Brief Non-Technical Summary of the Ideas in the Adjacent Paper
Published online in the International Journal of Astrobiology Aug. 3, 2017. www.cambridge.org/whitmire-08.17.
A short second adjacent paper below the main paper gives my response to an anthropic selection effect that has been raised regarding the predicted lifetime of the typical technological species. The associated paper can be found at the same journal under "First View".
For 37 years I taught a college introductory astronomy course. Toward the end of each semester I would cover the topic of Life in the Universe. In passing I would note that, among technological civilizations, we must be the dumbest guys in the Galaxy because our technological civilization is only about 100 years old, while other civilizations (assuming some exist) could be thousands or millions or billions of years more technologically advanced than us. This is a common view and I suspect that other astronomy professors made similar comments.
Then, finally, after 37 years I had an epiphany: Why should we be so extremely atypical? After all, since the time of Copernicus it has been usually assumed (and sometimes proven) that our place/status in the universe is not special but typical. The Earth is not the center of the universe, or the Galaxy, or the solar system, and we exist at a typical time during the period when it is possible for life to exist, and so on. This general idea that, in the absence of any contrary evidence, we should believe that we are a typical member of a given reference class is called the Principle of Mediocrity. It is a cornerstone of modern cosmology. This Principle is our best guide when there is only a single example, as is the case with our technological civilization.
If we assume that the Principle of Mediocrity applies to the reference class of all existing technological civilizations in the universe then they too, like us, will find that they are early in their technological evolution and also that they are the first such technological civilization to evolve on their planet. Then the questions are: Why only technologically young civilizations? and: Why is everybody first? The obvious answer to the first question is that the typical technological civilization is short-lived. The second question implies that there are no subsequent technological civilizations on a given planet, thus, as argued statistically in the paper, typically the demise of the first civilization coincides with the extinction of the planetary biosphere.