Summary of the Drake Debate

The ETI literature and related scientific research development indicate good reasons for optimism about the amount of life, even intelligent life, which has arisen in the galaxy. As Frank Drake like to put it: about one new intelligent civilization appears in the Mikly Way a year. The question remains: how much of this intelligent life is still around? In the Drake Equation this refers to the final term, L, the mean lifetime of an advanced civilization. This current author has been quite impressed with the insights of modern science in casting light on all the other factors of the Drake Equation. We know a great deal and we're advancing all the time. But this last factor, L, is almost a complete mystery. Sadly, all we can offer is a few tenuous guidelines.

Our galaxy was formed about 10 billion years ago, and it was at that time composed almost entirely of hydrogen and helium: no heavier elements, no heavy molecular clouds, no planets, no life. A significant but undetermined amount of time must have passed while the first generation stars built heavy elements in their cores, the larger stars exploded as supernovas, and these elements were dispersed to space. A great deal of this needed to happen before the "metallicity" of the galaxy would be high enough to allow formation of rocky terrestrial planets. For perhaps the first three billion years this process went on in the sterile galaxy. Perhaps seven billion year ago some solar systems outside the nucleus formed planets upon which the processes described earlier in this paper began. Two billion or so years later our own solar system was formed and we began the crawl up evolution's ladder.

If anything like the above picture was true, then some systems may have begun life-building a couple of billion years before our own. If so, and if Frank Drake's one civilization per year (essentially referring back to between one and ten sunlike stars per year) rule-of-thumb is anywhere near, then perhaps 2 billion civilizations have arisen before our own. The extremes are easily determined. If no civilization ever dies off (i.e., L=lifetime of galaxy), then all 2 billion or so are still out there. If civilization execute themselves immediately (i.e., L = 1), then there is only one. So one can be either form of extremist: pessimist or optimist. For the optimists one must admit that nearby supernovas or huge galactic nucleus events may scour some systems of life. For the pessimists one must admit that even our own erratic selves have managed to make it forty-plus years past the invention of nuclear weapons and are still staggering into the future. Intuition, all that we have on this issue, would seem to say: some make it, some don't. Even the most pessimistic scenarios would seem to be forced to the conclusion that there are advanced civilizations out there somewhere. And a little more faith in intelligence produces this:

There may be abundant groups of 10⁵ to 10⁶ worlds linked by a common colonial heritage. The radar and television announcement of an emerging technical society on Earth may induce a rapid response by nearby civilizations, thus newly motivated to reach our system directly rather than by diffusion [emphasis added] s1William Newman, UCLA, & Carl Sagan, Cornell, 1981.