It's no secret that searching for life beyond Earth, whether within our own Solar System or beyond, is very challenging work. For decades, scientists have sent robotic and crewed missions to other celestial bodies to search for evidence of past or present life.
In fact, with the Voyager probes, the Cassini-Huygens mission, and the New Horizons' spacecraft, all of the major bodies of our Solar System beyond Earth have effectively been explored to some degree. That's Mercury, Venus, the Moon, Mars, Ceres and Vesta, and Jupiter, Saturn, Uranus, Neptune, and Pluto (as well as some other larger moons).
And yet, all our best efforts have managed to turn up nothing — or at least, nothing conclusive. Now, why is that? Could it be that intelligent life is rare, hard to find, or not looking to be found? Or could it be humanity is alone in the Universe, staring out into a great black abyss with no one looking back?
The Search at Home
So far, the majority of our efforts to find extra-terrestrial life have been focused on Mars. The earliest efforts were the Viking 1 and 2 missions, which sent landers to the surface in 1976 (a few months apart). Unfortunately, these surveys were inconclusive, which is why Opportunity, Curiosity, and Perseverance rovers are continuing the search.
In the coming decade, NASA and the European Space Agency (ESA) are planning to send robotic missions to Europa to see if there really is life locked away beneath the moon's icy surface. Similar efforts are being proposed to explore other potential "Ocean Worlds" like Ceres, Callisto, Ganymede, Titan, Enceladus, Mimas, Triton, Pluto, and others.
While these efforts might reveal that life exists elsewhere in our Solar System (most likely in the form of microbes), it does very little to help in the Search for Extra-Terrestrial Intelligence (SETI). Here, scientists have spent decades monitoring deep space and other planets for indications of biological processes (biosignatures) and technological activity (technosignatures).
So far, the results have been equally discouraging, which has led many scientists and theorists to come up with various explanations for "the Great Silence."
Fermi’s Big Question
In 1950, while working at Los Alamos National Laboratory, physicist Enrico Fermi is said to have asked the question that launched a thousand possible answers. While having lunch with his colleagues and speaking on the subject of SETI, he famously asked: "Where is everybody?"
This became the basis of the Fermi Paradox, which addresses the discrepancy between the (supposed) statistical likelihood of life beyond Earth with the scarcity of evidence. This question reflected the state of SETI in Fermi's time. Unfortunately, things have not changed much since then.
And, the fact of the matter is this is surprising when you consider that, based on even the most conservative estimates, there should be at least some intelligent life out there. And given how long the Universe has been around (currently thought to be around 13.8 billion years), some of that life should have reached a very high level of technical development by now.
The Drake Equation
The principles and theories behind the Drake Equation are closely related to those of the Fermi Paradox. Named after the American astronomer Francis Drake, the equation was an attempt to formalize the theoretical parameters that SETI researchers had been operating within for decades.
In essence, the equation is a means for calculating the number of extraterrestrial civilizations in our galaxy that we should be able to communicate with at any given time. The equation is expressed as N = R* x fp x ne x fl x fi x fc x L, where:
N is the number of ETIs that we might be able to communicate with
R* is the average rate of star formation in our galaxy
fp is the number of stars that have a system of planets
ne is the number of planets that will be able to support life
fl is the number of planets that will develop life
fi is the number of planets that will develop sentient (aka. intelligent) life
fc is the number of civilizations that will develop advanced technologies
L is the length of time that these civilizations will have to transmit radio or other communications signals into space
Granted, many of the parameters Drake specified in his equation were subject to a significant degree of uncertainty. Even today, we still have no idea how to assign values to most of them. For example, astronomers have a pretty good idea of how many stars there are in our galaxy and the average rate of star formation — between 100 and 200 billion stars, with a handful being added every year.
Thanks to recent advances in extrasolar planet research, scientists can place constraints on the number of stars that have a system of planets (most will have at least 1) and the number of these that will be able to support life (aka. those that are "potentially habitable"). So, it's fair to say we have at least some idea of the values for the first, second, and third parameters.
Beyond that, however, we don't have the slightest idea. We have no idea how many potentially habitable planets will actually give rise to life, let alone how many of those will develop life capable of communicating with us, or how long such a civilization could be expected to live before some cataclysmic event or other fate causes their demise.
And how could we? At present, we know of only one planet where life exists (Earth) and only one species capable of communicating with radio waves or other parts of the electromagnetic spectrum (humanity). But that's not really the point of the Drake Equation.
In the end, Drake proposed this equation as a sort of statistical exercise, designed to show that even by the most conservative estimates, there ought to be at least a few civilizations out there right now that humanity could be hearing from.
Moreover, given the age of the Universe, at least a few of those civilizations should have been able to develop extremely advanced technology by now. This brings up another important concept known as...
The Kardashev Scale
In 1964, Soviet astronomer and SETI researcher Nikolai Kardashev proposed a classification method for grouping species based on their level of technological development. The resulting scale had three levels (or types) which classified species based on the amount of energy they could harness.
By definition, Type I civilizations (aka. "planetary civilizations") are those that have developed the means to harness and store all of their home planet's energy. According to Kardashev, this would amount to the consumption of 4 x 1019 erg/sec, which would likely be in the forms of fusion power, antimatter, and renewable energy on a global scale.
Next up are Type II civilizations ("stellar civilizations"), which evolved to the point where they could harvest all the energy emitted by their star — which Kardashev speculated would likely involve a structure like a Dyson Sphere. In this case, this would work out to a consumption of 4 x 10³³ erg/sec.
Type III civilizations ("galactic civilizations") are those that would be able to harness the energy of an entire galaxy, which would work out to energy consumption on the order of 4 x 1044 erg/sec.
Based on the fact that the Universe has been around for 13.8 billion years, and our Solar System has only existed for the last 4.6 billion years, some think it would seem likely that at least a few civilizations would have been able to achieve a Type III level of development. Even with our modest means, it would be very difficult for humans to miss the signs of such a civilization.
Again, we are forced to ask why we have found no signs of intelligent life in the cosmos. How it is that the odds of intelligent life seems so likely, but the evidence is so lacking? Here's where things get particularly interesting, frightening, and more than a little mind-blowing.
The Barrow Scale
Since Kardashev's time, many additions and extensions have been suggested for the scale he designed. While some theorists have speculated that there are even greater levels of development beyond the Type III classification (and others have suggested classifications that fall between the three types), others have suggested that the scale needs to be redrawn to emphasize other metrics.
Instead of energy, the famed science communicator and astronomer Carl Sagan suggested that civilizations could be classified based on "information mastery." That is to say, the more advanced the species, the more information they would have at their fingertips.
Robert Zubrin, another famous science communicator (and advocate for exploring Mars), suggested that a civilization could be measured by "planetary mastery." In this scenario, development is pegged to the number of planets (or star systems) a civilization can successfully colonize.
But it was John D. Barrow who introduced what is arguably the most radical reinterpretation, in his 1998 book, titled Impossibility: the Limits of Science and the Science of Limits. Using human history as a template, Barrow showed how technological progress has allowed humanity to extend its control over the environment to increasingly smaller scales.
Rather than characterizing a civilization by the amount of outer space it commands, Barrow ventured that more advanced species would eventually harness the power of inner space. From this, he created what is known as the Barrow Scale, a reverse classification that consists of seven Types:
- Type I-minus: capable of manipulating objects similar in size to themselves (building structures, extract ore, monuments, etc.)
- Type II-minus: capable of manipulating and altering the development of living things (organ transplants, studying DNA, genetic engineering, etc.)
- Type III-minus: capable of manipulating molecules and molecular bonds to create new materials
- Type IV-minus: capable of manipulating individual atoms, creating nanotechnologies and complex forms of artificial life
- Type V-minus: capable of manipulating atomic nuclei and engineering nucleons that compose them
- Type VI-minus: capable of manipulating the elementary particles (quarks and leptons)
- Type Omega-minus: capable of manipulating the basic structure of space and time
In short, advanced civilizations might focus on optimizing the space they have rather than expanding the amount of space they occupy. It also makes more sense for an intelligent species to increase the total amount of energy it could harvest from smaller and smaller units of matter rather than trying to acquire more of it.
Is Anybody Out There?
Now that we've covered all these big concepts, we can address the big question at the heart of this paradox. Given that the Universe is so incredibly large and the ingredients for life so common, then why don't we see evidence of it beyond Earth?
There's the obvious answer: that extra-terrestrial intelligence doesn't exist. This was the conclusion argued by Michael Hart, an American astrophysicist, in a paper he published in 1975 titled, "Explanation for the Absence of Extraterrestrials on Earth."
This argument was elucidated further by mathematician Frank J. Tipler in his 1979 study, "Extraterrestrial Intelligent Beings do not Exist." In what has come to be named the Hart-Tipler Conjecture, they argue that if any ETIs had developed the means for interstellar travel, they would have visited our Solar System by now.
Coincidentally, it was these papers that framed the Fermi Paradox as we know it today. In his time, Fermi never suggested that humanity was alone in the Universe or that the absence of a galactic empire (something Hart and Tipler claimed should have happened by now) was a strong indication that intelligent life didn't exist beyond Earth.
Nevertheless, this fact (Fact A) is central to the Hart-Tipler Conjecture and has become a major feature of the Fermi Paradox. If life should be plentiful, and a species should have left its mark on the galaxy by now, then why haven't we seen indications that either of these likely possibilities is true?
Beyond concluding that humanity is alone in the Universe, many hypotheses have been ventured to resolve how Fact A can coexist with the more optimistic appraisals of people like Frank Drake, Carl Sagan, and other "Contact Optimists."
The Great Filter
Another possibility was suggested by economic Robin Hanson in an online essay, "The Great Filter - Are We Almost Past It?" published in 1998. As he summarized his argument:
"Humanity seems to have a bright future, i.e., a non-trivial chance of expanding to fill the universe with lasting life. But the fact that space near us seems dead now tells us that any given piece of dead matter faces an astronomically low chance of begating such a future. There thus exists a great filter between death and expanding lasting life, and humanity faces the ominous question: how far along this filter are we?"
In Hanson's view, this "filter" must lie somewhere between life's starting point (abiogenesis) and the proliferation of advanced life beyond its home planet and star system. Using humanity as a template, he also outlined a nine-step process that life would need to follow in order to produce a complex and space-faring species. These included:
- Habitable star system (organics and habitable planets)
- Reproductive molecules (e.g., RNA)
- Prokaryotic single-cell life
- Eukaryotic single-cell life
- Sexual reproduction
- Multi-cell life
- Animals capable of using tools
- Industrial civilization
- Wide-scale colonization
According to the Great Filter hypothesis, at least one of these steps must be improbable. If it is an early step, then humanity's existence is a statistical rarity, and our future prospects would seem bleak. If it is a later step, then there would be many civilizations (past and present) that have reached our current level of development but not progressed further.
In any case, no species has reached, or at least maintained, the ninth step in our galaxy, or it would be teeming with evidence of their existence. So it is entirely possible that intelligent species don't survive the transition from step eight to step nine, which would coincide with a Type I to Type II level civilization.
As you might suspect, this is not good news for humanity. Given the environmental issues that have become evident since the latter half of the century — air and water pollution, waste, drought, ozone depletion, global warming, etc. — it is not a farfetched possibility that no species survives becoming advanced.
And with the threat of nuclear war still a possibility, it is also possible that intelligent species are destined to wipe themselves out. In this respect, the fact that we have not found evidence of any ETIs can be seen as a good sign. As Hanson indicated in his essay, there is a bright side to the fact that humanity has not found evidence of extra-terrestrial life yet:
"But contrary to common expectations, evidence of extraterrestrials is likely bad (though valuable) news. The easier it was for life to evolve to our stage, the bleaker our future chances probably are."
Beyond the Hart-Tipler Conjecture and the Great Filter, there are many other possible reasons why we haven't found evidence of intelligent life yet. Another popular explanation is that the reason we haven't found any evidence of ETIs is that they don't want to be found!
In 2001, famed science fiction author Stephen Baxter asserted as much in his seminal essay, "The Planetarium Hypothesis - A Resolution of the Fermi Paradox." In an attempt to resolve the Fermi Paradox, Baxter postulated that humanity's astronomical observations are actually an illusion created by a Type III Civilization that is keeping humanity in a giant "planetarium." As he put it:
"A possible resolution to the Fermi Paradox is that we are living in an artificial universe, perhaps a form of virtual- reality `planetarium,' designed to give us the illusion that the universe is empty. Quantum-physical and thermo-dynamic considerations inform estimates of the energy required to generate such simulations of varying sizes and quality. The perfect simulation of a world containing our present civilization is within the scope of a Type K3 extraterrestrial culture. However, the containment of a coherent human culture spanning ~100 light-years within a perfect simulation would exceed the capacities of any conceivable virtual-reality generator."
This concept is similar to the Simulation Hypothesis, which posits that the observable Universe is actually a massive holographic simulation. This idea has deep roots in mystic and empirical philosophy, which included the practice of questioning whether or not reality is actually real.
In this case, however, it is suggested that the purpose of keeping humanity in a simulation is to protect ourselves (and our hosts) from the dangers associated with "the first contact." Variations on this hypothesis generally state that ETIs are employing other forms of advanced technology to remain undetected (e.g. cloaking devices or other such things).
There's also the possibility that the Universe is too big, and relativity too challenging, for a species to colonize significant portions of space. This would certainly address Fact A, which is the absence of any "galactic civilization," or a visit from an interstellar explorer. And it makes sense considering how much energy traveling to even the nearest star would take.
Carl Sagan and William I. Newman suggested this in their 1981 study, “Galactic civilizations: Population dynamics and interstellar diffusion." In response to the Hart-Tipler hypothesis, they argued that alien signals and probes may simply not have reached Earth yet.
In 1993, NASA scientist Geoffrey A. Landis wrote a paper titled, "The Fermi paradox: an approach based on percolation theory." Here, Landis argued that as a consequence of relativity, an exo-civilization would only be able to expand a limited amount throughout the galaxy.
In short, Landis argued that interstellar settlement wouldn't happen in a uniform or consistent way. Instead, colonists would "percolate" outwards, eventually reaching a limit where the time-lag in communications and the costs of expansion would be too great.
Serbian astronomer and astrophysicist Milan M. Cirkovic made a similar argument in his 2008 study, "Against the Empire." For the sake of this study, Cirkovic used models to determine the behavior of a civilization — whether it would be expansion-driven ("Empire-State") or optimization-driven ("City State").
In the end, Cirkovic considered it to be a naive assumption that a species would be driven to create a galactic empire, and that more advanced (post-biological) species would be more interested in living in an optimized, spatially-compact environment. These findings anticipated what would later be argued with the "Transcension Hypothesis" (see below).
The Zoo Hypothesis
It has also been suggested that advanced species are deliberately keeping their distance from Earth, perhaps as part of a "quarantine" or "non-interference" protocol. The hypothesis was coined in 1973 by Harvard astrophysicist John A. Ball in a study of the same name.
In this study, Ball addressed some common assumptions made by SETI researchers. Among them is the belief that life will emerge where favorable conditions exist, that these conditions can be found all across the cosmos, that ETIs do exist, and we are unaware of “them.”
However, another inherent assumption Ball identified is the belief that “they” want to talk to us. In rebuttal, Ball argued what he termed the “Zoo Hypothesis”:
“I believe that the only way that we can understand the apparent non-interaction between “them” and us is to hypothesize that they are deliberately avoiding interaction and that they have set aside the area in which we live as a zoo.
“The zoo hypothesis predicts that we shall never find them because they do not want to be found and they have the technological ability to ensure this. Thus this hypothesis is falsifiable, but not, in principle, confirmable by future observations.”
In addition, Ball argued that it takes huge stretches of time for life to evolve and reach an advanced state. Using Earth as a template, we can see how it took no less than 4 billion years for life to advance from single-celled organisms (prokaryotes) to modern humans.
As such, Ball argued that it is statistically more likely that the majority of life is either early in its development or highly-advanced (much more than ourselves). From this, he reasoned that the "Great Silence" stems from the fact that much of the life out there either cannot contact us, or can, but doesn't want to.
As for why ETIs would want to avoid contact, Ball suggested that it could be due to a reverential attitude towards life and evolution and a desire to avoid harmful interference (a la "the Prime Directive" from Star Trek). It's also possible that advanced species believe less advanced ones to be chaotic and unpredictable, which means avoidance is an act of self-preservation.
The Transcension Hypothesis
Another fascinating possibility is one that scientists have hinted at for decades but has been more clearly elucidated since the turn of the century. The modern version of this hypothesis was proposed by John M. Smart, the CEO of Foresight University and founder of the Acceleration Studies Foundation.
In a paper titled "Answering the Fermi Paradox: Exploring the Mechanisms of Universal Transcension,” (2002) he argued that the “Great Silence” could be explained by a process of Evolutionary Development. In a 2011 essay, Smart presented an updated version of this argument and a synopsis of what the theory of transcension entails:
"The transcension hypothesis proposes that a universal process of evolutionary development guides all sufficiently advanced civilizations into what may be called “inner space,” a computationally optimal domain of increasingly dense, productive, miniaturized, and efficient scales of space, time, energy, and matter…"
Inspired in part by the Barrow Scale, Smart argued that advanced civilizations would eventually be drawn to black holes, to use as an ideal power source. Living close to black holes would also allow them to evade detection and conduct all kinds of extreme physical science.
This echoes sentiments previously shared by Russian scientist and "father of astronautics" Konstantin Tsiolkovsky. In his 1932 essay entitled “Is there a God?” — Tsiolkosvky theorized that a state of “perfect intelligence” lay in humanity’s future, which had already been achieved by other lifeforms in the Universe:
“Millions of milliards of planets have existed for a long time, and therefore their animals have reached a maturity which we will reach in millions of years of our future life on Earth. This maturity is manifest by perfect intelligence, by a deep understanding of nature, and by technical power which makes other heavenly bodies accessible to the inhabitants of the cosmos.”
This was followed by Tsiolkovsky's 1933 essay, “The Planets are Occupied by Living Beings," which was written in the form of a dialogue with himself. At one point, he poses the challenge that if there were any advanced species out there, they should have visited Earth by now. To this, he replied:
“Perhaps they will visit us, but time has not come yet for this. Aboriginal Australians and Native Americans of past centuries saw Europeans visit them – but many millenniums passed before they arrived. Similarly, we will see such a visit in some time. The powerful inhabitants of other planets, perhaps, have been visiting one another for a long time.“
It raises an interesting point. If we assume that any extraterrestrial intelligence currently out there is far more advanced than our own, then it would be foolish to assume that they would engage in activities that we would immediately recognize. Their capabilities, technology, and priorities would be vastly different than anything we are familiar with.
The Aurora Hypothesis
In 2019, researchers from NASA’s Nexus for Exoplanetary Systems Science (NExSS), the Center for Exoplanets and Habitable Worlds, and multiple universities released a study detailing what they called "the Aurora Effect." The name was inspired by the 2015 SF novel Aurora by author Kim Stanley Robinson.
In the novel, Robins tells the story of how a multi-generational crew must abandon its plans for colonization when they learn that the environment contains microorganisms (prions) that are deadly to humans. Based on the notion that organisms will not be able to habituate easily to alien environments, they drew the following conclusion:
“Often there is the assumption that any planet can be terraformed to the specific needs of the settling civilization. But the idea that the purpose of probes is to build habitable settlements and that all stellar systems are viable targets for such settlements goes to the agency of an exo-civilization; in our work we therefore relax this assumption.
“In addition, some stars may host indigenous forms life, which may preclude settlement for practical or ethical reasons… This theme was explored in (spoiler alert) the novel Aurora by Kim Stanley Robinson (Robinson 2015) in which even though a world was formally habitable it was not what we would call settleable. Thus we include the possibility that good worlds are hard to find – what we call the Aurora Effect.”
A simpler way of stating this is to say that even though a planet may be "habitable" by our standards, it is not "settleable." As a result, no species (regardless of how advanced) could be expected to colonize freely throughout our galaxy and any efforts to do so would be limited by biological factors.
The Dark Forest Hypothesis
This proposed resolution is a relatively recent addition to the Fermi debate. It takes its name from The Dark Forest, a 2015 science fiction novel by Chinese author Liu Cixin. The second installment in the award-winning Remembrance of Earth’s Past series, this novel takes place in the near future, where humanity is faced with destruction at the hands of a hostile extraterrestrial species.
The hypothesis emerges from a series of discussions between the main characters about the nature of "cosmic sociology," a premise that Liu invented for the sake of the series. To summarize, Liu claims that intelligence is common throughout the Universe, but mutual fear and the futility of launching attacks across interstellar space keeps them all in a state of silence. As he summarizes it:
"The universe is a dark forest. Every civilization is an armed hunter stalking through the trees like a ghost, gently pushing aside branches that block the path and trying to tread without sound. Even breathing is done with care. The hunter has to be careful, because everywhere in the forest are stealthy hunters like him.
"If he finds other life — another hunter, an angel or a demon, a delicate infant or a tottering old man, a fairy or a demigod — there’s only one thing he can do: open fire and eliminate them. In this forest, hell is other people. An eternal threat that any life that exposes its own existence will be swiftly wiped out. This is the picture of cosmic civilization. It’s the explanation for the Fermi Paradox."
In summary, alien civilizations are not broadcasting their existence because they fear what the response will be. This is illustrated by humanity's own search for intelligent life, which has overwhelmingly focused on passive listening (SETI) and very little on signaling (METI).
In an attempt to answer Fermi's challenge, other possibilities have been suggested that are too numerous to count. However, some of the more popular suggestions include the following:
Intelligent life is very rare:
It could be that our searches for evidence of ETIs have not yet succeeded because we haven't been looking for long enough. This certainly fits in with more conservative estimates using the Drake Equation. It's also bolstered by the number of exoplanets that appear to have a high mass-fraction that consists of water (aka. the Waterworlds Hypothesis).
Intelligent life is too far apart:
Addressing our failure to find evidence of radio signals and other transmissions technology is due to distance. Simply put, ETIs may be too distant in terms of space and time, since transmissions will only be discernible within a limited volume of space.
Similarly, it's also possible that civilizations are not around long enough to pick up on alien transmissions. In fact, in a recent study co-authored by Frank Drake, a team of scientists argued that any alien signals picked up by human observers will most likely have come from a civilization that went extinct a long time ago.
Intelligent life is hibernating:
This possibility was suggested by Oxford research associate Anders Samberg and colleagues from the Future of Humanity Institute (FHI). In their 2017 study titled, “That is Not Dead Which Can Eternal Lie: the Aestivation Hypothesis for Resolving Fermi’s Paradox, “they suggest that ETIs are engaged in "aestivation" — a prolonged state of torpor which organisms enter into during a particularly hot or dry period — and waiting for better conditions.
We don’t know what to look for:
As it stands, we know of only one planet that supports life (Earth) and only one example of technologically-advanced life (our own). For this reason, all of our searches for biosignatures and technosignatures are based entirely on what we are familiar with.
Perhaps that is the problem, and perhaps we should be casting a wider net. Unfortunately, that just isn't possible because our scientists wouldn't know where to begin. Given the limits of our technology, we are forced to look for "signatures," which makes looking for life "as we don't know it" impossible.
We're looking in the wrong places:
At present, humanity's efforts to find life beyond Earth are focused on terrestrial (rocky) bodies, and that includes the search at home. Most of our efforts to find evidence of life are aimed at Mars, while many feel that our efforts should be focused on places like Europa and other "Ocean Worlds."
This has raised the possibility that the most likely place to find life elsewhere in the Universe could be within icy bodies with interior oceans. This life would be unable to communicate with us since it would be contained within an icy shell and would be entirely aquatic.
We haven't been looking long enough:
In cosmological terms, humanity has been an "advanced" species for a very short time. Radio communications have only existed on Earth since the end of the 19th century, and radio telescopes have only existed since the 1930s. As such, it could be that not enough time has passed for distant aliens to pick up on our radio transmissions or for us to pick up on theirs.
Intelligent life is already here!:
Here's a possibility that no fan of science fiction will fail to recognize! Perhaps aliens not only exist but are moving among us and gathering information as we speak. You have to admit if we ever found an ETI and were capable of making contact, wouldn't we want to do a little investigating first to prevent any "cultural misunderstandings"?
Intelligent life destroys itself or others:
Here we have the extension of the Great Filter hypothesis. In this scenario, it could be that no intelligent species survives climate change, nuclear war, etc., or that more advanced species wipe out less advanced species — creating the illusion that intelligent life is rare.
Humanity is early to the party:
Another sobering suggestion is that humanity is actually one of the first intelligent species to emerge in our Universe and hasn't found any intelligent species because they haven't achieved our level of development yet. Harvard Professor Abraham Loeb and colleagues suggested this possibility in a 2016 study titled "Relative Likelihood for Life as a Function of Cosmic Time."
Exploring the possibility of life emerging in a star system as a function of time, they found that long-lived stars (such as low-mass, M-type red dwarfs) have the best odds of producing life-bearing planets. In this respect, it could be argued that humanity is actually an early arrival to the party, rather than a late one (as has generally been assumed).
Alas, all of these possibilities are informed by the same basic problem: we just don't know. Until we find examples of extra-terrestrial life and ETIs, we won't know with any confidence under what conditions life is able to emerge and evolve.
Possible Detections of ETIs
In the meantime, there is the possibility that humanity has found evidence of ETIs and simply didn't realize it. There have also been numerous instances where potential signals were detected, and we simply haven't been able to prove that they came from an extra-terrestrial source yet.
On August 15th, 1977, astronomers using the Big Ear radio telescope at Ohio State University detected a 72-second radio signal coming from the direction of the Sagittarius Constellation. This powerful signal, which quickly earned the nickname the “WOW! Signal”, was thought by some to be extra-terrestrial in origin.
Since then, the WOW! Signal has been an ongoing source of controversy among SETI researchers and astronomers. This is because all attempts to date to find a natural cause — which includes asteroids, exoplanets, stars, signals from Earth, hydrogen clouds, and comets — have been inconclusive. To date, it remains the strongest candidate for possible alien transmissions.
In September of 2015, citizen scientists with the Planet Hunters project noticed that the star KIC 8462852 (aka. Tabby’s Star) was experiencing a mysterious dip in luminosity. Located in the constellation Cygnus, roughly 1,470 light-years from Earth, this star experienced fluctuations and a drop of up to 22% in brightness.
Since then, observatories around the world have noted further incidents of dimming, and multiple studies have been conducted to try and offer a natural explanation for this behavior. These have ranged from a circumstellar debris disk, shattered comets, and asteroids to the presence of a giant planet, a planet with rings, or a planet that had been consumed in the past.
However, it was the proposal that the irregular dimming could be caused by the presence of alien megastructures that attracted the most attention. While no evidence has been produced to reinforce this idea, the fact that no natural explanation has been able to account for the star's behavior has kept it in the public mind.
Fast Radio Bursts (FRBs) on Repeat:
Here is yet another example of astronomical phenomena that appears to defy natural explanation. Basically, FRBs are short-lived radio pulses that last only a few milliseconds. Since the first was discovered in 2007 (known as the Lorimer Burst), only about two dozen have been detected (mostly in archival data), and only a handful have been found to be repeating.
In the case of one-off events, several theories have been offered for what causes them - ranging from exploding stars and black holes to pulsars and magnetars. However, no viable explanation has been offered to date for repeating FBRs, leading some to suggest that they might be evidence of alien radio transmissions.
On October 19th, 2017, the Panoramic Survey Telescope and Rapid Response System-1 (Pan-STARRS-1) in Hawaii announced the detection of an object named 1I/2017 U1 (aka. ‘Oumuamua, Hawaiian for "scout"). Unlike the many Near-Earth Objects (NEOs) that periodically pass near to Earth, 'Oumuamua was the first known object to have come from interstellar space.
After multiple follow-up observations were conducted, scientists were still unable to determine whether 'Oumuamua was an asteroid or a comet. On the one hand, its composition data indicated that it was likely to be icy, but it didn't form a tail like a comet. However, it then accelerated out of the Solar System like a comet would when experiencing outgassing.
Based on its behavior, two scientists from the Harvard-Smithsonian Center for Astrophysics — Shmuel Bialy and Prof. Abraham Loeb — speculated that 'Oumuamua might actually be an interstellar lightsail or the remains of an interstellar spacecraft. This not only would explain why it sped up as a result of radiation pressure from our Sun. It would also explain 'Oumuamua's orbit.
For starters, after entering the Solar System, 'Oumuamua passed within 0.25 AU of our Sun, which is a good orbit for intercepting Earth without experiencing too much solar irradiation. In addition, it came to within 0.15 AU of Earth, which could have been the result of orbital corrections designed to facilitate a flyby.
If this was indeed the case, then 'Oumuamua could be transmitting images of Earth to its home system as we speak! It's also possible that our Solar System is littered with the remains of many interstellar probes, since astronomers have deduced that objects like 'Oumuamua enter our Solar System on a regular basis.
Decades later, Fermi's Paradox continues to haunt and excite us. What's more, the Drake Equation continues to serve as a thought experiment where most of the parameters are still subject to major uncertainty. At the same time, the nature of our search is subject to a paradox of its own:
Until we find evidence of life beyond Earth, we won't know what to look for!
If this life takes the form of terrestrial beings that are reliant on technology for their survival, then Earth-like planets are a safe bet. If we find lifeforms that exist under "exotic" circumstances, we will be able to expand our search to other types of environments. Until that happens, we won't know if there's anyone out there, or if we're even looking in the right places.
But that's the cool thing about the Fermi Paradox: you only need to solve it once! The moment we find evidence of an extraterrestrial civilization (assuming we ever do), the Paradox will be solved for all time. And it really doesn't matter if the civilization is still alive or not.
One signal, one glimpse of a megastructure, or one confirmed spaceship sighting, and we will know with certainty that humanity is not alone in the Universe.
In the meantime, all we can do is wait and get better at searching for it!
- Wikipedia - Fermi Paradox
- Wikipedia - Great Filter
- NASA - Anyone Out There?
- SETI Institute - Fermi's Paradox
- Fermi's Paradox - Possible Answers
- NASA - "The Fermi Paradox: An Approach Based on Percolation Theory" by Geoffrey Landis
- George Mason University - "The Great Filter - Are We Almost Past It?" by Robin Hanson