CHAPTER EIGHT
The Search for Spock “It's difficult to work in a group when you are omnipotent.”
Q, upon joining the crew of the Enterprise, in “DŽjˆ Q”
"Restless aggression, territorial conquest, and genocidal annihilation ... whenever possible.... The colony is integrated as though it were in fact one organism ruled by a genome that constrains behavior as it also enables it.... The physical superorganism acts to adjust the demographic mix so as to optimize its energy economy.... The
austere rules allow of no play, no art, no empathy."
The Borg are among the most frightening, and intriguing, species of alien creature ever portrayed on the television screen. What makes them so fascinating, from my point of view, is that some organism like them seems plausible on the basis of natural selection. Indeed, although the paragraph quoted above provides an apt description of the Borg, it is not taken from a Star Trek episode. Rather it appears in a review of Bert Holldobler and Edward O. Wilson's Journey to the Ants, and it is a description not of the Borg but of our own terrestrial insect friends. 1 Ants have been remarkably successful on an evolutionary scale, and it is not hard to see why. Is it impossible to imagine a cognizant society developing into a similar communal superorgan-ism? Would intellectual refinements such as empathy be necessary to such a society? Or would they be a hindrance?
Gene Roddenberry has said that the real purpose of the starship Enterprise was to serve as a vehicle not for space travel but for story-telling. Beyond all the technical wizardry, even a techie such as myself recognizes that what makes Star Trek tick is drama, the same grand themes that have driven storytelling since the Greek epics love, hate, betrayal, jealousy, trust, joy, fear, wonder.... We all connect most closely with stories that illuminate those human emotions that govern our own lives. If warp drive were used merely to propel unmanned probes, if the transporters were developed merely to move soil samples, if medical scanners were utilized merely on plant life, Star Trek would never have made it past the first season.
Indeed, the “continuing mission” of the starship Enterprise is not to further explore the laws of physics but “to explore strange new worlds, to seek out new life and new civilizations.” What makes Star Trek so fascinating and so long-lived, I suspectis that this allows the human drama to be extended far beyond the human realm. We get to imagine how alien species might develop to deal with the same problems and issues that confront humanity. We are exposed to new imaginary cultures, new threats. It provides some of the same fascination as visiting a foreign country for the first time does, or as one sometimes gets from reading history and discovering both what is completely different and what is exactly the same about the behavior of people living centuries apart.
We must, of course, suspend disbelief for such entertainment. Remarkably, almost all alien species encountered by the Enterprise are humanlike, and they all speak English! (In their defense, the Star Trek writers invented, in the sixth season of The Next Generation, a rationale for this. The archeologist Richard Galen apparently discovers that a wide variety of these civilizations share genetic material, which was seeded in the primordial oceans of many different worlds by some very ancient civilization. This is a notion reminiscent of the Nobel laureate Francis Crick's [only partly] tongue-in-cheek theory of Panspermia.) 2 This has not escaped the notice of any trekker, and it was perhaps most colorfully put to me by the theoretical physicist and Nobel laureate Sheldon Glashow, who said of the aliens, “They all look like people with elephantiasis!” Nevertheless, he is willing to ignore, as are most trekkers, these plot contrivances in order to appreciate the Star Trek writers' exploration of alien psychologies. Hollywood screenwriters are generally neither scientists nor engineers, and thus it is natural to expect that most of their creative energy would go into designing alien cultures rather than alien biology.
And creative they have been. Besides the Borg and the omnipotent prankster Q, over two hundred specific life- forms populated the Star Trek universe at the point when I gave up counting. Our galaxy is apparently full of other intelligent civilizations, some more advanced and some less advanced. Somelike the Federation, the Klingons, the Romulans, and the Cardassianscontrol large empires, while others exist in isolation on single planets or in the emptiness of space.
The discovery of extraterrestrial intelligence could be, as emphasized by the practitioners of the ongoing search, the greatest discovery in the history of the human race. Certainly it is hard to imagine a discovery that might change our view of ourselves and our place in the universe more than this. Nevertheless, after three decades of concerted searching, we have yet to find any definitive evidence for any form of life outside our own planet. One might find this surprising. Certainly, if there is life out there, it seems inevitable that we should find it, just as many of the civilizations that independently emerged on several continents here on Earth eventually ran into each other, sometimes traumatically.
Nevertheless, when one thinks in some detail about the likelihood of discovering intelligent life elsewhere in the universe, the daunting nature of the search becomes clear. Consider, for example, that some other civilization in the galaxy was informed somehow of exactly where to look among the 400 billion or so stars in the Milky Way to find a planet that could support life. Say further that they were directed to look in the direction of our Sun. What is
the probability even then that they would discover our existence? Life has existed on Earth for much of the 4.5 billion years since it formed. Yet only in the past half century or so have we been transmitting any signals of our existence. Furthermore, only in the past 25 years or so have we had radiotŽlescopes sufficiently powerful to serve as radio beacons for observation by other civilizations. Thus, in the 4.5 billion years during which aliens might have been scanning the Earth from space, they could have discovered us only during the last half century. Assuming that an alien civilization chose to make its observations at some random time during the planet's history, the possibility of discovering our existence would be about 1 in 100 million. And I remind you, this applies only if they knew exactly where to look!
There have been whole books written about the possibility of life existing elsewhere in the galaxy, and also about the possibility of detecting it. Estimates for the number of advanced civilizations range from millions on the high side to one on the low side (liberally interpreting our own civilization as advanced). It is not my purpose to
review all the arguments in depth here. I would like, however, to describe some of the more interesting physical arguments related to the origin of the sorts of life the Enterprise was sent out to discover, and to discuss some of the strategies currently being employed here on Earth to search for it.
The a priori argument that life should exist elsewhere in our galaxy seems to me to be compelling. As noted, there are roughly 400 billion stars in our galaxy. It would seem truly remarkable if our Sun were the only one around which intelligent life developed. One can propose what on the surface seems like a more sophisticated argument to estimate the probability that life like ourselves occurs elsewhere, starting with obvious questions such as: “What is the probability that most stars have planets?” or “What is the probability that this [particular] star will live long enough to sustain life on a planetary system?” and then moving on to planetary matters, such as “Is this planet big enough to hold an atmosphere?” or “What is the likelihood of its having undergone sufficient early volcanism to produce enough water on the surface?” or “What is the probability of its having a moon either massive enough or close enough to produce tides sufficient to make tidal pools where life might originate, but not daily tidal waves?” While I will discuss some of these issues, the problem with trying to determine realistic probabilities is, first, that many of the relevant parameters are undetermined and, second, that we do not know how all the parameters are correlated. It is difficult enough to determine accurately the probability of everyday events. When one sets out to estimate a sequence of very small probabilities, the operational significance of such an attempt often becomes marginal.
One should also remember that even if one derives a well-defined probability, its interpretation can be pretty subtle. For example, the probability of any specific sequence of eventssuch as the fact that I am sitting in this specific type of chair typing at this specific computer (among all the millions of computers manufactured each year), in this specific place (among all the possible cities in the world), at this specific time of day (among the 86,400 seconds in each day) is vanish-ingly small. The same can be said for any other set of circumstances in my life. Likewise, in the inanimate world, the probability that, say, a radioactive nucleus will decay at the exact moment it does is also vanishingly small. However, we do not calculate such probabilities. We ask, rather, how likely it is that the nucleus will decay in some nonzero time interval, or how much more probable a decay is at one time compared to another time.
When one is attempting to estimate the probabilities of life in the galaxy, one has to be very careful not to overrestrict the sequence of events one considers. If one does, and people have, one is likely to conclude that the probability that life formed on Earth when it did is infinitesimally small, which is sometimes used as an argument for the existence of Divine intervention. However, as I have just indicated, the same vanishingly small probability could be assigned to the likelihood that the stoplight I can see out my window will turn red while I am waiting in my car there at precisely 11:57 A.M. on June 3, 1999. This does not mean, however, that such a thing won't happen.
The important fact to recognize is that life did form in the galaxy at least once. I cannot overemphasize how important this is. Based on all our experience in science, nature rarely produces a phenomenon just once. We are a test case. The fact that we exist proves that the formation of life is possible. Once we know that life can originate here in the galaxy, the likelihood of it occurring elsewhere is vastly increased. (Of course, as some evolutionary biologists have argued, it need not develop an intelligence.)
While our imaginations are no doubt far too feeble to consider all the combinations of conditions which might give rise to intelligent life, we can use our own existence to ask what properties of the universe were essential or
important in our own evolution.
We first begin with the universe as a whole. I have already mentioned one cosmic coincidence: that there was one extra proton produced in the early universe for every 10 billion or so protons and antiprotons. Without these extra little guys, matter would have annihilated with antimatter, and there would be no matter left in the universe today, intelligent or otherwise.
The next obvious feature of the universe in which we live is that it is old, very old. It took intelligent life about 3.5 billion years to develop on Earth. Hence, our existence requires a universe that accommodated our arrival by lasting billions of years. The current best estimate for the age of our universe is between about 10 billion and 20 billion years, which is plenty long enough. It turns out, however, that it is not so easy a priori to design a universe that expands, as our universe does, without either recollapsing very quickly in a reverse of the big banga big crunchor expanding so fast that there would have been no time for matter to clump together into stars and galaxies. The initial conditions of the universe, or some dynamical physical process early in its history, would have to be very finely tuned to get things just right.
This has become known as the “flatness” problem, and understanding it has become one of the central issues in cosmology today. Gravitational attraction due to the presence of matter tends to slow the expansion of the universe. As a result, two possibilities remain. Either there is enough matter in the universe to cause the expansion to halt and reverse (a “closed” universe), or there is not (an “open” universe). What is surprising about the present universe is that when we add up all the matter we estimate is out there, the amount we find is suspiciously close to the borderline between these two possibilitiesa “flat” universe, in which the observed expansion would slow but never quite stop in any finite amount of time.
What makes this particularly surprising is that as the universe evolves, if it is not exactly flat then it deviates more and more from being flat as time goes on. Since the universe is probably at least 10 billion years old today, and observations suggest that the universe is close to being flat today, then at much earlier times it must have been immeasurably close to being flat. It is hard to imagine how this could happen at random without some physical process enforcing it. Some 15 years ago, a candidate physical process was invented. Known as “inflation,” it is a ubiquitous process that can occur due to quantum mechanical effects in the early universe.
Recall that empty space is not really empty but that quantum fluctuations in the vacuum can carry energy. It turns out that it is possible, as the nature of forces between elementary particles evolves with temperature in the early universe, for the energy stored as quantum fluctuations in empty space to be the dominant form of energy in the universe. This vacuum energy can repel gravitationally rather than attract. It is hypothesized that the universe went through a brief inflationary phase, during which it was dominated by such vacuum energy, resulting in a very rapid expansion. One can show that when this period ends and the vacuum energy is transferred into the energy of matter and radiation, the universe can easily end up being flat to very high precision.
However, another, perhaps more severe, problem remains. In fact Einstein first introduced the problem when he tried to apply his new general theory of relativity to the universe. At that time, it was not yet known that the universe was expanding; rather, the universe was believed to be static and unchanging on large scales. So Einstein had to figure out some way to stop all this matter from collapsing due to its own gravitational attraction. He added a term to his equations called the cosmological constant, which essentially introduced a cosmic repulsion to balance the gravitational attraction of matter on large scales. Once it was recognized that the universe is not static, Einstein realized that there was no need for such a term, whose addition he called “the biggest blunder” he had ever made.
Unfortunately, as in trying to put the toothpaste back into the tube, once the possibility of a cosmological constant is raised, there is no going back. If such a term is possible in Einstein's equations then we must explain why it is absent in the observed universe. In fact, the vacuum energy I described above produces exactly the effect that Einstein sought to produce with the cosmological constant. So the question becomes, How come such vacuum energy is not overwhelmingly dominant in the universe today?or, How come the universe isn't still inflating?
We have no answer to this question. It is probably one of the most profound unanswered questions in physics.
Every calculation we perform with the theories we now have suggests that the vacuum energy should be many orders of magnitude larger today than it is allowed to be on the basis of our observations. There are ideas, based
on exotica like Euclidean wormholes, for how to make it vanish, but none of these ideas is firmly grounded.
Perhaps even more surprising, recent observations on a variety of scales all suggest that the cosmological constant, while much smaller than we can explain, may nevertheless not be zero today, and may therefore have had a measurable effect on the evolution of the universemaking it older than it might otherwise have been, for example. This is a subject of great interest, and in fact is occupying much of my own present research efforts.
Nevertheless, whatever the resolution of this problem, it is clear that the near flatness of the universe was one of the conditions necessary for the eventual origin of life on Earth and that the cosmological conditions favoring the formation of life on Earth hold elsewhere as well.
At a fundamental microphysical level, there is also a whole slew of cosmic coincidences that allowed life to form on Earth. If any one of a number of fundamental physical quantities in nature was slightly different, then the conditions essential for the evolution of life on Earth would not have existed. For example, if the very small mass difference between a neutron and proton (about 1 part in 1000) were changed by only a factor of 2, the abundance of elements in the universe, some of which are essential to life on Earth, would be radically different from what we observe today. Along the same lines, if the energy level of one of the excited states of the nucleus of the carbon atom were slightly different, then the reactions that produce carbon in the interiors of stars would not occur and there would be no carbon the basis of organic moleculesin the universe today.
Of course, it is hard to know how much emphasis to put on these coincidences. It is not surprising, since we have evolved in this universe, to find that the constants of nature happen to have the values that allowed us to evolve in the first place. One might imagine, for the purposes of argument, that our observed universe is part of a meta- universe that exists on a much larger scale than we can observe. In each of the universes making up this meta- universe, the constants of nature could be different. In those universes that have constants incompatible with the evolution of life, no one is around to measure anything. To paraphrase the argument of the Russian cosmologist Andrei Linde, who happens to subscribe to this form of what is known as the “anthropic principle,” it is like an intelligent fish wondering why the universe in which it lives (the inside of a fish bowl) is made of water. The answer is simple: if it weren't made of water, the fish wouldn't be there to ask the question.
Since most of these issues, while interesting, are not empirically resolvable at the present time, they are perhaps best left to philosophers, theologians, or perhaps science fiction writers. Let us then accept the fact that the universe has managed to evolve, both microscopically and macroscopically, in a way that is conducive to the evolution of life. We next turn to our own home, the Milky Way galaxy.
When we consider which systems in our own galaxy may house intelligent life, the physics issues are much more clear-cut. Given that there exist stars in the Milky Way which, from all estimates, are at least 10 billion years old, while life on Earth is no older than about 3.5 billion years, we are prompted to ask how long life could have existed in our galaxy before it arose on Earth.
When our galaxy began to condense out of the universal expansion some 10 billion to 20 billion years ago, its first generation stars were made up completely of hydrogen and helium, which were the only elements created with any significant abundance during the big bang. Nuclear fusion inside these stars continued to convert hydrogen to helium, and once this hydrogen fuel was exhausted, helium began to “burn” to form yet heavier elements. These fusion reactions will continue to power a star until its core is primarily iron. Iron cannot be made to fuse to form heavier elements, and thus the nuclear fuel of a star is exhausted. The rate at which a star burns its nuclear fuel depends on its mass. Our own Sun, after 5 billion years of burning hydrogen, is not even halfway through the first phase of its stellar evolution. Stars of 10 solar massesthat is, 10 times heavier than the Sunburn fuel at about 1000 times the rate the Sun does. Such stars will go through their hydrogen fuel in less than 100 million years, instead of in the Sun's 10-billion-year lifetime.
What happens to one of these massive stars when it exhausts its nuclear fuel? Within seconds of burning the last bit, the outer parts of the star are blown off in an explosion known as a supernova, one of the most brilliant fireworks displays in the universe. Supernovae briefly shine with the brightness of a billion stars. At the present time, they are occurring at the rate of about two or three every 100 years in the galaxy. Almost 1000 years ago, Chinese astronomers observed a new star visible in the daytime sky, which they called a “guest star.” This supernova created what we now observe telescopically as the Crab Nebula. It is interesting that nowhere in Western Europe was this transient object recorded. Church dogma at the time declared the heavens to be eternal
and unchanging, and it was much easier not to take notice than to be burned at the stake. Almost 500 years later, European astronomers had broken free enough of this dogma so that the Danish astronomer Tycho Brahe was able to record the next observable supernova in the galaxy.
Many of the heavy elements created during the stellar processing, and others created during the explosion itself, are dispersed into the interstellar medium, and some of this “stardust” is incorporated in gas that collapses to form another star somewhere else. Over billions of years, later generations of starsso-called Population 1 stars, like our Sunform, and any number of these can be surrounded by a swirling disk of gas and dust, which would coalesce to form planets containing heavy elements like calcium, carbon, and iron. Out of this stuff we are made. Every atom in our bodies was created billions of years ago, in the fiery furnace of some long dead star. I find this one of the most fascinating and poetic facts about the universe: we are all literally star children.
Now, it would not be much use if a planet like the Earth happened to form near a very massive star. As we have seen, such stars evolve and die within the course of 100 million years or so. Only stars of the mass of our Sun or less will last longer than 5 billion years in a stable phase of hydrogen burning. It is hard to imagine how life could form on a planet orbiting a star that changed in luminosity by huge amounts over the course of such evolution. Conversely, if a star smaller and dimmer than our Sun should have a planetary system, any planet warm enough to sustain life would probably be so close in as to be wracked by tidal forces. Thus, if we are going to look for life, it is a good bet to look at stars not too different from our own. As it happens, the Sun is a rather ordinary member of the galaxy. About 25 percent of all stars in the Milky Waysome 100 billion of themfall in the range required. Most of these are older even than the Sun and could therefore, in principle, have provided sites for life up to 4 billion to 5 billion years before the Sun did.
On to the Earth. What is it about our fair green-blue planet that makes it special? In the first place, it is in the inner part of the solar system. This is important, because the outer planets have a much higher percentage of hydrogen and heliummuch closer to that of the Sun. Most of the heavy elements in the disk of gas and dust surrounding the Sun at its birth appear to have remained in the inner part of the system. Thus, one might expect potential sites for life to be located at distances smaller than, say, the distance of Mars from a 1-solar-mass star.
Next, as Goldilocks might have said, the Earth is just rightnot too big or too small, too cold or too hot. Since the inner planets probably had no atmospheres when they formed, these had to be generated by gases produced by volcanoes. The water on the Earth's surface was also produced in this fashion. A smaller planet might well have radiated heat from its surface rapidly enough to prevent a large amount of volcanism. Presumably this is the case with Mercury and the Moon. Mars is a borderline case, while Earth and Venus have successfully developed an atmosphere. Recent measurements of radioactive gas isotopes in the terrestrial rocks suggest that after an initial period of bombardment, in which the Earth was created by the accretion of infailing material over a period of 100 million to 150 million years about 4.5 billion years ago, volcanism produced about 85 percent of the atmosphere within a few million years. So, again, it is not surprising that organic life formed on Earth rather than on other planets in the solar system, and one might expect similar proclivities elsewhere in the galaxyon Class M planets, as they are called in the Star Trek universe.
The next question is how quickly life, followed by intelligent life, might take to evolve, based on our experience with the Earth. The answer to the first part of the question is: Remarkably quickly. Fossil relics of blue-green algae about 3.5 billion years old have been discovered, and various researchers have argued that life was already flourishing as long as 3.8 billion years ago. Within a few 100 million years of the earliest possible time that life could have evolved on Earth, it did. This is very encouraging.
Of course, from the time life first began on Earth until complex multicellular structures, and later intelligent life, evolved, almost 3 billion years may have elapsed. There is every reason to believe that this time was governed more by physics than biology. In the first place, the Earth's original atmosphere contained no oxygen. Carbon dioxide, nitrogen, and trace amounts of methane, ammonia, sulfur dioxide, and hydrochloric acid were all present, but not oxygen. Not only is oxygen essential for the advanced organic life-forms on Earth, it plays another important role. Only when there is sufficient oxygen in the atmosphere can ozone form. Ozone, as we are becoming more and more aware, is essential to life on Earth because it screens out ultraviolet radiation, which is harmful to most life-forms. It is therefore not surprising that the rapid explosion of life on Earth began only after oxygen was abundant.
Recent measurements indicate that oxygen began building up in the atmosphere about 2 billion years ago, and reached current levels within 600 million years after that. While oxygen had been produced earlier, by photosynthesis in the blue-green algae of the primordial oceans, it could not at first build up in the atmosphere. Oxygen reacts with so many substances, such as iron, that whatever was photosyn-thetically produced combined with other elements before it could reach the atmosphere. Eventually, enough materials in the ocean were oxidized so that free oxygen could accumulate in the atmosphere. (This process never took place on Venus because the temperature was too high there for oceans to form, and thus the life-forming and life-saving blue- green algae never arose there.)
So, after conditions were really ripe for complex life-forms, it took about a billion years for them to evolve. Of course, it is not clear at all that this is a characteristic timescale. Accidents such as evolutionary wrong turns, climate changes, and cataclysmic events that caused extinctions affected both the biological timescale and the end results.
Nevertheless, these results indicate that intelligent life can evolve in a rather short interval on the cosmic timescalea billion years or so. The extent of this timeframe has to do with purely physical factors, such as heat production and chemical reaction rates. Our terrestrial experience suggests that even if we limit our expectations of intelligent life to the organic and aerobicsurely a very conservative assumption, and one that the Star Trek writers were willing to abandon (the silicon-based Horta is one of my favorites)planets surrounding several- billion-year-old stars of about 1 solar mass are good candidates.
Granting that the formation of organic life is a robust and relatively rapid process, what evidence do we have that its fundamental ingredientsnamely, organic molecules, and other planetsexist elsewhere in the universe? Here, again, recent results lead to substantial optimism. Organic molecules have been observed in asteroids, comets, meteorites, and interstellar space. Some of these are complex molecules, including amino acids, the building blocks of life. Microwave measurements of interstellar gas and dust grains have led to the identification of dozens of organic compounds, some of which are presumed to be complex hydrocarbons. There is little doubt that organic matter is probably spread throughout the galaxy.
Finally, what about planets? In spite of the fact that to date only one direct observation of a planetary system other than our own has been made, it has long been believed that most stars have planets around them. Certainly a fair fraction of observed stars have another stellar companion, in so-called binary systems. Moreover, many young stars are observed to have circumstellar disks of dust and gas, which are presumably the progenitors of planets. Various numerical models for predicting the distribution of planetary masses and orbits in such disks suggest (and I emphasize here the word “suggest”) that they will produce on average at least one Earthlike planet at an Earth-like distance from its star. Most recently, another planetary system was finally directly detected, 1400 light-years from Earth. Somewhat surprisingly, the system observed is one of the least hospitable places one might imagine for planets: three planets all orbiting a pulsarthe collapsed core of a supernovaat a distance closer than Venus is to our Sun. These planets could easily have formed after rather than before the supernova, but either way, this discovery indicates that planetary formation is probably not rare.
I do not want to lose the forest for the trees here. It is almost miraculous that the normal laws of physics and chemistry, combined with an expanding universe more than some 10 billion years old, lead to the evolution of conscious minds that can study the universe out of which they were born. Nevertheless, while the circumstances that led to life on Earth are special, they appear to be by no means peculiar to Earth. The arguments above suggest that there could easily be over a billion possible sites for organic life in our galaxy. And since our galaxy is merely one out of 100 billion galaxies in the observable universe, I find it hard to believe that we are alone. Moreover, as I noted earlier, most Population 1 stars were formed earlier than our Sun wasup to 5 billion years earlier. Given the time frame discussed above, it is likely that intelligent life evolved on many sites billions of years before our Sun was even born. In fact, it might be expected that most intelligent life in the galaxy existed before ours. Thus, depending upon how long intelligent civilizations persist, the galaxy could be full of civilizations that have been around literally billions of years longer than we have. On the other hand, given our own history, such civilizations may well have faced the perils of war and famine, and many may not have made it past a few thousand years; in this case, most of the intelligent life in the universe would be long gone. As one researcher cogently put the issue over 20 years ago, “The question of whether there is intelligent life out there depends, in the last analysis, upon how intelligent that life is.” 3
So, how will we ever know? Will we first send out starships to explore strange new worlds and go where no one
has gone before? Or will we instead be discovered by our galactic neighbors, who have tuned in to the various Star Trek series as these signals move at the speed of light throughout the galaxy? I think neither will be the case, and I am in good company.
In the first place, we have clearly seen how daunting interstellar space travel would be. Energy expenditures beyond our current wildest dreams would be neededwarp drive or no warp drive. Recall that to power a rocket by propulsion using matter-antimatter engines at something like 3/4 the speed of light for a 10-year round-trip voyage to just the nearest star would require an energy release that could fulfill the entire current power needs in the United States for more than 100,000 years! This is dwarfed by the power that would be required to actually warp space. Moreover, to have a fair chance of finding life, one would probably want to be able to sample at least several thousand stars. I'm afraid that even at the speed of light this couldn't be done anytime in the next millennium.
That's the bad news. The good news, I suppose, is that by the same token we probably don't have to worry too much about being abducted by aliens. They, too, have probably figured out the energy budget and will have discovered that it is easier to learn about us from afar.
So, do we then devote our energies to broadcasting our existence? It would certainly be much cheaper. We could send to the nearest star system a 10-word message, which could be received by radio antennae of reasonable size, for much less than a dollar's worth of electricity. Howeverand here again I borrow an argument from the Nobel laureate Edward Purcellif we broadcast rather than listen, we will miss most of the intelligent life-forms. Obviously, those civilizations far ahead of us can do a much better job of transmitting powerful signals than we can. And since we have been in the radio-transmission business for only 80 years or so, there are very few societies less advanced than we are that could still have the technology to receive our signals. So, as my mother used to say, we should listen before we speak. Although as I write this, I suddenly hope that all those more advanced societies aren't thinking exactly the same thing.
But what do we listen to? If we have no idea which channel to turn to in advance, the situation seems hopeless. Here we can be guided by Star Trek. In the Next Generation episode “Galaxy's Child,” the Enterprise stumbles upon an alien life-form that lives in empty space, feeding on energy. Particularly tasty is radiation with a very specific frequency1420 million cycles per second, having a wavelength of 21 cm.
In the spirit of Pythagoras, if there were a Music of the Spheres, surely this would be its opening tone. Fourteen hundred and twenty megahertz is the natural frequency of precession of the spin of an electron as it encircles the atomic nucleus of hydrogen, the dominant material in the universe. It is, by a factor of at least 1000, the most prominent radio frequency in the galaxy. Moreover, it falls precisely in the window of frequencies that, like visible light, can be transmitted and received through an atmosphere capable of supporting organic life. And there is very little background noise at this frequency. Radioastronomers have used this frequency to map out the location of hydrogen in the galaxywhich is, of course, synonymous with the location of matterand have thus determined the galactic shape. Any species intelligent enough to know about radio waves and about the universe will know about this frequency. It is the universal homing beacon. Thirty-six years ago, the astrophysicists Giuseppe Cocconi and Philip Morrison proposed that this is the natural frequency to transmit at or listen to, and no one has argued with this conclusion since.
Hollywood not only guessed the right frequency to listen to but helped put up the money to do the listening. While small-scale listening projects have been carried out for more than 30 years, the first large-scale comprehensive program came on line in the autumn of 1985, when Steven Spielberg threw a big copper switch that formally initiated Project META, which stands for Megachannel Extra Terrestrial Array. The brainchild of electronics wizard Paul Horowitz at Harvard University, META is located at the Harvard/Smithsonian 26-meter radiotŽlescope in Harvard, Massachusetts, and funded privately by the Planetary Society, including a $100,000 contribution from Mr. ET himself. META uses an array of 128 parallel processors to scan simultaneously 8,388,608 frequency channels in the range of 1420 megahertz and its so-called second harmonic, 2840 megahertz. More than 5 years of data have been taken, and META has covered the sky three times looking for an extraterrestrial signal.
Of course, you have to be clever when listening. First, you have to recognize that even if a signal is sent out at 1420 megahertz, it may not be received at this frequency. This is because of the infamous Doppler effecta train whistle sounds higher when it is approaching and lower when it is receding. The same is true for all radiation
emitted by a moving source. Since most of the stars in the galaxy are moving at velocities of several hundreds of kilometers per second relative to us, you cannot ignore the Doppler shift. (The Star Trek writers haven't ignored it; they added “Doppler compensators” to the transporter to account for the relative motion of the starship and the transporter target.) Reasoning that the transmitters of any signal would have recognized this fact, the META people have looked at the 1420 megahertz signal as it might appear if shifted from one of three reference frames: (a) one moving along with our local set of stars, (b) one moving along with the center of the galaxy, and (c) one moving along with the frame defined by the cosmic microwave background radiation left over from the big bang. Note that this makes it easy to distinguish such signals from terrestrial signals, because terrestrial signals are all emitted in a frame fixed on the Earth's surface, which is not the same as any of these frames. Thus terrestrial signals have a characteristic “chirp” when present in the META data.
What would an extraterrestrial signal involve? Cocconi and Mor-rison suggested that we might look for the first few prime numbers: 1,3,5,7,11,13.... In fact, this is precisely the series that Picard taps out in the episode “Allegiance,” when he is trying to let his captors know that they are dealing with an intelligent species. Pulses from, say, a surface storm on a star are hardly likely to produce such a series. The META people have searched for an even simpler signal: a uniform constant tone at a fixed frequency. Such a “carrier” wave is easy to search for.
Horowitz and his collaborator, the Cornell astronomer Carl Sagan, have reported on an analysis of the 5 years of META data. Thirty-seven candidate events, out of 100,000 billion signals detected, were isolated. However, none of these “signals” has ever repeated. Horowitz and Sagan prefer to interpret the data as providing no definitive signal thus far. As a result, they have been able to put limits on the number of highly advanced civilizations within various distances of our Sun which have been trying to communicate with us.
Nevertheless, in spite of the incredible complexity of the search effort, only a small range of frequencies has actually been explored, and the power requirements for a signal capable of being detected by the META telescope are rather largecivilizations would have to use broadcast powers in excess of the total power received on Earth from the Sun (about 10 17 watts) in their transmitters to produce a detectable signal. Thus, there is yet no cause for pessimism. It is a difficult task just to listen. The META group is now building a bigger, better (or BETA) detector, which should improve the search strength by roughly a factor of 1000.
The search goes on. The fact that we have not yet heard anything should not dissuade us. It is something like what my friend Sidney Coleman, a physics professor at Harvard, once told me about buying a house: You shouldn't get discouraged if you look at a hundred and don't find anything. You only have to like one.... A single definitive signal, as improbable as it is that we will ever hear one, would change the way we think about the universe, and would herald the beginning of a new era in the evolution of the human race.
And for those of you who are disheartened at the idea that our first contact with extraterrestrial civilizations will not be made by visiting them in our starships, remember the Cytherians, a very advanced civilization encountered by the Enterprise who made outside contact with other civilizations not by traveling through space themselves but by bringing space travelers to them. In some sense, that is exactly what we are doing as we listen to the signals from the stars.