2 Radio SETI experiments

In 1960, Cornell University astronomer Frank Drake performed the first modern SETI experiment, named " Project Ozma", after the Queen of Oz in L. Frank Baum's fantasy books. Drake used a 25-meter-diameter radio telescope at Green Bank, West Virginia, to examine the stars Tau Ceti and Epsilon Eridani near the 1.420 gigahertz marker frequency. A 400 kilohertz band was scanned around the marker frequency, using a single-channel receiver with a bandwidth of 100 hertz. The information was stored on tape for off-line analysis. Nothing of great interest was found.

The first SETI conference took place at Green Bank in 1961. The Soviets took a strong interest in SETI during the 1960s, and performed a number of searches with omnidirectional antennas in hopes of picking up powerful radio signals beginning in 1964. In 1966, the legendary American astronomer Dr. Carl Sagan and Soviet astronomer Iosif S. Shkolovskii published in collaboration the pioneering book in the field, Intelligent Life in the Universe.

In 1971, the US National Aeronautics and Space Administration (NASA) funded a SETI study that involved Drake, Bernard Oliver of Hewlett-Packard Corporation, and others. The report that resulted proposed the construction of an Earth-based radio telescope array with 1,500 dishes, known as " Project Cyclops ". The price tag for the Cyclops array was $10 billion USD, and unsurprisingly Cyclops was not built.

In 1974, a largely symbolic attempt was made to send a message to other worlds. To celebrate a substantial upgrading of the 305 metre Arecibo Radio Telescope in Puerto Rico, a coded message of 1,679 bits was transmitted towards the Globular Cluster M13, some 25,000 light years away.

The pattern of 0s and 1s contained in the message defined a 23 × 73 grid which when plotted revealed some data about our location in the Solar System, a stylised figure of a human being, chemical formulae and an outline of the radio telescope itself.

The 23 by 73 grid was chosen because both 23 and 73 are prime numbers and it was thought that this could aid any hypothetical alien listener to recognize the grid representation.

Given the limitations of the speed of light, no reply would be possible for 50,000 years and hence has been dismissed by some as a publicity stunt. A controversy arose because the transmission raised the serious question of whether a small group should be allowed to speak for Earth.

In 1979 the University of California, Berkeley launched a SETI project named " Search for Extraterrestrial Radio from Nearby Developed Populations (SERENDIP)". In 1980, Sagan, Bruce Murray , and Louis Friedman founded the US Planetary Society, partly as a vehicle for SETI studies.

In the early 1980s, Harvard University physicist Paul Horowitz took the next step and proposed the design of a spectrum analyzer specifically intended to search for SETI transmissions. Traditional desktop spectrum analyzers were of little usefulness for this job, as they sampled frequencies using banks of analog filters and so were restricted in the number of channels they could acquire. However, modern integrated-circuit digital signal processing (DSP) technology could be used to build "autocorrelation" receivers to check far more channels.

This work led in 1981 to a portable spectrum analyzer named "Suitcase SETI" that had a capacity of 131,000 narrowband channels. After field tests that lasted into 1982, Suitcase SETI was put into use in 1983 with the 25-meter Harvard/Smithsonian radio telescope at Harvard, Massachusetts. This project was named "Sentinel", and continued into 1985.

Even 131,000 channels weren't enough to search the sky in detail at any fast rate, and so Suitcase SETI was followed in 1985 by Project "META", for "Megachannel Extra-Terrestrial Array". The META spectrum analyzer had a capacity of 8 million channels and a channel resolution of 0.5 hertz.

The project was led by Horowitz with the help of the Planetary Society, and was partly funded by moviemaker Steven Spielberg. A second such effort, META II, was begun in Argentina in 1990 to search the southern sky. META II is still in operation, after an equipment upgrade in 1996.

Also in 1985, Ohio State University began their own SETI program, named Project "Big Ear", which later received Planetary Society funding. The next year, in 1986, UC Berkeley initiated their second SETI effort, SERENDIP II, and has continued with two more SERENDIP efforts to the present day.

In 1992, the US government finally funded an operational SETI program, in the form of the NASA "Microwave Observing Program (MOP)". MOP was planned as a long-term effort, performing a "Targeted Search" of 800 specific nearby stars, along with a general "Sky Survey" to scan the sky.

MOP was to be performed by radio dishes associated with the NASA Deep Space Network, as well as a 43-meter dish at Green Bank and the big Arecibo dish. The signals were to be analyzed by spectrum analyzers, each with a capacity of 15 million channels. These spectrum analyzers could be ganged to obtain greater capacity. Those used in the Targeted Search had a bandwidth of 1 hertz per channel, while those used in the Sky Survey had a bandwidth of 30 hertz per channel.

MOP drew the attention of the US Congress, where the program was strongly ridiculed, and was cancelled a year after its start. SETI advocates did not give up, and in 1995 the nonprofit "SETI Institute" of Mountain View, California, resurrected the work under the name of Project "Phoenix", backed by private sources of funding.

Project Phoenix, under the direction of Dr. Jill Tarter, previously of NASA, is a continuation of the Targeted Search program, studying 1,000 nearby Sunlike stars, and uses the 64-meter Parkes radio telescope in Australia.

Backers believe that if there is any alien civilization among those thousand stars broadcasting toward us with a powerful transmitter, the search should be able to detect it.

The Planetary Society is now pursuing a follow-on to the META project named "BETA", for "Billion-Channel Extraterrestrial Array". This is a dedicated DSP box with 200 processors and 3 gigabytes of RAM. BETA is about a trillion times more powerful than the signal processing equipment used in Project Ozma.

BETA actually only scans 250 million channels, with a range of 0.5 hertz per channel. It scans through the microwave range from 1.400 to 1.720 gigahertz in eight hops, with two seconds of observation in each hop.

The SETI Institute is now collaborating with the Radio Astronomy Laboratory at UC Berkeley to develop a specialized radio telescope array for SETI studies, something like a mini-Cyclops array. The new array concept is named the "Allen Telescope Array (ATA)" (formerly, One Hectare Telescope [1HT]). It will cover 100 meters on a side.

The array will consist of 350 or more Gregorian radio dishes, each six point one (6.1) meters in diameter. These dishes will essentially be commercially available satellite television dishes. The ATA is expected to be completed by 2005 at a very modest cost of $25 million USD. The SETI Institute will provide money for building the ATA while UC Berkeley will design the telescope and provide operational funding.

Berkeley astronomers will use the ATA to pursue other deep space radio observations. The ATA is intended to support a large number of simultaneous observations through a technique known as "multibeaming", in which DSP technology is used to sort out signals from the multiple dishes. The DSP system planned for the ATA is extremely ambitious.

Another interesting UC Berkeley effort called SETI@home began in May 1999. The existence of the SETI@home project means that anyone can get involved with SETI research by simply downloading screen saver software over the Internet. The software performs signal analysis on a downloaded 350 kilobyte "work unit" of SERENDIP IV SETI radio survey data, and then reports the results back over the Internet.

Over 5 million computer users in hundreds of countries have signed up for SETI@home and have collectively contributed with over 14 billion hours of computer processing time. The project is widely praised in the computer press as an interesting exercise in home-grown distributed computing. As of June 22 2004 a follow-on SETI@home II based on the Berkeley Open Infrastructure for Network Computing (BOINC) was released.

3 Optical SETI experiments

While most SETI sky searches have studied the radio spectrum, some SETI researchers have considered the possibility that alien civilizations might be using powerful lasers for interstellar communications at optical wavelengths. The idea was first suggested in a paper published in the British journal Nature in 1961, and in 1983 Charles Townes, one of the inventors of the laser, published a detailed study of the idea in the US journal Proceedings of the National Academy of Sciences.

Most SETI researchers were cool to the idea. The 1971 Cyclops study discounted the possibility of optical SETI, reasoning that construction of a laser system that could outshine the bright central sun of a remote star system would be too difficult. Now some SETI advocates, such as Frank Drake, have suggested that such a judgement was too conservative.

There are two problems with optical SETI, one of which is easy to deal with, the second of which is troublesome. The first problem is that lasers are highly "monochromatic", that is, they only emit light on one frequency, making it troublesome to figure out what frequency to look for. However, according to Fourier analysis, emitting light in narrow pulses results in a broad spectrum of emission, with the frequencies becoming higher as the pulse width becomes narrower, and an interstellar communications system could use pulsed lasers.

The other problem is that while radio transmissions can be broadcast in all directions, lasers are highly directional. This means that a laser beam could be easily blocked by clouds of interstellar dust, and more to the point, we could only pick it up if we happened to cross its line of fire. As it is unlikely an alien civilization would focus an interstellar laser communications beam on Earth deliberately, we would have to cross such a beam by accident.

However, as discussed earlier, the power requirements for omnidirectional interstellar radio broadcasts are tremendous, and narrow-beam radio communications are technically more plausible. As SETI researchers have adjusted to the idea that interstellar radio communications may be over narrow beams, the idea of hunting for interstellar laser beams has become no more troublesome.

In the 1980s, two Soviet researchers conducted a short optical SETI search, but turned up nothing. During much of the 1990s, the optical SETI cause was kept alive through searches by Stuart Kingsley , a British dedicated amateur living in the US state of Ohio.

Now the SETI old-timers have warmed to the concept of optical SETI. Paul Horowitz of Harvard and researchers with the SETI institute have conducted simple optical SETI searches using a telescope and a photon pulse detection system, and are considering further searches. Horowitz says: "Everyone's been mesmerized by radio, but we've done that experiment a lot and we're a little tired of it."

Optical SETI enthusiasts have conducted paper studies of the effectiveness of using contemporary high-energy lasers and a ten-meter focus mirror as an interstellar beacon. The analysis shows that an infrared pulse from a laser, whose light output is not bound by the inverse-square law of light emitted from a hot body like the Sun, would appear thousands of times brighter than the Sun to a distant civilization in the beam's line of fire. The Cyclops study proved incorrect in suggesting a laser beam would be inherently hard to see.

Such a system could be made to automatically steer itself through a target list, sending a pulse to each target at a rate, say, of once a second. This would allow targeting of all Sun-like stars within a distance of 100 light-years. The studies have also described an automatic laser pulse detector system with a low-cost, two-meter mirror made of carbon composite materials, focusing on an array of light detectors. This automatic detector system could perform sky surveys to detect laser flashes from civilizations attempting to contact us.

Several optical SETI experiments are now in progress. A Harvard-Smithsonian group that includes Paul Horowitz designed a laser detector and mounted it on Harvard's 155 centimeter (61 inch) optical telescope. This telescope is currently being used for a more conventional star survey, and the optical SETI survey is "piggybacking" on that effort.

Between October 1998 and November 1999, the survey inspected about 2,500 stars. Nothing that resembled an intentional laser signal was detected, but efforts continue. The Harvard-Smithsonian group is now working with Princeton to mount a similar detector system on Princeton's 91-centimeter (36-inch) telescope. The Harvard and Princeton telescopes will be "ganged" to track the same targets at the same time, with the intent being to detect the same signal in both locations as a means of reducing errors from detector noise.

The Harvard-Smithsonian group is now building a dedicated all-sky optical survey system along the lines of that described above, featuring a 1.8-meter (72-inch) telescope. The new optical SETI survey telescope is being set up at the Oak Ridge Observatory in Harvard, Massachusetts.

The University of California, Berkeley, home of SERENDIP and SETI@home, is also conducting optical SETI searches. One is being directed by Geoffrey Marcy, the well-known extrasolar planet hunter, and involves examination of records of spectra taken during extrasolar planet hunts for a continuous, rather than pulsed, laser signal.

The other Berkeley optical SETI effort is more like that being pursued by the Harvard-Smithsonian group and is being directed by Dan Wertheimer of Berkeley, who built the laser detector for the Harvard-Smithsonian group. The Berkeley survey uses a 76-centimeter (30-inch) automated telescope and an older laser detector built by Wertheimer.

4 Probe SETI and SETA Experiments

The possibility of using interstellar messenger probes in the search for extraterrestrial intelligence was first suggested by Ronald N. Bracewell in 1960, and the technical feasibility of this approach was demonstrated by the British Interplanetary Society's starship study Project Daedalus in 1978. Starting in 1979, Robert Freitas advanced arguments [1] [2] [3] for the proposition that physical space-probes are a superior mode of interstellar communication to radio signals, which, if true, would favor a more solarcentric Search for Extraterrestrial Artifacts (SETA) [4]. Much like the "preferred frequency" concept in SETI radio beacon theory, the Earth-Moon or Sun-Earth libration orbits [5] might therefore constitute the most universally convenient parking places for automated extraterrestrial spacecraft exploring arbitrary stellar systems. A viable long-term SETI program may be founded upon a search for these objects.

In 1979 Freitas and Valdes [6] conducted a photographic search of the vicinity of the Earth-Moon triangular libration points L4 and L5, and of the solar-synchronized positions in the associated halo orbits, seeking possible orbiting extraterrestrial interstellar probes, but found nothing to a detection limit of about 14th magnitude. The authors conducted a second more comprehensive photographic search for probes in 1982 [7] that examined the five Earth-Moon Lagrangian positions and included the solar-synchronized positions in the stable L4/L5 libration orbits, the potentially stable nonplanar orbits near LI/L2, Earth-Moon L3, and also L2 in the Sun-Earth system. Again no extraterrestrial probes were found to limiting magnitudes of 17-19th magnitude near L3/L4/L5, 10-18th magnitude for L1/L2, and 14-16th magnitude for Sun-Earth L2.

In June 1983, Valdes and Freitas [8] used the 26-m radiotelescope at Hat Creek Radio Observatory to search for the tritium hyperfine line at 1516 MHz from 108 assorted astronomical objects, with emphasis on 53 nearby stars including all visible stars within a 20 light-year radius. The tritium frequency was deemed highly attractive for SETI work because (1) the isotope is cosmically rare, (2) the tritium hyperfine line is centered in the SETI waterhole region of the terrestrial microwave window, and (3) in addition to beacon signals, tritium hyperfine emission may occur as a byproduct of extensive nuclear fusion energy production by extraterrestrial civilizations. The wideband- and narrowband-channel observations achieved sensitivities of 5-14 x 10^-21 W/m²/channel and 0.7-2 x 10^-24 W/m²/channel, respectively, but no detections were made.

5 Where are they? / The interstellar Internet

SETI experiments performed so far have not found anything that resembles an interstellar communications signal. Says Frank Drake of the SETI Institute: "All we know for sure is that the sky is not littered with powerful microwave transmitters."

The great Italian physicist Enrico Fermi suggested in the 1950s that if there was an interstellar civilization, its presence would be obvious once we bothered to look. This is known as the Fermi paradox. While faster than light, or "superluminal", flight is ruled out by contemporary physics, no law of physics absolutely rules out interstellar flight at "subluminal" speeds, though the physical requirements are formidable.

Assuming that stars are on the average about ten light-years apart; that an interstellar mission can be conducted at a speed of 10% of the speed of light; and that it takes four centuries for an interstellar colony to grow to the point where it can launch a pair of interstellar missions, then the "doubling time" of the interstellar colonies created by this advanced civilization would be 500 years. This would allow colonization of the entire galaxy in five million years.

Even limiting an interstellar mission to 1% of the speed of light and assuming it takes a millennium for a society to get to the point where it can mount two interstellar missions, this still means the galaxy would be completely populated in 20 million years. That is a very short interval on a cosmic time scale.

Given the lack of observable signals, as well as the lack of any persuasive evidence that extra-terrestrials have ever visited this planet, Fermi's argument suggests that there is no such interstellar civilization. This depressing argument is called the " Fermi paradox".

The fact that SETI searches have not come up with anything very interesting so far is not cause for despair. As the previous sections of this document show, trying to find another civilization in space is a difficult proposition, and we have only searched a small fraction of the entire "parameter space" of targets, frequencies, power levels, and so on.

The negative results do place limits on the proximity of certain "classes" of alien civilizations, as specified in a scheme proposed by Soviet SETI researcher Nikolai S. Kardashev in the early 1960s called the Kardashev scale and later extended by Carl Sagan. In this scheme, a "Type I" civilization is one capable of using all the sunlight falling on the surface of an Earthlike planet for an interstellar signal; a "Type II" civilization is capable of harnessing the power of an entire star; and a "Type III" civilization is capable of making use of an entire galaxy. Intermediate civilizations can be numerically defined on a logarithmic scale.

Assuming that an alien civilization is actually transmitting a signal that we could pick up, the searches so far rule out a Type I civilization within a spherical radius of 1,000 light-years, though there may be many civilizations comparable to our own within a few hundred light years that have remained undetected.

A similar analysis using the same assumption shows that there is no detectable Type II civilization in our Galaxy. In the early days of SETI, researchers assumed that such advanced civilizations were very common in our Galaxy. It is discouraging that this does not seem to be so.

However, it is important to emphasize that our SETI hunts have been based on assumptions on communications frequencies and technologies that may be laughable to alien societies, if they have the concept of humor. The lack of results do not say that alien civilizations don't exist. They only say that if they do, our most optimistic assumptions for getting in touch with them have proven unrealistic.

There is another issue that hints as to why we don't see evidence of a large number of alien societies. That issue is time.

Our Sun is not a first-generation star. All first-generation stars are either very small and dim, or have exploded, or have burned out. This first generation synthesized the heavy elements needed to create planets and lifeforms. Later generations of stars, including our Sun, have been born and have died or will die in their turn.

Our galaxy is more than 10 billion years old. Intelligent life and technological societies may have arisen and died out many times during this ten billion years. Assuming that an intelligent species survives for ten million years, that means that only 0.1% of all societies that have arisen during the history of our galaxy are in existence now.

Science writer Timothy Ferris has suggested that since galactic societies could be only transitory, then if there is in fact an interstellar communications network, it consists mostly of automated systems that store the cumulative knowledge of vanished civilizations and communicate that knowledge through the galaxy. Ferris calls this the "Interstellar Internet", with the various automated systems acting as network "servers".

Ferris suspects that if such an Interstellar Internet exists, communications between servers are mostly through narrow-band, highly directional radio or laser links. Intercepting such signals is, as discussed earlier, very difficult. However, the network could maintain some broadcast nodes in hopes of making contact with new civilizations. The Interstellar Internet may be out there, waiting for us to figure out how to link up with it.

6 Criticism of SETI

Not all scientists think that SETI is proper science, labelling it pseudoscience. While many of the arguments against SETI are themselves pseudoscience of the best sort, describable, charitably, as metaphysics at best, there is a line of attack on SETI which has credibility and has yet to be adequately answered in the opinions of many scientists.

The core issue in this line of attack is falsifiability. Karl Popper's criterion for distinguishing science from pseudoscience is short and succinct:

"Scientific methodology exists wherever theories are subjected to rigorous empirical testing, and it is absent wherever the practice is to protect a theory rather than to test it."

It is argued that SETI, by this criterion, is not science. (Extreme versions of this criticism label it a religion, an accusation which is aided somewhat by nigh-spiritual levels of importance attached to the finding of extraterrestrial intelligence by prominent SETI advocates present and past.) The key problem with SETI, according to its detractors, is that it lacks falsifiability. Predictions of positive results of the SETI experiments are explained at great length in a wide variety of ways and places, but a definitive failure condition is not provided.

Ironically, the need for such a failure condition can even be found in a former SETI advocate's own writing. Dr. Carl Sagan, a famous scientist, prominent skeptic and affable science populist, in his book , provides his so-called "Baloney Detection Kit". SETI critics have sometimes charged that the following entries of this mental kit are violated by SETI:

• Arguments from authority carry little weight (in science there are no "authorities").
• "Occam's razor" - if there are two hypotheses that explain the data equally well choose the simpler.
• Ask whether the hypothesis can, at least in principle, be falsified (shown to be false by some unambiguous test). In other words, is it testable? Can others duplicate the experiment and get the same result?

Aside from the falsifiability issue, the first item is also often violated--ironically enough, by invoking Dr. Sagan as an authority whose belief in SETI vindicates the SETI programmes. The second is violated by the very conception of SETI. All available data from astronomy research is adequately explained by an assumption that there is no detectable intelligent life in our (or any) galaxy. SETI is, by this criticism, asking a question which does not arise from available data.

SETI advocates respond to this criticism by pointing out that no mainstream scientist claims proof of intelligent life. The advocates merely claim that it is possible or likely that it exists. By such reasoning, any search for anything not yet discovered can be called unscientific. In terms of Occam's Razor, it is debatable whether it is indeed a simple hypothesis that Earth is the only planet in the entire universe that harbors intelligent life.

Given SETI's popularity in the public arena, it is unlikely that its critics will prevail. Nevertheless, the criticisms associated with SETI have made it difficult for the project to acquire public funding, so it relies primarily on private donors.

7 See also

• SETI@home
• Extraterrestrial life
• Xenobiology
• Drake equation

8 External links

• SETI Institute
• SETI FAQ
• SETI@home, the distributed computing project
• First Contact Within 20 Years: Shostak – From SpaceDaily.com, 22 Jul 2004 (based on calculations to be published in Acta Astronautica )
• SETI: Searching for Life – Article series in Sky & Telescope magazine
• Searching for Good Science: The Cancellation of NASA's SETI Program ( PDF) – By Stephen J. Garber , NASA History Office . Journal of the British Interplanetary Society, Vol. 52, pp. 3-12, 1999. Provides more details on the elimination of SETI funding by the US Congress in 1993.
• Open SETI Initiative – Gerry Zeitlin's site concerned with reforming SETI's approach
• NASA JPL Terrestrial planet finder
• NOAO sky survey program
• The Search For Extraterrestrial Intelligence (NASA SP-419, 1977)

This article (or previous versions of it) are based on v1.0.2 / 01 jan 02 / gvgoebel@yahoo.com / public domain

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