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I have heard often about the Drake equation, but have never heard estimates of how soon SETI expects some kind of contact in the event these Drake estimates are accurate. If there are indeed 10,000 communactive civilizations in the Milky Way galaxy alone, shouldn't the sky be alive with non-natural radio signals? What does SETI's lack of success to this point say about the Drake equation? At what point does experimental evidence (no contact) affect the theory (Drake Equation)? YV (via email)
The Doctor Responds: The Universe may very well be abuzz with intelligently generated radio signals from those 10,000 civilizations. But there are some 400,000,000,000 stars in the Milky Way, so when you point your antenna at one of them, there's only a one in 40,000,000 chance (that's 0.00000025%) of picking up signals. And this assumes you're tuned to the right frequency and listening at the right moment. Our most sensitive searches, the targeted ones, cover only a few hundred nearby stars, not the whole sky. Then there's the problem of range. It's likely to be hundreds of light years to the nearest civilization, and signal strength diminishes with the square of distance. Our very best current systems can probably detect out to perhaps 1000 light years, if they happen to know exactly where to point (but they don't, of course). So we really need better sensitivity. I wouldn't call our outcome to date a lack of success, rather a lack of instantly verifiable results. We're just in the shakedown phase of developing our capabilities. Nobody seriously expected that we would have detected any signals by now. That would be like Orville and Wilbur having expected to fly supersonic. (Of course, we have a better chance of getting lucky than they did!) The lack of firm evidence to date more or less confirms the theory that communicative civilizations are modestly rare (after all, 10,000 out of 400,000,000,000 is not a huge ratio). The late NASA all-sky survey only observed for a total of 1000 hours before Congress pulled the plug. The privatized searches are doing a little better, but we still have a very long way to go. Not only have we not yet scratched the surface, we haven't even felt the itch.
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