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What Is the Fermi Paradox?

The story behind the Fermi Paradox begins in the summer of 1950 in Los Alamos. The Italian-American physicist Enrico Fermi was having lunch there with a few colleagues. The group had been talking about extraterrestrial life and the possibility of technologically advanced civilizations when Fermi eventually asked, more or less: Where is everybody? That relatively simple question later developed into what we now know as the Fermi Paradox.

The reason we call it a paradox lies in the contradiction between two observations. On the one hand, our galaxy is enormous, very old, and full of stars and planets. If intelligent life emerges even occasionally, and some of those civilizations eventually develop interstellar travel, then at least some of them would theoretically have had plenty of time to spread across large parts of the Milky Way. By various estimates, even relatively slow expansion would require only a few tens of millions of years. That sounds like an incredibly long time, but compared with the age of our galaxy, it really isn't.

On the other hand, we have seen none of that so far. There are no confirmed extraterrestrial signals, no clearly artificial structures around other stars, and of course no aliens that have ever shown up here.

And that is exactly where the paradox lies. The Milky Way has theoretically had more than enough time and opportunities for other technological civilizations to emerge and spread. So far, however, we have found no convincing trace of them.

Why Should Anyone Be Out There?

Until just a few decades ago, we did not even know whether planets around other stars were common. The first confirmed exoplanets were only discovered in the 1990s. Today, we know of more than 6200 of them, and NASA estimates that our Milky Way alone contains hundreds of billions of stars and possibly trillions of planets. So a lack of planets probably is not the problem.

But even if there are enormous numbers of them, that does not mean life could exist on all of those worlds. Even if a planet lies within the so-called habitable zone, that initially only means liquid water could exist on its surface under the right conditions. Whether there actually is water, whether the rest of the conditions are suitable, or whether life ever emerges there is a completely different question.

And that question is not exactly easy to answer, because so far we know of only one planet where life has definitely emerged: Earth. So we know that it is possible. What we do not know is how likely that step actually is. Maybe life appears fairly quickly once the conditions are good enough. Or maybe Earth was simply an incredibly rare lucky case.

Things get even more uncertain when it comes to intelligent life. Earth is around 4.5 billion years old, and life has existed here for well over three billion years. Our own species, by comparison, has only been around for roughly 300,000 years, and we have only had the ability to send radio signals into space or deliberately search for other civilizations with telescopes for a tiny fraction of that time. So there is a damn long history between the first forms of life and a species capable of wondering whether anyone else is out there.

The Drake Equation

To approach all of these probabilities in a slightly more structured way, American astronomer Frank Drake developed an equation in 1961 that is now known as the Drake Equation. Drake was already involved in the search for extraterrestrial radio signals at the time, and the equation was meant to estimate how many technologically advanced civilizations might exist in our Milky Way whose signals could, in principle, be detectable by us.

The equation consists of seven factors. These include the rate at which suitable stars form, the fraction of those stars that have planetary systems, the average number of potentially habitable planets per system, the probability that life actually emerges on those worlds, the probability that intelligent life develops from it, the probability that such life develops detectable technology, and finally the amount of time a civilization continues transmitting signals or remains detectable in some other way. Multiply all of these factors together, and you get an estimate for the number of technological civilizations in the Milky Way that could be detectable at the same time.

The only problem is pretty obvious: Nobody actually knows the correct values for several of these factors. We can estimate some of them much better today than Drake could in 1961. We now know, for example, that planets are very common. But for some of the other variables, we still have practically no idea. We do not know how likely it is for life to emerge, how often intelligence develops from it, or how long a technological civilization remains detectable on average.

Depending on what values you assume for these unknown factors, you can end up with a huge number of civilizations or with the possibility that we might actually be alone. So the Drake Equation gives us less of a concrete answer and more of a pretty good overview of just how many things we still do not know.

And because these numbers are actually pretty fun to play around with, I also built a small Drake Equation Calculator for this article. You can try out a few preset scenarios or change the individual parameters yourself and immediately see how much the result shifts. But maybe finish the article first and take a proper look at the tool afterwards. Don't worry, I've linked it again at the end.

Maybe Life Is Just Really Rare

One possible answer to the Fermi Paradox would be pretty simple. One of the crucial steps on the way from a habitable planet to a technological civilization might just be extremely rare.

Even if simple life emerges relatively often, it might barely develop beyond simple organisms on many worlds. In that case, maybe the emergence of life itself is not the big exception, but complex life is. Or perhaps intelligence, technology, and the ability to remain detectable for a longer period of time are so unlikely that only very few worlds ever reach that point.

A planet full of microorganisms would of course still be an enormous discovery for us. But it would not really answer the main question behind the Fermi Paradox. After all, we are not only wondering whether any kind of life exists out there, but especially why we have not found any traces of other technological civilizations.

So if there really is an extremely unlikely step somewhere along this path, the next question is pretty obvious: Which one is it? And this is where the so-called Great Filter comes in.

The Great Filter

The idea of the Great Filter goes back to American economist Robin Hanson. In his 1998 essay "The Great Filter: Are We Almost Past It?", he basically asked the same question that follows from the previous section. If the path from non-living matter all the way to a technological civilization is possible, why does nobody else seem to have made it far enough to eventually become noticeable somewhere in the galaxy?

Somewhere along this long path from non-living matter to a technological civilization, there would have to be at least one step that is extremely unlikely. Maybe life itself emerges only very rarely. Maybe simple organisms almost never develop into more complex life forms. Maybe intelligence or technology is the real bottleneck. Or perhaps several unlikely steps together make it so difficult that hardly any world ever produces a technological civilization.

And this is where the idea also becomes interesting for us. After all, we have not completed this entire path either. We may be a technological civilization, but we are still pretty far away from becoming a species that permanently spreads across other star systems.

The better possibility for us would therefore be that the most unlikely steps are already behind us. Maybe the emergence of the first life was the Great Filter. Maybe it was the development of complex cells, multicellular organisms, or eventually an intelligent species. In that case, we would at least have already made it through some of the most difficult hurdles.

But it could just as well be that an important filter is still ahead of us. Maybe technological civilizations rarely survive long enough to take the next major step. Wars, environmental destruction, or dangerous technologies could be possible reasons, along with risks we may not even know about yet.

And that is exactly why the discovery of even simple extraterrestrial life would be so interesting when it comes to the Great Filter.

There are potential candidates for such life even within our own solar system. On Mars, scientists are already searching for traces of ancient life. Jupiter's moon Europa and Saturn's moon Enceladus also contain vast oceans beneath their icy surfaces, where conditions could at least potentially be suitable for life.

If we discovered microorganisms in one of these places and could show that this life had originated independently from life on Earth, it would obviously be one of the biggest scientific discoveries ever made.

But for the Great Filter idea, such a discovery would have a second meaning as well. If life had emerged independently at least twice within our own solar system, then the origin of simple life would suddenly look a little less like the extremely unlikely step. The actual filter might then lie somewhere later, perhaps with complex life, intelligence, technology, or even something that comes after all of that.

That would not automatically mean that the most dangerous part is still ahead of us, or that there even has to be one clearly defined Great Filter in the first place. But one possible early explanation for the cosmic silence would at least become a little less convincing.

So the discovery of simple extraterrestrial life could somehow be incredibly good news and, at the same time, just a little bit unsettling.

Maybe Civilizations Stay Local

The Fermi Paradox also relies on the assumption that a sufficiently advanced civilization would eventually start spreading farther and farther across the galaxy. But.... does it really have to?

That is exactly the assumption Jacob Haqq-Misra and Seth Baum questioned in their 2009 paper The Sustainability Solution to the Fermi Paradox. Part of their argument was that we cannot simply assume an advanced civilization would keep growing exponentially forever and therefore inevitably colonize large parts of the Milky Way.

Maybe such a civilization would find its own star system entirely sufficient. It could settle multiple planets and moons, build enormous space stations, use energy on a massive scale, and be far more advanced than we are without ever feeling the need to turn that into a galactic empire.

Maybe interstellar travel is simply too expensive, too slow, or at some point just not interesting enough, even for a very advanced civilization. Or maybe it reaches a stage where stability and the further development of its own system become more important than endless growth and expansion.

And in the end, our idea of what a highly advanced civilization would actually do is based on exactly one example of intelligent technological life. Us.

A civilization that is millions of years ahead of us might have completely different priorities. Maybe it would rather invest its energy in research, virtual worlds, massive habitats within its own system, or technologies we cannot even imagine yet, instead of colonizing countless other star systems.

That would not fully solve the Fermi Paradox, of course. If there were many technological civilizations, then in theory it would only take a single one of them to be highly expansionist for its traces to eventually spread across large parts of the galaxy. But it does show that there is no reason to assume every advanced civilization would automatically want to keep expanding forever.

Maybe We're Just Not Very Good at Looking Yet

And yeah, there is of course also the possibility that we are simply not particularly good at searching for extraterrestrial life in the first place.

The modern SETI search began in 1960. Back then, Frank Drake used a radio telescope as part of Project Ozma to observe the two nearby stars Tau Ceti and Epsilon Eridani and search for possible artificial radio signals.

At the time, the search therefore mainly consisted of listening for signals like these. Today, things have become much broader. The term technosignatures refers to possible traces of a technological civilization. Besides radio signals, these could include targeted laser pulses, unusual electromagnetic signatures, or other anomalies that are difficult to explain through natural processes. But even with these methods, we still have not found any confirmed evidence of extraterrestrial technology.

More than 60 years have passed since Project Ozma. What matters here, however, is less the number of years and more the enormous size of the actual search space. After all, we have not been continuously monitoring the entire sky across every possible frequency. We would have to look in the right direction at the right time, observe the right frequency range, and then find a signal that is strong enough and that we can actually recognize as artificial. There is also the possibility that another civilization uses a technology we are not even looking for.

And searching for simple life is not necessarily any easier. With distant exoplanets, we obviously cannot just take a closer look and see whether anything is living there. Instead, we have to search for indirect clues.

Telescopes such as the James Webb Space Telescope can, for example, study the atmospheres of certain exoplanets. Put simply, when a planet passes in front of its star from our point of view, a small portion of the star's light is filtered through the planet's atmosphere on its way to us. Certain gases absorb specific wavelengths of that starlight and leave characteristic patterns in the spectrum that eventually reaches our telescopes. From these patterns, scientists can draw conclusions about which molecules might be present in the planet's atmosphere.

This allows us to study molecules such as water vapor, carbon dioxide, or methane. That already sounds pretty impressive, but of course it does not mean that James Webb directly spots a bacterium somewhere and then sends out a push notification saying "Aliens found!"

Individual molecules are still far from being proof of life. Many of them can also be produced by processes that have nothing to do with life at all. A convincing biosignature therefore requires several clues that fit together, additional observations, and most importantly, the right context.

K2-18 b is a pretty good example of this. James Webb detected methane in its atmosphere, among other things. In 2025, possible signs of additional molecules also made headlines because some of them are associated with biological processes on Earth. Later analyses, however, could not convincingly confirm those signals. So no, life has not been discovered on K2-18 b.

We are basically looking for two different things here. With biosignatures, we are searching for possible signs that some form of life might exist somewhere at all. With technosignatures, we are looking for traces of a civilization that has already developed technology. One could eventually lead us to a planet full of microorganisms. The other would actually be someone who could, at least theoretically, answer us.

And yeah, then there is the whole problem of time.

Two technological civilizations could emerge in the same part of the Milky Way and still completely miss each other. Maybe a civilization somewhere sent signals into space a million years ago and has long since disappeared. Or intelligent life might not emerge there until a million years from now, when perhaps nothing detectable remains of our own civilization either.

And even if two civilizations exist at the same time, they do not see each other in real time. Light and radio signals can travel no faster than the speed of light. A civilization 1000 light-years away would therefore receive light from Earth today that started its journey 1000 years ago. The signals from our modern technological civilization would not even have reached them yet. And if we observe a planet 1000 light-years away, we are likewise only seeing the light that left that world 1000 years ago.

So it is not even enough for someone to simply exist somewhere out there. In a way, we also have to be online at the same cosmic time and exist long enough for our signals to actually reach each other.

The Zoo Hypothesis: Maybe They Know We're Here

And then there is, of course, a completely different possibility. Maybe other technological civilizations exist, already know that we are here, and simply do not want to make contact with us.

That is exactly the idea behind the so-called Zoo Hypothesis. American astronomer John A. Ball proposed it in a scientific paper published in the journal Icarus back in 1973. His idea was that advanced extraterrestrial civilizations might deliberately leave younger species like us alone.

The name comes from the idea that, to such a civilization, we might be something like animals in a protected reserve are to us. You know they are there, maybe even observe them, but try not to interfere with their development.

That already sounds a lot like science fiction, but it was actually proposed as a possible scientific explanation for the Fermi Paradox. The problem is pretty obvious, though. We have absolutely no evidence that anyone is actually watching us or deliberately avoiding contact.

So the Zoo Hypothesis remains exactly that: a hypothesis.

The Dark Forest Hypothesis: Maybe Nobody Wants to Be Found

There could also be a completely different reason why a technological civilization might stay quiet. Maybe it simply does not want to be found in the first place.

That is the basic idea behind the so-called Dark Forest Hypothesis. Unlike the Zoo Hypothesis, however, its modern name and best-known form did not come from a scientific paper, but from Liu Cixin's science fiction novel The Dark Forest.

The basic idea is that no civilization can ever really know whether another one is peaceful or might eventually become a threat. The enormous distances between the stars make things even more complicated because communication takes an incredibly long time. A civilization that seems harmless today could already be technologically completely different by the time a reply even arrives.

Under those conditions, it might simply be safer to stay as quiet as possible and avoid revealing your own location. That is also where the name Dark Forest comes from. Every civilization moves as silently as possible through this dark forest because it has no idea who else might be hiding between the trees or what could happen if it gets noticed.

The idea is now also discussed in scientific and philosophical contexts, for example using game theory. Mathematical models can be used to explore what decisions different players might make when none of them really knows how the others are going to behave.

But that still does not make the Dark Forest Hypothesis a proven explanation for the cosmic silence. We would have to make assumptions about fear, risk tolerance, and strategic decisions made by civilizations we do not even know exist.

In theory, though, the idea could explain why a technological civilization might deliberately avoid making itself noticeable. So far, however, we have no evidence that the Milky Way actually works this way.

Or Maybe We're Really Alone

After all of these possible explanations, there is still one pretty simple possibility left. Maybe we really are alone.

With hundreds of billions of stars and possibly trillions of planets in our Milky Way alone, that might sound pretty unlikely at first. But an enormous number of possible worlds does not guarantee that a second technological civilization has to emerge somewhere. If the probability per planet is extremely small, even huge numbers can still add up to zero.

This is exactly where Anders Sandberg, Eric Drexler, and Toby Ord started in their 2018 paper Dissolving the Fermi Paradox. With the Drake Equation, values inevitably have to be assigned to many of the unknown factors, even though for some of them we barely know what order of magnitude they should be in. So the three authors looked at what happens when this enormous uncertainty itself is taken much more seriously in the calculation.

And that makes a huge difference. Depending on which values within those wide uncertainty ranges turn out to be correct, there could be a large number of technological civilizations. But it can also be consistent with what we currently know that we are the only technological civilization in the Milky Way, or possibly even in the observable universe.

So the sheer size of the universe alone is not enough to say that there simply has to be someone else out there.

Because when you really get down to it, we only know two things so far. There are an unbelievable number of planets, and life has emerged at least once.

Everything after that is still extrapolation.

Closing Words

Personally, I think it is only a matter of time before we discover at least simple extraterrestrial life somewhere, or find truly convincing evidence for it. And if I had to bet on it, I would also say that we are not the only technological civilization. Maybe there is another one somewhere in our Milky Way, or maybe we would have to look much farther out.

Actually making direct contact with one during our lifetime, though, seems much less likely to me. But hey, who knows.

And so, more than 75 years later, Fermi's pretty simple question from 1950 still leaves us with one rather unsatisfying answer: We don't know.