Are we alone? It’s one of those big questions that humans have pondered for centuries. All the way back in 1600AD, Giordano Bruno was burned at the stake for, among other things, believing that there were many other inhabited worlds; some of the early pioneers of the telescope expected to see animals and plants on the moon with their inventions; and, as late as 1909, Percival Lowell claimed to see canals and vegetation on Mars. Unlike most of the big philosophical questions that are a matter of pure speculation, we’ve gotten more and more information about the universe around us, as our technology has advanced. We now know for certain that there are no animals on the moon, canals on Mars, or jungles on Venus; but, at the same time, our view of the universe has expanded. We now know that we live in a galaxy with hundreds of billions of stars, in a universe with hundreds of billions of galaxies; but, in all that space, we have not found one single definitive sign of intelligent life.

This is the beginning of a 10 part series where I will discuss my thoughts on the Fermi Paradox. This is the apparent contradiction between the fact that the universe is incredibly vast and full of other places where intelligent life could arise, and the absence of any evidence of its existence. This is by no means the first or most detailed attempt to systematically deduce a solution to this paradox. Many scientists and other thinkers have attempted to do so before, and I will lean heavily on their work in my analysis.

This problem has an attraction like no other because, at least in principle, it would allow one to determine the environment humans might encounter as we spread out into the stars. As far as we know right now, we could find a barren galaxy devoid of life, or a set of vast alien empires that might either welcome us into a galactic community or try to exterminate us. If we could determine which one of these possibilities, among many others, is most probable, we would start to have a better picture of what the future course of humanity could be like.

However, for the Fermi Paradox to truly be a paradox in need of a solution, we need to be confident that if alien life existed, we would be able to detect it. Our inability to detect certain types of activity that we might expect from alien civilizations prevents us from eliminating many possibilities for what the galaxy at large is like. Therefore, I will start by discussing the methods that we currently have at our disposal to detect alien civilizations and their activities, then try to figure out if we can rule out the possibility that they have already visited us.

SETI Searches

The Search for Extraterrestrial Intelligence (SETI) Institute is an organization dedicated to searching for signals from alien civilizations. Their Allen Telescope Array (ATA) is a set of 42 radio telescopes located in northern California that spends twelve hours a day scanning the sky for radio signals. The ATA has already scanned tens of thousands of stars over a wide range of frequencies. Its primary goal is to scan a million of the closest stars for transmissions with a strength similar to what we could create with existing radio telescopes. However, there are hundreds of billions of stars in the galaxy, and an alien transmitter would have to be pointed right at us continuously for us to notice it. Either that, or it would have to be far more powerful than anything we have now. Either way, they would need to be intentionally announcing their presence to civilizations at or near our level of development.

The ATA does scan larger areas of the sky when looking for narrow-band transmissions, but these searches rely on a guess for what frequency the aliens would be broadcasting on, usually frequencies corresponding to the ionization energy of hydrogen or the dissociation energy of water. If aliens were sending narrow-band transmissions toward us at a different frequency, then we might never notice it. Even with very thorough surveys by the most advanced radio telescopes, we will still not be able to rule out the possibility that there are alien civilizations intentionally sending signals our way.

But should we even expect them to send signals our way? The Milky Way is approximately 100,000 light years in diameter, and if an alien civilization were more than 200 light years away, they would see no evidence of civilization on Earth: no radio signals, no spectral signatures of artificial compounds in the atmosphere, and minimal artificial lighting on the night side of the planet. I think that there are really only three scenarios in which we would expect to detect a radio signal from an alien civilization.

The first scenario is the existence of a civilization right next door to us in galactic terms, that wants to make contact with us as soon as we start building large radio telescopes. I think we can safely rule out this possibility, since nearby stars have been intermittently monitored with radio telescopes for decades, and it’s not difficult to send a detectable signal over a distance of 100 light-years or so.

The second scenario is the existence of a civilization spending enormous amounts of power (millions of gigawatts) on an omnidirectional radio beacon to announce its presence to any civilization in the galaxy at our level of technology or higher. If the signal from such a beacon is not strong enough to be detected by radio telescopes that have done all-sky surveys, it could go unnoticed by us. However, if there were many of these beacons in the galaxy, it stands to reason that one of them would be close enough that it would be impossible for us not to notice.

The third scenario is if this civilization directed narrow-beam transmissions at planets that they know to have life. We are just getting to the point that our most powerful telescopes can get information about the atmospheric composition of the atmospheres of some exoplanets, so it is conceivable that a highly advanced civilization could find all life-bearing planets in the galaxy by looking for gasses like oxygen and methane in their atmospheres. Just how much power they would save over an omnidirectional beacon would depend on just how common life is in the galaxy.

Another type of SETI search is Optical SETI, which uses visible-light telescopes to search for laser emissions. Lasers are much better than radio for interstellar communications; so, if an alien homeworld is communicating with starships or colonies that are directly between it and Earth, we could conceivably detect their signal with a large enough optical telescope.

Additionally, one of the most credible propulsion systems for interstellar travel is the laser-driven lightsail. Lasers meant to propel spacecraft would be vastly more powerful than anything used for communication, and therefore much easier to detect over interstellar distances. In a 1984 paper, Robert L. Forward estimated that a crewed mission to a nearby star using lightsails would require 43 petawatts (million gigawatts) of laser power, powerful enough for the laser to be visible across vast stretches of the galaxy.

Optical SETI is in a much earlier stage than its radio counterpart, with surveys so far having only covered a few thousand stars and only being sensitive enough to detect intentional contact signals and the (much more powerful) laser propulsion beams. The Large Synoptic Survey Telescope (LSST), expected to come on-line in 2020, will have a much better chance of detecting extraterrestrial laser signals. It is an eight-meter telescope tasked with performing general sky surveys of the southern hemisphere, with an emphasis on finding anything anomalous. However, even a thorough survey of sky would never find a laser signal that was not directly lined up with Earth. Therefore, the LSST would still not be able to completely rule out the possibility that there are alien civilizations in the galaxy actively using lasers for communication or propulsion. However, just like with radio beacons, we can be fairly confident that this kind of activity is not common. If there were millions of worlds sending laser-propelled spacecraft in all directions, one of our existing survey telescopes would almost certainly have found something.

Transits and Megastructures

There is another kind of alien activity that we could detect with existing technology. The most successful method to date for detecting extrasolar planets is to continuously watch a set of stars and look for dips in brightness as planets pass in front of or transit their stars. The Kepler Space Telescope discovered over 3,000 planets using this method, while only looking at one small patch of the sky. The intent was to essentially take a cross section of the galaxy, finding out how common various types of planets are. The Transiting Exoplanet Survey Satellite (TESS), which is the follow-up mission to Kepler, will scan the entire sky looking for planetary transits, but focus on the most nearby stars.

If an alien civilization were to build enough solar arrays around its star to collect a significant portion of the star’s energy, telescopes looking for planetary transits, or just doing spectrographic surveys could flag the star as an anomaly. Follow-up observations would confirm that the star is too dim for its spectral type and is emitting an anomalously large amount of infrared light. This is because no solar collector can be 100% efficient and they would emit most of the energy they absorbed as infra-red waste heat. If this civilization kept building collectors until they were capturing almost all of the available solar energy, they would form a Dyson sphere (or more accurately a Dyson swarm). This star would be an obvious anomaly on most stellar surveys. However, the most extensive surveys in the infra-red have only covered a tiny fraction of the hundreds of billions of stars in the galaxy and there could still be many Dyson swarms out there that have gone unnoticed.

The idea of looking for this type of megastructure became much more popular recently with Kepler’s discovery of anomalous behavior in the star KIC-8462852, aka “Tabby’s Star.” The star was found to regularly dim by as much as 20%, indicating that something very big was orbiting it. If such an object was artificial, then it would truly be worthy of the term “megastructure.” However, data from follow-up observations seems to be much more consistent with a very strange dust cloud than any solid structure or set of solid structures. Even so, several other stars have since been found with similar behavior that are currently under investigation. What we do know for sure is that megastructures are not so common that they are obvious: i.e. they do not make up a significant portion of the Milky Way’s total energy output. The European Space Agency’s Gaia space telescope has surveyed over a billion stars, but only in visual light wavelengths. A follow-up mission called Gaia-NIR (Near Infra-Red) has been proposed, and would have a much better chance of detecting alien megastructures.

If an entire galaxy had been colonized by a civilization that built enough Dyson swarms that they were utilizing a large fraction of their galaxy’s total energy output, we would be able to detect them over vast distances. One analysis of data from the WISE space telescope attempted to do just that. It found no anomalous IR radiation from the vast majority of a set of 100,000 galaxies. Subsequent analyses of those few that did emit greater than 25% of their energy in the infra-red had unusually large amounts of dust or other obviously natural features that explained the unusual IR radiation. This result means our consideration of the Fermi Paradox needs to take into account other galaxies, as well as our own.

The Hart Conjecture

All of these SETI methods focus on remotely detecting alien civilizations, but if they have the capability to travel to other star systems, then it’s possible for them to come to us. The question of how long it should take for them to get here was considered by Michael Hart in a 1975 paper. He modeled a wave of colonization propagating across the galaxy, with each colony founded launching its own colonization ships, on and on until the entire galaxy is full. He calculated that this would take less than a million years, if the new ships are launched almost immediately after colonies are established. Carl Sagan and William Newman published a response paper in 1981, using much more conservative assumptions for population growth and the associated turnaround time for colony ships. They found that it would take tens of millions of years for every viable planet in the galaxy to be colonized, which is still a short time in cosmic terms. So, if aliens began colonizing the galaxy at a time further in the past than 30 million years or so, they would have reached Earth by now, and had plenty of time to grow a population large enough to completely dominate the planet.

Several other teams of researchers have since applied different mathematical modeling approaches to this problem. The most recent attempt, by Johnathan Carroll-Nellback and collaborators, indicates that the absolute maximum time to colonize the galaxy is around 300 million years, based on the fact that stellar drift will spread civilizations around, even if they, themselves, rarely travel far from home. However, with a more realistic set of assumptions about the speed, range, and launch rate of starships, their analysis indicates that the galaxy could fill up in less than 10 million years, unless civilizations have lifespans on a similar timescale.

Hart used the conclusion from his model to argue that intelligent life does not exist in our galaxy, because if it arose at any point, it is highly likely that that point was far enough in the past that Earth could not avoid being colonized. The argument goes that by all rights, we shouldn’t even be here. Earth should be a planet fully populated by aliens who keep the native dinosaurs in zoos. Therefore, the fact that we exist at all strongly suggests that they don’t exist.

Later in this series of articles, I will get into reasons why civilizations might not colonize the galaxy in general, or Earth specifically. What we do know for sure is that if any civilization has arisen more than a few tens of millions of years ago, they did not colonize Earth, or any of the other major bodies in our solar system. The question remains, can we be sure that they never came here for some reason that did not involve expanding their population and resource base.

The Zoo Hypothesis

If our solar system is somehow set aside as a special preserve to allow life to develop on its own, that brings us to the other major reason why an alien civilization would choose to visit our solar system: scientific research. The high concentrations of oxygen and methane in Earth’s atmosphere have been broadcasting the presence of life for over a billion years. Even a very distant civilization could send a probe or scientific expedition in that time.

The question is, can we rule out the possibility that a probe or even a crewed starship exists in our solar system today? The answer, somewhat disconcertingly, seems to be no. It is estimated that there are hundreds of thousands of asteroids over a kilometer in size in the main asteroid belt, and only a fraction of these have been catalogued. Any of them (excluding those few that have been closely studied) could be hiding an alien spacecraft or base the size of some of the smaller space habitats envisioned by Gerard O'Neill, which were designed to satisfy all the needs of a population in the thousands. If they were using nuclear or fusion reactors for power, they would never have to leave the interior. If they arrived more than a century or two ago, they would have plenty of time to build such a facility before we developed the technology to detect spacecraft near Earth.

Further out into the solar system, in the Kuiper belt and Oort cloud, there are even more objects that are both large and difficult to track. Even though it may be uncomfortably close to some of the ideas of UFO enthusiasts and ancient alien conspiracy theorists, we can not rule out the possibility that there are aliens in our own solar system monitoring us.

What we do know

As it turns out, there are quite a few ways that alien civilizations could go unnoticed by us, but there are also some scenarios that we can rule out even with our limited capabilities for astronomical observation. A few things we can be fairly certain about alien civilizations are:

  1. If they are close enough to know we have technology, they don’t want to contact us.
  2. It is not a common practice in our galaxy to broadcast messages in hopes of contacting a civilization as soon as it arises.
  3. If interstellar propulsion is common in our galaxy, it is not laser-based.
  4. Megastructures are rarely ever constructed in large numbers.
  5. If alien civilizations ever arose in our galaxy, they either:
    a. Evolved less than a few tens of millions of years ago
    b. Never colonized the galaxy to the point of saturating it
    c. Made a special exception for Earth and did not colonize it
    d. Died out in the past

There are a lot of possibilities left open by these conditions, but some are going to be much more likely than others. In future installments of this series, I will examine these possibilities and try to figure out which ones are most likely. The Fermi Paradox is only a paradox if the number civilizations that we expect to exist is large enough that it is inconceivable that not one of them would violate one of conditions 1 through 5. From now on, I will call these the non-observability conditions, since any civilization that violated one of them would be observed by us.

In the next few articles, I will work toward estimating the number of habitable planets in the galaxy. First looking at the structure of the galaxy itself, then moving on to the characteristics of different types of stars, and finally, the planets themselves. I’ll try to find the most up to date research on these topics, and put it together to get a picture of exactly how many places there are in the galaxy (and beyond) where intelligent life could arise. After that, I’ll get into the Fermi paradox solutions based on biology and sociology. If it turns out that there are plenty of habitable planets, then there must be some factor that either keeps intelligent life from arising on these planets or keeps intelligent species from violating the non-observability conditions. There are lots of ideas out there for what this factor might be, and choosing between them is largely a matter of speculation and personal opinion. However, I’ll try to come up with some sort of systematic way of comparing them.


SETI Institute

R. Forward (1984). Roundtrip Interstellar Travel using Laser-Pushed Lightsails. Spacecraft. 2,21.  

P. Horowitz, et al. (2001). Targeted and All-Sky Search for Nanosecond Optical Pulses at Harvard-Smithsonian SETI at Harvard.

M. H. Hart, (1975). An explanation for the absence of extraterrestrials on Earth, Quarterly Journal of the Royal Astronomical Society, 16:128-135.

W. I. Newman and C. Sagan (1981). Galactic Civilizations: Population Dynamics and Interstellar Diffusion., Icarus,

J. Carrol-Nellback, et al. (2020). The Fermi Paradox and the Aurora Effect: Exo-civilization Settlement, Expansion, and Steady-States. The Astronomical Journal.

J. T. Wright, et al. (2014). The G-HAT Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies II. Framework, Strategy, and First Result. The Astrophysical Journal

M. A. Garrett. (2015). Application of the mid-IR radio correlation to the G-HAT sample and the search for advanced extraterrestrial civilizations. Astronomy and Astrophysics.