant planets, without any equivalent developments in defensive technology that can sufficiently prevent said destruction. While in our current age, it is easy to see this as plausible (nuclear weaponry is a powerful destructive technology that has no clear defensive equivalent), it is not necessary that said pattern will exist in the future, thought it certainly seems logical, as a general theory, that it is usually easier to destroy than to preserve.
The second is that there is no guarantee or high probability of a detection system that would allow for second strike capacity. That is, the deployed offensive weaponry cannot be traced back to its owner. Therefore, MAD is not in effect, and the attacker has no fear of a retaliatory attack. This could be easily supposed, that in the vastness of space, it would be extremely difficult to pinpoint the exact origin point of a weapon, especially as an offensive power would take steps to ensure such (at the very least, “curving” the trajectory of any physical weapon, if we in our primitive state can suppose that it would be such).
Third, as already discussed, there would have to be a perception of threat from the other alien civilization. The larger space-wide system would have to exist in anarchic conditions, with no superordinate governing body, and therefore in a Hobbesian sense, in “a time of war, where every man is an enemy to every man… [with] continual fear and danger of violent death.” As part of this, there would have to be the psychological sense of self-preservation that, combined with the assessment of the threat, would lead to action. For the former, I think we can assess with a high probability that any biological entity resulting from the (universal?) mechanism of long-term natural selection would have ingrained self-preservation instincts. To further develop the sense of threat perception, game theory operates on what seems to be universal mathematical principles. Should the rules of the game be what we described above, the set actions of the players would be, at the very least, heavily incentivized (I do worry about making that statement and making a very universalistic claim with our very limited and narrow human knowledge).
So, I find this theory plausible — given certain assumptions. Again, each theory relies on assumptions — some more than others. That’s all we can do when we don’t, well, know anything.
There’s only one takeaway I can offer from this, and one that I cannot emphasize enough. To all the scientists and governments who think it’s a good idea to send messages out into deep space -
STOP BROADCASTING OUR EXISTENCE TO THE GREAT UNKNOWN!
5. The Great Filter
This hypothesis is simple. Something happens, or doesn’t happen, to almost any alien species that prevents intelligent life from colonizing the galaxy or emerging at all.
Several of the hypotheses we’ve already discussed could be an example of a “Great Filter.” For example, if very very few planets ever develop the chemical and physical environment to support life, that could be the “Great Filter.” If alien life does evolve, but for some reason very few ever reach intelligence, that too could be the “Great Filter.”
Basically, it is anything that reduces the rather large number of n intelligent life that we would expect, given the number of habitable planets out there and the long history of the universe, to the zero n that we know of right now.
More frighteningly, it could be something that hasn’t happened to us yet. Some devastating technology that we have yet to discover (but will soon) that wipes out most intelligent species on their paths of development.
That’s why many scientists say the worst news we could uncover would be evidence of intelligent life at our stage of development or earlier. It means that the bad thing that seems to be happening might have not happened to us yet.
The fact that we seem to be alone… could be a good thing. It means we’ve already passed the bottleneck.
Kind of like we know a hurricane is expected sometime in the area we’re currently driving through… but all the houses are trim and proper, with no evidence of wreckage or devastation. You shouldn’t be comforted. You should be terrified. It means the storm hasn’t hit yet.
6. Space Travel is Really Hard and Also a Terrible Investment
If you haven’t had the chance to read Kim Stanley Robinson’s science fiction novel Aurora, it basically encapsulates this theory. Robinsons got a fair bit of heat for pointing out the many logistical, technological, financial, practical, and other hurdles that humans (or aliens) would have to surmount to ever colonize another planet outside of our solar system.
They’re absolutely staggering, and far beyond our current abilities. Even if/when they do become possible, they will (likely?) be quite resource intensive… which begs the question — why spend all that money and time if you have everything you need already right in front of you, on your own planet, which is habitable? Sure, more space could be nice, but at what cost? Maybe most rational, advanced species develop their home planet or solar system instead of spending precious capital to colonize far distant/potentially uninhabitable planets.
Food for thought.
I’m not sure that this would explain everything — for example, we haven’t picked up on any signals, and those are relatively cheap. Secondly, probes would be too, and we haven’t seen any of those either.
So no, I don’t think this theory can explain the Fermi Paradox in and of itself. It’s a useful reminder of how resource intensive any space endeavor has/would be, but that’s a cost/benefit analysis, and organic behavior is not always purely “rational” in that sense, even if choosing to not colonize far away places is the “rational” choice.
7. Extraterrestrial Life Is Too Alien
What does this theory mean? Well, that alien life is out there — even intelligent life — but it is too alien/different to notice. That is, things we think would be proof of intelligent life — probes, radio waves, etc. — are based completely on our human conception that is too limited or too different to pick up on the signs that are already there.
Take a thought experiment. If a spider saw a car, would it recognize it as a car? Obviously, it would not. The spider has no pattern-recognition engine or understanding that a physical shape with those properties is a car. Likewise, there could be signs all around us of alien life, but we’re the spider. We can’t see the car for what it is.
We either can’t identify the advanced technology for what it is, or we’re not looking for the right signals — (who’s to say that aliens would communicate in the same way we use our signals)?
Maybe we’ve grown up in a world and galaxy already shaped by intelligent life, but because that’s what we grew up in we do not recognize it for what it is. We have no priors from which to judge. If humanity was wiped out, and another species inherited the planet, would they be able to recognize the artificial from the natural?
Well… maybe. I have a hard time believing that complex hardware would ever be regarded as a result of a natural process (once a species discovers some technology). It just seems too complex. But perhaps any life intelligent enough to leave evidence of their existence in space is so far beyond us/different from us that we cannot recognize it. After all, pattern-recognition depends on comparison, and alien life is, well, alien.
It’s an interesting theory, but hard to evaluate. How do you evaluate the probability of a theory that rests on the level of your ignorance? All I’d say is that in the grand scheme of things, humans don’t know much, and we shouldn’t arrogantly assume that we know what we should be looking for.
So there is a summary of several major theories!
Of course there are many more, ranging from conspiracy theories (the government is covering it up!) to more philosophical explanations (we live in a simulation). However, those are theories of a different nature, and I wanted to focus on the more science-focused ones in this post.
Before I wanted to conclude, there was a very interesting paper from Oxford a few years back assessing the probability of alien life. The researchers took an innovative and creative tack, where they incorporated the lack of evidence of alien life as evidence itself into the analysis. After all, the lack of data is data. To summarize a long and complex paper, they determined that humans are likely the only intelligent species in the entire galaxy. Now, this was controversial and definitely not the final say on the matter (after all, we just really don’t have a lot of the information we would need to run any full-proof assessment), but their paper was innovative and took a look at the paradox through a different lens, so I thought it would be worth including. You can find the paper here.
I hope you all find it interesting! Let me know if you have any thoughts in the chat.