v1.0.3 / 01 jan 04 / greg goebel / public domain
* Interstellar flight is a common theme in science-fiction stories, but the obstacles to such journeys are in practice enormous. An alternative approach to interstellar exploration is to survey the sky in hopes of finding transmissions from a civilization on a distant planet, but such a "Search for Extra-Terrestial Intelligence (SETI)" effort is faced with obstacles as well. This document outlines the history, current efforts, and future prospects for SETI.
* Visiting another civilization on a distant world be fascinating, but at present such a trip is beyond our capabilities. However, it is perfectly within our capabilities to develop a communications system using a powerful transmitter and a sensitive receiver, and using it to search the sky for for alien worlds whose citizens have a similar inclination.
SETI is still no trivial task. Our Galaxy is 100,000 light-years across, and contains a hundred thousand million stars. Searching the entire sky for some faraway and faint signal is an exhausting exercise.
Some simplifying assumptions are useful to reduce the size of the task. One is to assume that the vast majority of life-forms in our Galaxy are based on carbon chemistries, as are all life-forms on Earth. While it is possible that life could be based around atoms other than carbon, carbon is well known for the unusually wide variety of molecules that can be formed around it.
The presence of liquid water is also a useful assumption, as it is a common molecule and provides an excellent environment for the formation of complicated carbon-based molecules that could eventually lead to the emergence of life.
A third assumption is to focus on Sun-like stars. Very big stars have relatively short lifetimes, meaning that intelligent life would not likely have time to evolve on planets orbiting them. Very small stars provide so little heat and warmth that only planets in very close orbits around them would not be frozen solid, and in such close orbits these planets would be tidally "locked" to the star, with one side of the planet perpetually baked and the other perpetually frozen.
About 10% of the stars in our Galaxy are Sun-like, and there are about a thousand such stars within 100 light-years of our Sun. These stars would be useful primary targets for interstellar listening. However, we only know of one planet where life exists, our own. There is no way to know if any of the simplifying assumptions are correct, and so as a second priority the entire sky must be searched.
Searching the entire sky is difficult enough. To find a radio transmission from an alien civilization, we also have to search through most of the useful radio spectrum, as there is no way to know what frequencies aliens might be using. Trying to transmit a powerful signal over a wide range of wavelengths is impractical, and so it is likely that such a signal would be transmitted on a relatively narrow band. This means that a wide range of frequencies must be searched at every spatial coordinate of the sky.
There is also the problem of knowing what to listen for, as we have no idea how a signal sent by aliens might be modulated, and how the data transmitted by it encoded. Narrow-bandwidth signals that are stronger than background noise and constant in intensity are obviously interesting, and if they have a regular and complex pulse pattern are likely to be artificial. However, while studies have been performed on how to send a signal that could be easily decoded, there is no way to know if the assumptions of those studies are valid, and deciphering the information from an alien signal could be very difficult.
There is yet another problem in listening for interstellar radio signals. Cosmic and receiver noise sources impose a threshold to power of signals that we can detect. For us to detect an alien civilization 100 light-years away that is broadcasting "omnidirectionally", that is, in all directions, the aliens would have to be using a transmitter power equivalent to several thousand times the entire current power-generating capacity of the entire Earth.
It is much more effective in terms of communication to generate a narrow-beam signal whose "effective radiated power" is very high along the path of the beam, but negligible everywhere else. This makes the transmitter power perfectly reasonable, but the problem then becomes one of having the good luck to be in the path of the beam.
Such a beam might be very hard to detect, not only because it is very narrow but because it could be blocked by interstellar dust clouds or garbled by "multipath effects", the same phenomenon that causes "ghosted" analog TV images. Such ghosts occur when TV transmissions are bounced off a mountain or other large object, while also arriving at our TV antenna by a shorter, direct route, with the TV picking up two signals separated by a delay. Similarly, interstellar narrowbeam communications could be bent or "refracted" by cosmic gas clouds to produce multipath effects that could obscure the signal. If interstellar signals are being transmitted on narrow beams, there is nothing we can do at this end to deal with this problem other than to be alert.
* Modern SETI efforts began with a paper written by physicists Guiseppe Cocconi and Philip Morrison and published in the science press in 1959. Cocconi and Morrison suggested that the microwave frequencies between 1 and 10 gigahertz would be best suited for interstellar communications. Below 1 gigahertz, "synchrotron radiation" emitted from electrons moving in galactic magnetic fields tends to drown out other radio sources. Above 10 gigahertz, radio noise from water and oxygen atoms in our atmosphere tends to also become a source of interference. Even if alien worlds have substantially different atmospheres, quantum noise effects make it difficult to build a receiver that can pick up signals above 100 gigahertz.
The low end of this "microwave window" is particularly attractive for communications, because it is in general easier to generate and receive signals at lower frequencies. Lower frequencies are also desireable because of the "Doppler shifting" of a narrow-band signal due to planetary motions.
Doppler shifting is a change in the frequency of a signal due to the motion of the source of that signal. If the source is approaching, the signal will be shifted up in frequency, while if the source is moving away, the signal will be shifted down in frequency. The rotation of a planet and its orbit around a star causes a Doppler shift in the frequency of any signal generated from that planet, and over the course of a day the signal can drift in frequency far out of its intended bandwidth. The problem gets worse with higher frequencies, and so lower frequencies are preferred.
Cocconi and Morrison suggested that the frequency of 1.420 gigahertz was particularly interesting. This is the frequency emitted by neutral hydrogen. Radio astronomers often search the sky on this frequency to map the great hydrogen clouds in our Galaxy. Transmitting a communications signal near this "marker" frequency would improve the chances of its detection by accident. This frequency is sometimes called the "watering hole" by SETI enthusiasts.
* In 1960, Cornell University astronomer Frank Drake performed the first modern SETI experiment, named Project "Ozma", after the Queen of Oz in Frank L. 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 beginning in 1964 performed a number of searches with omnidirectional antennas in hopes of picking up powerful radio signals. In 1966, American astronomer and pop-science figure 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 & 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, an attempt was made to send a message to other worlds. A 1,679 bit message was transmitted from the 305-meter Arecibo radio observatory dug into the ground in Puerto Rico toward the globular star cluster M13, about 25,000 light-years away. The pattern of 1s and 0s defined a 23 by 73 pixel "bitmap" image that included numbers, stick figures, chemical formulas, and a crude image of the Arecibo radio telescope itself. The 23 by 73 grid was chosen because as prime numbers, they are the only integer values that can be multiplied together to get 1,679, and hopefully alien listeners would be able to factor the number of pixels and recognize that they represented a grid. Given the distance to M13 and the cryptic appearance of the bitmap, the experiment amounted mostly to a publicity stunt.
In 1979, Sagan, Bruce Murray, and Louis Friedman founded the US Planetary Society, partly as a vehicle for SETI studies. That same year, the University of California at Berkeley launched a SETI project named "Search for Extraterrestial Radio from Nearby Developed Populations (SERENDIP)".
* 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 (Megachannel Extra-Terrestial 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 has used the the 64 meter Parkes radio telescope in Australia, the 43 meter telescope at Green Banks, and currently the Arecibo dish. 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 (Billion-Channel Extraterrestial 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 was originally named the "One Hectare Telescope (1HT)", because it is to cover 100 meters on a side, but it is now known as the "Allen Telescope Array (ATA)", named after donor Paul Allen, previously of Microsoft.
An initial ATA was in operation with ten 6-meter radio dishes in 2002, but when it is completed in 2005, it will consist of 350 radio dishes. Final cost is expected be a modest of of $26 million USD. The SETI Institute is providing money for building the ATA, while UC Berkeley is building the telescope and will 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. It is being seen as a prototype of sorts for an even more ambitious system, the "Square Kilometer Array (SKA)", which of course will be a kilometer on a side and will be a powerful tool for radio astronomy and SETI studies.
* Another interesting SETI Institute effort called "SETI@home" began in May 1999 and ran through 2001. SETI@home allowed hobbyists to get involved with SETI research by simply downloading screen saver software over the Internet. The software performed 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. Millions of computer users in hundreds of countries signed up for SETI@home and provided hundreds of thousands of hours of computer processing time. The project was widely praised in the computer press as an interesting exercise in home-grown "distributed computing". A follow-on project was initiated after the first effort, and it now appears that SETI@home is an ongoing thing.
* 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 range becoming wider 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 a dedicated amateur named Stuart Kingsley, a Britisher 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 have been conducted or 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 was being used for a more conventional star survey, and the optical SETI survey "piggybacked" 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 has worked with Princeton to mount a similar detector system on Princeton's 91-centimeter (36-inch) telescope. The Harvard and Princeton telescopes were "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 has built 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 set up at the Harvard observatory site in Oak Ridge, Tennessee.
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.
* 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. 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-terrestials 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 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 are probably 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 probably still maintains 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.
* SETI is a very appealing idea. It is well within the capabilities of current technology, and compared to space missions can be performed at relatively low cost.
It seems contrary that governments are willing to fund space projects costing tens of billions of dollars and whose benefits are debatable, but find the idea that searching for signals from other civilizations silly and not worth a fraction of that effort. On the other hand, given that SETI can be performed at relatively low cost, it is debatable that governments should be involved with SETI research, or that it would be good for the field it they were.
A coherent, long-term SETI program can be pursued on a modest scale with private funding. A large-scale government-funded program would suffer from fluctuating support, and besides, as ground-based telescope builders have demonstrated over the last few decades, limited budgets can often lead to surprising ingenuity that is lost when more money is available.
SETI may prove to be a dead end, and any SETI effort has to accept that fact up front. That's the question, anyway: is there someone else out there, or are we the only ones? Either way, as has been said, the implications are staggering.
* Sources include:
Various SETI websites were consulted as well, and the History Channel TV program THE 20TH CENTURY WITH MIKE WALLACE also ran a program on extraterrestial life that discussed the SETI effort.
The TV show did not provide many details and was out of date, but it did have interviews with Paul Horowitz, Frank Drake, Jill Tarter, Louis Friedman, and of course the late Carl Sagan that were interesting from a personality point of view. I am afraid, however, I can never see a video of Dr. Sagan without wanting to say "billyuns and billyuns!"
The show was also amusing because it had segments dealing with the UFO community at Roswell, New Mexico, and elsewhere. The last segment of the program discussed the relationship between the SETI community and the UFO community, showing it as fundamentally antagonistic, with the SETI people fearing the UFO groups were undermining the credibility of SETI efforts, and the UFO groups resenting being treated as cranks by the SETI people.
* Revision history:
v1.0 / 01 jul 00 / gvg
v1.1 / 01 oct 00 / gvg / Comments on Fermi paradox, narrowbeam radio.
v1.0.2 / 01 jan 02 / gvg / Minor cosmetic update.
v1.0.3 / 01 jan 04 / gvg / Minor cosmetic update.
However, I kept on picking up new details. The original document started to grow, and so I changed my mind and split it into separate documents on interstellar exploration and SETI. The interstellar exploration document ended up as part of the SPACEFLIGHT PROPULSION document.