© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

24.2.3  Acquisition and Artificiality Criteria

There are many different kinds of possible alien transmissions we might receive -- local radio traffic, beamed messages to regular correspondents, beacon signals designed to attract attention, and so on. We are most likely to pick up beacons first because these should be comparatively more powerful. But xenologists disagree on the exact nature of the beacon signals we may detect.³¹⁷⁹ Artificiality criteria have not yet been worked out in full detail, but certainly not for want of trying. A variety of radioastronomical criteria have been proposed over the years which turn out to be insufficient: Small angular dimensions of the source, characteristic spectral density distribution of intensity, time-variability or flux pulsations in time, circular polarization, alternating left- and right-hand polarization pulses, corrected orbit-induced or planetary axial-rotation-induced Doppler shifts, and so forth. (See Kaplan,²⁹ Konstantinov and Pekelis,²⁵ and Tovmasyan.²⁸) According to Vsevolod S. Troitskii, a well-known Russian radioastronomer, there do seem to be two basic requirements for all beacon signals that xenologists can agree upon:

1. The signal should not leave any doubt as to its artificial origin. The artificial signal should be distinctly different in its properties from natural radiations.

2. The signal should carry some information about the transmitting civilization.³¹²⁴

The second of these criteria -- information content -- is highly significant. So far as we know, only the processes of life, intelligence and culture are able to impress large quantities of information onto packets of photons. Perhaps we should try to decide if there is some minimum information content than a given transmission must possess before we may regard it as artificial. As Philip Morrison points out:

A little more information content is needed than just the existence of a {stellar spectral} line of something rather rare or a very regular pulse, because if you look for a sufficiently long time, you are bound to find rare things just in the course of events. This is a very valuable lesson. Although rare things occur rarely, it is also true that rare things occur rarely.³¹²⁷

Soviet astrophysicist Nikolai S. Kardashev believes that a signal should contain at least 10-100 bits of information before we may feel confident that it is artificial in origin.³¹²⁶ Terry Winograd’s artificial intelligence computer program consists of about 10⁶ bits, but Marvin Minsky estimates that a message with as few as 10⁴-10⁵ bits, properly situated, could be considered "intelligent."²²

Hopefully the information will not be too difficult to extract. According to E.C. Shannon’s statistical theory of coding, the most efficient signals will appear indistinguishable from random thermal noise.^3186,3187 That is, a maximally information-saturated message looks like noise -- unless the recipient happens to know the correct translation code. Xenologists suspect that ETs may not attempt to achieve maximum information content in interstellar beacon devices. Signals that look like noise don’t attract much attention. If such highly efficient beacon transmissions are used, they probably will be accompanied by special attention-getting messages. These are often referred to as "call signals," and may be expected to satisfy Troitskii’s first criterion on the previous page.*

* Specific search strategies have been proposed by Bihary,³¹⁵⁸ Dixon,¹²⁶⁶ Gray,³¹⁵⁰ Haviland,¹¹⁴⁷ Morrison, Billingham and Wolfe,²⁸⁶⁵ Oliver and Billingham,⁵⁷ Ridpath,^3154,3257 Sagan,²² Shklovskii and Sagan,²⁰ and Walker.¹⁵⁸ Timing of the call is also of paramount importance -- we must know when to look as well as where and how. The interested reader is referred to the timing schemes offered by Gindilis,²² Makovetskii,³²⁶³ McLaughlin,²⁷¹⁹ Pace and Walker,⁶⁵¹ Tang,¹⁶¹³ and Tovmasyan.²⁸

Last updated on 6 December 2008