Where are they, then?

Sixty years ago Enrico Fermi asked this question when the discussion of the prevalence of extraterrestrial intelligence came up. It's a fair question.

Over the past fifty years or so, various writers, some very carefully examining all the known facts, and others perhaps a bit less careful, have addressed the question of whether it's realistic for the human race to expect to encounter alien technological civilizations (usually the expectation being that they will be contacted by radio). Frank Drake came up with the famous Drake Equation in the early 1960s to attempt to give a number to the technological civilizations in the Galaxy (let's call them, rather, spacefaring, meaning that they have made themselves detectable from outside their own home star system). The problem with Drake's "equation," as variously treated by different authors, is that virtually all of its terms, beyond the basic physical facts about the Galaxy and its stars, are pretty much informed (at best) guesses, some usually pretty clearly colored by wishful thinking. The factors are designed so that the number of spacefaring civilizations will equal a fraction of the number of stars in the Galaxy, by estimating how close to unity various factors are. Some of these are: How many (i.e., what proportion of) star systems have planets with living organisms? How many of these develop complex multicellular life? How many of these develop intelligent life? How many of these develop outer-space technology? (E.g., there's no sign that Earth's dolphins, who may well be as intelligent as we, will ever do so; which begs the question of just what is intelligence, anyway, and how biased is our view of what it essentially is by our own characteristics as a species?) How long does an average spacefaring civilization last? (This of course determines how likely we would be to encounter any given civilization in a given period of time). The original Drake equation ignored the physical evolution of the Galaxy, whereby in its earlier phases the likelihood of intelligent life was presumably zero, since the interstellar medium from which stars formed was pretty much devoid of the necessary elements for the evolution of life for the first billion or more years of the Universe's (and therefore necessarily any particular galaxy's) lifetime.

In any case, there are no definitive answers to any of these questions, and estimates have tended to be on the rosy side. See this post for reasons to believe that complex life, and therefore intelligent life, may be a good deal rarer in the universe at large than formerly thought. Many of the Drake factors could well be very low, in which case spacefaring civilizations could be quite rare... or we could even be unique in all the Galaxy. I think it cannot be dismissed that there may be some number well above zero (one, really, counting us), on the other hand, but the fact is we don't have a lot of evidence from which to make informed estimates, so people's proclivities and wishes tend to infect their estimations. 

Most scientifically minded people allow that conditions similar to the early Earth are probably not all that rare, and that life is likely to arise in such conditions, although even this is debatable. There are those who, not resorting to any kind of religious argument or metaphysics at all, nonetheless conclude, using Occam's Razor reasoning with regard to the likely permutations of molecules, that life may be a fluke, so unlikely to arise that it is probably unique to Earth in all the universe. Personally, I'm convinced by explanations for why a purely probabilistic analysis of how likely it is, for example, for DNA molecules to arise spontaneously from simple organic precursors, are not realistic. See, for example, the excellent work of Christian René de Duve on the origin of life. I think there's a good intellectual case to be made that basic living systems, comparable to bacteria, at least, will arise in time wherever the conditions are present and stable long enough for it to do so.   

I have not yet read Jack Cohen and Ian Stewart's Evolving the Alien, but from what I can gather their thesis is that we need to expand our thinking about how and where life may evolve, and not merely assume that a planet much like Earth is the only possible place of origin. Fair enough, but I don't see how most of the observations I talk about here are affected much by widening the scope of exobiology a bit; the facts about evidence or lack thereof of its presence remain. I am not arguing that intelligent beings do not likely exist elsewhere than Earth. I am only arguing for a certain perspective and realism in discussing this subject. 

It's generally been accepted by most who have written about this subject that the Earth and its Solar System bear no evidence of ever having been actually physically visited by alien beings (or by their machines), UFO true believers notwithstanding. I would have to call this an open question. We have hardly explored the Solar System in sufficient depth, and our knowledge of the form that alien technology might take is sufficiently paltry, that no definitive statement to the effect that we have never in our planet's 4½ billion year history been visited by alien intelligence, is possible. Still, there is no positive evidence either, so at least we can say that there doesn't appear to be any active visitation going on. Of course it isn't possible to absolutely rule out the Galactic Zoo hypothesis, whereby we are under clandestine observation by aliens careful to conceal their presence, but in the absence of actual evidence for this, it remains purely speculative. To my personal prejudices, I consider this idea just short of silly and not at all likely.

Technically, indeed, as discussed above, the existence of any life of any kind anywhere other than this planet is speculative, but given what is known about the depth and breadth, as well as the essential sameness of the stars and galaxies which comprise the vast universe, I take it as a reasonable supposition that life, if it could evolve once, must surely have done so countless times, in sufficiently similar contexts; and that, however rarely, nonetheless somewhere and at some times, it must have evolved into beings to some arbitrary degree comparable to ourselves. People with certain religious--or other philosophical-- views (e.g. Ray Kurzweil) will reject this notion out of hand, but holding an opposite view, it seems to me, is what requires special pleading: it is actually hard to imagine that what has happened here has not happened, in essentials, elsewhere. The essential, really unanswerable, question, is how often? Are beings comparable to ourselves common in the universe, vanishingly rare, or somewhere in between? The fact is, we just don't know.

So what can be said? Not a great deal, definitively, but there are some additional factors that need to be taken into account.

What about machines visiting the Solar system in the past and having left evidence? As noted, this can't be ruled out. But how likely is it that some past civilization would have actually done this? The brilliant theoretician John von Neumann is credited with first conceiving of self-replicating machines, sometimes called Von Neumann Machines. Elaborating on this a bit, the idea is that a sufficiently advanced technological civilization is likely to wish to explore the galaxy, and to do so, eventually, it is likely to build spacecraft which use Von Neumann's idea: they would be capable of using raw materials they encounter in space to construct copies of themselves, to proceed further, possibly in exponentially increasing numbers, to explore star after star, reporting back to 'home base' (the programming is reproduced as well). It is a pretty simple calculation to realize that, even without fantasizing about unknown physics allowing faster than light travel, or particularly exotic energy technologies, such systems would be at least theoretically capable of visiting every last one of the 300 billion stars in the Galaxy is a small fraction of the age of the Galaxy. I've seen estimates of perhaps one to no more than a few million years for such a project, to explore a galaxy that (like all galaxies) is something on the order of 13 billion years old. Is this possible? Since it's never actually been done, as far as we know, I don't know that anyone can really say. Maybe even if some particularly relentless and determined explorer species tried this, their program might fail just from the inevitable incursion of noise into the programming. But the multiply redundant code of DNA in living genes suggests that sufficiently detailed and exact information can be preserved sufficiently intact over tens or hundreds of millions of years, so this doesn't seem to be a legitimate objection, at least in principle.

The Scottish science fiction writer Iain M. Banks has publicly dismissed the whole Von Neumann Machine scenario on the grounds of aesthetics. It's just so mechanical, and ultimately, boring, he would argue. What conclusion can be drawn from any of this? I would say only that advanced civilizations capable of building self-replicating automatic exploration machines, if they have ever existed in this galaxy, are (in all likelihood) not currently active and have not left any very noticeable traces here in the Solar System. Can this scenario, having happened in the past, or likely to happen in the future, be ruled out? No, but beyond that there really isn't much to say about it.

So what about "SETI," the search for extraterrestrial radio communications? SETI enthusiasts always say that their search is just barely begun, and that it isn't realistic to expect them to have succeeded by now, because the Galaxy is so vast, and the available bandwidths and modulation potentials for communication so various and extensive. But is this really a reasonable point of view? It seems to me that the whole premise of SETI is that other civilizations will be actually a lot like us, using electromagnetic waves to communicate by superimposing artificial patterns on them, and further that any such civilizations will continue to do this for long periods of time, so that there would be a reasonable probability that they would exist and be doing this concurrently with our existence, and we might detect this activity passively. I think this is at best an untested guess. We may well have advanced in the relatively near future to the point where we no longer use leaky EM broadcast signals for any form of communication. Maser or Laser communications (tight-beam) would probably not be detectable by anyone not their intended recipients over any great distances of space, and this technology already exists. To my mind, the fact that we have not detected other civilizations from their communications is not surprising, but it also doesn't tell us much about how likely other civilizations are to exist, and not, or not only, for the reasons the SETI folks suppose. It may just be that they are on a fool's errand.

What does the fact that there's no obvious evidence of visitation to Earth by extraterrestrials at any time in the past tell us about the likely potential for space travel? Here, I think, the implications are clear, and they are not what many space enthusiasts like to believe. Let's make some assumptions and see where they lead us. Assume that there have been in the Galaxy some substantial number of civilizations which have well exceeded our level of technology, over the at least few billion years that stars a lot like the Sun have existed, such that at least potentially a civilization comparable to ours could have arisen. Assume that some reasonable fraction of these persisted, or continue to exist, for long periods of time, such as at least many thousands of years (making it likely that some exist now). Next, assume that we have not learned all the fundamental laws of physics, and, after all, despite all the sound reasons what we have learned tells us otherwise, it turns out that it is feasible to build starships, wonderful engines of technology that can transport beings and materials from one star to another at speeds faster than light. And at least some fraction of these putative past and present civilizations have discovered this secret, and actually built such things. Star Trek!

Well, there is a serious problem with the implications of these assumptions in combination. It's the Von Neumann implication all over again, only moreso. If a civilization with even a very modest rate of natural growth and stable technology capable of traveling between stars at faster than light speed (if that were even possible) existed, and if it persisted for a few tens of thousands of years, it would have plenty of time to visit every single star in the Galaxy. If it persisted for millions of years, it would have had time to visit a few of the nearer outer galaxies as well. And surely, with all that awesome power, it would, somewhere in all the vast cosmos under constant observation by thousands of terrestrial astronomers, have left some evidence... some radiation that didn't fit a natural cause; some star whose energy signature was obviously artificially modified, some artifact somehow detectable using the powerful telescopes and other means of detection at our disposal. To my mind, all this seriously strains credulity. The fact is that nothing has ever been discovered which is even seriously suspected of being the product of an advanced technological civilization. I think the fact that we live in a quiet galaxy, where to all appearances our Solar System has remained undisturbed for billions of years, speaks volumes, and what it tells us, if we have ears to hear, is that no technology exists anywhere which can transport matter or even signals faster than light. This is an actual limitation, a real barrier, imposed on us by the nature of reality in our universe, that is and has always been an effective limit on the ability of any organisms anywhere to travel or propagate themselves in space. No amount of scientific or technological progress is going to change this fact, ever.

To those who might object, "what if they just didn't want to visit every star?," I'd have to say, well, maybe so, but surely in all of the Galaxy, and in all that time, if this were possible, and if such civilizations existed in any numbers, someone would have done this. I think the conclusion is all but inescapable that a Star Trek type universe, with tens or hundreds of concurrently existing spacefaring civilizations, all capable of traveling faster than light and zipping around the galaxy in small fractions of their individual lifetimes, will forever remain a fairy tale.  

The fact that, for reasons I don't have time or space to go into here, travel at speeds faster than light is the exact functional equivalent of time travel, and that both of them violate fundamental laws of causality, is at least suggestive that there are very good reasons why faster than light travel is and always will be impossible.  

The fact is that stars are very, very far apart (see this), and travel between them takes a very long time, in any vehicle likely to be developed, even by advanced technologies. It is also likely to remain expensive, in terms of fraction of resources that a civilization would likely as soon devote to other uses, although a sufficiently advanced civilization may have resources to freely spend without much care. So, generally, star travel is likely to remain essentially infeasible, except, conceivably, for one-way colonization and/or exploration missions. Extremely long lifespans of the travelers could change this, but it seems to be a real limitation. This is a subject of much speculation, which I will defer for another discussion, except to say that these same considerations would apply, and would have applied, to other civilizations as much as to our own.

Another subject of much speculation is whether machine civilizations might come into existence. There has been a fair amount of speculative fiction written on this subject. I have rather definite views on the possibility of machine consciousness (not), but I will defer that subject as well, and merely observe that whether you are talking about biological civilizations, machine civilizations, or hybrids thereof, makes little difference for this discussion. Assuming, that is, that machine civilizations must necessarily arise originally as byproducts of the civilizations of living beings. 

I will also leave alone any metaphysical speculations on the existence of nonphysical entities, whose intelligence and presence may not be observable conventionally. This is a fascinating topic, but it has no direct bearing on the issue, which assumes ordinary physical existence as a starting point for discussion.  

What about advanced civilizations' artifacts? Freeman Dyson and science fiction writer Larry Niven postulated, respectively, "Dyson spheres," huge spherical artifacts which contain a star at their center and host living space on their inner surfaces; and "ringworlds," bandlike rings orbiting stars at roughly earth-distance, with habitable inner surfaces. Iain M. Banks modified the ringworld idea to postulate what he calls an "orbital," a rotating ring about 3 million km. in diameter, orbiting a suitable star, which could be spun to have approximately Earthlike centrifugal gravity equivalent on its inner surface, and, as it works out, approximately a 24 hour cycle of light and dark, just like a planet. These fictional future technologies (or current or past technologies of hypothetical advanced civilizations elsewhere) are, to say the least, conjectural, but even if they exist, in large numbers, would they be detectable? I'm not sure of the answer to that. I've seen it speculated that a Dyson sphere, at least, would be noticeable within a certain range. It would be a dully radiating infrared source, emitting all the energy of its concealed star. Such an object would be detectable for quite large distances, and would be sufficiently unlike any natural objects to raise some red flags. Conceivably, the smaller versions might exist and go unnoticed, but this at least suggests that civilizations that use all the energy of their home stars are not common, if they exist or have previously existed at all.

The Russian scientist Nikolai Kardashev pursued these ideas to a logical extreme in the 1960s, postulating the existence of different levels of super-advanced civilizations, with Type I consuming all the energy of a single star, all the way up to Type IV, which would consume the equivalent of all the energy of the entire universe. On this scale, we aren't even at Type I -- far from it. Looking out into space, we do not see any obvious evidence of civilizations which are using all the energy of their star, and re-emitting it as infrared. Still less are there any reasons to suppose that any of the fascinating objects observed in the Galaxy or beyond is the glow of the waste heat of a supercivilization. To my mind, all of this is pure fantasy, with the possible exception of advanced cultures that use some considerable fraction of the light of a star, and might eventually be detectable as a result. It seems to me looking for such things might actually be worthwhile. Certainly actually finding one would be of earthshaking importance. But the idea that there are civilizations anywhere using all the energy of a galaxy seems to me fanciful in the extreme, particularly in the complete absence of any actual evidence for even the lesser 'classes.'

So what can we conclude from all this? Where, in fact, are they? I think from what we observe we can conclude that advanced technological civilizations, at our level or above, are not, and have not been in the past, very abundant. If they were, over long periods of time, it seems to me reasonable to assume that there would already be some evidence, somewhere, of their existence.

For all intents and purposes the Galaxy has been more or less as it is now for billions of years, so other worlds could have formed earlier than Earth (or later, for that matter, since rates of evolution are not necessarily uniform)... and reached an equivalent of Earth's current evolutionary phase many millions of years ago. Therefore, there is no reason to believe, at least in terms of hundreds of thousands or even millions or tens of millions of years, that there is anything special about this time, as opposed to some earlier time. So civilizations could have arisen and surpassed our technological level, persisting for long periods of time, at least some tens or even hundreds of millions of years ago. Had any significant number of them at any time reached a level where their presence was likely to leave unmistakable traces... artifacts sent roaming the Galaxy to search for life; huge space arcologies, whatever... it seems likely that some residual evidence of that fact would persist and be detectable, somewhere in the vastness of the Galaxy. Is there such evidence? We can't be sure, because our technology for detection remains of uncertain prowess. We have not exhaustively explored places like the Moon, where there is no erosion, and where small artifacts from an actual visit remote in time could remain. Also, the works of a civilization at any distance, particularly one which has ceased to function, could well be completely undetectable. But still, the absence of evidence, while, in the famous formulation (attributed to Carl Sagan), not being evidence of absence, is nonetheless suggestive: it seems to me that we should accept as an operating assumption, since there is literally nothing in all the observational history of our world that can reasonably be ascribed to extraterrestrial life, that many of the factors in the Drake equation are a good deal less than one, and that advanced civilizations are not really all that common, nor necessarily all that long-lived when they do arise.

To my mind the facts point to a tentative conclusion that space is vast, and likely mostly empty of beings (to whatever arbitrary degree) "like" ourselves. I find it hard to imagine that intelligent beings haven't existed, and don't continue to exist, elsewhere in space, but space is really, really big, so we could well be alone in a huge volume, which could mean communication with or even detection of extraterrestrial civilizations may just not be effectively possible. Maybe someday we (or our remote descendants) will find and somehow converse with our like out there somewhere, but there doesn't seem to be any good reason to expect this to happen any time soon. 
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See further thoughts here.