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SETI - The Search for Extraterrestrial Life
"Kardashev (1964) established a general criteria regarding the types of activities of extraterrestrial civilizations which can be detected at the present level of development. The most general parameters of these activities are apparently ultra-powerful energy sources, harnessing of enormous solid masses, and the transmission of large quantities of information of different kinds through space.
"According to Kardashev, the first two parameters are a prerequisite for any activity of a supercivilization. In this way, he suggested the following classification of energetically extravagant civilizations:
TYPE I: A level "near" contemporary terrestrial civilization with an energy capability equivalent to the solar insolation on Earth, between 1016 and 1017 Watts.
TYPE II: A civilization capable of utilizing and channeling the entire radiation output of its star. The energy utilization would then be comparable to the luminosity of our Sun, about 4x1026 Watts.
TYPE III: A civilization with access to the power comparable to the luminosity of the entire Milky Way galaxy, about 4x1037 Watts."
"Kardashev also examined the possibilities in cosmic communication which attend the investment of most of the available power into communication. A Type II civilization could transmit the contents of one hundred thousand average-sized books across the galaxy, a distance of one hundred thousand light years, in a total transmitting time of one hundred seconds. The transmission of the same information intended for a target ten million light years distant, a typical intergalactic distance, would take a transmission time of a few weeks. A Type III civilization could transmit the same information over a distance of ten billion light years, approximately the radius of the observable Universe, with a transmission time of just three seconds."
- Guillermo A. Lemarchand in The Electronic Journal of the Astronomical Society of the Atlantic (Volume 5, Number 5 - December 1993)
"The power of an ET transmitter broadcasting in all directions at once, just visible from Arecibo with NASA's SETI analyzers, is
where R is the distance of the transmitter in light-years and P is in megawatts. The observation time is one hundred seconds. For a star twenty light-years away, (there are about twenty Sun-like stars that close), the required power is four billion watts. This is the total output of two large nuclear power plants; possible, but not trivial either."
If "the alien civilization beamed directly at Earth [and] used an antenna like Arecibo, concentrating their energy a millionfold, the required transmitter power would be only four thousand watts. This is a power level many radio amateurs can produce with their rigs at home now, though in the U. S. it is illegal to use this much. There could be an intermediate case: a transmitter placed in the galactic plane, broadcasting an annular beam pattern that just irradiates the disk, might cut power requirements by factors of five to twenty-five times. Even a three-meter dish could receive a four-billion-watt signal from their Arecibo equivalent."
- Cullers and Alschuler in First Contact
(2) SETI Sites
"In 1985 astronomer Paul Horowitz of Harvard University began a...project he called META (Megachannel Extraterrestrial Assay). His equipment scans the entire sky along eight [8.4] million channels near a chosen central frequency, such as that of hydrogen [1.4 gigahertz]. or hydroxyl [1.7 gigahertz]. By searching millions of frequencies at the same time, project META has eliminated the problem of the Doppler shift, since its wider range would be able to capture a signal even if it were slightly shifted from its original frequency. But the most comprehensive search of all will come from NASA in the 1990s. The agency's equipment will cover eight million very broad channels and scan the microwave spectrum from 1 to 10 gigahertz."
- Life Search
META "is demonstrating the real-time high-speed parallel-processing concept behind NASA's SETI project."
- Linda Billings in First Contact
"META has now (fall, 1989) run reliably for four years, during which it has covered the sky several times, at both 21 cm and its second harmonic. Apart from a few interfering signals, nothing has been found."
- David Brin in First Contact
Horowitz claims to have found 37 signals "which survived all our cuts" and cannot be identified. On September 10, 1988 the 84 foot radio dish detected "an enormous spike which was 750 times noise. If you converted the radio signal into audio it would sound just like a tone. It would sound like a flute." All signals, however, were single events and never heard again.
- 48 Hours, CBS
An array processor, SERENDIP II, allowed the simultaneous analysis of about 65,000 separate channels of data from the 92-meter transit telescope at Green Bank beginning in 1987. "What we find to be truly surprising is that after analyzing our several trillion data samples, we have only about one hundred that stand out from the statistical noise, and that we cannot clearly reject as manmade."
- Stuart Bowyer in First Contact
"The Nançay decimetric telescope, in Sologne, France, is well adapted to the search for artificial signals. With a collecting surface of 200 X 35 meters and a receiver tuned to the wavelengths of 18 and 21 centimeters, it can detect a 10-million-megawatt monochromatic signal emitted 40 light-years away from Earth in 1 second. The receiver's 1,024 channels let us simultaneously analyze the same number of 50-hertz-wide frequency bands. To detect a weaker signal, we just have to listen (or 'integrate', in radio astronomers' jargon) longer."
"The radio telescope of Ohio State University is of the same type as that at Nançay. Since 1969, it has been used only to search for artificial signals - which explains it nickname, 'the big ear'. In August 1977 a remarkable signal was registered at the telescope, the most intense in 8 years, but this 'Wow!' signal [at right ascension 19h17m and declination -27¼ 3'] never appeared again on the computer charts, and was never explained."
- Emmanuel Davoust, The Cosmic Waterhole
"There are a number of anomalous signals that have some characteristics of what the SETI researchers think might be real ET signals....They have never been repeated and further investigation has found no repeats. Happens all the time in all science experiments. They're called outliers."
- Mark North (north@watop.nosc.mil)
The "Wow!" signal "was recorded on a computer printout of radio noise intensity from 50 frequency channels at varying sky positions."
"It was narrow-band, and matched the antenna pattern exactly, indicating it had to be at least at lunar distance. (A signal from a nearer object would show a wider pattern.) But is was not coming from the direction of the moon, or any planet, or even any particular known star or galaxy.....A check of man-made satellite data showed that no publicly known Earth satellites were anywhere near the position of the signal source. Furthermore, the frequency of the signal was near the 1420 MHz hydrogen line, where all radio transmissions are prohibited everywhere on and off the Earth by international agreement."
- Robert S. Dixon in First Contact
SETI buzz words: "search space", "nine-dimensional hay stacks", "terabits"
(3) The canceled NASA effort
The prototype SETI equipment was housed in a small white trailer which could be linked up to a radio telescope and used as an operational base. "Inside, the trailer was as dark and as cool as a cave. There were no people. It was filled with machines. Jackson led the way toward the far corner of the trailer. Lined in a short row were four pieces of electronic equipment. They were ominously nondescript; no telltale wires, no dancing display screen, not even any 'start' buttons. Three were waist-high; the last was taller than Jackson. All of them were covered in the sort of bumpy white enamel that appliance manufacturers boast is smudge-free."
"He...pointed out the Sun 100 Workstation, the power conditioner, the VAX 11/750; and then, as he came to the tall machine, the Multichannel Spectrum Analyzer....'We just call these things the trash compactor, the washer, the dryer, and the refrigerator'."
"One group, working out of the Ames Research Center, will employ a 'target' approach. It plans to zero in on about eight hundred stars in the Milky Way similar to our Sun, using the largest antennas available (such as the thousand-foot dish at the Arecibo Observatory in Puerto Rico). This strategy emphasizes sensitivity; each star will be carefully scrutinized throughout the water hole frequency range."
- Howard Blum, Out There
"Data from twenty-[eight] million frequency channels will flash through the digital processors as custom-designed microprocessor 'chips' and commercially available electronics process the data at blinding speed. Several dozen of the most powerful general purpose supercomputers (e.g., the CRAY X-MP/18) would be required to match the speed of this special purpose hardware, which will complete tens of billions of mathematical computations each second."
- Michael J. Klein in First Contact
The DFT chip developed at Stanford University sponsored by NASA "can perform more than a thousand 1000-point spectra per second; one chip has the computational power of a first-generation supercomputer (80 million instructions per second). We believe we can build a 1000 million channel spectrometer, with 100 MHz total bandwidth, for a cost comparable to what it cost to construct META. Such a system would have 250 times the bandwidth of META, and would permit us to cover the 'water hole' in three seconds. Such a search strategy would eliminate all requirements on Doppler shifts, covering as it would a range of frequencies for greater than that caused by any interstellar motions."
"The primary task of the targeted search will be to examine nearby stars that are similar to our Sun. Approximately eight hundred of these stars have been identified within a radius of 80 light-years, which is the limit of the current star catalogues. Automated receiving systems designed to search for signals at 1 to 3 GHZ in the microwave band of frequencies (which includes the water hole) will be used with the world's largest and most sensitive radio telescopes. The 305-meter-diameter antenna at Arecibo will produce the most sensitive search, covering the one-third of the sky that can be observed at that equatorial site. Other large antennas located at radio astronomy observatories and at NASA's Deep Space Communication complexes [34-meter-diameter] will be used to complete the targeted search at more northerly and southerly latitudes."
- David Brin in First Contact
"At the core of its hardware is a device called a multichannel spectrum analyzer (MCSA) which divides the incoming radio noise into 14 million narrow-band channels. The MCSA also combines the signals from several adjacent channels to create another 14 million broader bandwidths, just in case the extraterrestrials use them."
"Any kind of signal - continuous, pulsed, polarized, even drifting signals that appear in different channels at different times - can be intercepted by this system."
- Drake and Sobel, Is Anyone Out There?
"In searching the sky forty five years over a wide band of frequencies, the total amount of data processed will be thousands of terabits (a terabit is 1012 bits). Each second, NASA instruments will process the information equivalent of several entire Encyclopedia Britannicass (several gigabits - 109 bits - per second). This information [is] almost entirely cosmic noise..."
"Noise in general is broad banded, with many frequencies present in about equal amplitude."
"We can imagine carving up a batch of data into time segments and running an FFT on each one. The data is them represented by the amount of sine wave amplitude for each frequency at a particular time. The sine wave amplitude is the amount of a particular frequency needed, during a particular time interval. to synthesize the data there. The output of the Fourier Transform can thus be viewed as an array of numbers with rows representing time and columns representing frequency. Each number is the sine wave amplitude in its own little frequency-time rectangle."
NASA's SETI "uses knowledge of signal processing to carve up the frequency and time dimensions in pieces likely to contain large signals but little noise. This search concentrates on continuously present or regularly recurring simple signals. Such searches are good at finding certain types of regularly pulsed radar signals as well as amplitude-modulated radio (AM) and TV transmissions with a narrow carrier wave component. Other transmission types, such as commercial FM stereo, are very poorly matched to NASA's search. Since the information in an FM signal is represented as frequency changes, the signal covers many channels in an irregular pattern determined by the transmission's content."
NASA's High Resolution Microwave Survey (HRMS) was terminated by Congress in October 1993 due to budget pressures. A privately funded organization, Project Phoenix, is continuing The Targeted Search portion of HRMS.
Regarding amateur ETI detection:
"If he knows the frequency of ETI, a big if, an amateur can purchase software that does better signal processing than that planned by the professionals at NASA-Ames and JPL. This can increase amateur sensitivities for some particular star and frequency so that his antenna disadvantage is overcome completely."
"...AM radio and TV have a single sinusoidal component (the carrier wave) containing over half the power of the transmission. Though this unchanging continuous wave part of the transmission has no information content, it makes the construction of cheap receivers possible.
"If carriers are commonly used by technological civilizations, they are like no natural sources known. They can be made very pure, covering less than .001 cycles per second. A long Fourier Transform of 1,000 seconds produces frequency channels well matched to such transmissions. Any carrier is enormously large in its channel, and the noise is small."
"If the signal is drifting because of the motion-induced Doppler effect, your detection attempt will fail (unless you have a drift algorithm to match it) since the frequency will change while the analysis is being carried out. On the other hand, an extraterrestrial might intentionally stabilize his signal. He could either take out the Doppler shifts in the direction he is transmitting, just as we do for receiving, or he could transmit from a stable platform in deep space."
"...Any complex signal sent out is likely to be leakage, not a beacon, and thus weak to begin with. In addition, the modulation that carries the complex information will be weaker than the carrier, if there is one. So recognizing a 'picture' seems unlikely."
- Cullers and Alschuler in First Contact
(4) The Transmission
"...narrow-band signals get broader as they pass through moving clouds of free electrons in interstellar space. When radio waves of a single frequency pass through such an electron cloud, the motion of the cloud has a Doppler effect on them, changing their frequency. The waves then travel along separate curved and changing paths through space. As a result, a signal that starts out from a distant star as a single narrow-band frequency arrives at Earth over a small range of frequencies, having been spread out by its passage through the electron clouds.
"Multifrequency, or broad-band, signals also arrive from space spread out in time by transiting electron clouds. The highest-frequency part of the signal arrives first, followed in succession by the middle and lower frequencies. A signal with a very broad bandwidth may take several minutes to complete its arrival."
"The clouds thus put a lower limit on the narrowness of the bandwidth we can expect to receive from extraterrestrials. That natural restriction has since been named the 'Drake-Delou Limit'. It stipulates that no interstellar radio signal, no matter how narrow it is when transmitted, can reach Earth with a bandwidth narrower than a few hundredths of a hertz."
- Drake and Sobel, Is Anyone Out There?
"There are 'key' ranges of radio frequencies in which radio astronomers often observe for astrophysical purposes. They have nicknamed 'the water hole' a frequency band between 1,4000 and 1,8000 megahertz (i.e., a wavelength of 18 to 21 centimeters) of natural emissions of the components of water: hydrogen and the hydroxyl radical (OH)."
"One strategy would be to transmit easy-to-detect but messageless signals over several frequencies, and to broadcast the message itself over the central frequency. A listener would be intrigued by a series of signals that get closer together and more intense as he sweeps over the range of frequencies, then weaker and further apart. If he is clever enough to examine what is happening in the center of this system of symmetrical signals, he will probably detect a meaningful signal."
- Emmanuel Davoust, The Cosmic Waterhole
"...The optimum range...where the signal-to-noise ratio reaches its maximum" is "at the frequency of 56 gigahertz, 5.35 millimeters wavelength.."
- V. S. Troitsky in Communication with Extraterrestrial Intelligence
"As a civilization becomes more efficient, the transmitted signals look more and more like noise, as demonstrated in Shannon's statistical theory of coding. The most efficient communications are indiscernible from pure Gaussian noise unless you know the code word."
- Marvin Minsky in Communication with Extraterrestrial Intelligence
"...The understanding of any symbol system requires that the symbols be repeated in situations we know but in different practical contexts; that is the only clue to decoding their message."
- B. I. Panovkin in Communication with Extraterrestrial Intelligence
To transmit an image "one would just have to draw on successive lines a sequence of signals and silences. The end of each line would be identified by a special sign - a longer signal, for example."
"A monochromatic signal oscillates regularly in amplitude with time; its form is sinusoidal. However, with a wide-band signal, the time modulation can be any way at all. It can also be altered by its passage through the interstellar medium. If the form of the wave is not known beforehand, the computation time needed to detect the wide-band signal becomes very long."
"The search for monochromatic radio signals requires the technical development of utrasensitive multichannel or scanner-type receivers and the treatment of the intercepted signals in real time."
- Emmanuel Davoust, The Cosmic Waterhole
The likely characteristics of a transmission for the purposes of Chimera include:
1.) Three channels - phase or differential phase modulation
2.) Hierarchical signal
(a) acquisition or beacon (a rapidly repeating sequence)
(b) decoding (map of the symbols into some reference)
(c) "language lesson" (complex structure, wide band < 100 megacycles in bandwidth.)
3.)Binary-coded prime numbers to determine frame parameters?
4.)Intensity, frequency; and time coordinates to produce a topological image
For an interstellar beacon "the most likely choices are either a pure carrier or a regular train of pulses."
Narrow band carriers have the following advantages:
1.) "The are easier to detect, using straightforward Fourier spectrum analysis techniques;
2.) "They stand out a clearly artificial, being a fraction of a hertz wide in a Universe where the narrowest natural spectral feature are at least a kilohertz wide;
3.) They are efficient beacons, permitting excellent received signal/noise ratios through coherent integration;
4.) They permit effective discrimination against terrestrial interference, owing to the particular time-varying Doppler shift signature that is impressed upon a true extraterrestrial signal by the effect of the Earth's rotation."
"Interstellar velocities are quite large - tens to hundreds of kilometers per second, within our galaxy alone - and cause corresponding frequency shifts (which, unlike the 'chirp' caused by the Earth's rotation, are unchanging with time) of a few megahertz, at microwave frequencies...One possibility is to build spectrometers of large enough bandwidth to cover all reasonable Doppler shifts; another is to assume the senders have transmitted their signals at a frequency precompensated for their motions relative to a guessable frame of reference. In the first case we need very large numbers of channels - several MHz total bandwidth, divided by channel widths of order 0.21 Hz, or several tens of millions of channels..."
- Horowitz and Alschuler in First Contact
"...There is no technical difficulty whatever in getting information rates of 107 or 108 bits [10 mb] per second over 500 to 1000 light years."
- B. M. Olliver in Communication with Extraterrestrial Intelligence
"If the message was in binary code, that meant that every cycle of the signal - every flick of the dancing line on our screen - carried a bit of information. The signal's wavelength was 5000 Angstroms; there are a hundred million Angstrom units to the centimeter; figuring the speed of light...the signal could carry, in theory at least, something like 6000 trillion bits of information per second."
- Ben Bova, "Answer, Please Answer"
Deciphering the code "was a task that required the obscure, finely honed skills of a Fort Meade crypto specialist."
- Howard Blum, Out There
"No mere symbol string, no set of mathematical symbols alone would be the content of the message, but a very rich three-dimensional, moving, carefully scaled cinema."
- P. Morrison in Communication with Extraterrestrial Intelligence
"A computer is an absolute topological device and if we send pictures - and I prefer the proposal of Drake for television in prime number format rather than this frequency modulated form of television - we can send a series of Drake pictures which explain how to build a program or, rather, a computer with the program....I believe by sending Boolean logic diagrams, one avoids most of the possible ambiguities."
- Marvin Minsky in Communication with Extraterrestrial Intelligence
"Larger atomically precise structures with complex three-dimensional shapes can be viewed as a connected sequence of small atomically precise structures."
- J.Storrs Hall, "Overview of Nanotechnology" (from papers by Merkle and Drexler)
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The Search for Extraterrestrial Intelligence at Home
An experiment to harness hundreds of thousands of Internet-connected computers in the search SETI Institute Home for scientific research in the general field of Life in the Universe - including Project Phoenix |