Yesterday we looked at Milan Ćirković’s paper “The Temporal Aspect of the Drake Equation and SETI” (Astrobiology Vol. 4 No. 2, pp. 225-231), and pondered whether there might not be a ‘communications window’ — an interval for any society between when it reaches the technological capacity for interstellar communication and the point when it becomes a ‘supercivilization’ unlikely to use conventional SETI methods to contact us or anyone else. If so, that ‘window’ would have a profound effect on how many civilizations we might be able to contact via SETI, and would thus change our answers to the Drake Equation.
But there are other kinds of assumptions built into the equation that may be problematic. Ćirković notes that the equation assumes a more or less uniform physical and chemical history of our galaxy, but uniformitarianism doesn’t work well in astrophysics or cosmology (think of the Steady State theory — uniformitarian — vs. the Big Bang, which introduced the concept of epochs never experienced by observers). As Ćirković notes, the development of the galaxy places constraints on when and how life could develop. From the paper:
Obviously, the history of the Galaxy divides into at least two periods (or phases): before and after sufficient metallicity for the formation of Earth-like planets has been built up by global chemical evolution. But this reflects only the most fundamental division. It is entirely plausible that the history of the Galaxy is divided still finer into several distinct periods with radically different conditions for life. In that case, only weighted relative durations are relevant, not the overall age.
Indeed, there may be a regulation mechanism preventing the growth of technological civilizations early in the galaxy’s history. An outstanding candidate for this would be gamma-ray bursts, which could have caused biological extinctions over much of the galactic habitable zone. Now apply this to our thinking about the Drake Equation — if the idea of continuous habitability is invalid, then the results we derive from the equation will be flawed. In fact, the equation may cause us to seriously underestimate the number of technological civilizations now existant, if we assume a galactic history with sharp boundaries on when intelligent life is possible, and then factor in our own existence at this particular epoch of that history.
Image: M31, the Andromeda Galaxy. Is the development of life in such galaxies regulated by catastrophic events that limit how and when intelligence can arise? Credit: Adam Block/NOAO/AURA/NSF.
The galaxy may, in fact, be populated by technological civilizations in a state of development far more constrained than previous models suggest. And here is where the arguments of the SETI optimists and the ‘contact pessimists’ may be united. Ćirković again:
Intuitively, it is clear that in such phase-transition models it is a very sensible policy for humanity to engage in serious SETI efforts: We expect practically all extraterrestrial intelligent societies to be roughly of the same age as ours, and to be our competitors for Hart-Tiplerian colonization of the Milky Way. This class of models underlines the essential weakness in the “contact pessimist” position; as Tipler (1980) wrote: “[pessimist] argument assumes that the . . . probabilities of the Drake equation do not vary rapidly with galactic age.” Phase transition is exactly such a “rapid variation.” The price to be paid for bringing the arguments of “optimists” and “pessimists” into accord is, obviously, the assumption that we are living in a rather special epoch in galactic history, i.e., the epoch of phase transition.
Note too that this assumption is in accord with the ‘rare Earth’ hypothesis advocated by Peter Ward and Donald Brownlee in their book Rare Earth: Why Complex Life is Uncommon in the Universe (New York: Springer, 2000). Here, then, are two factors we must take account of: 1) evolutionary effects in galactic history, such as the periods before and after sufficient metallicity to create the formation of Earth-like planets and other more subtle phase shifts; and 2) catastrophic regulation (as through gamma-rays or other mechanisms) of habitability within the galaxy. A non-uniform galactic history offers surprising and positive news for SETI hunters by explaining why life may be widespread but not necessarily advanced to the level of ‘supercivilizations.’