Which biological laws, if any, might be universal?

Biological life is an evolving complex dissipative structure, in the sense introduced by Prigogine. The question is considered how life in an advanced stage of evolution manifests itself to an outside observer. Fundamental uncertainties are inherent to the process of self-organization as manifested by dissipative structures. Their evolution is determined by stochastic fluctuations in far from equilibrium conditions, where instabilities arise that cause bifurcations in the behavior of the process system. Since no specific physical or chemical properties of extraterrestrial life can be predicted, general characteristics based on fundamental thermodynamic principles should guide us in our SETI programs. A biological system necessarily must be macroscopic, open with regard of the rest of the universe, and far from thermodynamic equilibrium. The processes within the system must be strongly nonlinear. We should therefore look for life in environments that are in a staticnary far from equilibrium state, such as planetary surfaces and cool interstellar molecular clouds, where an extreme nonequilibrium exists between radiation and kinetic temperature. In any case an advanced form of life should manifest itself by a large entropy emission, possibly carried by low-grade energy radiation, and propably not by information-rich communication channels intended for internal process regulation.

This paper reviews all recent SETI observing programs that the author is aware of from the literature and private communication. The technical parameters of all known SETI efforts are summarized in an appendix containing references whenever possible.

We have been searching the northern sky for ultra-narrowband carriers of intelligent extraterrestrial origin at 21cm wavelength. The dual-polarization 65,536-channel receiver has a resolution bandwidth of 0.03Hz, matched to the Drake-Helou spreading of the interstellar medium. This maximizes signal/noise ratio, and simultaneously rejects carriers of terrestrial origin. The results of 15 months of observation are summarized.

A full-time dedicated SETI progam has been in operation at Ohio State University since 1973. A 50-channel narrowband filter bank is in use near the 21 cm. hydrogen-line. All data is processed in real time and permanently recorded. A large portion of the northern sky has been surveyed and analyzed for large-scale structure. The only obvious non-random structure is caused by isolated narrowband pulses, which are anti-correlated with galactic latitude, and congregate in two specific areas. The origin of these pulses is unknown, although they may be partially due to instrumental effects.

Huge space power plants (Dyson Spheres) utilizing most of a star's energy should be detectable as infrared or microwave sources. A recent far infrared all-sky survey has revealed many sources with a spectrum peaking on this region which is characteristic of the thermal emission of the hypothetical Dyson spheres. The possibility of confusing them with thick circumstellar dust shells around evolved red giant stars is discussed. Microwave detection of cool extended Dyson Spheres by all-sky surveys searching for microwave background fluctuations is also considered.

A summary is made of a program to scan slowly a band of sky in search of strongly linearly polarized radio signals from E.T.I. communication relays. Despite this and numerous other observational searches, no signals have been found. An optimist approach to this great silence (Fermi's Paradox) is developed.

Terrestrial radio leakage as reflected off the moon has been observed with the Arecibo antenna. We find that military radars and television transmitters are main contributors in the 150–500 MHz range, as predicted in the model of Sullivan et al. (1978). The earth indeed is revealing itself (eventually) to any interstellar eavesdropper with an Arecibo-like antenna at distances up to 30 light years, and with a Cyclops-like system up to fifteen times farther.

Detection of other civilizations by “eavesdropping” on accidental radiation from television and other self-oriented signals is proposed. One-bit spectral analysis techniques to implement this are discussed. These can utilize existing general-purpose computers, and provide flexibility of strategy in the analysis. The first derivative of doppler shift from an unknown planet can be compensated for with increased integration times possible.

Three groups, one at Ames Research Center, one at the Jet Propulsion Laboratory, and one at Stanford University are currently engaged in the research and development phase of the NASA SETI program Other papers in this session will describe in greater detail some of the work going on at each of these centers. It is my intent here to give an overall picture of the program so that the other papers may be viewed in context.

An 8.4 million channel narrowband spectrum analyzer is nearing completion, and will be used to expand the frequency coverage of the ongoing search at Oak Ridge by a factor of 200. The new system – project META – will cover 420kHz at 0.05Hz resolution, utilizing a swept receiver to cancel the effect of the earth's rotation. The increased bandwidth will permit observation of CW beacons transmitted at magic frequencies in any of three preferred frames: the local standard of rest, the galactic barycenter, and the cosmic blackbody rest frame.

The MCSA is a special-purpose digital signal processor. Its main function is to filter a wide-band signal into many narrower bands, so that each of the output bands has a bandwidth that is a better match to the signal being searched for.

The basic MCSA provides simultaneous output bandwidths of approximately 1 HZ, 32 Hz, 1024 Hz, and 74 kHz over a spectrum that is about 8 MHz wide. The input to the MCSA consists of a complex signal sampled at 10 MHz, and the outputs consist of either complex samples or power (square-law-detected) samples. In addition, the MCSA provides an accumulator for taking the integral of the power of the output bands for periods up to 1000 sec.

The MCSA hardware is constructed using wire-wrap technology. The implementation of the hardware is done with the aid of a computer program developed specifically for the design of the MCSA. Care has been taken in the MCSA design to ensure that engineering tradeoffs do not adversely affect the performance of the system.

The NASA Multichannel Spectrum Analyzer (MCSA) supplies data in several formats and in a wide range of frequency resolutions. To decide if an extraterrestrial signal is present, this data is searched using detection algorithms particularly sensitive to signals concentrated in frequency and/or time. The algorithms, whose ultimate sensitivities are determined by theoretical considerations, are also constrained by available computing power. At present, practical detection of repetitive pulses is possible at an average power about five times lower than that for incoherent CW detection. After a period of field testing, the best detection algorithms, presently implemented in software, will be converted to hardware to increase their speed.

The targeted search component of the NASA SETI program is limited to time sharing in the usual fashion on existing large radio telescopes. Unless the consequences of this restriction are compensated by increased capability in the SETI electronic systems, an undesirable loss in sensitivity must be accepted in order to prevent a more costly and humanly unattractive lengthening of the projected five-year observing program to a duration of some decades.

NASA's microwave observing program for SETI is presented. This strategy is composed of a high sensitivity, narrow frequency coverage, Target Search and a low sensitivity, broad frequency coverage, Sky Survey. The complementary nature of this dual mode search strategy is discussed. An overview is given of ongoing work in the development of the search strategy for the Sky Survey.

One component of the NASA search for microwave signals of extraterrestrial intelligent origin will be an all sky survey at a significantly low limiting flux over a broad frequency range. We are currently designing an overall strategy which will permit this survey to be: (1) carried out using existing antennas in less than 3 years of observation time, (2) uniform in sensitivity (within 0.5 db) over the celestial sphere for any given frequency, and (3) complete to 6×10−23 W/m2 or better over the frequency range 1.2GHz⩽ν⩽10GHz with a frequency resolution of 32 Hz.

The relative probability of detecting randomly distributed CW transmitters as a function of the fraction of the sky which is searched (in a fixed time interval) is given. It is shown that the probability of detecting such a class of transmitters with a given receiving system is a maximum if the entire sky is searched, provided that the receiving system is sufficiently sensitive to detect the nearest transmitter in the allocated time and that the integration time - bandwidth product in a specified direction is greater than 8.

We assume that the density of sites of technical civilizations emitting suitable signals (whether purposeful or unintentional) is proportional to the stellar density at any location in our Galaxy, as modelled by Bahcall and Soneira (1980). A wide variety of possible radio luminosity functions Ø(L) for these civilizations is then assumed and for each the number of detectable signals per square degree over the sky is calculated. We find that most detectable signals occur at galactic latitudes of 10° or less and longitudes within 90° of the galactic center, a region which covers only 9 per cent of the entire sky. This result holds for a wide range of Ø(L) types, including Gaussian distributions and power law functions with slopes less than 2.5, or any combination of these. The Milky Way is much less preferred, but still advantageous, for cases of steep power law functions (slopes greater than 2.5) or Gaussian functions with mean luminosities so low that any existing civilizations can only be detected at distances less than 0.5 kpc. The only cases where low galactic latitudes are not advantageous are (1) for frequencies of operation less than 600 MHz where the deleterious effects on signal-to-noise ratios of the natural galactic background emission become dominant, and (2) in searches for narrowband (⩽ 1 Hz) signals at frequencies less than 2 GHz where significant interstellar broadening of signals occurs over distances of ≳10 kpc. Furthermore, all of the above results have broader applicability: they are equally valid for searches for any type of natural radio phenomenon if its probability of occurrence is proportional to stellar density.