Given how much we do not know about everything from abiogenesis to the lifetime of technological civilizations, what can we say about SETI’s chances for success? Henry Cordova, a Centauri Dreams regular, is a long-time SETI enthusiast who has nonetheless been revising his thinking on the discipline’s prospects. Our one useful sample, Earth, tells us how long it took for life just to become multi-cellular, much less to reach the tiny window opened by our technological society. And need we assume that intelligence will inevitably arise even with complex biology to support it? A retired geographer and mapmaker currently living in southeast Florida, Henry served in the US Navy and was originally trained as an astronomer and mathematician. Amateur astronomy, celestial navigation and collecting star atlases occupy his time when he’s not pondering questions like how civilization might arise without technology, or whether Dysonian strategies — looking not for beacons but evidence in the form of engineering suggested by anomalies in our data — may or may not be the best way for SETI to proceed. One thing’s for sure: The questions SETI raises offer many solutions, not all of which lead to contact.
by Henry Cordova
The last few years have been kind to SETI enthusiasts. As hard scientific facts continue to accumulate, the factors in the Drake equation seem a little less unknowable and the evidence, although circumstantial, is starting to look very compelling. We know now that the molecular clouds where solar systems form are seeded with the chemical precursors to life and with the comets and meteoroids which can transfer this chemistry to planetary surfaces. The detection of organic compounds in meteorites and in the spectra of celestial bodies, as well as the serious proposal of fossils of Martian microbes, all argue that science is comfortable with the idea that life arises spontaneously wherever conditions are favorable. Our planet’s own geological history corroborates this; it appears that life began here almost as soon as the primitive Earth cooled. The non-photosynthetic food chains found in submarine vents and the recent discovery of deep crustal microbial communities have made scientists comfortable with speculation of life on Europa and other solar system sites. It now seems likely that life can gain a foothold in any suitable environment and will arise spontaneously in many places throughout the universe.
We may add to this evidence the results of one of the more mature branches of astronomy: stellar structure and evolution. Stable and long-lived stars suitable for nurturing life seem to be the rule, not the exception, and planets are probably a normal by-product of the formation of these systems. The recent detection of many extrasolar planets, albeit non-earthlike, suggests our own solar system is not unique. All the evidence is not in, but it is not unreasonable to assume that life is quite common in the Galaxy. Our own planet’s past supports the conclusion that living worlds are self-regulating and quite capable of surviving all but the most devastating of cosmic catastrophes. The combined speculations of astronomy, geology, and biology are in agreement that SETI is on the right track. In my opinion, there are tens of millions of life-bearing worlds at this moment in our galaxy, give or take an order of magnitude.
And now, the bad news… The evidence for extraterrestrial intelligence, that is, extrasolar species capable of constructing communications devices, is totally non-existent. We have very little to go on other than our own example. The Earth has been the abode of life for at least 3 billion years, but for most of that time it was represented by only the most primitive micro-organisms. For a substantial portion of Earth’s history life consisted solely of anaerobic microbes. The development of photosynthesis appears to be a bottleneck in evolution, but not as severe as the appearance of multi-cellular organisms, which did not appear until about half a billion years ago. If this pattern is not peculiar to Earth and is typical of life-bearing worlds, the consequences for SETI are troubling.
SETI requires reasonably complex and active life forms as participants–on both sides. We must expect that candidate species share an oxygen metabolism and a multicellular architecture comparable to vertebrates in sophistication. We know that intelligent primates, and possibly intelligent cetaceans, appeared on Earth only a half-billion years after the first appearance of multicellular creatures, a relatively short time by astronomical standards. But we should not conclude that intelligence was inevitable, simply awaiting a mammalian nervous system of sufficient capacity to support it. Mammals have been around for as long as the reptiles and have spent most of this time without developing any technical abilities. The dinosaurs and birds never chose intelligence as a strategy, and the cetaceans prove that even intelligence does not necessarily lead to tool-making. Primate cooperative technical intelligence, aided by language, arose only a heartbeat ago in the cosmic time scale. Its late arrival and explosive growth suggests it was not inevitable, and probably accidental. We cannot even conclude from this history that intelligence is a survival trait; it is possible that its very advantage to a species is destabilizing to the biosphere as a whole. I shall omit at this point the usual cautionary remarks about nuclear war and environmental pollution… Neither should we forget that highly advanced civilizations need not necessarily be technological, even if they start out that way.
If we determine that technical societies capable of interstellar communication are common and not prone to self-destruction; it is still not obvious that they will be eager to communicate with us. Many will be cautious about talking to strangers and choose only to listen, others may tire of SETI after the first success–just as I suspect we will, as much as I hate to admit it! In fact, the more likely a culture is to engage in SETI, the more likely that it already has all the correspondents it wants. Once a SETI-prone species makes contact, it may prefer to cancel expensive searches using energy-inefficient methods and concentrate on upgrading its existing communications and refining its technologies. In other words, if CETI is common and widespread in the Galaxy, newcomers will probably not be sought out very aggressively.
The limiting factor in interstellar communication is not distance, but time. Even neighboring civilizations might be highly separated in time, time also affects the conduct of individual contacts. Centuries have to pass before each transmission’s success can even be evaluated. If we assume that during the course of its existence the Galaxy has hosted a million technical societies, and each of them survived an average of a million years, at any one time we could expect only a hundred active cultures in the entire Galaxy–about one for every billion stars. These numbers are not very encouraging, the average age of a civilization is one of the big teasers in the Drake Equation. For ETIs to be common enough to find easily this value has to be inordinately high. It seems inescapable that even if civilizations are very common, at any one time they are few and far between.
It is risky to make predictions about ETIs, but let’s try a few very liberal assumptions in order to further explore some issues in SETI targeting. Assume that there are a million species in the Galaxy right now capable of and interested in communicating with their neighbors using microwave or laser technology (the only ways we know how!). With about a hundred billion stars in the Galaxy, this works out to about one civilization for every hundred thousand stars. The stellar density in the solar neighborhood is such that there are approximately that many stars within two hundred light-years of Earth. This line of reasoning suggests that civilizations are at least several hundred light-years apart and that for them to have the slightest chance of contacting their neighbors they must be prepared to transmit (and listen) to hundreds of thousands of stars for millions of years just to have a fighting chance of making a one-way contact. Even if our hypothetical ETI is capable of using advanced astronomical techniques to eliminate unsuitable stars, the magnitude of this task is enormous.
Any society contemplating a passive search for the first time (like us) must hope that its neighbors have been carefully tracking thousands of targets and transmitting continuously at them for millions of years in order to have even the remotest statistical chance of intercepting a signal. Passive SETI is easy, but it depends primarily on what the other guy is doing. If the passive party further expects the active member to be transmitting at him in a narrow, energy-efficient beam, and has designed its listening strategy accordingly, in my opinion, they are wasting their time and squandering their budget.
Most SETI passive strategies are based on high signal-to-noise ratio directed searches of nearby stars, selected for their high probability of harboring intelligent life (old and stable Pop I disc stars on the main sequence). But as we have demonstrated, even a perfectly suitable world is highly unlikely to be transmitting at us when we look at it. To have a good chance of intercepting a signal, we must simultaneously observe large numbers of stars, i.e., large volumes of space. The active partner will also realize this, and will know that transmitting directly at nearby sunlike stars on narrow beams will be energy-efficient but highly ineffective. To listen (or transmit) using narrow “spotlight” methods allows weak signals to be heard or sent immense distances, but does little to maximize the number of stars reached. A “floodlight” or broadcast approach will saturate small volumes of space with signal, but is very wasteful and reaches few stars for the energy expended.
I suspect advanced SETI species will adopt an intermediate “searchlight” methodology where a wide beam, on the order of a radian in diameter, will be aimed at regions where large numbers of stars are found. I envision active transmitters to be aimed at the galactic plane, the signal footprint wide enough to encompass the thickness of the entire Milky Way and nearby disk stars. The best way to listen for these signals would be to concentrate on wide areas near the galactic equator, and to worry about the location where the signal originates only after it has been detected. Design receivers for sensitivity, not resolution. In other words, sweep the galactic plane first, particularly in areas of high galactic longitude. ETI will most likely be transmitting towards the nucleus, the direction where most of the old stars are. Those civilizations at low galactic longitudes might even be transmitting across our line of sight, towards the galactic center, and we would not be able to hear them at all.
We have just recently learned, in cosmic terms, how to conduct SETI. At present, we cannot develop the antenna power to transmit a searchlight signal that would stick out above the noise for more than a few light years. We can only hope that others have been at this a lot longer than we have and will be able to do so.
The remarks above apply to the traditional thinking on SETI that has been the operational paradigm since the introduction of radio astronomy. The only way we know to make our presence known across galactic distances is with electromagnetic radiation, radio, or possibly, laser signalling devices. This may not necessarily be the case, but we have little choice but to proceed as if it were. We are forced to conclude that the only way to find ET is with physics-based technology similar to our own.
But what if there may be highly advanced civilizations with no technology at all? I suspect that high degrees of sophistication and culture are possible with no technology whatsoever. This may be no surprise to anyone reading this, but has the consequent reduction of the statistical probability of communicating civilizations sunk in to the average SETI enthusiast?
I must confess it has become apparent to me only lately. As you get older, you get wiser. Not necessarily smarter, but wiser.
And what about technical civilizations whose development proceeded among lines very different from ours? A cetacean-like creature may have evolved along acoustic lines, not mechanical or electro-magnetic. A commensal colonial organism might have concentrated on genetic technologies, breeding living organisms for practical use, rather than building machines. Some societies may have highly advanced chemistry based on biological concepts, rather than in vitro tech. Alien civilizations may not even be recognizable to our eyes as civilizations; we may only see them as complex ecosystems, rain forests or coral reefs (Gaia and Solaris-type entities come to mind) than as communities. Some civilizations may have evolved on permanently cloudy worlds, or under water, or in water under ice under clouds.
Even if we could realize them for what they are, they may be operating on vastly different timescales than we do. Those of us who read science fiction no doubt are familiar with these possibilities, but do we ever really consider how that rarest of cosmic accidents, the intelligent alien species, is made even rarer still by the fact maybe they never discovered fire, electricity, glass, metals? No matter how many of them there are, if they can’t build radio telescopes or spacecraft, how will they find us? How will we even recognize them if we find them first?
And of course, there is always the possibility they will have long ago stumbled onto technologies that make radio telescopes and spacecraft unnecessary. Broadcasting microwaves at nearby stars may be like Amazonian tribesmen trying to signal jet contrails by beating drums or lighting fires. There are lots of things allowed by nature that we do not know about, that perhaps we will never know about.
Indirect evidence suggests that SETI-capable civilizations will be highly separated in space and even more so in time. There may be a much smaller probability than previously thought to make contact with these cultures. In order to maximize this probability, we may have to rely on search strategies which are counter-intuitive. We can only hope that these civilizations will also anticipate these conditions and modify their procedures accordingly. I wish to go on record as saying that conducting passive searches for alien communications is a waste of resources. The aliens have probably realized this and are not actively signalling us anyway. Although I hesitate to suggest that any form of research be discouraged, perhaps our resources could be directed in other, more productive directions. At the very least, our searches could be piggybacked onto other research programs with a higher probability of success. It is also worthwhile to seek evidence of extraterrestrial industrial activities; navigational beacons, planetary defense radars, weapons testing, industrial accidents, experimental or accidental energy releases, or perhaps even deliberate attempts to make their presence known, such as the dumping of short-lived radio-isotopes into the local sun. It might not be worthwhile to search specifically for such speculative events, but they might very well turn up in the course of other investigations.
It may be a long time before we make contact, perhaps never. We are almost certainly not alone, but we may never be able to know for sure.