12 - Fraction of civilizations that develop a technology that releases detectable signs of their existence into space, fc, 1961 to the present

Summary

Abstract

We consider estimates of the value of f_c – the fraction of intelligent species that can make themselves visible to other societies – by examining the recent capabilities of terrestrial technology. The value of this Drake Equation parameter is important for evaluating SETI experiments because the development of radio or other technology that would permit interstellar communication is hardly inevitable, even when intelligence is present. For example, note that Homo sapiens lacked advanced technology throughout most of its history. In estimating f_c, we are attempting to gauge whether intelligent species frequently become detectable.

Unfortunately, any evaluation of f_c must deal with the fact that communication depends on both the technology of the listener and the speaker, and therefore even deciding whether our own species should be counted among those that are visible depends on assumptions about the receiving capabilities of others. For consistency, we assume those capabilities are similar to our own.

We consider the strongest microwave emissions from Earth and find that – with the exception of the Arecibo radar – our own broadcasts into space would be too weak to be found by our current SETI experiments at a distance of 100 light-years. Similarly, neither our inadvertent optical emissions (street lighting) nor the type of large-scale artifacts we've built on Earth are detectable by terrestrial telescope technology at this distance. Therefore, we conclude that Homo sapiens has not yet attained a value of f_c = 1. We could not find our own society at the distances of even relatively nearby stars.

This situation might be short-lived, however, and in any case should not dissuade us from doing SETI experiments. This is because both our visibility and our sensitivity to signals from others will likely increase in the near future. In other words, although our communication abilities are still inadequate in the context of interstellar signaling, even a modest extrapolation of today's technology suggests that the terrestrial value of f_c will change from 0 to 1 within a few centuries. If we assume that our species will survive that long – and that most other societies can do at least as well – we conclude that the best estimate for the cosmic value of f_c is 1.