February 4, 2009
New theory may explain why radio SETI has failed
It's been nearly 50 years since scientists first started scanning the heavens for radio signals that would indicate the presence of intelligent extraterrestrial life. And in all those 50 years we haven't heard so much as a peep. The apparent failure of radio SETI thus far has provided much of the fuel that now powers the Fermi Paradox, the observation that we haven't encountered signs of ETI's when we probably should have by now.
But a new theory may explain part of the problem and why radio SETI has so far proved fruitless. Reginald Smith from the Bouchet-Franklin Institute in Rochester argues that there is a limit to how far a radio signal from ET can travel before it becomes too faint to hear. If true, this could have interesting ramifications for radio SETI and our expectations for the project.
In his article, "Broadcasting but not receiving: density dependence considerations for SETI signals,"Smith applies his idea to the Drake Equation and derives a minimum density of civilizations below which contact is improbable within a given volume of space. The calculation depends on factors such as the lifetime of a civilization and the distance that it might be possible to communicate via radio signals. His thesis has produced some interesting scenarios.
"Assuming the average communicating civilization has a lifetime of 1,000 years, ten times longer than Earth has been broadcasting, and has a signal horizon of 1,000 light-years, you need a minimum of over 300 communicating civilization in the galactic neighborhood to reach a minimum density," writes Smith.
Consequently, if there are less than 300 advanced civilizations in our galaxy, the chances are that they’ll never notice each other. And a figure of N being less than 300 is not out of the question given recent insights into cosmology, astrobiology and the rise of the Rare Earth Hypothesis.
Smith's theory has led some to suggest that the Fermi Paradox has been solved. But their thinking here is flawed.
Our (apparent) inability to detect radio signals does not wipe away other means for extraterrestrial communication (quantum communication schemes, Bracewell probes) or detection (Dysonian SETI, interstellar calling cards, etc.), nor does it answer the issue as to why the Galaxy hasn't been colonized by now (i.e. self-replicating Von Neuman colonization probes).
So, while Smith's theory may explain the failure of radio SETI, it doesn't erase the problem of the Great Silence.