The Search for Life in the Universe

Just some small thoughts on the unlikeliness of finding an extra-terrestrial civilization that is not deliberately trying to contact unknown civilizations.

If a civilization is not trying to contact others, we would have to detect their actions. Over interplanetary distances, the only thing we can detect is electromagnetic radiation (light, radio waves ...). If we could take detailed photos of a planet surface, we might see cities or other evidence of civilization. We might also detect their attempts to communicate with each other (think radio, TV, cell phones...). In both cases the frequencies we would look for are those that are not absorbed by the planet atmosphere. In all cases, detection involves the recognition of patterns in the signal that indicate intelligence. Planets are next to suns. Suns transmit huge amounts of energy that mask planetary signals. Without traveling close to the solar system, we will probably never get detailed enough information to isolate sources from a particular region of a planet. That is, we won't be able to resolve images that would show us cities.

There is a chance that something like a radio signal might be regular enough and powerful enough to be distinguished from the background, but this is likely to occur only briefly in a civilization's history. Our own experience indicates that communication feeds on itself. The tendency is to try to increase the amount of information transmitted. Over time, all bandwidth that can be used will be used and that every given bandwidth will be saturated. In our case, we have compressed signals, which reduces the direct intelligibility. We have used both time and frequency multiplexing which smears signals together. Finally, we have reduced the power for any given communication to a minimum both to increase the number of devices that can communicate and to reduce interference.

The end result is that our civilization's internal conversations, and likely any other technologically advancing civilization, will be essentially undetectable. There is an infinitesimal increase in heat at frequencies transparent to the atmosphere, but no possibility of detecting intelligible transmissions.

If a civilization inhabits multiple planets or stars, we might detect traces of those signals, but this too is extremely unlikely. Such signals have a known destination and are likely to be highly directed. That makes eavesdropping difficult. Even if we received the signal, we probably could not recognize it as such because, again, available bandwidth will probably be saturated and the signals, in effect, disguised by compression and multiplexing.

That leaves only communications from civilizations that are deliberately trying to contact unknown civilizations.