Those who believe that extraterrestrial intelligent life does not exist argue that the conditions needed for life—or at least complex life—to evolve are rare or even unique to Earth. This is known as the Rare Earth hypothesis, which attempts to resolve the Fermi paradox by rejecting the mediocrity principle, and asserting that Earth is not typical, but unusual or even unique. While a unique Earth has historically been assumed on philosophical or religious grounds, the Rare Earth Hypothesis uses quantifiable and statistical arguments to argue that multicellular life is exceedingly rare in the universe because Earth-like planets are themselves exceedingly rare and/or many improbable coincidences have converged to make complex life on Earth possible.

[From the period 3000 BC to 0 AD] the average length of time that a civilization lasts is 349.2 years. The median is 330 years.

It has been proposed to generalize the Drake equation to include additional effects of alien civilizations colonizing other star systems. Each original site expands with an expansion velocity v, and establishes additional sites that survive for a lifetime L. The result is a more complex set of 3 equations.

The Drake equation may furthermore be multiplied by how many times an intelligent civilization may occur on planets where it has happened once. Even if an intelligent civilization reaches the end of its lifetime after, for example, 10,000 years, life may still prevail on the planet for billions of years, permitting the next civilization to evolve. Thus, several civilizations may come and go during the lifespan of one and the same planet. Thus, if nr is the average number of times a new civilization reappears on the same planet where a previous civilization once has appeared and ended, then the total number of civilizations on such a planet would be 1 + nr, which is the actual reappearance factor added to the equation.

The factor depends on what generally is the cause of civilization extinction. If it is generally by temporary uninhabitability, for example a nuclear winter, then nr may be relatively high. On the other hand, if it is generally by permanent uninhabitability, such as stellar evolution, then nr may be almost zero. In the case of total life extinction, a similar factor may be applicable for fl, that is, how many times life may appear on a planet where it has appeared once.

Where:

• N is the number of planets with detectable biosignature gases
• N* is the number of stars within the sample
• FQ is the fraction of quiet stars
• FHZ is the fraction with rocky planets in the habitable zone
• FO is the fraction of observable systems
• FL is the fraction with life
• FS is the fraction with detectable spectroscopic signatures

Now, it’s important to note that this equation is not an update to the Drake Equation per se — it’s more like a parallel equation that can work in tandem with the original version. Rather than come up with a formula to predict the predominance of intelligent life, Seager is interested in predicting our chances of detecting any kind of life within the next ten years.

“Just like on Earth where we have satellites that look down to measure gas concentrations, we can use space telescopes to look at the atmospheres of planets far away,” she explains. “We’re going to look for gases that essentially don’t belong — gases that may be produced by life.”

Seager gives the example of oxygen on Earth.

“Oxygen fills our atmosphere to 20% by volume. But without life we actually wouldn’t have oxygen at all — we’d have about 10 billion times less oxygen,” she says. “So plants and photosynthetic bacteria are creating oxygen in our atmosphere, and so, if aliens were to look at us from far away, and using optical wavelength telescopes — rather than radio telescopes — they would see all this excess oxygen and they would hopefully know that that it doesn’t belong here — that it should be attributed to life.”

Another unique element of Seager’s equation is the addition of so-called quiet stars.

Stars vary in terms of their activity. Our sun, for example, is currently in a solar maximum phase, so it’s giving off more solar flares than usual. But some active stars can be super active, and in ways that are not good.

“Active stars are way more active than quiet stars, and there’s a kind of concern that certain active stars could be harmful to life,” Seager told io9.

Another problem is that active stars vary in brightness, which often makes it hard to find exoplanets. Flare stars pose another problem.

Very active stars also have high ultraviolet radiation flux — and that’s a problem for biosignature gases. UV radiation sets off a chain of chemical reactions that often ends up destroying a lot of gases. Thus, it’s hard for biosignature gases to accumulate on those planets.

Hence the focus on quiet stars.

According to Seager’s own calculations, the value of N equals two — which is not as pessimistic as it sounds.

The pessimists' most telling argument in the SETI debate stems not from theory or conjecture but from an actual observation: the presumed lack of extraterrestrial contact. A civilization lasting for tens of millions of years might be able to travel anywhere in the galaxy, even at the slow speeds foreseeable with our own kind of technology. Furthermore, no confirmed signs of intelligence elsewhere have been recognized as such, either in our galaxy or in the observable universe of 2 trillion galaxies. According to this line of thinking, the tendency to fill up all available territory seems to be a universal trait of living things, so the Earth should have already been colonized, or at least visited, but no evidence of this exists. Hence Fermi's question "Where is everybody?".

[…] These lines of reasoning lead to the Great Filter hypothesis, which states that since there are no observed extraterrestrial civilizations, despite the vast number of stars, then some step in the process must be acting as a filter to reduce the final value. […]

A large number of explanations have been proposed to explain this lack of contact; a book published in 2015 elaborated on 75 different explanations. In terms of the Drake Equation, the explanations can be divided into three classes:

Few intelligent civilizations ever arise. This is an argument that at least one of the first few terms, R∗ · fp · ne · fl · fi, has a low value. The most common suspect is, fi, but explanations such as the rare Earth hypothesis argue that ne is the small term.

Intelligent civilizations exist, but we see no evidence, meaning fc is small. Typical arguments include that civilizations are too far apart, it is too expensive to spread throughout the galaxy, civilizations broadcast signals for only a brief period of time, it is dangerous to communicate, and many others.

The lifetime of intelligent, communicative civilizations is short, meaning the value of L is small. Drake suggested that a large number of extraterrestrial civilizations would form, and he further speculated that the lack of evidence of such civilizations may be because technological civilizations tend to disappear rather quickly. Typical explanations include it is the nature of intelligent life to destroy itself, it is the nature of intelligent life to destroy others, they tend to experience a technological singularity, and others.

It is possible that complex life may evolve through other mechanisms than those found specifically here on Earth, but the fact that in the history of life on the Earth only one species has developed a civilization to the point of being capable of space flight and radio technology lends more credence to the idea of technologically advanced civilizations being rare in the universe.

For example, the emergence of intelligence may have been an evolutionary accident. Geoffrey Miller proposes that human intelligence is the result of runaway sexual selection, which takes unpredictable directions. Steven Pinker, in his book How the Mind Works, cautions that the idea that evolution of life (once it has reached a certain minimum complexity) is bound to produce intelligent beings, relies on the fallacy of the "ladder of evolution": As evolution does not strive for a goal but just happens, it uses the adaptation most useful for a given ecological niche, and the fact that, on Earth, this led to technological intelligence only once so far may suggest that this outcome of natural selection is rare and hence by no means a certain development of the evolution of a tree of life.

Another theory along these lines is that even if the conditions needed for life might be common in the universe, that the formation of life itself, a complex array of molecules that are capable simultaneously of reproduction, of extraction of base components from the environment, and of obtaining energy in a form that life can use to maintain the reaction (or the initial abiogenesis on a potential life-bearing planet), might ultimately be very rare.

Additionally, in the nondirectional meandering from initial life to humans, other low-probability happenings may have been the transition from prokaryotic cells to eukaryotic cells (with separate nucleus, organelles, specialization, and a cytoskeleton allowing the cell to take on different shapes) and the transition from single-cellular life to multicellular life, which was recorded in the Cambrian Explosion of 530 mya when significant numbers of organisms had evolved hard body parts, although multicellular life perhaps first started to evolve a couple of hundred million years before that. Single celled life emerged c. 3.5 billion years ago, and for most of Earth's history and for reasons not fully understood there have only been single-celled creatures.

And there are many other potential branching points. For example, perhaps the transition from ocean creatures to land-dwelling creatures crucially depends on an unusually large moon and significant tides. Many astronomers refer to our Earth Moon pairing as a double planet. This ratio between parent and satellite is rare in our planetary system. There is no observational data on the numbers of 'double planets' in other planetary systems. And even fundamental conditions such as the chemical composition of the nursery nebula from which a planetary system forms could have unusual or detrimental consequences for the emergence and survival of life.

It is also possible that intelligence is common, but industrial civilization is not. For example, the rise of industrialism on Earth was driven by the presence of convenient energy sources such as fossil fuels. If such energy sources are rare or nonexistent elsewhere, then it may be far more difficult for an intelligent alien race to advance technologically to the point where humans could communicate with them. There may also be other unique factors on which our civilization is dependent. Or, on a water world, where the intelligent creatures are something like dolphins, it may be difficult to build fire and forge metals.

Another possibility is that Earth is the first planet in the Milky Way on which industrial civilization has arisen. However, critics note that according to current understanding, many Earth-like planets were created many billions of years prior to Earth, so this explanation requires repudiation of the mediocrity principle.

So where is everybody?

Welcome to the Fermi Paradox.

We have no answer to the Fermi Paradox—the best we can do is “possible explanations.” And if you ask ten different scientists what their hunch is about the correct one, you’ll get ten different answers. You know when you hear about humans of the past debating whether the Earth was round or if the sun revolved around the Earth or thinking that lightning happened because of Zeus, and they seem so primitive and in the dark? That’s about where we are with this topic.

In taking a look at some of the most-discussed possible explanations for the Fermi Paradox, let’s divide them into two broad categories—those explanations which assume that there’s no sign of Type II and Type III Civilizations because there are none of them out there, and those which assume they’re out there and we’re not seeing or hearing anything for other reasons:

Explanation Group 1: There are no signs of higher (Type II and III) civilizations because there are no higher civilizations in existence.

Those who subscribe to Group 1 explanations point to something called the non-exclusivity problem, which rebuffs any theory that says, “There are higher civilizations, but none of them have made any kind of contact with us because they all _.” Group 1 people look at the math, which says there should be so many thousands (or millions) of higher civilizations, that at least one of them would be an exception to the rule. Even if a theory held for 99.99% of higher civilizations, the other .001% would behave differently and we’d become aware of their existence.

Therefore, say Group 1 explanations, it must be that there are no super-advanced civilizations. And since the math suggests that there are thousands of them just in our own galaxy, something else must be going on.

This something else is called The Great Filter.

The Great Filter theory says that at some point from pre-life to Type III intelligence, there’s a wall that all or nearly all attempts at life hit. There’s some stage in that long evolutionary process that is extremely unlikely or impossible for life to get beyond. That stage is The Great Filter.

If this theory is true, the big question is, Where in the timeline does the Great Filter occur?

It turns out that when it comes to the fate of humankind, this question is very important. Depending on where The Great Filter occurs, we’re left with three possible realities: We’re rare, we’re first, or we’re fucked.

1. We’re Rare (The Great Filter is Behind Us)

One hope we have is that The Great Filter is behind us—we managed to surpass it, which would mean it’s extremely rare for life to make it to our level of intelligence. The diagram below shows only two species making it past, and we’re one of them.

This scenario would explain why there are no Type III Civilizations…but it would also mean that wecould be one of the few exceptions now that we’ve made it this far. It would mean we have hope. On the surface, this sounds a bit like people 500 years ago suggesting that the Earth is the center of the universe—it implies that we’re special. However, something scientists call “observation selection effect” says that anyone who is pondering their own rarity is inherently part of an intelligent life “success story”—and whether they’re actually rare or quite common, the thoughts they ponder and conclusions they draw will be identical. This forces us to admit that being special is at least a possibility.

And if we are special, when exactly did we become special—i.e. which step did we surpass that almost everyone else gets stuck on?

One possibility: The Great Filter could be at the very beginning—it might be incredibly unusual for life to begin at all. This is a candidate because it took about a billion years of Earth’s existence to finally happen, and because we have tried extensively to replicate that event in labs and have never been able to do it. If this is indeed The Great Filter, it would mean that not only is there no intelligent life out there, there may be no other life at all.

Another possibility: The Great Filter could be the jump from the simple prokaryote cell to the complex eukaryote cell. After prokaryotes came into being, they remained that way for almost two billion years before making the evolutionary jump to being complex and having a nucleus. If this is The Great Filter, it would mean the universe is teeming with simple prokaryote cells and almost nothing beyond that.

There are a number of other possibilities—some even think the most recent leap we’ve made to our current intelligence is a Great Filter candidate. While the leap from semi-intelligent life (chimps) to intelligent life (humans) doesn’t at first seem like a miraculous step, Steven Pinker rejects the idea of an inevitable “climb upward” of evolution: “Since evolution does not strive for a goal but just happens, it uses the adaptation most useful for a given ecological niche, and the fact that, on Earth, this led to technological intelligence only once so far may suggest that this outcome of natural selection is rare and hence by no means a certain development of the evolution of a tree of life.”

Most leaps do not qualify as Great Filter candidates. Any possible Great Filter must be a one-in-a-billion type thing where one or more total freak occurrences need to happen to provide a crazy exception—for that reason, something like the jump from single-cell to multi-cellular life is ruled out, because it has occurred as many as 46 times, in isolated incidents, just on this planet alone. For the same reason, if we were to find a fossilized eukaryote cell on Mars, it would rule the above “simple-to-complex cell” leap out as a possible Great Filter (as well as anything before that point on the evolutionary chain)—because if it happened on both Earth and Mars, it’s clearly not a one-in-a-billion freak occurrence.

If we are indeed rare, it could be because of a fluky biological event, but it also could be attributed to what is called the Rare Earth Hypothesis, which suggests that though there may be many Earth-likeplanets, the particular conditions on Earth—whether related to the specifics of this solar system, its relationship with the moon (a moon that large is unusual for such a small planet and contributes to our particular weather and ocean conditions), or something about the planet itself—are exceptionally friendly to life.

2. We’re the First

For Group 1 Thinkers, if the Great Filter is not behind us, the one hope we have is that conditions in the universe are just recently, for the first time since the Big Bang, reaching a place that would allow intelligent life to develop. In that case, we and many other species may be on our way to super-intelligence, and it simply hasn’t happened yet. We happen to be here at the right time to become one of the first super-intelligent civilizations.

One example of a phenomenon that could make this realistic is the prevalence of gamma-ray bursts, insanely huge explosions that we’ve observed in distant galaxies. In the same way that it took the early Earth a few hundred million years before the asteroids and volcanoes died down and life became possible, it could be that the first chunk of the universe’s existence was full of cataclysmic events like gamma-ray bursts that would incinerate everything nearby from time to time and prevent any life from developing past a certain stage. Now, perhaps, we’re in the midst of an astrobiological phase transition and this is the first time any life has been able to evolve for this long, uninterrupted.

3. We’re Fucked (The Great Filter is Ahead of Us)

If we’re neither rare nor early, Group 1 thinkers conclude that The Great Filter must be in our future. This would apply that life regularly evolves to where we are, but that something prevents life from going much further and reaching high intelligence in almost all cases—and we’re unlikely to be an exception.

One possible future Great Filter is a regularly-occurring cataclysmic natural event, like the above-mentioned gamma-ray bursts, except they’re unfortunately not done yet and it’s just a matter of time before all life on Earth is suddenly wiped out by one. Another candidate is the possible inevitability that nearly all intelligent civilizations end up destroying themselves once a certain level of technology is reached.

This is why Oxford University philosopher Nick Bostrom says that “no news is good news.” The discovery of even simple life on Mars would be devastating, because it would cut out a number of potential Great Filters behind us. And if we were to find fossilized complex life on Mars, Bostrom says “it would be by far the worst news ever printed on a newspaper cover,” because it would mean The Great Filter is almost definitely ahead of us—ultimately dooming the species. Bostrom believes that when it comes to The Fermi Paradox, “the silence of the night sky is golden.

Explanation Group 2: Type II and III intelligent civilizations are out there—and there are logical reasons why we might not have heard from them.

Group 2 explanations get rid of any notion that we’re rare or special or the first at anything—on the contrary, they believe in the Mediocrity Principle, whose starting point is that there is nothing unusual or rare about our galaxy, solar system, planet, or level of intelligence, until evidence proves otherwise. They’re also much less quick to assume that the lack of evidence of higher intelligence beings is evidence of their nonexistence—emphasizing the fact that our search for signals stretches only about 100 light years away from us (0.1% across the galaxy) and has only been going on for under a century, a tiny amount of time. Group 2 thinkers have come up with a large array of possible explanations for the Fermi Paradox. Here are 10 of the most discussed:

Possibility 1) Super-intelligent life could very well have already visited Earth, but before we were here.* In the scheme of things, sentient humans have only been around for about 50,000 years, a little blip of time. If contact happened before then, it might have made some ducks flip out and run into the water and that’s it. Further, recorded history only goes back 5,500 years—a group of ancient hunter-gatherer tribes may have experienced some crazy alien shit, but they had no good way to tell anyone in the future about it.

Possibility 2) The galaxy has been colonized, but we just live in some desolate rural area of the galaxy.* The Americas may have been colonized by Europeans long before anyone in a small Inuit tribe in far northern Canada realized it had happened. There could be an urbanization component to the interstellar dwellings of higher species, in which all the neighboring solar systems in a certain area are colonized and in communication, and it would be impractical and purposeless for anyone to deal with coming all the way out to the random part of the spiral where we live.

Possibility 3) The entire concept of physical colonization is a hilariously backward concept to a more advanced species.* Remember the picture of the Type II Civilization above with the sphere around their star? With all that energy, they might have created a perfect environment for themselves that satisfies their every need. They might have hyper-advanced ways of reducing their need for resources and zero interest in leaving their happy utopia to explore the cold, empty, undeveloped universe.

An even more advanced civilization might view the entire physical world as a horribly primitive place, having long ago conquered their own biology and uploaded their brains to a virtual reality, eternal-life paradise. Living in the physical world of biology, mortality, wants, and needs might seem to them the way we view primitive ocean species living in the frigid, dark sea. FYI, thinking about another life form having bested mortality makes me incredibly jealous and upset.

Possibility 4) There are scary predator civilizations out there, and most intelligent life knows better than to broadcast any outgoing signals and advertise their location.* This is an unpleasant concept and would help explain the lack of any signals being received by the SETI satellites. It also means that we might be the super naive newbies who are being unbelievably stupid and risky by ever broadcasting outward signals. There’s a debate going on currently about whether we should engage in METI (Messaging to Extraterrestrial Intelligence—the reverse of SETI, which only listens) or not, and most people say we should not. Stephen Hawking warns, “If aliens visit us, the outcome would be much as when Columbus landed in America, which didn’t turn out well for the Native Americans.” Even Carl Sagan (a general believer that any civilization advanced enough for interstellar travel would be altruistic, not hostile) called the practice of METI “deeply unwise and immature,” and recommended that “the newest children in a strange and uncertain cosmos should listen quietly for a long time, patiently learning about the universe and comparing notes, before shouting into an unknown jungle that we do not understand.” Scary.

Possibility 5) There’s one and only one instance of higher-intelligent life—a “superpredator” civilization* (kind of like humans are here on Earth)—who is far more advanced than everyone else and keeps it that way by exterminating any intelligent civilization once they get past a certain level. This would suck. The way it might work is that it’s an inefficient use of resources to exterminate all emerging intelligences, maybe because most die out on their own. But past a certain point, the super beings make their move—because to them, an emerging intelligent species becomes like a virus as it starts to grow and spread. This theory suggests that whoever was the first in the galaxy to reach intelligence won, and now no one else has a chance. This would explain the lack of activity out there because it would keep the number of super-intelligent civilizations to just one.

Possibility 6) There’s plenty of activity and noise out there, but our technology is too primitive and we’re listening for the wrong things.* Like walking into a modern-day office building, turning on a walkie-talkie, and when you hear no activity (which of course you wouldn’t hear because everyone’s texting, not using walkie-talkies), determining that the building must be empty. Or maybe, as Carl Sagan has pointed out, it could be that our minds work exponentially faster or slower than another form of intelligence out there—e.g. it takes them 12 years to say “Hello,” and when we hear that communication, it just sounds like white noise to us.

Possibility 7) We are receiving contact from other intelligent life, but the government is hiding it.* This is an idiotic theory, but I had to mention it because it’s talked about so much.

Possibility 8) Higher civilizations are aware of us and observing us but concealing themselves from us* (AKA the “Zoo Hypothesis”). As far as we know, super-intelligent civilizations exist in a tightly-regulated galaxy, and our Earth is treated like part of a vast and protected national park, with a strict “Look but don’t touch” rule for planets like ours. We wouldn’t be aware of them, because if a far smarter species wanted to observe us, it would know how to easily do so without us noticing. Maybe there’s a rule similar to the Star Trek’s “Prime Directive” which prohibits super-intelligent beings from making any open contact with lesser species like us or revealing themselves in any way, until the lesser species has reached a certain level of intelligence.

Possibility 9) Higher civilizations are here, all around us, but we’re too primitive to perceive them.* Michio Kaku sums it up like this:

Lets say we have an ant hill in the middle of the forest. And right next to the ant hill, they’re building a ten-lane super-highway. And the question is “Would the ants be able to understand what a ten-lane super-highway is? Would the ants be able to understand the technology and the intentions of the beings building the highway next to them?”

So it’s not that we can’t pick up the signals from Planet X using our technology, it’s that we can’t even comprehend what the beings from Planet X are or what they’re trying to do. It’s so beyond us that even if they really wanted to enlighten us, it would be like trying to teach ants about the internet.

Along those lines, this may also be an answer to “Well if there are so many fancy Type III Civilizations, why haven’t they contacted us yet?” To answer that, let’s ask ourselves—when Pizarro made his way into Peru, did he stop for a while at an anthill to try to communicate? Was he magnanimous, trying to help the ants in the anthill? Did he become hostile and slow his original mission down in order to smash the anthill apart? Or was the anthill of complete and utter and eternal irrelevance to Pizarro? That might be our situation here.

Possibility 10) We’re completely wrong about our reality.* There are a lot of ways we could just be totally off with everything we think. The universe might appear one way and be something else entirely, like a hologram. Or maybe we’re the aliens and we were planted here as an experiment or as a form of fertilizer. There’s even a chance that we’re all part of a computer simulation by some researcher from another world, and other forms of life simply weren’t programmed into the simulation.