12 One-way Communication
One-way communication of extraterrestrial intelligence may by accidental or intentional. Accidental communication may occur by reception of leakage from normal communications within a planetary system. On Earth, for example, signals from satellite ground stations, broadcast television, navigation beacons, and high power radar signals leak into space, giving any civilization within a few light years an indication of our presence. Similarly, intentional communication occurs when a signal from earth carrying deliberate information content is beamed at a specific star system from a radio astronomy antenna. Television signals may be detectable a few light years from Earth; high-powered radar, up to about 1,000 light-years with a sufficiently powerful antenna.
A deliberate signal from a powerful radio astronomy antenna may be detectable out to 1,000 light years. Conversely, a similarly powerful signal from another civilization, accurately beamed at Earth would be detectable from the same distance. The problem in both cases is the extremely wide spectrum of potential communication frequencies and the astronomical spread of possible pointing directions for the narrow beam that is required. These combine to make a successful random search for such a communication link difficult to achieve within a reasonable time. Nevertheless the effort is well worthwhile because the detection of an extraterrestrial consciouness would be of great significance to our understanding of the nature of the universe we find ourselves in. Not to pursue such a search against the high odds would be a guarantee of failure.
The alternative to receiving directed signals would be to pick a signal being sent out in all directions: a beacon announcing the presence of intelligent life at a particular location. A radio or light signal of this type would probably not be detectable over more than a 100 light years if continuously broadcasting omnidirectionally. The alternative of using a scanning pattern like a surveillance radar so that the signal would be picked up intermittently might extend the range to 1,000 light years.
If we omit exotic solutions such as neutrino communications, we can turn again to the average spacing of civilizations to see under what lifetime conditions we might expect in the near term to receive one-way signals. An omnidirectional beacon is the most likely source that will be detected. Tables 2 shows the situation for various numbers of extraterrestrial civilzations. It looks as if the best chance exists if the average lifetime of civilizations is 100,000 years or more.
Table 2 Likelihood of One-way Communication
| Number of Civilizations | 33,170 | 3,317 | 332 | 33 |
| Spacing, light-years | 619 | 1,539 | 4,864 | 6,187 |
| Communication Likelihood | Low | Zero | Zero | Zero |
| Number of Civilizations | 1,000,000 | 100,000 | 10,000 | 1,000 |
| Spacing, light-years | 199 | 428 | 923 | 2,802 |
| Communication Likelihood | Good | Fair | Low | Zero |
These results suggest that the probability of detecting signals by random scanning is not very high. It is more profitable to scan stars in our own vicinity based on estimation of the probability of civilizations developing around specific stars rather than around stars in general.