The search for extraterrestrial intelligence inherently assumes that any civilization that we make contact with will have gained a certain level of intelligence. Intelligence can have many different connotations, and in the context of SETI it has a specific meaning: the civilization is able to send out signs or signals that indicate it is an advanced civilization. A good way to start thinking about this is by asking when the Earth became an advanced intelligent civilization that sent out signals indicative of technology. We will start there and see how SETI searches have evolved from our planet’s first radio transmissions to modern surveys that scan the skies over broad bands to look for that cosmic needle in the haystack.

The Beginnings of Radio Astronomy

Huge strides in understanding the physical nature of electromagnetic radiation in the mid- to late-1800s led to the first electronic search for radio signals in 1899. Nikola Tesla was born in Croatia and emigrated to the United States in 1884. In 1899, while working on a project that could enable a global wireless communication network (indeed, Tesla was way ahead of the times in realizing that wi-fi networks were quite feasible), Tesla detected a repeating signal that he eventually attributed to Mars:

“Twenty-two years ago, while experimenting in Colorado with a wireless power plant, I obtained extraordinary experimental evidence of the existence of life on Mars. I had perfected a wireless receiver of extraordinary sensitiveness, far beyond anything known, and I caught signals which I interpreted as meaning 1–2–3–4. I believe the Martians used numbers for communication because numbers are universal.”  — Nikola Tesla, 1923, Albany Telegram.

Another pioneer of radio communication and a contemporary of Tesla, Guglielmo Marconi received the first trans-Atlantic radio transmission in 1901. Marconi detected an anomalous radio signal in 1921 and believed that it was a Martian response to signals that Marconi had sent out earlier. Today, we know that neither Tesla nor Marconi’s detections were of a Martian origin, but at that time Percival Lowell’s sketches of Mars and claims of inhabitants, as well as H.G. Wells’ publication of The War of the Worlds, were still fairly recent (the New York Times published a story on December 9, 1906 with the headline “There is life on the planet Mars: Prof. Percival Lowell recognized as the greatest authority on the subject, declares there can be no doubt that living beings inhabit our neighbor world.”).

Early SETI Searches

The year 1959 can be considered a turning point in the history of SETI, as that is when an article appeared in the journal Nature by Giuseppe Cocconi and Philip Morrison “Searching for Interstellar Communications.” In this paper, the authors suggested searching around the radio frequency of 1420 MHz, as this is the frequency associated with neutral hydrogen (the most common element in the universe), specifically the amount of energy emitted by the atom when the electron changes its spin state (this is called the hydrogen “spin flip”). The thinking was that other civilizations in an “early stage of the development of radio astronomy” would also recognize this frequency. Cocconi and Morrison end the paper with this memorable quote regarding searches for interstellar signs of communication:

“The probability of success is difficult to estimate; but if we never search, the chance of success is zero.”

SETI searches formally began in 1960, when Frank Drake used the 85-foot Green Bank radio telescope to observe the nearby stars Tau Ceti and Epsilon Eridani in a project that he dubbed “Project Ozma.” To minimize confusion from background sources, the radio search concentrated on a very narrow frequency band near 1420 MHz. Drake observed the stars for four months, always with a negative result until he found a surprising signal on April 8, 1960. An extraordinarily careful scientist, Drake followed up and ultimately discovered that this was a false signal from a high-flying aircraft. But, the realization of what it would feel like to detect another civilization started him on a lifelong search for technological signatures (coined “technosignatures” by Jill Tarter). At this early stage, these searches were referred to as CETI, for Communication with ExtraTerrestrial Intelligence.

The early 1960s also marks the formal beginning of optical SETI searches. In 1961, a paper was published in Nature that proposed searching for signals from interstellar and interplanetary communication by looking for laser pulses. One of the authors of this paper, Charles Townes, had recently invented the maser, which then led to lasers (Townes and a colleague patented the laser in 1960). While there are legitimate reasons for conducting optical searches — for example, signal processing in the optical regime is far simpler and requires less computing power than in the radio, information can be transmitted more efficiently, and there is virtually no optical interference to deal with — the power needed for laser pulse transmissions made this idea seem technically unfeasible. However, thanks to the natural doubling in technology every 2 years (Moore’s Law), lasers caught up and several optical SETI programs were up and running by 2000, notably at Harvard and the University of California at Berkeley.

Other ways of searching for extraterrestrial intelligence were circulating and being published. In 1960, physicist Ronald Bracewell published a paper in Nature, “Communications From Superior Galactic Communities,” in which he suggested we look for probes in our solar system sent by other advanced civilizations. Bracewell considered a galaxy-wide network of advanced civilizations who cooperate and send these probes out to stars nearest to their own. Also in 1960, Freeman Dyson published an article in Science titled “Search for Artificial Stellar Sources of Infrared Radiation,” which proposed that a very advanced civilization could build a giant structure surrounding its planetary system. These mega-structures would be detectable via their waste heat in the infrared.

Searches were also underway in the U.S.S.R. at the same time. Nikolai Kardashev proposed a scheme for classifying intelligent civilizations based on how much energy they could use (the Kardashev Scale) and published results from a study of two quasars in 1964. One of the sources that Kardashev speculated was possibly from an extraterrestrial civilization, CTA-102, caught the attention of the U.S. rock band The Byrds and they recorded the song CTA-102 in 1967, reflecting the interest from the general public in SETI. A book co-written by Soviet astronomer Iosef Shklovskii and American astronomer Carl Sagan called “Intelligent Life in the Universe” was published in 1966. It is a remarkable collaboration considering that this was right in the middle of the Cold War. SETI collaborations between American and Soviet scientists continued, and an international conference was held at the Byurakan Astronomical Observatory in Soviet Armenia in 1971. At the 1971 Byurakan conference, CETI was renamed SETI, since no communication had actually been detected yet.

More dedicated SETI searches were conducted in the 1970s using various radio telescopes, including the Arecibo dish in Puerto Rico and Ohio State’s Big Ear facility. On August 15, 1977, an astronomer observing at Big Ear picked up a very intense signal right around the hydrogen spin-flip frequency at 1420 MHz. When an astronomer working as a volunteer at Big Ear saw the print-out showing the signal, he wrote the word Wow! in red ink to convey his excitement (Figure 1). The “Wow! signal” has never repeated and the same region is still searched from time to time. This is a similar detection to Frank Drake’s “false alarm” in 1960, which turned out to be an aircraft, but the signal remains a subject of minor fascination to SETI scientists. In 1971, scientists Bernard Oliver and John Billingham, through their work for Project Cyclops, proposed extending radio SETI searches from 1420 MHz out to around 1700 MHz, to include radio emissions from the hydroxyl group (-OH); they eventually coined the term “water hole” for this range of frequencies. In 1979, on the 20th anniversary of the Cocconi and Morrison paper, Jill Tarter published the results of her radio search of 201 nearby stars using the 91-meter radio antenna at the National Radio Astronomy Observatory.

Want to know more: The Kardashev Scale

If we do detect communication signals from a technologically advanced extraterrestrial civilization, what should we expect? This is a vague question with really anything you can imagine being a valid answer, as we have never encountered any life outside of Earth. However, Nikolai Kardashev thought about this seriously and came up with a classification system for extraterrestrial civilizations based upon how much energy they are able to harness from their surroundings. Energy consumption can be thought of as a proxy for how technologically advanced a civilization is. He published this in 1964 in the journal Soviet Astronomy in a paper titled “Transmission of Information by Extraterrestrial Civilizations” and suggested three civilization types:

Type I civilizations use all of the energy available from their planet. Kardashev initially wrote that a Type I civilization would be at a technological level close to Earth’s level in 1964. This would include energy from natural phenomena such as weather. Residents of planet Earth are not yet at Type I.

Type II civilizations use all of the energy available from their star. One example of a technosignature from a Type II civilization is a Dyson Sphere.

Type III civilizations use all of the energy available from their galaxy. How to imagine this again brings us back to speculation, but we could think about a civilization harnessing massive amounts of energy from a supermassive black hole at the center of a galaxy.

 

More recent modifications to the Kardashev Scale also encompass Type 0 to Type 4, which includes biological civilizations (Type 0) and civilizations that can use all of the energy available in the universe (Type 4).

In 1980, The Planetary Society was founded — Carl Sagan was one of the original co-founders and today the Society is led by the American science communicator Bill Nye — to promote SETI to the public and to provide a means for non-governmental funding for SETI projects. By 1981, The Planetary Society had funded “Suitcase SETI,” a portable observing kit that was used at Arecibo and eventually morphed into the META SETI projects. The SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations) searches began at the University of California Berkeley in the early 1980s and took advantage of commensal observations, meaning that several projects could run at the same time. The non-profit SETI Institute was founded around this time, in 1984, with Jill Tarter conducting its first SETI search.

Modern SETI

The 1990s were a challenging time for SETI, as government funding for SETI was axed in 1993. Given the fascination with aliens in Hollywood science fiction movies, one would think that the public and the government would consider SETI to be an exciting and worthwhile area of research. This is a research field that costs little but has faced active opposition. The federally funded SETI program was started in 1992, and just a year later, Senator Richard Bryan of Nevada argued that funding should be shut down: “…As of today, millions have been spent and we have yet to bag a single little green fellow. Not a single Martian has said ‘take me to your leader,’ and not a single flying saucer has applied for FAA approval.”

Scientists have been funded to search for elusive dark matter particles for more than 40 years, but not a single one has yet been found.  Searches for black holes went on for decades before the first candidates were found. The hunt for exoplanets dates back a few centuries, but the first exoplanet was not detected around a Sun-like star until 1995. Transformational science requires that we push the boundaries.

Fortunately, SETI pursuits continued on in the wake of the federal funding being cancelled. UC Berkeley’s SERENDIP programs continued into the 2010s and the SETI@home project came online in 1999. After more than a decade of planning, the Allen Telescope Array (ATA) came online in 2007 at the Hat Creek Radio Observatory in northern California, and this was the first telescope built from the ground up to perform SETI searches. Major funding for the ATA came through a donation from Paul Allen, the co-founder of Microsoft.

Breakthrough Listen

A major milestone for SETI came in 2015 with the launch of the Breakthrough Listen initiative. Like the SETI Institute, Breakthrough Listen is supported by both scientists and other philanthropists. Funding was provided by Yuri Milner, and physicist Stephen Hawking was involved in the initial launch. Breakthrough Listen is being funding $10 million a year for 10 years. The goals of Breakthrough Listen are to search for technological signatures of intelligent life in the universe and answer the question “Are we alone?”. The science goals are to observe 1 million nearby stars and 100 galaxies using radio telescopes. Breakthrough Listen is also surveying the Milky Way’s dense disk and central region in both the radio and optical parts of the electromagnetic spectrum (the central region includes the Milky Way’s super-massive black hole, which a Type III civilization could potentially be pulling energy from). All of the data collected is immediately made available to anyone interested in using it. This is the largest dedicated SETI search to date and is currently centered at the University of California at Berkeley and led by Andrew Siemion.

Breakthrough Listen is a global collaboration using the largest and most sensitive radio telescopes in the world, including: the 64-m Parkes Telescope (Murriyang) in Australia; the MeerKAT array in South Africa, which has sixty-four 13.5-m dishes that can be used in tandem; the 100-m Green Bank Telescope in West Virginia; and the 500-m FAST Telescope in China. Some of the telescopes used by Breakthrough Listen, such as the Green Bank and Parkes, are steerable, meaning that they can point directly at any object of interest that may arise at some point. Observations for Breakthrough Listen began right away in 2016 using the Parkes and Green Bank telescopes.

Optical SETI (OSETI) searches are being conducted by Breakthrough Listen through a partnership with the VERITAS telescope array in Arizona. VERITAS is looking for high-energy gamma ray pulses that arrive as very short bursts, and this strategy can be co-opted to search for optical SETI signals. This is aligned with OSETI’s strategy of looking for fast (on the order of nanoseconds) laser pulses that were sent as beacons from another technologically advanced civilization. Breakthrough Listen is also doing a deep search for laser beacons by analyzing spectra of stars taken by the Automated Planet Finder telescope at Lick Observatory. These signals would appear in a star’s spectrum as a very strong emission line.

In addition to stars and galaxies, Breakthrough Listen is also looking at other objects of interest to SETI. For example, an object, now known as Oumuamua, passed close to Earth in October 2017 and was detected by the Pan-STARRS mission. Oumuamua immediately garnered attention from the astronomical community as its motion suggested that it did not come from our solar system but was an “interstellar” visitor. What kind of an interstellar visitor, exactly? Natural origins, such as an asteroid or comet, were considered, but it was also suggested that Oumuamua was a piece of alien technology sent our way for communications. Breakthrough Listen used the Green Bank Telescope to observe Oumuamua in December 2017 and found that there were no radio transmissions emanating from the object, evidence against the alien artifact theory.

Earth’s Technosignature: Voyager Spacecraft

How can we have confidence that any search strategy will succeed if we haven’t yet detected any signs of extraterrestrial intelligent life? One proof-of-concept for our SETI radio searches is to detect one of our own artifacts. The Voyager I spacecraft was launched in 1977 and is currently coasting through the Kuiper Belt out beyond 160 AU. In 2020, as part of Breakthrough Listen, the Green Bank Telescope pointed at the Voyager I spacecraft and recorded data for 5 minutes. Figure 5 shows the signal as recovered in the GBT data. The receiver at the GBT picked up Voyager’s carrier signal which broadcasts at a frequency of 8415 MHz.

Many other technosignatures from Earth are regularly picked up, such as the rovers on Mars or the New Horizons mission that is exploring the Kuiper Belt.

Messages from Earth

The discussion of SETI above has focused on searches that humans have carried out to look for signs of extraterrestrial intelligence. We can also ask what another civilization would find if they pointed their detectors and receivers at Earth. This is in many ways a speculative question as we must assume that another civilization will search in ways similar to our own, but it is still a valuable exercise to think about this in a broader context and consider how our technology — and our species — are evolving.

Unintentional

Earth has been a technologically advanced civilization for a little over a century. Radio and TV broadcasts have been traveling away from the Earth since the 1920s and 1930s. We can think of this type of transmission as part of Earth’s radio leakage, or everyday radio transmissions that someone or something “eavesdropping” on Earth would detect. Other types of radio leakage from Earth include military radar, and, more modernly, cell phones. You can see exactly how radio frequencies in the U.S. have been allocated by the NTIA (National Telecommunications and Information Administration) here — the takeaway is that we are sending out an immense amount of radio waves!

Intentional

Arecibo Radio Message

After the Green Bank Conference in 1961, Frank Drake began seriously thinking about what information to encode into a radio message sent out from Earth. Drake decided on a grid where each square was either filled or empty, meaning set to a binary value of either ‘0’ or ‘1’. This grid could be filled in to convey information (as much as can be done in this simple way) and then transmitted out toward another potentially technologically advanced civilization. Despite the fact that only one of his colleagues from the Green Bank Conference could decode a test message he put together (that person was Bernard Oliver), Drake got some input from others, including Carl Sagan, and in 1974 there was a fortuitous opportunity to actually send the message.

The Arecibo Observatory in Puerto Rico opened in 1963 and the radio dish underwent a major upgrade in 1974. To help publicize the newly gained power of the Arecibo dish, Drake and Sagan decided to transmit a radio message in the direction of the globular cluster M13 using the refurbished dish. The message was a grid with 23 columns and 73 rows, leading to a total of 1679 binary digits (Figure 6). These numbers were deliberately selected because each is a prime number and it is thought that perhaps math is a universal language and other civilizations might recognize these as special numbers. To transmit the message, two frequencies very close to each other were used, with one of the frequencies representing zeroes and the second representing ones. It took a little under 3 minutes to send out this message using 3 TW (3 trillion Watts) of power. No response is expected and the event was really intended to show just how much power the transmitter at Arecibo could send out.

Artifacts

The 1970s were a prolific decade for exploring our solar system. The Pioneer and Voyager missions were both successfully launched to study the outer planets in our solar system. Pioneer 10 left the Earth in 1972 and Pioneer 11 in 1973. Pioneer 10 and 11 each had an identical metal plaque attached to it. The plaque was designed by Carl Sagan and meant to convey some information about Earth and show some symbols of our civilization. A sketch showing the spin-flip frequency of hydrogen was included, as this was the dominant frequency that humans used to search for other civilizations.

Voyager 1 and Voyager 2 were launched in 1977 and both included a relic of our humanity. This time, instead of a plaque, the object was a phonograph record. It may seem outdated now to send out a record, but this was the dominant musical medium at that time; the first commercial compact disc (CD) wasn’t pressed until 1982. (If you are curious, the first CD was released in Japan and was American rock artist Bruce Springsteen’s album “Born in the USA.”) The records are made of gold, vinyl being deemed too flimsy to withstand the interstellar journey. The records also contain a stylus — the part of the arm on a record player that holds the needle — and instructions on how to play the record are etched onto the aluminum cover (Figure 7).

The information on the records describes our biochemical make-up, including and the nucleotide bases in our DNA, and provides iconic cultural images, circa 1977. In addition to greetings in more than 50 languages, the Voyagers are carrying 90 minutes of music, including the haunting “Dark was the Night, Cold was the Ground” by Blind Willie Johnson and Chuck Berry’s “Johnny B. Goode.”  A 1977 Saturday Night Live skit portrays extraterrestrials finding the recording and sending back the message: “Send more Chuck Berry.”

The Voyager spacecraft are now destined to orbit the center of the galaxy, like all of the stars in the Milky Way. They may well live outlive our Sun and our solar system. Perhaps in the very distant future, another civilization will intercept these spacecraft. If they decode our message, they will know that once upon a time in the galaxy, there was another civilization with the generosity to send a message: “You are not alone. Here is the code for carbon-based life that lived on our planet. Even if our species is never able to learn if we are alone in the vast universe, we hope that this message will give you some answers.”

What to do if a signal is detected?

Extraordinary claims require extraordinary evidence! The first step is to confirm the detection using the most sensitive detector available, and the next is to be sure that the signal is not a false positive from an astrophysical source or human interference. This is explored more deeply in the next chapter on Technosignatures and Biosignatures.

Review Questions

Summary Questions

1. Why did the first SETI searches focus on radio waves (as do the majority of searches today)?
2. Why is the frequency of 1420 MHz often searched around?
3. How do optical SETI searches differ from radio SETI searches?
4. How were SETI searches funded after the U.S. government cut off federal funding in 1993?
5. What are some of the strategies that modern SETI searches use?
6. Why is the Voyager I spacecraft considered a technosignature from Earth?
7. What are some artifacts that the Earth has sent out?
8. What information was contained in the Arecibo Message and why was it sent out?

Exercises

1. You can look through the Breakthrough Listen data and find the Voyager 1 signal! The Berkeley SETI Research Center provides a tutorial and link to the data at https://seti.berkeley.edu/listen/data.html.
2. Open the link to the United States Frequency Allocation Chart. Look at the region around the “water hole” from about 1420 MHz to 1670 MHz (1.42 GHz to 1.67 GHz). What other types of transmissions are being broadcast around the water hole? Do you think this is a concern if other civilizations are listening to Earth?
3. Can you decode the Arecibo Message? The top 4 rows are a primer, showing the symbols for the numbers 1 through 10.
4. The Earth is estimated to be 0.72 on the Kardashev scale. What do you think it would take to bring Earth to 1.0?
5. Design your own “Arecibo message” using binary digits (0s and 1s). If you are including words, do you think another civilization will understand them?
6. Design you own artifact to send out. Describe what you would include and where you would send it.