Part III: SETI: The Search for Extraterrestrial Intelligence

May 28, 2022

VII. SETI: The Beginning:

SETI is the acronym for “search for extraterrestrial intelligence.” In this post we will describe the use of messages sent out through space in the hope that they will be received and understood by an intelligent culture, together with efforts to detect any messages being sent from outer space. Other attempts to detect alien life in the universe involve searches for space craft that might have originated on other planets. That field, called searches for UFOs or “unidentified flying objects,” is a separate effort that we will not cover in this post.

One might trace the beginning of searches for signs of intelligent alien life to 1865. That was the year that James Clerk Maxwell predicted that electro-magnetic (EM) waves could travel through space at c (300 million meters/second), the speed of light. In 1886, the German physicist Heinrich Hertz demonstrated that electromagnetic waves could be produced; he also showed that they took the form of transverse electromagnetic waves (the electric and magnetic fields oscillate perpendicular to the direction of the wave), and he measured that these waves traveled through space at speed c.

The experimental physicist Nikola Tesla produced EM waves that could travel through space. Tesla was a pioneer in wireless transmission of EM waves. He also built receivers that could receive EM waves traveling through the Earth’s atmosphere (we have reviewed Tesla’s work in a blog post on the controversy between Tesla and Thomas Edison regarding the superiority of alternating current or direct current sources of electric power). Tesla had built a facility in Colorado Springs to test his high-voltage sources of electricity. In 1899, he observed a strange signal in his detectors, which he attributed to EM signals from Mars. His reasoning was based on his belief that the signals he received stopped after Mars set in the night sky.

Figure VII.1: Nikola Tesla, a physicist who made seminal advances in our ability to create and manipulate electromagnetic fields.

We know that Tesla had not received a signal from Mars, but it is still not clear what Tesla was observing. At the time, Guglielmo Marconi was also producing and detecting wireless EM waves. In 1901, Marconi claimed that he had detected signals that had been sent from England at Signal Hill in Newfoundland, Canada (although some recent scientific studies assert that it was impossible for Marconi to have detected a trans-Atlantic signal with technology that existed in 1901). If Marconi was correct, this was the first demonstration that radio waves could be transmitted overseas. Some have speculated that Tesla may have detected Marconi’s signals that had been sent from England. In any case, Tesla’s claims inspired other scientists to attempt to communicate with Mars by wireless EM waves traveling through the atmosphere and outer space.

Figure VII.2: Part of Nikola Tesla’s laboratory in Colorado Springs, Colorado. It was here that Tesla claimed to have received radio signals from Mars.

In 1920 Marconi wrote “I have encountered during my experiments with wireless telegraphy [a] most amazing phenomenon. Most striking of all is receipt by me personally of signals which I believe originated in the space beyond our planet. I believe it is entirely possible that these signals may have been sent by the inhabitants of other planets to the inhabitants of earth.”

Now that we have landed rover vehicles on Mars, it seems quite clear that there are no radio messages originating from that planet. However, attempts to detect signals from intelligent life elsewhere in the universe have continued, and over time have become much more sophisticated. We will discuss those ventures in this post.

Arguments in Favor of Extraterrestrial Intelligence:

There are logical arguments suggesting that extraterrestrial intelligence quite likely exists elsewhere in the universe, particularly in our own Milky Way galaxy. The chain of reasoning goes as follows:

The Milky Way contains billions of stars that are similar to our Sun
Several of these stars have planets similar to Earth, that exist in a habitable zone relative to their star. When these arguments were first made, this was a conjecture. However, in recent decades searches for exoplanets have found thousands of planets in orbit around stars in our Milky Way, as has been discussed in the first two sections of this post. Some of these exoplanets appear to have orbits that might produce environments suitable for developing life.
Many of these stars are significantly older than our Sun; thus, their Earth-like planets have had ample time for intelligent life to develop.
Some of these civilizations may have developed interstellar travel; alternatively, they may have sent messages announcing themselves or probes to search for other intelligent life.
Since many of these Sun-like stars are billions of years older than our Sun, Earth should either have been visited by extraterrestrial civilizations, or their probes or messages should have reached the Earth.

These arguments formed the basis for searches for extraterrestrial intelligence. We will review attempts to find messages that have been sent through space, and our own efforts to send signals or space probes that might be received by an advanced alien civilization.

VIII: SETI — The Search for Evidence of Extraterrestrial Life

Over the past century there have been many efforts to search for intelligent life elsewhere in the universe. Because of the large distances to extrasolar planets, our searches have mainly concentrated on civilizations in our Milky Way galaxy. We will first summarize efforts to detect signals in electromagnetic waves originating from outside our solar system. EM waves will travel faster than any massive spacecraft, so such messages would reach us sooner than other means of communication.

SETI Searches: The Electromagnetic Spectrum:

The first attempts to discover messages from outer space involved studying radio waves that reach Earth. Around 1900, both Nikola Tesla and Guglielmo Marconi were sending and receiving radio waves that were transmitted through Earth’s atmosphere. In fact, both of these scientists claimed to have received radio waves from outer space, and both of them speculated that these waves may have originated from Mars. These claims were similar to many other pseudo-scientific interests of the times. At the end of the 19^th century, electric power was still rather new and somewhat mysterious. Also, at this time there was a great deal of interest in parapsychology. It was fashionable to attend a séance where practitioners (quacks, actually) would use parlor tricks to claim that they were communicating with the dead.

Many well-known figures were attracted to this field. Arthur Conan Doyle, the author of the Sherlock Holmes stories, was a staunch proponent of spiritualist phenomena. The gullible Conan Doyle was convinced that some British schoolgirls had taken photographs of fairies in their garden. And Thomas Edison claimed to have invented a telephone that would communicate with the dead. So, the claims by Tesla and Marconi to have received communications from Mars fit right in with the Spiritualist craze of the time. On the other hand, the great magician Harry Houdini spent a great deal of time unmasking charlatans and frauds in the field of parapsychology.

The first attempts to search for signs of extraterrestrial intelligence also scrutinized radio waves. Radio waves are produced at frequencies that travel rather easily through the Earth’s atmosphere; because they are not absorbed in passing through the atmosphere, they are particularly good candidates for searches involving terrestrial telescopes. In 1960, Frank Drake carried out an experiment using a 26-meter diameter radio telescope at Green Bank, West Virginia. Drake looked for signals emanating from one of two stars, Tau Ceti and Epsilon Eridani, in the vicinity of a single frequency, 1.42 Gigahertz (GHz, or billions of cycles per second). Drake scanned a 400-kiloHertz band around the central frequency using a single-channel signal analyzer; he found no interesting signals.

The choice of 1.42 GHz for the scanned frequency was based on the criteria for obtaining and analyzing electromagnetic (EM) energy from space. At much higher frequencies, EM radiation is blocked by interstellar dust, and will not survive to reach us. To interpret a signal, it must be much stronger than the “background noise” of EM radiation in our atmosphere. At frequencies below about 1 GHz, the noise is dominated by synchrotron radiation of our galaxy, which is emitted by accelerating charged particles. This falls rapidly with increasing frequency (it is the left-hand band in Fig. VIII.1). At frequencies above about 60 GHz, the dominant background noise is from quantum receiver noise; this increases linearly with frequency. Superimposed on this rise are absorption lines corresponding to states in water vapor and oxygen, that re-radiate photons. In Fig. VIII.1 one sees that there is a region between about 1 GHz and 10 GHz where the background noise is a minimum and is roughly constant. Two absorption lines from Hydrogen and the Hydroxyl radical are shown as sharp spikes just above 1 GHz. This is the region that Drake chose to look for extraterrestrial signals. Many subsequent SETI searches with radio waves have also focused on this frequency region.

Figure VIII.1: Sources of noise in the electromagnetic spectrum vs frequency in GHz. Left band: synchrotron radiation from the galaxy, which falls rapidly with increasing frequency. Right band: quantum receiver noise, which increases linearly with frequency. Superimposed on the right-hand band are states in water vapor and oxygen. These re-radiate photons and produce the broad peaks seen in the figure. The noise spectrum is small and relatively constant in the region between 1 and 10 GHz. In this region, two very narrow peaks corresponding to a hydrogen spectral line at 1.420 GHz and a hydroxyl radical line at 1.662 GHz are shown as sharp spikes on this plot.

Since the presence of water is strongly correlated with life on Earth, and the hydrogen and hydroxyl spectral lines are associated with reaction products of water, and since this region corresponds with a minimum in the radio noise spectrum in our galaxy, most of the searches for intelligent life signals have focused on the frequency region between those two spectral lines.

After an electromagnetic wave is emitted from a point in the galaxy, the observed power will decrease in inverse proportion to the square of the distance from the source. In Figure VIII.2 the flux measured on Earth is plotted vs. the distance of the source in light-years. Since the relationship between observed flux and distance obeys a power law, a plot of the logarithm of the flux vs. the logarithm of the distance will be a diagonal straight line, as shown in Fig. VIII.2. The vertical line marked “SS” shows the typical sensitivity achieved by a full sky search in 1980; the line marked “TS” shows the sensitivity achieved in a targeted search. The left-hand vertical axis shows the number of Sun-like stars within a given distance from Earth.

Figure VIII.2: A plot of the flux observed on Earth (in Watts/m²) vs. the distance of the source from Earth (in light years), and the power of the source. The vertical line marked “SS” represents the typical sensitivity achieved by a full sky search in 1980, while the vertical line marked “TS” is the typical sensitivity achieved by a targeted search.

From Fig. VIII.2 we see that if the power of the emitted message was 10¹³ Watts, then in a full sky search we would have been able to receive a signal from sources within about 30 light years from Earth; from the left-side vertical axis we see that there are roughly 10 Sun-like stars at that distance from Earth (note: the number of Sun-like stars in Fig. VIII.2 is based on what was known in 1977. From part I of this post, many more Sun-like stars have been discovered in the past decade).

During the 1960s, Soviet scientists performed many experiments where they scanned the sky using omnidirectional antennas. They could have picked up radio signals from space if they were sufficiently powerful, but once again they found nothing. Their efforts were summarized by Josef Shklovskii and Carl Sagan in their best-selling 1966 book Intelligent Life in the Universe. For many years, Sagan was the most influential astronomer advocating for SETI searches.

SETI searches using radio telescopes were then taken up by Ohio State University. They were awarded a grant from the National Science Foundation to build the Ohio State University Radio Observatory telescope. In 1977 Jerry Ehman, a volunteer working for OSU, recorded an extremely powerful signal. Spotting the signal on a computer printout, Ehman wrote “Wow!” on the printout. This single signal is the strongest evidence to date of an unusual transmission from space. It has never since been replicated.

Figure VIII.3: The “Wow!” signal observed by Jerry Ehman from the Ohio State University Radio Observatory in 1977. This was a uniquely strong radio signal from outer space. Ehman circled the signal and wrote “Wow!” in the margin of his computer printout. The “Wow!” signal has never again been observed.

In 1971, NASA funded a SETI study that proposed building a radio telescope array containing 1,500 receiver dishes. The effort was dubbed “Project Cyclops.” This was an extremely ambitious proposal that would have cost $10 billion. The Cyclops project was summarized in an influential Scientific American article by Carl Sagan and Frank Drake. Although the Cyclops project was never funded, it did form the basis for several future SETI efforts. In the 1980s, Harvard physicist Paul Horowitz designed a spectrum analyzer specifically to search for SETI transmissions. In 1981, Horowitz used what was then state of the art integrated-circuit digital signal processing techniques to produce a portable spectrum analyzer that could search for correlations between 131,000 narrow-band channels. This device was put into service in searches for SETI transmissions.

In 1985, an improved spectrum analyzer was developed and used for Project META (Megachannel Extra-Terrestrial Assay). The META spectrum analyzer had a capacity of 8.4 million channels with a resolution of 0.05 Hertz. This spectrum analyzer was used with the 26-meter Harvard/Smithsonian radio telescope at Oak Ridge Observatory in Massachusetts. META was partly funded by the Planetary Society, which had been founded by Carl Sagan and collaborators, with a contribution from filmmaker Steven Spielberg. Spielberg’s interest in SETI searches may have arisen from the work on his 1977 film, Close Encounters of the Third Kind. In 1995 yet another spectrum analyzer upgrade produced the BETA project (Billion-Channel Extraterrestrial Assay). The BETA system utilized 21 personal computers that had been equipped with digital signal processing boards. These boards used fast Fourier-transform methods to receive 250 million simultaneous channels with a resolution of 0.5 Hertz per channel.

Both the META and BETA projects were supervised by Paul Horowitz. Horowitz and Sagan reported that the META project had identified 37 anomalous signals that passed all of the tests for potential extraterrestrial signals. In particular, one signal on Sept. 10, 1988 was 750 times the noise rate. However, just like the “Wow!” signal, every one of the 37 candidate events occurred only a single time and was never repeated. Unfortunately, in 1999 the 26-meter radio telescope was severely damaged by high winds, which ended the BETA project.

Volunteer SETI Searches:

Initially, many of the SETI searches were funded by government agencies such as the Department of Energy (DOE), National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). However, enthusiasm for federal funding of SETI searches has waxed and waned over time. In the late 1970s, for example, Senator William Proxmire became a vocal critic of what he considered frivolous spending by science agencies. Proxmire regularly published what he called “golden fleece awards,” a list of what he claimed were wasteful scientific projects. In 1978, Proxmire included the SETI searches in his list of worthless projects. By 1981, Congress had removed federal funding for SETI; however, funding was restored in 1982 after Carl Sagan managed to convince Proxmire that SETI searches were worthwhile.

The next major federal program was to be the NASA Microwave Observing Program, or MOP. The plan was to combine input from the National Radio Astronomy Observatory, the Arecibo Telescope in Puerto Rico (with a radius of 300 meters, Arecibo was then the world’s largest radio telescope), and radio antennas from the Deep Space Network. This was an ambitious program using state-of-the-art digital spectrum analyzers; the program would combine targeted searches of 800 stars with a general survey of the sky. Unfortunately, after just one year of funding Congress cancelled the program.

Figure VIII.4: The Arecibo radio telescope in Puerto Rico. It had a 300-meter dish, and at the time it was the largest single-aperture telescope in the world. It searched for radio signals from outer space, and sent out a message to potential intelligent cultures in the Milky Way. In 2020, two of the cables supporting the receiver platform collapsed, and in Dec. 2020 the 900-ton receiver platform fell into the telescope dish. The left and right-hand images show the Arecibo telescope before and after the fall of the receiver platform.

At this point, SETI searches had to be carried out using funds donated by private individuals, and in addition SETI used volunteer citizens to assist in data-taking efforts. In 1995, the SETI Institute initiated a program that was inspired by the MOP project. The SETI Institute is a non-profit center that coordinates SETI efforts. The Institute carries out SETI research, but it also provides information on space science aimed at educators and students, and it issues regular bulletins on advances in SETI technology. The SETI Institute search was named Project Phoenix. The founding director of the SETI Institute was Dr. Jill Tarter, an astronomer whose research interests were focused on SETI searches.

The initial plan was to develop a specialized array of radio telescopes for SETI searches. The plan was to construct 350 radio dishes each 6.1 meters in diameter. The SETI Institute had an enviable record of fund-raising both from the public at large and from major donors. The proposed group of radio telescopes was named the Allen Telescope Array (ATA) after the principal donor Paul Allen, the co-founder (with Bill Gates) of the Microsoft Corporation. To date, an array of 42 telescopes (ATA-42) was constructed by the SETI Institute in collaboration with the Berkeley SETI Research Center. It was located at the Hat Creek Radio Observatory in northern California. From 2007 until 2011, SETI searches were carried out in a collaboration of the SETI Institute and the University of California at Berkeley. In 2011, this collaboration was terminated. From that time until the present, the SETI Institute has carried out searches using ATA-42.

Figure VIII.5: A photograph of some of the radio dishes in the Allen Telescope Array, located at the Hat Creek Radio Observatory. This facility is dedicated to searches for extraterrestrial intelligence.

Jill Tarter and her colleagues at the SETI Institute had significant success in raising funds from donors in order to carry out their research. Although the support for this project has fluctuated over the years, they have continued to search for signals of intelligent life elsewhere in the galaxy. In addition to their dedicated searches using the Allen Telescope Array, SETI research has also carried on at other radio telescopes. A program called SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations) was introduced in 1979 by the Berkeley SETI Research Center. The SERENDIP program made use of data taken by collaborations at radio telescopes such as the Green Bank Observatory in West Virginia and the Arecibo radio telescope in Puerto Rico. SETI researchers would analyze deep-space data collected for other purposes, and search for anomalous signals.

Breakthrough Listen:

In 2015, Russian entrepreneur Yuri Milner donated $100 million to fund the Breakthrough Listen program. This operation is based at the Berkeley SETI Research Center. This program has forged agreements with the Green Bank Observatory and the Parkes Observatory in Australia. Special hardware was constructed at these observatories to mine the data that was received for evidence of signals of intelligent life.

Another SETI project enlisted volunteers who participated in an effort called SETI@home. Participants download a software program that sifts through observatory data for intelligent-life signals. The program is run on personal computers when they are not active. The results are then reported back to SETI@home servers that are located at Berkeley. Although the project ran for over 20 years, from March 2020 the SETI@home project is currently in hibernation. The dedicated Sun Enterprise servers used to collect data have been sent to salvage. Although some analysis is still undertaken using previously-acquired data, it looks like this program has essentially ended.

Figure VIII.6: Some of the Sun Enterprise servers that were used for the SETI@home project. This was a joint project of the Berkeley SETI Research Center and the SETI Institute. These Sun servers have now been sent to salvage as the SETI@home project is now in hiatus, perhaps permanently.

SETI at Laser Frequencies:

Charles Townes was a physicist who would later win the Nobel Prize for his development of masers (an acronym for microwave amplification by stimulated emission of radiation). These devices generated coherent electromagnetic waves using the principle of stimulated emission of radiation, a process that had been predicted by Einstein. The use of masers led directly to the invention of lasers. In 1961 Townes and Schwartz suggested that intelligent civilizations might use lasers to send signals into space. Lasers tend to emit EM radiation in wavelengths in the region of visible light. Thus, SETI searches have been extended to search for signals in the optical region.

Townes’ suggestion was somewhat controversial, as others speculated that optical laser signals from space might be difficult or impossible to detect. First, although radio signals can be sent in all directions, lasers essentially emit radiation only in a single direction. Thus, observers on Earth could only detect laser signals that were beamed directly at our planet. Lasers typically emit only a single frequency of light and unlike the situation for radio waves, there is no particular reason for SETI searchers to choose a particular frequency. However, formation of an extremely narrow laser beam requires a spread of frequencies, which would make the beam easier to detect. Another issue is that it would require an extraordinarily powerful laser to be detected on Earth; and the beam would have to out-shine the light from the nearest star.

Despite these potential drawbacks, optical SETI searches have been carried out. Paul Horowitz at the Harvard/Smithsonian observatory constructed a laser detector that has been used for conventional astronomy research, and a team of researchers is using the laser detector for SETI searches. And researchers at Berkeley are performing optical surveys using equipment designed for exoplanet searches. A particular star that was the focus of study was called Tabby’s Star (star KIC 8462852 in the Kepler Input Catalog). That star had exhibited some unusual fluctuations in the starlight that it emitted, and some had suggested that the fluctuations might be the result of manipulation by intelligent life. However, detailed data of light curves from Tabby’s Star revealed no properties that suggested its light had been manipulated by an extraterrestrial civilization.

Extraterrestrial Artifacts:

Some scientists have argued that it makes more sense for an advanced civilization to launch space probes, rather than issuing electromagnetic signals. The space probes could either carry messages from the source planet, or they could be used to observe other cultures, for example Earth. Since a space probe will move much slower than the speed of light, a probe would make more sense if launched from a relatively nearby star. If an advanced civilization wanted to observe activity on Earth, a logical place to “park” a space probe would be at one of the Lagrange points for the Earth-Sun system or the Earth-Moon system. A probe located at one of those points would be in a stable or quasi-stable equilibrium and could remain “locked” in place relative to Earth, only requiring comparatively small stabilizing forces.

Therefore, in 1979 Freitas and Valdes conducted searches of the Earth-Moon system by scanning photographs taken at the L₄ and L₅ Lagrange points for the Earth-Moon system (see Fig. III.10 in Part I of this series of posts for the layout of Lagrange points). They found nothing to the limits of their photographs (the 14^th-magnitude in brightness). They later repeated this process with more sensitive photographic images, and extended their search to all five Lagrange points in the Earth-Moon system, and to several Lagrange points in the Earth-Sun system. They found no evidence of any space probes, up to their limits of detection.

Since 2016 a group of UCLA students have been searching for “techno-signatures” in radio signals from outer space, using the Green Bank Telescope. They have focused on likely targets including the Kepler field, TRAPPIST-1 and stars similar to our Sun (the TRAPPIST-1 system is described in detail in Sect. IV in Part II of this post). They are sensitive to transmitters with power similar to the Arecibo radio telescope to a distance of 420 light years from Earth, and power 1000 times the Arecibo telescope to 13,000 light years from Earth.

Another distributed research effort is Project Argus led by the SETI League. This is a network of small radio telescopes built by amateurs. The aim is to eventually achieve real-time coverage of the entire sky. At present there are 143 radio telescopes located in 27 different countries. Their current sensitivity is about 10^-23 Watts/meter²; this is roughly equivalent to the sensitivity of the Ohio State telescope that received the “Wow!” signal in 1977.

The Voyager 1 and Voyager 2 spacecraft were launched from Earth in 1977. Each spacecraft carried a golden record, whose contents were chosen by a NASA committee chaired by Carl Sagan. The record contained 115 images of life on Earth and a variety of sounds, including bird song and whale song. They added spoken words including greetings in 55 different languages. A preface to the record included greetings from President Jimmy Carter: “This is a present from a small, distant world, a token of our sounds, our science, our images, our music, our thoughts and our feelings. We are attempting to survive our time so we may live into yours.” The record included a selection of music, ranging from Bach, Beethoven and Stravinsky to Chuck Berry (Johnny B. Goode). It is rumored that the committee which determined the contents of the record wanted to include “Here Comes the Sun” by the Beatles, but the copyright holder EMI demanded an obscene amount of money in order for the song to be included.

Figure VIII.7: The Voyager Golden Record, a record carried by both the Voyager 1 and Voyager 2 spacecraft that were launched in 1977. The record carried 115 images of life on Earth, a variety of sounds, greeting messages in 55 different languages, an introduction by President Jimmy Carter, and an eclectic collection of music.

Figure VIII.8: The cover of the Voyager Golden Record.

Sending out messages into space, or including records on spacecraft launched into space, is a controversial subject. A number of scientists have cautioned that advanced extraterrestrial civilizations might turn out to be hostile, in which case it could be extremely dangerous to advertise our presence. In a similar vein, many scientists believe that we should not respond to signals from alien life, lest we put ourselves in harm’s way. In response, the International Academy of Astronautics has formed a SETI Permanent Study Group (SPSG). A SETI Post-Detection Science and Technology Taskgroup was formed in 2005 “to act as a Standing Committee to be called on at any time to advertise and consult on questions stemming from the discovery of a putative signal of extraterrestrial intelligent (ETI) origin.”

One of the issues being reviewed by the SPSG group is the possibility that a genuine discovery of a signal of extraterrestrial origin might be covered up by a government. In addition, there have been concerns that the discovery of extraterrestrial life might have profound negative consequences for religions across the globe. In 2015 a Breakthrough Message program announced a competition to design a digital message that could be transmitted to an extraterrestrial civilization. The prize for the best message was $1 million U.S. dollars. But the Breakthrough Message group also pledged “Not to transmit any message until there has been a wide-ranging debate at high levels of science and politics on the risks and rewards of contacting advanced civilizations.”

IX: Results of SETI Searches

As we have shown, SETI searches have been carried out in a number of different areas. The most ambitious searches for signals from intelligent civilizations have been undertaken in the radio spectrum. EM waves at radio frequencies pass rather easily through Earth’s atmosphere, which makes this a very promising area for searches utilizing ground-based radio telescopes. It is also the case that one particular frequency region (between 1 and 10 GHz) has the smallest background noise (as shown in Fig. VIII.1), and so is technically the most desirable region to search for radio messages that originated from extraterrestrial civilizations.

However, a number of searches in other areas have been pursued at the same time. Optical searches using high-powered lasers and detectors in the optical region have also been mounted. There are also searches for signs of intelligent life on exoplanets, particularly those that appear to have favorable conditions for sustaining life. And there are also searches for spacecraft from other planets that might be observing life on Earth.

Researchers have developed a set of criteria that must be satisfied to verify that a signal has arisen from a distant site in our galaxy. The signal must:

Have arisen from a single point in our galaxy;
Be much larger than background noise;
Be sustained (it must last longer than some pre-set criterion);
Be reproducible (it must be detected multiple times);
It must not represent signals generated on Earth;
It must be non-random.

Over the years, the technical complexity of the searches has increased enormously. What were initially searches at one frequency from nearby stars has evolved into very sophisticated searches that utilize banks of radio telescopes acting in a coordinated fashion. The sensitivity of the searches has also increased by orders of magnitude. Unfortunately, none of these searches has definitively identified signals that clearly originate from deep space and apparently carry signals of intelligent life. There are presently no signals that cannot be explained as of terrestrial origin, and none of the candidate signals has been repeated.

What can we conclude from our failure to identify signals from other planets? It has been emphasized that the question of the existence of extraterrestrial intelligence is not falsifiable (in the sense proposed by Karl Popper). An editorial from Nature in 2009 stated, “Regardless of how exhaustively the Galaxy is searched, the null result of radio science doesn’t rule out the existence of alien civilizations. It means only that those civilizations might not be using radio to communicate.” It is even possible that an advanced civilization is transmitting messages by radio, but they may be too distant for us to detect; or they may be transmitted at rates much faster or slower than we can process.

The failure to detect signs of extraterrestrial life may explain why federal funding for SETI searches has oscillated over time. During “optimistic” periods, Congress may be convinced that a search for intelligent life in the Universe is a bold and visionary enterprise with enormous implications should it succeed. Remember that “optimists” analyze the Drake Equation (see Fig. X.3, later in this post) and conclude that there may be millions of intelligent civilizations in our galaxy alone. The “pessimists”, on the other hand, use the same equation and conclude that there may be only a single advanced civilization (our Earth culture) in the entire visible universe. As far as federal funding for SETI searches goes, a single influential Congressperson might be able to convince their colleagues that these searches are worthwhile – or the opposite, a single person in Congress might be able to block federal funding for such searches (at one time Senator William Proxmire was able to block federal funding for a series of projects that he deemed frivolous).

At the moment, the pessimists appear to be carrying the day. Federal funding for SETI searches has essentially dried up. However, some persuasive scientists such as Jill Tarter have managed to convince wealthy donors (such as Steven Spielberg, Yuri Milner and the late Paul Allen) that the possibility of a successful search is worth a personal investment of several million dollars. And they have also convinced a large cadre of individual volunteers to provide space on their personal computers to carry out data analysis.

X: “But Where Is Everybody?” Questions About Extraterrestrial Intelligence

Despite arguments that extraterrestrial intelligence should exist in many areas of the universe, we have no evidence that Earth has been visited by alien civilizations. Furthermore, ambitious monitoring of electromagnetic waves reaching Earth has found no evidence of any extraterrestrial signals. This conundrum – the fact that arguments suggest intelligent life should be fairly common in the Universe and therefore we should have been contacted by extraterrestrial civilizations, but there is no solid evidence of any other advanced civilizations – is called the “Fermi Paradox,” after physicist Enrico Fermi. We will discuss the origin of this paradox with Enrico Fermi, and then list possible explanations for why we find no evidence of these extraterrestrials.

The Fermi Paradox:

A major event in modern searches for extraterrestrial intelligence came from a conversation with physicist Enrico Fermi. Fermi was one of the greatest physicists of the 20^th century. While nearly all contemporary physicists choose either an experimental or theoretical career, Fermi excelled in both areas. He performed seminal work in statistical physics, where he incorporated the Pauli exclusion principle to determine the statistics of particles in an ideal gas. He also noted that in a process called beta decay, where an unstable atomic nucleus emits an electron, energy did not seem to be conserved. Therefore, Fermi hypothesized that an undetected neutral particle was also emitted in beta decay. Fermi called the particle a neutrino; several years later the neutrino was detected in reactor experiments.

Figure X.1: Physicist Enrico Fermi. The failure to find any signs of intelligent civilizations in our galaxy, despite arguments that intelligent life might be abundant in the universe, is called the “Fermi Paradox.”

Fermi was one of the first physicists to use neutrons to probe the atomic nucleus. In some of his experiments, where neutrons were absorbed by heavy atomic nuclei, Fermi detected unusual reaction products. It was believed that his neutron-capture experiments had produced superheavy elements, elements that had more protons than any known elements, and which were unstable. Fermi received the 1938 Nobel Prize for this discovery; he was only 37 when he won the Nobel Prize. Unfortunately, it was later shown that Fermi had not created superheavy elements, but instead had observed nuclear fission. Fission turned out to be the key to producing chain reactions; this principle was used in the Manhattan Project to manufacture atomic bombs. Enrico Fermi was an outstanding mentor. In 1926, at the age of 24, he was named professor of physics at the Sapienza University of Rome. Fermi proceeded to recruit an incredibly talented group of young Italian physicists. Eight of the people who were students or junior fellows in Fermi’s group went on to win Nobel Prizes.

Fermi emigrated from Italy to the U.S. in 1938, after his native country passed laws that discriminated against Jews (Fermi’s wife was Jewish). At the University of Chicago, Fermi led a team that produced the first reactor facility, that used controlled nuclear fission to produce power. Fermi also played a significant role on the Manhattan Project, the World War II scientific effort that developed the atomic bomb. Fermi was named an Associate Director of Los Alamos Laboratory, the secret laboratory that developed the first atomic bombs.

Fermi’s contribution to extraterrestrial intelligence came from a lunch-time conversation in 1950 with three other physicists: Edward Teller; Herbert York and Emil Konopinski (note from TL: Konopinski was a physicist in my department at Indiana University. I knew Emil well, but never discussed this incident with him). While walking to lunch, the four of them were discussing recent reports of possible unidentified flying objects (UFOs), and also the possibility of travel at speeds faster than light. When the four of them were at lunch, Fermi suddenly interjected something like “But where is everybody?” It is not certain what Fermi’s exact words were, but everyone at the table understood that Fermi was talking about extraterrestrial civilizations. While his colleagues were talking amongst themselves, Fermi had been estimating the magnitude of a series of terms that would enable him to estimate the probability that alien civilizations might exist, and had come to the conclusion that the universe should contain many such intelligent civilizations. His question meant “So why don’t we see evidence of those civilizations?”

Fermi was famous for his ability to obtain qualitative estimates of events. He claimed that on any given topic, he could rapidly obtain an estimate of a quantity that was accurate within a factor of two. It might take a year to pin down this quantity to within one percent; but Fermi asserted that in many cases, the exact number did not matter, and the qualitative result would be sufficient. Fermi’s use of this method allowed him to check the results of experiments against his estimate. If the measured result differed greatly from his estimate, then either the experiment was faulty, or Fermi’s estimate contained some major error.

Fermi also tried to teach his colleagues to perform similar qualitative estimates. When he took part in committees to review Ph.D. candidates, Fermi would typically ask a question like “How many barbers are there in the city of Chicago?” He would then go through the logical steps to obtain a good estimate of this number. Questions that seem difficult or impossible to answer, but which can be approximated by making a series of reasonable assumptions, became known as Fermi questions or Fermi problems. For many years, Fermi problems were given as a part of the Science Olympiad.

In any case, Fermi’s estimate that extraterrestrial intelligence might be common in other parts of the Universe helped spur efforts to locate and perhaps communicate with alien civilizations.

Carl Sagan and Frank Drake:

One of the places most active in searches for extraterrestrial intelligence was Cornell University. Carl Sagan was the Director of the Laboratory for Planetary Studies at Cornell. Sagan had an extraordinary ability to explain arcane concepts in physics and astronomy to the general public, and he wrote and produced many best-selling books on science, including Planets, The Demon-Haunted World and Dragons of Eden. But probably Sagan’s most impressive accomplishment was the PBS television series Cosmos. Co-written and narrated by Sagan, Cosmos was the most-watched series in the history of American public television at the time. It has since been viewed by 500 million people in 60 countries. And a book describing the PBS series became a world-wide best-seller.

Figure X.2: Astronomer and Astrobiologist Carl Sagan. Sagan was both an influential astronomer and a skilled communicator who explained astronomical concepts to the general public.

Frank Drake was an astronomer at Cornell University and a colleague of Carl Sagan. In 1961, he produced an equation that would estimate the number of advanced civilizations in our Milky Way galaxy; this is known as the Drake Equation, which is given in Fig. X.3.

Figure X.3: The Drake Equation. The quantity N, the number of advanced civilizations in the Milky Way Galaxy, is given by the product of the seven quantities on the right-hand side.

In the Drake Equation, N is the number of advanced civilizations in the Milky Galaxy. Figure X.3 provides the meaning for each of the seven quantities that enter there. The Drake Equation is a formal equation that essentially repeats the calculations that Fermi did in his head to estimate the number of advanced civilizations in our galaxy. The number of those civilizations is the product of all the quantities on the right-hand side of Fig. X.3.

Part I of this post showed that we now have much more precise estimates of some of the terms in the Drake Equation. We know the number of planets surrounding an average star, and we know that roughly 4% of exoplanets are in a “habitable zone” orbit around a star. The advantage of the Drake Equation is that scientists can compare their estimates of these quantities with those of other researchers. The disadvantage of the Drake Equation is that the final four quantities are essentially completely unknown. Thus, estimates of N from different groups can vary by hundreds of orders of magnitude.

Frank Drake and Carl Sagan organized the first meeting on searches for extraterrestrial intelligence or SETI. The meeting had ten participants, and they estimated that the Milky Galaxy contained somewhere between 1,000 and 100 million advanced civilizations. However, later estimates by Frank Tipler and John Barrow used much less optimistic estimates, and they concluded that the average number of advanced civilizations was much less than one per galaxy. The astronomical community has divided between “optimistic” groups who believe that N is significantly greater than one, and “pessimists” who argue that N is much less than one. In fact, there are astronomers who argue there is a very high probability that, apart from ourselves, there is no other intelligent life in the visible universe.

XI: Possible explanations for the Fermi Paradox

The Wikipedia article on the Fermi Paradox lists many arguments as to why we have not found evidence of intelligent life elsewhere in our galaxy. We will summarize a few of those here, and the remainder can be found in the Wiki article. These arguments are not mutually exclusive; and several of them are inter-related. At present, all these arguments are purely hypothetical.

Other intelligent life in the Universe is rare or non-existent.

Earth may have what is called a “Great Filter”: one or a few processes that exist on Earth, but are extremely rare in other environments, and that prevents intelligence from arising commonly in extraterrestrial habitats. One of these processes is abiogenesis, or the series of steps by which life arises from non-living matter. Figure XI.1 shows some of the steps by which living matter is believed to arise from simple organic compounds. It may be that some of the steps that lead to living organisms are very rare in extraterrestrial environments. If this is the case, intelligent life would seldom occur on other planets. Another way to phrase this is that arguments suggesting that intelligent life is common in the Cosmos assume that Earth is a “typical” planet (this is known as the “mediocrity principle”). If Earth is highly atypical, then development of extraterrestrial life could be very rare.

Figure XI.1: A schematic picture of the steps that are believed to take place in abiogenesis. This is the process by which organic compounds on Earth (or brought to Earth from other environments) eventually evolve to living organisms.

The assumption that biological complexity is extremely uncommon in other environments is known as the “rare Earth hypothesis.” Some of the circumstances involved in this hypothesis include a sufficiently large galactic habitable zone; the role of Jupiter and our Moon in shielding Earth from asteroid impacts; Earth’s magnetosphere and plate tectonics; and the chemistry of the atmosphere and our oceans. The combination of these conditions may be extremely rare on other planets. (As detailed in Part II of our post on The Search for Other Earths, we may have a pretty good idea of how rare Earth may be by mid-century, given current plans for sophisticated new space telescopes probing promising exoplanet candidates in our Milky Way galaxy.)

If there is intelligent life on other planets, these civilizations may not send out signals into the Universe.

Alien intelligent life may not develop advanced technologies. For example, large-brained animals on Earth include primates, whales, dolphins and octopi. But sea-faring creatures are quite unlikely to communicate via electromagnetic signals. Dolphins appear to have developed highly intelligent cultures but show no signs of building radio telescopes. “Optimists” look at Earth and see that one species has developed a culture that has produced technological advances, and is space-faring. They conclude that similar results are likely to occur on other planets with similar features. “Pessimists,” on the other hand, note that only one Earth species out of a million has developed these qualities; furthermore, other intelligent species such as dolphins and octopi have “water world” cultures that show no signs of attempting to communicate with other planets. They thus conclude that human-like species on other planets are probably extremely rare.

Another possibility is that the condition most likely to produce and sustain alien life is a buried ocean; a situation where a water environment exists below the surface of a rocky planet. Such civilizations would be extremely unlikely to communicate by electromagnetic waves. An example of such an environment is Europa, a moon of Jupiter, which has an ocean buried under a thick shell of ice.

Alien civilizations may develop advanced technologies and even send out messages, but only for a short period of time. One hypothesis is that extraterrestrial intelligent life may move rapidly from communication by EM waves to more advanced methods, such as direct communication by “thought to thought” processes. These processes would be essentially impossible for us to detect.

It is also possible that advanced alien civilizations choose to listen but not to communicate. These aliens may consider it too dangerous to announce their presence to others. This would make it extremely difficult for us to detect them.

Intelligent civilizations exist, but they do not survive long enough to send out signals that we can detect.

Intelligent life could evolve elsewhere in the galaxy, but might be destroyed by extinction events before they develop the capability of communicating with other planets. Earth, for example, has experienced a number of major extinction events, including asteroid impacts, major volcanic eruptions or extensive glaciation. And stars can evolve to produce lethal bursts of radiation, wiping out life on planets.
It may be that it is the nature of intelligent societies to destroy themselves. If this is the case, then societies will not exist for sufficiently long periods to send out cosmic messages to announce their existence. Some have argued that technologically ‘advanced’ civilizations result from an urge to dominate one’s environment. This drive to dominate could very likely result in the society destroying itself. In our current world, a number of countries now possess nuclear weapons capable of annihilating much of human life on the planet. Other threats could also imperil humanity: these include climate change moving our environment past a tipping point, or the depletion of resources, or accidentally poisoning ourselves from chemicals in our environment. The issue here is whether technological ‘innovations’ make our Earth more resilient, or more vulnerable.
Another hypothesis assumes that it is the nature of intelligent societies to destroy others. Currently, humans appear to be taking actions that deplete the Earth’s resources such as animals, fish and forests. Some observers believe that an intelligent civilization will necessarily try to destroy extraterrestrial life once it is observed. There are several arguments that an intelligent society will destroy another civilization. One is that an advanced society will assume that other intelligent societies have the same urge to dominate as our own. Many motives would favor these actions, including greed and aggression. Religious motives could also enter into such actions; when Europeans first came into contact with native Americans, they tended to slaughter them on the grounds that God had intended the Europeans to expand and dominate non-Christian societies.

Another possibility is that alien civilizations exist but are “too alien” for us to discover them.

Intelligent extraterrestrial societies may communicate by means that are extremely difficult or impossible for us to detect. Societies that communicate much faster or slower than Earth communications may emit signals that appear to us simply as noise. Or societies may communicate without emitting EM radiation; once again this would be difficult for us to detect.

The messages sent by aliens have too small a range for us to detect. Conversely, messages that we send may have too small a range. For example, in 1974 a message was sent by the Arecibo radio telescope, and directed to the M13 globular cluster (this group of stars was large, relatively close to Earth and in the sky at the time the message was transmitted). The process of sending out messages to intelligent cultures in our galaxy, rather than attempting to receive messages from them, is called “active SETI.” The Arecibo message, which would take 25,000 years to reach the M13 cluster. is shown in Figure XI.2. The message was designed by Frank Drake in collaboration with Carl Sagan.

The Arecibo message contained (in digital form, from top to bottom) the numbers one to ten; the elements that make up the DNA molecule; the chemical groups that comprise the DNA molecule; a picture of the double helix form of DNA; a cartoon that shows a human, together with information about an average human height; a picture of the planets in our Solar System; and a graphic representing the Arecibo radio telescope. However, if alien civilizations were outside the reach of the Arecibo telescope or its successors, we would be unable to communicate with them.

Figure XI.2: The message sent by the Arecibo radio telescope in 1974.

We need more sensitivity with our detection instruments in order to detect messages from alien civilizations. These messages may either originate from too far a distance, or they may be too faint for us to decipher at present. Also, we may need some luck; alien intelligences need to transmit in the frequency range of our instruments.

Aliens are already present on Earth. One possibility is that aliens are able to escape detection. They may be “hidden” from us. Alternatively, they may be here and have been detected, but this information is being concealed from us. Several conspiracy theories claim that the U.S. or world governments have evidence of an alien presence but choose to hide this. This has been the premise of several movies and TV series; one of the most successful was the TV series The X-Files, which ran from 1993 to 2002 on Fox TV. There are detailed theories of alien encounters; one of the most famous claims is that a “flying saucer” (and possibly also alien beings) was captured in Roswell, New Mexico in 1947.

Figure XI.3: The front page of the July 8, 1947 Roswell Daily Record, announcing the “capture” of a “flying saucer” at the Army Air Field in that town.

It has also been claimed that the government has been suppressing information about UFOs from the public. In June 2021, the Dept. of Defense released results from inquiries regarding unexplained aerial phenomena (the government’s term for what are commonly termed unidentified flying objects) over several decades. The report was released because the 2020 federal spending bill that included COVID spending contained a clause that required federal agencies to submit a report on “Unidentified aerial phenomena … including observed airborne objects that have not been identified.”

The DOD report lists a number of sightings that have not been adequately explained. Of course, conspiracy theorists jumped to the conclusion that this government release validates their hypotheses that we have been contacted by alien civilizations, and that the government has previously suppressed this information. There is no definitive evidence that UFO sightings are actually extraterrestrial spacecraft. But this does not stop conspiracy theorists from making hypotheses about the Roswell incident, or the purpose of the Area 51 government site in Nevada, or many other unfounded assertions. For example, a 2019 Gallup Poll found that one-third of adults in the U.S. agreed with the statement “Some UFOs have been alien spacecraft visiting Earth from other planets or galaxies.”

Figure XI.4: One example of the sighting of a strange moving object that has not yet received a satisfactory explanation. The object was observed in 2015 by a naval aviator. It apparently has a disc shape, and it was traveling against the wind.

Summary of SETI Searches

The search for extraterrestrial life in the Universe is a major sub-field of astronomy. One of the factors motivating those searches is our increasing awareness that forms of life on Earth exist in very extreme conditions. The first discovery was that there are abundant forms of life that cluster around hydrothermal vents in the deepest trenches of our oceans. In addition, scientists have discovered an astonishing amount of biological diversity in mines deep below Earth’s surface. In many cases, life has evolved in conditions very different from those at Earth’s surface. These discoveries on our own planet have led scientists to extend the requirements for the development of life.

It appears quite possible that we will discover current or former life forms on other planets or asteroids in our own Solar System. We now have had several rovers land on Mars; the latest two that landed in 2021 (the American rover Perseverance and the Chinese rover Zhurong) are still performing analyses of surface and sub-surface samples to determine whether microbial life exists on Mars, or whether it did in the past.

It would be surprising if various probes or rovers did not discover some evidence of past or present organic life elsewhere in the Solar System. However, intelligent life is another matter altogether. Evolutionary biology shows us that the development of intelligent life on Earth required a series of steps. Stephen Jay Gould stated that “The problem with the history of life is that we only have one experiment … Without [a second independent experiment in the origin of life], it’s all speculation.” Gould goes on to remind us that in the 4.5 billion-year history of the Earth, only one species (out of a total of approximately 200 species of primates, 4,000 species of mammals and about 1 million species of life forms) developed intelligence as we understand the term. Also, Gould points out that completely random processes such as mass extinctions played an important role in the emergence and domination of mammals on Earth. SETI searches deal with a major unknown quantity – we have no idea how common or rare intelligent cultures are in the Universe. We are making incredible progress in some of the quantities that arise in the Drake Equation, but there are other quantities in that formula whose value is completely unknown.

Despite current failures to find irrefutable evidence of alien civilizations, we view the SETI searches as inspiring. They are quite literally sampling input from the Cosmos in the hope of catching a glimpse of information from afar. We do not know the odds of finding anything –in particular, we don’t know the form that information might take. However, SETI searches have become more and more technically sophisticated. The first SETI search by Frank Drake utilized a single-channel analyzer at one single frequency. Now we have arrays of dedicated telescopes sharing information with each other via extremely complex multi-channel analyzers. In the future, we can imagine detectors aimed more narrowly at particular exoplanets whose atmospheres are found to contain chemical signatures indicative of life.

Roughly a decade ago, federal funding for SETI searches dried up. The lack of any probable signals from deep space, with no hints that further searches would yield positive signals, led government agencies to withdraw support for such ventures. However, that situation is rapidly changing. As documented in the earlier sections of this post, searches for planets orbiting other stars in our galaxy have been extraordinarily successful. We have now discovered over 5,000 exoplanets, and a number of space telescopes will greatly expand our ability to discover exoplanets (see Fig. V.1 for a list of current and future space telescopes from both the American and European space agencies). A number of SETI searchers have joined on to exoplanet research teams; thus we expect that SETI research teams will soon have access to the latest data from space telescopes to resume their searches. And SETI researchers will be able to direct their attention to exoplanets that exist in a “habitable zone” of their star.

We have yet to identify a single unmistakable and reproducible signal from intelligent life. Furthermore, we have sent out space probes that contain messages that might be received and analyzed by extraterrestrial civilizations elsewhere in our galaxy. If we were to obtain such information, it would be truly revolutionary. It would forever change our understanding of the world around us; at the same time, it would pose a major challenge to many of our major religions. Making contact with alien civilizations might also have very negative implications. But whether or not such contact leads to a favorable outcome, one would have to respond to contact with intelligent life outside our planet by repeating Jerry Ehman’s reaction to his 1977 radio signal – “Wow!”