So Where Are They?
One of the questions bruited about in SF circles is the Fermi paradox: Where are the aliens? In a universe with billions of galaxies, galaxies having a hundred billion stars, it is implausible that this planet is the only abode of intelligent life, that there aren't quite a few planets around with intelligent life forms on them. Given intelligent aliens it is not likely that we lead the pack as far as technology and science are concerned. In fact, if one thinks of the billions of years involved, there should be races that are millions of years ahead of us. When we think of what our own race has managed to achieve in the past few hundred years of technological development we boggle at what could be achieved in millions of years by these hypothetical aliens. Surely their technology must be like magic to us, as far beyond our comprehension and the laser are beyond the comprehension of a caveman.
So where are they?
Why aren't they here now? Why haven't they been here already for millions or even billions of years. Why hasn't anyone been here before? If they are here now (or have been in the past) why aren't we stumbling over their artifacts?
Some years ago in Astounding Science Fiction there was an extensive discussion of the problem. Since then it has been the subject of much speculation, both in SF circles and in those groups concerned with space travel. Proposed answers include:
Theories 1-4 are different versions of "there is nobody else besides us at the moment." If there are advanced extraterrestrial races we need to account for their absence. Some more theories:
Theories 5-7 are different versions of the thesis that other intelligent races may exist but there is no travel or communication between them because the technical problems imposed by interstellar distances are simply too great. The first group of theories says that there is nobody to talk to; the second says that there is no way to talk to anybody. We have a third group of theories which says that for one reason or another people are talking all right - they just aren't talking to us. Some theories:
In theories 8-13 communication is possible; we just haven't achieved it yet. There are probably a few other major possibilities that I have overlooked in this enumeration. I believe, however, that the three general categories are complete.
This is not the first time that the problem has been considered. (Scarcely!) However much of the speculation on this topic has been published in the Science Fiction arena. This would be all right except that SF is biased by the demands of having good stories and by a number of standardized conventions. For example it is very common in SF to assume that there are a large number of intelligent races, all of them young in terms of recorded history. If one considers the vast amounts of time involved in the development of an intelligent race this is highly improbable unless some kind of common factor is involved. (It should be pointed out, though, that it is possible that our initial contacts will be with cultures of own level.)
The problem is that if one is going to try to make an attempt to answer the question one has to be prepared to speculate not only on whether or not there are technological societies in existence but also on the potentialities of advanced technology. This is one of the staples of SF and we can mine it for ideas. We have to be careful though. A notion whose consequences are inconsistent with what is known is quite permissible in SF.
Besides SF there is a rapidly growing body of work in this whole area. It is an academically respectable topic. International conferences of scholars. Good examples are books by Carl Sagan, Freeman Dyson, and Tippler and Barrow. Exobiology, like cosmology, is an attempt to understand the big picture - the rather large universe of which we are such a minute part. Although they are working with highly uncertain guesswork they are trying to do the best that they can.
We can sharpen the problem by considering it as a paradox. Current thought suggests that it is quite unlikely that we are the only intelligent race in the galaxy - that there are others besides us now and that we have predecessors who have achieved science and technology millions and even billions of years before us. (That's American billions, of course.)
Let us suppose that interstellar travel is possible. Mind you, we don't have to suppose that it is particularly fast. In Science Fiction stories it common to postulate faster than light travel. Unfortunately we have no real reason to believe that FTL travel is possible and a great deal of reason to believe that it is not. Let us suppose that speeds upward of 300 kilometers/second (.001c, 186 mps) are attainable. At these speeds it would take about two hundred million years to traverse the galaxy. This may seem like a long time; to us it is a very long time. In the history of the galaxy it would be a short noticeable time - rather like one year in the life of a man aged 75.
Note: In the original I remarked speeds of an order or two greater would be reasonable. I am now skeptical about that because of the energy requirements; kinetic energy goes up with the square of the velocity.
In any case the key point is that any race which can or will colonize other planetary systems will fill up the galaxy at approximately the rate at which it can travel. If it takes two hundred million years to cross the galaxy it takes two hundred million years to fill it end to end. The reasoning is that the ships reach a world, colonize it, and that world also sends out ships. The colonies expand in all directions at a rate that includes travel time between stars and refitting time. (Frank Tippler makes this argument; however he assumes that colonizing is done by Von Neumann machines.)
In short, on the interstellar time scale, colonization is a flash phenomenon. Once a race acquires the capability and the will for colonization it will take over the galaxy in short order. To summarize: (a) It is probable that there are other intelligent races and that they have been in existence for billions of years. (b) Interstellar travel is feasible, if somewhat expensive. (c) Out of all of these races at least one was imperialistic and it went out and colonized the stars. This happened a long time ago. (d) When this race arrived on Earth there was no life on land. (Animal life on land didn't occur until 300 million years ago.) The aliens proceed to seed the surface with their own plant and animal forms and colonize the land. Native life forms never come out of the sea except as airborne bacteria. Native land plants and animals never developed. In particular the human race never happened. (e) You and I don't exist.
That conclusion has been reached before, although usually with a somewhat different line of argument. I am willing to concede that it is correct - it is hard to conceive of any way of proving the matter once it is open to question. However believing in one's nonexistence turns out to be a very poor basis for action. It may be logically correct to assert that since one does not exist one does not have to eat. It is an economical line of argument. Despite the claims of logic and economy, however, it turns out that if you don't eat you get hungry. Since I like to eat I refuse to believe in my own non-existence. Therefore I am inclined to reject conclusion (e) above and pretend it is all wrong.
If it is granted that we exist then there must be something wrong in the line of argument that leads to point (e). The obvious point to reject is point (a). However I don't believe (a) is wrong. Consider:
There are about one hundred thousand million stars in our galaxy. Most of them are about the size of our sun, a little larger or somewhat smaller. There are a small minority of very bright or very large stars and a large minority that are substantially smaller than our sun. Somewhere between 25% and 40% could be homes for life if they had planets in the right orbits.
Planetary systems appear to be common. (Note: The original essay speculated that they would be common. That was in 1976. We have learned a lot since then.) It isn't clear how many of those systems are like ours; large planets near the primary are unexpectedly common. Planets suitable for life as we know it must meet a number of requirements. The main ones are: (a) They must be in the life zone (not too close and not too far from the primary.) For our solar system Mars and Earth are in the life zone; Venus may have been originally. The life zone is the region in which the combined effect of solar input and the greenhouse effect produces a mean temperature in which water is liquid. (b) It must be a small rocky planet. It must be large enough to hold an atmosphere and so large that it will keep the primordial Hydrogen atmosphere. Mars is too small. (c) It must have extensive bodies of surface water and must be able to keep them.
If current planetary formation theories are correct there should be large numbers of suitable planets. As a general rule dim stars have very small life zones and very bright ones don't last very long. Hence most of these will be around stars not too different from our sun in luminosity. I estimate that there about one billion suitable planets in our galaxy (this estimate is based on vigorous hand waving.) The number might be as small as ten million; I doubt it would be higher than ten billion.
Given that there some millions of planets which are candidates for bearing life, what are the chances that life actually developed. My guess (we're doing hard science here) is that the chances are pretty good. Life seems to have developed fairly quickly (less than two hundred million years) after the Earth cooled down from formation and the early bombardment ceased. We know rather more about how life originated now than when this article originally appeared (and rather less than I thought we knew.)
Having life doesn't mean that there is intelligent life with technology. What are the chances of that? I suggest that they are very good.
It might be argued that it took several billion years on Earth for intelligence to develop and that, therefore, intelligence is a rare and improbably occurrence. I submit that this line of thought is fallacious. There are two precursor events that must happen if intelligent technological life is to develop: (a) Complex (multi-cellular life forms) must evolve. (b) Land must be invaded.
The first of these requirements is the most time consuming. Multi-cellular life forms appear in the late pre-Cambrian (Ediacarian) age, about 600 million years ago. The triggering event is almost certainly the rise in oxygen levels in the atmosphere. (Gould disputes this - his main argument seems to be that physicists and other such ilk should not comment on evolution.) The rise in O2 levels occurred when the Earth had been, so to speak, rusted out. On Earth this took about 3 billion years. This is probably typical, i.e., it takes a few billion years of single celled life before multi-cellular life can develop. I will take it as highly likely that multi-cellular life will develop when it can.
I am inclined to believe that it is quite unlikely that technology (at least of space-going variety) would be developed by intelligent races which live underwater. Ergo land must be colonized by multi-cellular life. This implies that there is land to be colonized which may in turn imply that there be tectonic plate activity. In turn this implies the presence of significant amounts of radioactive metals. It may mean that early planetary systems are water worlds, i.e., they have little land surface - erosion having removed it with no replacement.
On Earth it took about 200 million years for land to be colonized and about 400 million years for us to show. The colonization of land I will also take as being highly likely. The evolution of a species with the capacity for technology is another matter.
The eminent evolutionary theorist, Ernst Mayr, has expressed the opinion that the evolution of intelligent species is highly unlikely. The general reasoning is that our particular evolutionary history depends on a large number of contingent and improbable events.
That is certainly true as far as our species is concerned. It is even true as far as the evolution of major phyla are concerned. The fact that vertebrates are the dominant land animals is a contingent event. However I think that he is wrong.
The division into plants and animals is not an accidental event; it is a consequence of the way the world is - of the general availability of niches. Likewise the existence of large land animals is not an accident - the niches are there to be occupied. Again, the division into herbivores and carnivores is not an accident. (It has been argued that the reason amniotes and not amphibians are land herbivores is because the amniote egg permits larger young; perhaps large herbivores are not inevitable.)
On Earth there has been an irregular but definite increase in the maximum encephalization quotient (EQ) of vertebrates over time. It seems to me that this is inevitable - that herbivore and carnivore are in a long term arms race in which increased EQ gives an advantage. Ergo the same pattern should occur on other worlds.
Evolutionarily speaking, human beings are improbable freaks. They are extreme outliers on the EQ distribution. However as EQ increases over time the likelihood of such outliers steadily increases over time. For this reason I think that it is inevitable on life bearing worlds that sooner or later an intelligent species will occur. It is unlikely that they will look much like us; they may look like giant lobsters or centaurs but that is unimportant; as long as they can use tools and create artifacts they are to be reckoned with.
In short, I think that intelligent life on Earth was no sort of accident at all. Something like us was certain to develop once there was animal life on land. Animal life on land was certain once the oxygen level was high enough. I suspect that Earth's time table for evolving an intelligent race is typical, give or take a billion years.
As a post-script there is a good argument to be made that the development of language is a rare event because (a) the investment in language specific neural capacity is high and (b)there is little specific benefit until a threshold is reached. See my review of The Symbolic Species for a feel for some of the issues.All of which leaves us with a Galaxy in which a lot of intelligent races have appeared over time - millions of them with a new one coming up every thousand years or so. I repeat:
Where are they?
One answer, which is ominously plausible, is that the average life span of technological races is short - most of them just don't make it. In view of our current problems it seems all too likely that we are one of the vast majority of species that won't make it.
And yet ...
What would it mean "not to make it?" One answer is that we might indulge in a themonuclear war which did enough damage to the biosphere to eliminate most of the life on land - including us. That, however, would only postpone the issue for a few hundred million years until the next intelligent race came along. I suppose there must be a fair number of intelligent (?) races that commit suicide in that particular fashion.
What about ecodoom? What about the problems that are pressing on us right now? Population pressure, exhaustion of irreplaceable resources, pollution, contamination and destruction of the ecology, et cetera? Will we be done in by these?
In the long run I think that the answer is almost certainly no.
This is not to say that the next few hundred years might not be fairly grim. Our times and those of our children look to be fairly exciting. One recalls the ancient curse, "May you live in interesting times." These are interesting times.
To be sure, there are going to be a lot of people starving to death in the next one hundred years. We may well have a species dieback in the coming century. But a population explosion and mass starvation does not mean the end of the human race. Even if our present industrial society collapses it does not mean the end of the human race. It does not even mean the end of technology and science.
The exhaustion of resources is not an important issue - in the long run. (Be it noted, however, that we don't live in the long run.) In the nature of things extractive industries such petroleum and mining are transient phenomena. In the near future - five hundred years or less - we shall learn to do quite well without them. Recycling and conservation only delay the inevitable; the real question is what we will do when all of the irreplaceable resources run out.
I only see two options. One is an agricultural society which does without science and technology. The other is a technological society in which all of the primary sources are the inexhaustible resources - solar energy, hydrogen fusion, minerals from the sea, et cetera - and which has developed a stable ecology that includes both technology and the biosphere.
I suspect that the agricultural option must be fairly common. It need not be as bad as the feudal societies of the past. We have learned a lot in the past few thousand years. We have learned a lot about learning in the past few hundred. All of that knowledge would still be available. With the population under control, with advanced biochemistry, and with good medicine it might not be too bad. It might look a lot like modern China. I don't think I would like it....
However I suspect that the human race will eventually find its way to a high level of technology that is stable. I say this because I don't think that the knowledge will disappear. Without the resources that we are so recklessly squandering it will be much harder to build the requisite computers, fusion plants, et cetera. Given the existence of the knowledge, though, I think the temptation to try will always be present and that one ruling elite after another will make the attempt until one finally succeeds.
What of the chances of ecodeath - of fouling up our environment to such an extent that the ecology collapses? I think it is improbable. To be sure we can and are doing a lot of damage; we are pouring poisons into the environment and destroying ecosystems. We may well pay a very high price for what we have done and continue to do in the next century. But we do learn. You don't use such and such technology because the side effects are just too expensive. Many more species will be lost. There will be disasters. In the long run, though, these kinds of problems will be solved.
There are more potential hazards. Here are three that I can think of right off the bat. In the next hundred years we will probably go in for genetic engineering in a big way. There is a reasonable chance that we will make an irretrievable mistake. In the next hundred years we will probably develop artificial intelligence and use it in a big way. The mad computer that destroys the world is a possibility. And then there is the possibility that a world encompassing high level technology, an incredibly complicated interconnected network, has an unpredictable instability and the whole world crashes.
There are a lot of ways to go to hell in a handbasket....
A lot of races which develop technology probably don't make it....
Unless, of course, they get help from the stars....
Even if the chances of surviving the first thousand years of technological civilization are poor there should be quite a few survivors. Those survivors should last for a long time. Remember, all that is required is that there be one race that develops interstellar travel and has a taste for expansion; they will take over the entire galaxy in due time. On the evidence there isn't even one.
Perhaps interstellar travel is impractical? I don't think so. Relativistic speeds (.1C and greater) are impractical. They require too much energy in even very optimistic scenarios. The interstellar rocket is ruled out by the rocket equation (carrying your reaction mass loses big time.) However rockets are not the only possibility; energy can be transferred to a vehicle via lasers and linear accelerators. (FTL is by means ruled out - GR permits it. Physicists have come up with excessively exotic possibilities.) The real difficulty is time.
Consider a ship travelling at .001C. It will take thousands of years to transit from one star to another. To us this is grossly impractical. Suppose, however, that our lifespans were twenty thousand years and more. Then such trips are no longer inconceivable. Technological civilizations which have been around for millions or even billions of years would have a different perspective.
Which leaves us with the original problem. If life is highly likely, if there are a multitude of intelligent races with technology, and if interstellar travel is feasible, then where are they?
I don't know.
I thought about the possibility that nobody considers interstellar conquest as being worth the trouble. After all, if you have managed to solve the problems of living on your home world there are no real motives for interstellar colonization except that of ensuring racial survival by scattering your seed. To survive at all intelligent races must have devised societies that are non-expansive. Having done so, they've lost their motives for expansion. The trouble with this line of though is that a race might well wish to launch a campaign of conquest and colonization for purely abstract reasons which are not economic.
Another possibility that I have played with is that Arthur C. Clarke is right. The future belongs to the intelligent machines. Perhaps the galaxy is girdles with intelligent machines talking to each other. Perhaps they know all about us and are watching and waiting right now. However they don't care about us. What they are waiting for is for us to develop the intelligent machines that will succeed us. A somewhat disquieting thought.
I don't think that is too likely. Maybe I'm overly optimistic....
There is another possibility which struck me as I was writing this. We have argued that there must be quite a few intelligent extraterrestrial races with advanced technology. We have also argued that interstellar travel is feasible but rather pointless. On the other hand interstellar communication is quite practical for long lived, high technology races. There should be a good deal of it. We have also argued that advanced races probably have no interest in interstellar colonization. Finally, it seems unlikely that there haven't been any potential interstellar conquerors.
Consider the threat presented by a paranoid race with a mature technology. Suppose they wanted to destroy other intelligent races. They probably could do so. What they would do would be to send ships to the stars of other races, ships equipped with a device that would cause the stars to become novae.
Any intelligent race present an unpredictable danger to all other races. Even worse, the danger is probably not from one of the races which is willing to communicate but rather from one which is not - one which is totally unknown.
When intelligent life first appeared in the galaxy, say seven billion years ago, the danger must have been obvious. It seems reasonable that they would have done something about the possibility. And this is what they might have done....
Well beyond Pluto, in a cometary orbit that takes it in past the Sun every twenty thousand years, is a ship. It is ancient, that ship. It has been there for three billion years. But it is not dead, that ship. Its builders had the resources of a galaxy wide culture four billion years old to draw upon. They built very well. Inside that ship is an intelligent machine. For billions of years that machine has drowsed. Every twenty thousand years it makes a quick pass at the Sun and renews its energy supplies. It is patient, that machine. It has to be.
Lately that machine has come very much alive. It watches our planet for one particular thing. It knows that there is a galaxy wide communication net. It knows that sooner or later we will begin to listen to the net and that when we do we will be warned about it.
It waits. It knows that we are there because our planet is radiating radio energy like a star. It waits for one of two things. One is an attempt to destroy it. The other is the launching of an interstellar ship without announcing it and its destination on the galactic net. If either of these events that it has waited so patiently for occurs it does its thing.
And the Sun goes nova....
Richard Treitel has a page containing links to several articles on the Fermi paradox. There is a fair bit of material on the web on the subject.
Geoffrey Landis has an interesting page which shows that, under fairly reasonable assumptions, there is a strong likelihood that the colonization of the galaxy by an intelligent species would leave holes, i.e., unvisited systems.
This page was last updated July 17, 2003.