In modern society, stargazing is a lost art.

Though the lights that web our planet testify to the reach of our technology, they have a downside as well. There are relatively few places left on Earth where the night sky has not been dimmed by the extravagance of artificial light we wastefully pour into it. In many urban and suburban areas, the light pollution is so severe that barely any stars are visible at all. Even in more rural areas, where the light pollution is less severe, many fainter stars are still washed out, and the sight of the Milky Way is lost. Only in the very few wild and unspoiled places left on this planet, on pristine, dark nights far from the busy glow of civilization, can the misty, sky-spanning band of the galactic plane be seen.

It wasn't always this way, of course.

We grew up under a night sky vast and untamed, an endless starry heaven unmarred by artificial light. For most of our history, the stars were our familiar companions, the governors of the cycles of our lives. By their rising and setting we navigated, steered our ships, charted the year, decided when to plant and when to harvest, cast our fortunes, and found in them the stories and mythic figures that formed the background to our everyday lives. And, though fewer records exist of this, I like to think that throughout our species' history we have simply admired the stars for their beauty and majesty. They have always had the power to stir in us the most profound sensations of awe and wonder.

But the companion emotion to wonder is curiosity, the bright-eyed inquisitiveness that is inherent to our nature. When some phenomenon excites us to awe, it is invariably not long before we begin to ask questions; and looking into the cosmos, there is one question that it seems almost natural to ask. There have been many people throughout the ages who have entertained the outlandish proposition that each of those twinkling pinpricks in the night sky is a far-distant sun like our own, and that some of these may even have worlds of their own. But if there are other worlds, why not other people? Is there intelligent life out there, among the stars? Are we truly alone in the universe, or are there fellow minds that it might someday be our fortune to meet?

For many centuries the question could be no more than that, but today, in this age, we are finally acquiring the tools that will enable us to investigate and potentially answer it. We have sent robot probes to study and explore the other worlds of our solar system; some of these distant voyagers carry messages, written in the universal language of science, that anyone who one day finds them will hopefully be able to understand. We have constructed great radio telescopes to scan the heavens, listening patiently for signals of other civilizations, and transmitting a few of their own in turn. And we continue to close in on solving one of the greatest mysteries of all time - how life first began on this blue and white Earth billions of years ago - a question whose answer, hopefully, will provide calibration to help answer the further question of how common life is likely to be elsewhere in the cosmos.

Regrettably, the outcome of our search has so far been negative. Our messages have gone unreciprocated; our scans have found no signals of indisputable intelligent origin. Our biosphere is a single musical theme in the vast and lonely dark, and if there are other voices, other melodies, they have so far remained beyond our hearing. Imagination inspires us, but solid fact is still lacking, if indeed there is any to be found.

Granted, there are some who feel the question of whether there is other life in the universe has already been answered resoundingly in the affirmative, and that that other life has already visited our planet and made contact with humans. I cannot share this conclusion. Although I would joyfully welcome proof of intelligent extraterrestrial beings, a claim so extraordinary cannot be supported by anything less than the very highest quality of evidence, and the evidence provided by UFO believers so far falls far short of that standard. In essence, UFO belief is nothing but a modern-day religion, and like other religions, claims validation solely on the basis of unexplained natural events and the personal testimony of believers, while failing to produce any of the positive evidence that would be needed to support its fantastic claims. A day may come when we will truly make contact with life beyond our world, but that day has not happened yet.

However, even if it is not here, what is the probability that extraterrestrial life simply exists?

Although we cannot know the answer to this question with any degree of confidence, we do have a framework that can be used to estimate. The Drake Equation, named for the astronomer Frank Drake who created it, expresses the number of intelligent species in our galaxy as the product of a variety of factors. Unfortunately, at present we can only fill in a few of these values with any certainty, and for many of the rest we have little more than guesswork. Based on what these missing values turn out to be, we could be alone, or there could be thousands or even millions of communicating civilizations in our galaxy alone. At present it is impossible to know.

Leaving the Drake Equation aside, though, I prefer to look at things in a more intuitive way. Consider our galaxy, the Milky Way. Compared to most galaxies, it is relatively large. The best estimates we have place the number of stars in the Milky Way between 200 and 400 billion, but the scale runs all the way from magnificent large spirals such as our own to the more numerous dwarf irregular galaxies, which may have only a few hundred million. Assume for the moment that we can take 100 billion as the average number of stars in a given galaxy.

How many galaxies, then, are there in the universe? This question is more difficult to answer. Light travels at a finite speed, and thus it can only have traveled so far since the beginning of the universe, which has been determined with high precision to have been 13.7 billion years ago; therefore, the farthest objects we can see are those that are 13.7 billion light-years away or less. The universe could literally be infinite, but we would never know it, because we can only see, at most, all the objects within a sphere of radius 13.7 billion light-years centered on our galaxy. Everything beyond that horizon is invisible to us, because its light has not had time to reach us and likely never will.[1]

But assume for the moment that the observable universe is all there is. In that case, current estimates are in the neighborhood of 100 billion galaxies. Therefore, the number of stars in the universe is approximately 100 billion * 100 billion, or 10²² (10,000,000,000,000,000,000,000) stars. This is a number so huge that no human mind can fully grasp its scale. However, an analogy may be helpful.

Estimate the size of a grain of sand as one cubic millimeter. Assume, as well, that there are 1,000,000 kilometers of beach coastline on this planet, each one kilometer in width and packed with sand to a depth of 10 meters. Based on these admittedly very rough calculations, there are approximately 10²² sand grains in total. Thus, we see that the number of stars in the universe is roughly comparable to the number of all the grains of sand on all the beaches of planet Earth. Think of this the next time you pick up a fistful of sand - consider how many suns are spilling through your fingers. Consider how many of these may have worlds of their own.

Is it really reasonable to believe that out of all these vast multitudes of stars, there is not one besides ours with a planetary system that harbors intelligent life?

It is, of course, possible. It could be that the appearance of life is a fluke - a one-in-a-googol chance collision of molecules, and our planet orbiting our sun in our galaxy just happened to be the lucky one. Or perhaps life is plentiful, but the evolution of intelligent life is the fluke, and human beings are the only reasoning, thinking species in a universe teeming with alien microbes and bacteria. Or - a far more dismal scenario - perhaps intelligent life arises all the time, but there is something inherent in the nature of intelligence that causes civilizations to destroy themselves through war or resource exhaustion before they ever make contact with others. All of these scenarios are conceivable; and yet, I do not believe that any of them is actually the case. I do not think of this as faith, but rather confidence in the laws of probability. As celestial neighborhoods go, ours seems quite mundane: an ordinary yellow star in an outer arm of a spiral galaxy. And the basic molecules that make up our bodies, the chemical reactions that sustain our vital processes, are not fundamentally different from the rest of the cosmos; indeed, the basic ingredients of life are common throughout the universe. There does not seem to be anything privileged about our existence, nothing that does not seem as if it could not be duplicated somewhere else. It is difficult to believe that out of all this inconceivably vast expanse, the conditions that made it possible for intelligent life to arise on Earth exist nowhere else. And even if some intelligent species arise and then destroy themselves, surely not all of them do. Although the same dangers would surely threaten any civilization, there must be at least some that have the wisdom to see the danger and take steps to avert it in time. Human beings themselves are very near this point.

In summary, I do believe it is likely that there is intelligent life elsewhere in the universe, and I think theists should agree. Granted, some hold to the type of arrogant worldview which insists in the face of all the evidence that God made the universe just for us. However, for those who do not, I think this conclusion is almost inescapable. If one believes that God has a purpose for everything he does, it seems deeply implausible that he would have created a universe so immense if he only intended life to form on one single, tiny planet. Surely these billions of galaxies and quadrillions of suns, if they were made for a purpose, were made for a purpose greater than serving as a cosmic fireworks display for our telescopes.

Of course, even if there are other intelligent beings in the universe, that does not necessarily mean we will ever meet them or know them. If the speed of light is truly an insurmountable barrier, then our chances of doing so seem bleak indeed. If they live in the next galaxy over, or even on the other side of this one, then even a mere message, sent by radio waves, could take thousands or millions of years to reach them, and just as long for their reply to return to us. Any physical craft carrying ambassadors would necessarily travel much slower, and could take thousands of years to reach even the very nearest stars. In the time it would take us to receive a message, send one to them in return, and wait for their acknowledgment, the human species might well go extinct.

And all this is assuming that they are sending a signal that we can detect and decode. It assumes that they think enough like us to send a message that we can translate, and that they use radio waves for their transmissions in the frequency bands where we're listening for them - around 1.4 GHz, the characteristic emission frequency of hydrogen, the most plentiful element in the universe. Perhaps they are so far advanced, or their thinking is so radically different from ours, that they use some other means of communication entirely, something we do not even know about yet, something we are not looking for. Perhaps they do not even have any desire to communicate. But even if they do want to communicate, if they live at any appreciable distance from us - more than a few tens of light-years - then unless their transmitter is broadcasting at phenomenal power, by the time their signal reaches us it will be so attenuated not even our most sensitive radio telescopes will be able to detect it.

But even if all these difficulties are surmounted, all the signals in the universe do no good if we are not listening. Although the receipt of an extraterrestrial message would be the most momentous discovery in the history of our species, some politicians ridicule the idea of spending money to listen to the stars; apparently, they feel that such money could more usefully be spent developing another powerful new weapon system to add to our destructive capabilities here at home. As a result, projects such as SETI are chronically underfunded, understaffed, and in need of telescope time, and huge sections of the sky are not being covered at all. We could have missed hundreds of signals already.

All in all, barring some radical advances in physics and possibly in human nature as well, the chances of our ever coming in contact with extraterrestrial civilizations seem very poor indeed. It is the cruelest of ironies to imagine a universe brightened by life, populated by intelligent species filling the planetary systems of millions of stars - yet none of these civilizations will ever meet, indeed will never even know the others exist, because they are all separated from each other by the vast, unbridgeable gulfs of space and distance and the iron laws of physics.

And yet, perhaps that is for the best. It is sad but true: at this stage in its cultural evolution, humanity is probably not prepared for the shock of such a discovery. If the existence of intelligent extraterrestrials was confirmed tomorrow, it is conceivable that it would lead us to recognize what we all have in common and become more unified, peaceful and mature as a people; however, the far more likely result would be mass panic, widespread chaos, an epidemic of cults and apocalyptic hysteria, perhaps violence and war. And would they be any more eager to meet us, if they knew exactly who we are and what we do? Would they look with admiration on a planet where a privileged few hoard extreme wealth and possess limitless material comfort, while millions more are desperately poor and thousands die each day of hunger, disease, and other easily preventable ills? Would they accept our explanations for why we continue to wage war, suffer under oppressive governments, and harm and kill others in the name of faith or profit? Or, more likely, would they consider us primitive and savage?

But on the other hand, perhaps I give humanity too little credit. For all our faults, we are a marvelously diverse, clever and adaptable species. Though more challenges await, we have overcome so many of the difficulties and prejudices that faced us in the past - and it may make these triumphs more noteworthy, not less, that we have done it in spite of all that holds us back. Trying to view the world as an alien visitor would see it, I find, helps form a very clear picture both of what we have achieved and what faults we still suffer from.

And of course, I am assuming that they would be more advanced socially than us, and perhaps that is an unwarranted assumption. It could be that they would be more like us than we might expect. It could be that they still make war on each other. It could be that many of them still live under oppressive governments and tyrannical theocracies. It could be that the majority of them suffer in poverty while a few wealthy nations fund programs to communicate with or travel to other worlds. It could even be that they themselves still have practices like slavery that we abolished long ago.

Nevertheless, the sheer vastness of the cosmos gives hope here. Even if some extraterrestrial species are at or below the moral level of humans, surely others must surpass it. Somewhere out there, circling some distant star, there must be a planet of beings infinitely more beautiful, peaceful, wise and noble than humans. The potential implications for theism are immense. Could it be that we are not God's chosen people after all? Could it be that his true children are an alien race so far away that the light from their star will never even reach us, while we, a forgotten, primitive experiment in life on a distant, insignificant world, strut about proudly, convinced in our pride that we are the pinnacle of creation?

Not only is this a humbling thought, I think it would explain a lot.

So many mysteries could be resolved by the discovery of an extraterrestrial civilization. Biologists and biochemists would be thrilled - for the first time, we could retrace the course of evolution on a planet entirely different from our own and see whether it had gone the same way. Would they be carbon-based as we are? Would water still be the essential molecule of life, or would it be some other polar solvent? Would other life still use nucleic acids as its basic informational code? Would they be bilaterally symmetric like us? Would their senses be similar to ours, or would they perceive entirely different bands in the electromagnetic spectrum? The possibilities are countless. Scientists and engineers in all fields would no doubt be thrilled by the possibility of exchanging knowledge with them - seeing what new laws, new equations and new technologies we could learn from them, and perhaps what we could teach them in return. Psychology and the social sciences would be infused with a flood of new data as well - would they be like humans, sometimes wise and sometimes foolish, or would they be far more intelligent and rational than we are? Would they have religion?

The latter possibility and the potential repercussions of the answer on both atheism and theism are alluded to in "The Theist's Guide to Converting Atheists". Were we to contact an extraterrestrial species and discover that they had a religion completely identical to some human religion, down to fine details, that would almost certainly convince me to convert. For example, Muslims claim that the Qur'an as we possess it is a reflection of a perfect and eternal book in Heaven; therefore, if we were to contact an alien civilization and find that they had a holy book that, when translated, contained the same concepts in the same order as the Qur'an, I would very likely convert to Islam. However, the sharing of such basic building-block concepts as monotheism, or prophets inspired by the gods, would not in itself persuade me - it may well be that these elements are natural parts of the progression of all religions. It also would not convince me to discover alien societies whose religions were all completely different from those of Earth - that would merely strengthen the arguments from religious confusion and locality discussed in "The Cosmic Shell Game" and "The Argument from Locality" respectively.

On the other hand, if we discovered that alien beings did not have religion or theistic belief, that would be a huge triumph for atheism. It would decisively confirm what we have been saying all along, that there is no inherent knowledge of God in everyone's hearts. Granted, since most varieties of theism are designed to be impossible to disprove with any evidence, such a discovery likely would not cause many theists to deconvert. If anything, it would probably lead many of them to denounce the extraterrestrials as demonic illusions. However, I will still hold out the challenge to theists: If we were to contact an alien species and found that they had no belief in God, would it cause you to reconsider your beliefs?

Whether other civilizations had religion or not, the effects on theism of discovering them would be enormous. For one thing, the remnants of the geocentric view - the foolish conceit that we are the center of the cosmos, figuratively if not literally - would be destroyed once and for all time. The belief that past religions were inspired by alien rather than supernatural beings would doubtless gain a great deal of credence, although I do not believe there is any good reason to attribute those origins to anything other than human factors. On the other hand, extremist factions among traditional religions would almost certainly gain in strength as well, fed by an upsurge in apocalyptic fervor caused by the inevitable belief that this discovery was a sign heralding the end of the world. However, if we could survive this period of adjustment without falling prey to suicidal madness, normality would reassert itself, as it inevitably does. Apologists would scramble to find proof that their holy books predicted extrasolar civilizations, liberal clergy would be calling for interfaith sessions with the aliens, and evangelicals would soon be forming outreach ministries dedicated to converting them. (Would they be met at the door by extraterrestrial evangelists wielding their own sacred scriptures?) Theologians would debate such weighty questions as whether the aliens were affected by original sin. The doctrinal disputes would be heated; numerous new sects would splinter off, and entire creeds might have to be rewritten. It would be a day well worth waiting to see.

Sadly, for the time being, all speculation about the effects of the discovery of intelligent extraterrestrial life on human society must remain only that. We remain, as far as we know, alone in the universe. Among the scientific community, the absence of evidence of other civilizations has given rise to what is known as the Fermi Paradox: If there are intelligent alien beings, where are they? Why do we not see evidence of their existence?

Other than the answers already posited in this essay, there is another which I have not seen mentioned in other discussions of this question: maybe we are the first. However plentiful life may be, there has to be one species that develops intelligence and high technology before all others, and perhaps by a stroke of astonishing coincidence, humanity is that species. A weaker version of this hypothesis is that we are not the first, but other civilizations have existed for a comparatively short time compared to the number of light-years between our solar system and theirs, and therefore their messages, even traveling at light speed, would not yet have had time to reach us.

However, assuming that we are one of the very first intelligent species violates a basic principle of science that can be summed up as "Assume mediocrity" - that is, do not assume that there is anything privileged or special about one's own vantage point without good evidence in favor of that proposition. Repeated experience has taught us not to disregard this rule without good reason, and in this particular case there is no such reason. We know that the Earth alone has been hospitable to life for close to four billion years, and in other solar systems it could have emerged even earlier. Likewise, human beings did not appear until relatively late in Earth's history, and even so the time it took us to develop high technology once our species evolved was, in geological terms, an eyeblink. There is every reason to believe that we are not the first or even one of the first technological species to emerge in the universe.

Why, then, are we not aware of the existence of extraterrestrial civilizations? In this essay, I have advocated a different solution: that life may indeed be plentiful throughout the universe, but that the inherent difficulties in communicating combined with a lack of political will have kept us from detecting that so far. It is possible that we will never detect it at all. But that will not stop us from imagining, as the mysterious, majestic sight of the starry night sky seems almost to invite us to do. How many of those distant twinkles, I wonder, harbor other worlds where the spark of life and intelligence burns? Is it possible that, as you look up at the stars on a warm summer evening or a snowy winter midnight and wonder if humanity is alone in the universe, at that same instant, somewhere unimaginably far away, someone else on a distant world is looking up at the same night sky and pondering the very same question?

Perhaps one day we will know for sure.

Footnotes

[1] The experienced user will note that this is an oversimplification: due to cosmic expansion, light from objects more distant than 13.7 billion light-years does fall within our horizon. However, we have not yet detected astronomical objects more distant than this in any case.