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SETI and Drake: Part II

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.

M31 galaxy

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.’

Comments on this entry are closed.

  • Joseph October 22, 2005, 13:34

    Very good points. The galactic enviroment is extroadinarily lethal. The odds of life developing are high in my assesment but the duration needed for life to select for intelligence is long enough that random disasters can easily erase that promising trend.

    Add to this of course the tendency for an aggressive life form to turn upon itself and you’ve further lowered the potential number of technologically advanced civilizations that could be present. Now please no laughter but has the concept of the self extinguishing predator been contemplated? By this I’m refering to concepts normally used for storylines by scifi writers producing the genre known as the “space opera.” While it is of course possible for non-aggressive civilizations to develope I would make the assumption that aggressive ones would be the norm. A truly aggressive species, that didn’t initially extinguish itself, could spread through galactic areas laying waste to other life forms as a matter of principle. Such a civilization would probably turn upon itself eventually but would basically sanitize large areas before doing so.

    We’ll probably have to wait until our decendents actually start poking around for an answer. It is interesting to speculate though that given even a very minute number of spacefaring civilizations it would seem at least a few would have continued spreading over the eons (we ourselves actually are at the point of having the technology to contemplate it) and produced some signature we could percieve. So what is the limiting factors?

  • Administrator October 22, 2005, 13:48

    The writer that comes immediately to mind when talking about highly aggressive species is Fred Saberhagen, though as I recall the machines he unleashes on the universe were the creation of a long-dead race. Given what we are learning about the potential for machine self-reproduction (and we’ve discussed automated, self-replicating probes here at various times), the coupling of aggression with rogue machinery could be deadly indeed!

  • jon_lomberg October 22, 2005, 16:13

    A thought to share about today’s entry. If we ask, who is motivated to transmit, rather than how many silent societies there are, it is possible to make different choices for targets used in our listening strategies. Examples might be:

    1) It is harder to conceive of a vaster tragedy than a powerful explosion in the core of a galaxy filled with civilizations. Societies in such a galaxy might be highly motivated to send out signals, just to mark their existence so they are not entirely forgotten (a similar idea on a smaller, planetary scale, was used in one of the very best episodes of Star Trek: TNG). For some reason very few people do extragalactic SETI, perhaps because of the large power requirements for transmitters and the fact that the travel times preclude any dialog, even for long-lived ETs. But a dying society might expend whatever energy is required to make a strong beacon, a shout in the dark before dying.

    2) Another non-uniform galactic phase-transition event that might impact SETI is the fact that large galaxies cannibalize small ones, as our galaxy is now doing with the Sagittarius Dwarf Galaxy on the far side of the Milky Way. Such incorporations are very common in galaxy clusters. While there is little danger of stellar collisions, the tidal forces that deform both absorber and absorbee could alter the galactic environment in ways that would affect worlds in both galaxies– for example by adding or removing dust clouds in a given world’s galactic orbit Again, this provides an incentive and motivation for such worlds to contact somebody else. We might target an entire small galaxy in the process of its merger with a larger one.

    For any individual world, there is profound danger inherent in the passage through a galactic cloud of dust or gas. Physicist and science fiction writer Gregory Benford recently published a story based on this premise called “The Hydrogen Wall” which details how our Solar Systems passage through a dust cloud or nebula would pose a serious risk to our biosphere. If that is true across the galaxy, we might target stars whose orbits dictate an imminent passage through such clouds.

  • abraxas365 October 22, 2005, 16:37

    At an intergalactic level we’re now learning of significant chemical evolution differences between the galaxies – this would go a long way towards explaining James Annis’ non-observation of galaxies steadily being wrapped in Dyson shells/clouds by Type III civilizations. As for memorials of dying civilisations a ‘shout’ might be missed, but messages-in-a-bottle might be more likely to be found. Perhaps they expend their efforts launching hardened message-probes into long duration orbits around their home-worlds or even between the stars, pushed by light-sails?

  • Administrator October 22, 2005, 17:00

    A fascinating possibility, and parallel in its way to the difference between conventional SETI and SETA — the Search for Extraterrestrial Artifacts.

    I’m not familiar with Annis’ work and if you have a citation handy and wouldn’t mind e-mailing or posting here, I’d like to look at it, perhaps for review on Centauri Dreams.