Radio Leakage and Eavesdropping

Abstract

In addition to searches for purposeful signals, those attempting interstellar communication should also consider the possibility of eavesdropping on radio emissions inadvertently “leaking” from other technical civilizations. To understand better the information which might be derivable from radio leakage, the case of planet Earth is considered. The most detectable and useful escaping signals arise in a few ultra-powerful military radar systems and in normal television broad-casting. A model including over 2,000 television transmitters is used to demonstrate the wealth of astronomical and cultural information available from a distant observer’s careful monitoring of frequency and intensity variations in individual video carriers (program material is not taken to be detectable). Observations of the Earth’s leakage radiation, observed at Arecibo as reflected from the Moon, are also presented. It is concluded that, given our present modest understanding of the cultural and technical evolution of civilizations, any initial interstellar radio contact has a priori even chances of placing us in the role of eavesdropper or of intended recipient.

One can make either of two basic assumptions about our first contact with an extraterrestrial civilization: (i) that it will arise through a purposeful attempt, perhaps through the use of an interstellar radio beacon, or (ii) that a civilization will be detected through no special efforts of its own. The latter hypothesis, often called eavesdropping, is concerned with the extent to which a civilization can be unknowingly detected through the by-products of its daily activities. While much thought has gone into the idea of purposeful contact, eavesdropping has been somewhat neglected; we will argue that it deserves more attention.

The overall likelihood of contact through eavesdropping depends on the nature and intensity of the civilization’s “leakage”, as well as on how long that leakage continues. Very general arguments (see Oliver and Billingham (1973)) show that radio waves provide the most economical and reliable means of contact at interstellar distances. This is true not only for intentional contact, but probably for eavesdropping as well. In any case, there can be no argument with the fact, first discussed in print by Webb (1963) and by Shklovskii (1966), that the presence of humans can already be detected at interstellar distances as a result of the complex communities and transportation network spread over our globe. Of course, we do not know how applicable our present situation is to the more general case of all galactic civilizations over all time. It may be that our present “leaky” state will soon be terminated by advancing technology, but on the other hand it may continue for a very long time, perhaps even longer than any period in which we might have the perseverance to send out purposeful messages. If we are at all typical, then we should perhaps be also looking for unintentional signals from others at least as diligently as for intentional ones. In order better to understand the principles involved, let us now examine the appearance of the Earth from interstellar distances.