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The Search for Extra-Terrestrial Intelligence

In 1877, Italian astronomer Giovanni Schiaparelli observed a network of lines on the planet Mars and called them “canali,” meaning “channels.” This was widely misinterpreted as “canals,” and a number of people took this to mean that intelligent life on Mars was building the so-called canals. Among them was the prominent American astronomer Percival Lowell.

The canal theory has since been disproven, though subsequent exploration by robot probes has indicated there might indeed have been water on Mars carving natural channels in its surface. In any event, the story illustrates the human curiosity about extraterrestrial intelligence and the quest to find evidence of their technology.

Cosmological Principles

In previous units, we have established that astronomers have known for some time there are billions of stars in each galaxy and billions of galaxies in the universe. More recently, the Kepler mission has suggested that there are billions of Earth-like planets circling sun-like stars and red dwarfs in our galaxy alone.

These large numbers have led some cosmologists to embrace what is called “the principle of mediocrity.” This notion suggests that there is nothing special about our Earth, solar system, galaxy, or ourselves, and that the conditions leading to life and intelligence on our home planet are likely being reproduced throughout the universe. The principle of mediocrity points to the possibility of there being thousands, or even millions, of advanced technological civilizations “out there.” With such a high number of possibilities, the probability of intelligence life existing also seems very high.

By contrast, advocates of the “anthropic cosmological principle” argue that probability is not the same as certainty: the fact that life arose once does not mean that life is common, even with so many opportunities for it to arise.

They suggest that when we consider all the elements of the Drake Equation, it may be quite rare that every variable -- from having a habitable planet to having the conditions required for life to life actually emerging, to intelligence appearing -- will be present only in rare cases.

Fermi’s Paradox

In 1950, while having lunch with some colleagues, Enrico Fermi, a physicist who worked on the atomic bomb and won the Nobel Prize, contemplated the apparent abundance of opportunities for advanced technical civilizations to arise in the universe, and asked: “Where is everybody?”

In other words, if the probabilities are so high that there are hundreds, thousands, or even more advanced technological civilizations out there, why haven’t they sent us a radio signal, or showed up in our solar system? Even if they can’t travel faster than light, the universe is old enough that someone might have found their way to us, or at least sent a robot probe our way (as we ourselves have done with the Voyager spacecraft, which recently left the solar system and headed into interstellar space.)

Fermi asked this question more than 60 years ago, and the answer is the same as it was then: we don’t know.


We haven’t just been waiting to hear from the aliens; we have been actively listening for them. As early as 1960, scientists began laying the intellectual and practical groundwork for pursuing what has come to be called SETI, or the Search for Extraterrestrial Intelligence. They, and their successors, have been using radio telescopes to sweep the skies for signals displaying a regularity indicating that they are not merely cosmic background noise.

For a brief period, the United States government funded a SETI project, but when those funds were withdrawn, a group of scientists formed a private entity, the SETI Institute, which continues the work to this day.

SETI researchers have developed a protocol for determining the validity of a radio signal arriving from the cosmos. It requires that any promising signal must be independently verified and it must repeat itself on subsequent observations. There have been many candidates over the five decades that this effort has been under way, but none have met the strict criteria for verification.

The most famous of these was the “WOW” signal, so named because Jerry Ehman, who was monitoring transmissions at the Big Ear radio telescope lab in Ohio in 1977, wrote that word on the printout when he saw it. The signal stood out and seemed clearly to be artificial and non-terrestrial in origin, but it never repeated itself after being monitored for 72 seconds by the Big Ear telescope. Many researchers looked for the signal in subsequent years, but it was never again observed.

Those who follow the mediocrity principle have tried to respond to Fermi’s Paradox with a number of possible answers as to why advanced extraterrestrial civilizations have not contacted us, assuming they are out there. For example, most of these civilizations could have destroyed themselves when they reached a level of technological capability high enough such that they could communicate with us.

However, these are all speculations about the psychology of civilizations that may or may not exist, hypotheses at best that cannot be scientifically verified. The fact is that Fermi’s simple question remains unanswered, at least by this search methodology.


Another approach began to develop in the 1960s in parallel with that of communication SETI, and it has come to be called “Artifact SETI,” or the “Search for Extraterrestrial Technology” (SETT). Soviet astronomer Nikolai Kardashev was an early proponent of SETT. He suggested that there might be three different types of civilizations on exoplanets, classified by the amount of energy they could harness:

While Type II and III civilizations might seem beyond belief for us at our present level of development, we should bear in mind that terrestrial civilization is close to being a Type I, and we were nowhere near that level 100 years ago. Imagine a civilization even just 1000 years older than ours, and what its capabilities might be.

Around the same time that Kardashev posed his theory, an American scientist, Freeman Dyson, suggested that an advanced civilization might break up some of the planets in a solar system and create a sphere that would enclose its sun, thereby preventing the solar energy from escaping and providing a continuous source of energy for a Type II society. While many questioned the ethics and practicality of the so-called “Dyson Sphere” at the time, the idea offered a plausible explanation of how Type II extraterrestrials might function.

The artifact approach to the search for aliens avoids all the speculation about why we have not heard from them. It is based on the simple observation that energy does not disappear from the universe once used. According to the Second Law of Thermodynamics, it dissipates as heat. Since this is so, we should be able to detect the heat residue of the energy used by extraterrestrial civilizations. All we need is a means of doing so.

Richard Carrigan, scientist emeritus at Fermilab in Illinois, pioneered the search for “Dyson Spheres” using the Infrared Astronomical Satellite (IRAS) and found a few objects within several hundred light-years of Earth that might indicate the presence of such objects. When his team looked for radio signals in those locations, they heard nothing.

More recently, Assistant Professor Jason Wright of Pennsylvania State University and his team have begun the G-HAT (Glimpsing Heat from Alien Technologies) and have made some progress towards either confirming or denying the validity of this theory.

Wright believes that the waste heat created by a Type I civilization would not be sufficient to be seen in comparison with the luminosity of its star. However, the heat generated by a Type II or Type III civilization would show up in the infrared and would therefore be detectable as something other than the natural luminosity of a star or galaxy. G-Hat has initiated its search on a galactic scale, reasoning that a Type III Dyson Sphere would stand out by blocking some percentage of the light emanating from its home galaxy.

Wright and his team have been using the WISE (Wide-Field Infrared Survey Explorer telescope in their artifact SETI searches, and have found no evidence so far of areas where 50-100% of a galaxy’s light is being blocked. They are now looking for evidence of regions where 20-30% is being obscured.

Wright views artifact SETI as complementary to communication SETI and expects that the two techniques will be used in tandem to search for extraterrrestrial civilizations.

We still do not have an answer to Enrico Fermi’s provocative question, but new and inventive efforts are on the horizon.

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