Interstellar Spaceflight How can our civilization search for extraterrestrial life? By what means can we attempt to make this evolutionary leap forward? One obvious way is to develop the capability to travel far outside our Solar System. By involving many nations in the space programs begun by the United States and the Soviet Union, our civilization might be expected eventually to develop the means to travel through interstellar space. However, such technology will not be achieved easily; it may not even be a practical possibility.

A basic problem with interstellar space travel is the amount of fuel needed for long-duration flights. The damaging effects of galactic radiation is another issue, as are the loneliness and boredom of generations of humans having to spend their entire lives aboard a spacecraft. Consider, for example, a trip to the nearest star system beyond our Solar System, namely Alpha Centauri ~4.3 light-years away. With a constant flight velocity of 50 km/s, which approximates the speed of our fastest robotic space probes, this trip would take ~25,000 years. That's a fast enough velocity to escape the Solar System, yet 250 centuries would still be needed to reach the closest star system. Contrast this duration with, for instance, ~15 centuries since the fall of the Roman Empire or ~50 centuries since the construction of the Pyramids. Even if fuel for the spacecraft and food for the inhabitants were plentiful, such a flight would take an incredibly long time by human standards.

This example assumes a spacecraft traveling at a velocity much less than the velocity of light. In fact, 50 km/s equals less than 0.02% of light velocity. We might imagine that our civilization could someday become smart enough to achieve flight velocities close to 100% of light velocity, which might dramatically reduce travel times by means of special relativistic effects—but we would probably be fooling ourselves, given the following application of basic physics that is hard to ignore.

Relativistic space flight seems nice in theory, for flight durations could seemingly be diminished, as has been suggested by legions of science-fiction writers. But in practice, there’s a problem. Spacecraft traveling at speeds close to light velocity cannot be easily refueled. A reasonably sized craft of, for example, 10⁴ kg (10 tons) would need more than a billion kilograms (a million tons) of hydrogen fuel just to accelerate it to 10% of light velocity. These numbers assume that such a spacecraft derives its energy from a hydrogen-helium fusion reaction, not simply chemical fuels now used for rocketry; the helium would exit the rear and the spacecraft would lurche forward via Newton's third law. However, to reach relativistic speeds, namely >90% of light velocity, would require so much on-board fuel as to make this proposition ridiculous.

The idea of using interstellar hydrogen gas as a spacecraft moves through space—refueling as needed while traveling—seems equally futile. To accelerate to 10% of light velocity would require a gigantic scoop kilometers across, given the low density of H in interstellar space. Such a scoop need not be made of physical material; rather, it could be a huge electromagnetic field, capable of harvesting charged protons needed for fuel. Yet to reach relativistic speeds would require such a scoop to be of light-year dimensions. Furthermore, according to the proven rules of relativity, the spacecraft's mass would increase relative to the interstellar medium from which it captured its fuel, thereby causing the craft to need even more fuel and so on, making this a self-defeating proposition. Given the laws of physics as we currently know them, relativistic space flight seems destined to remain science fiction. Probably no intelligent civilization would or could do it.

These arguments don’t necessarily prohibit traveling through interstellar space at much slower speeds. If our descendants can overcome harmful radiation and severe boredom during long, long journeys, interstellar travel might someday become feasible with modest fuel supplies and speeds of a few percent of light velocity. To do so, however, would mean not just generations of space travel, but flights lasting thousands and perhaps even millions of years.

Equally possible, alas, these practical problems might never be solved, or perhaps our civilization will not survive long enough even to attempt interstellar space flight. Future generations of Earthlings might well conclude that it's utterly impractical to travel over large galactic distances. If so, our species will be forever confined to our Solar System and at best a handful of nearby stars.

Accordingly, it seems unlikely that human space flight is a useful means, at least anytime soon, to seek contact with extraterrestrials. Even with the most optimistic estimates of the many factors in the Drake equation, galactic civilizations (if they exist at all) are probably spread out like small islands within the vast sea of galactic space. For example, with all the interior factors of our equation maximized and the average technological lifetime equal to 1000 years, we might conclude that 1000 advanced civilizations currently reside in our Galaxy. Given the size and shape of the Milky Way, ~3000 light-years would then separate any two adjacent civilizations. Such large distances force the word "neighboring" to take on new meaning.

Even if the average lifetime of galactic civilizations is 1 million years, our most optimistic estimates suggest that each is separated by ~300 light-years. To have a reasonable hope of successful contact, hundreds, perhaps thousands, of sorties would need to be launched toward candidate star systems. All in all, interstellar space flight is both impractical and uneconomical either at the present time or in the foreseeable future. It may never become feasible.

Interstellar Probes Other methods might be used to search for extraterrestrial intelligence in the Galaxy. Imagine, for example, launching many robot space probes, each with a velocity sufficient to escape our Solar System. Each probe would eventually reach a star system judged a good candidate for intelligent life, where it would orbit the star, looking and listening for evidence of life on one of its planets. Such a probe, for example, could be programmed to detect the leakage of electromagnetic radiation arising from the daily activities of a technological civilization. It might succeed immediately upon arriving, should an advanced civilization already be thriving. Or the probe might need to "bug" an alien star system for thousands of years before seeing or hearing any type of planetary activity resembling our radio, television, military radar, wasted energy, or whatever. Once the robots did detect any sign of intelligence, they would send a radio signal back to Earth, letting us know that a new technological civilization has been found.

Robot probes offer a couple of advantages in the search for extraterrestrials: They would be neither bored by the long duration of the flight, nor harmed by the harsh radiation of interstellar space. But a disadvantage with this method of contact is that, once again, it would seem uneconomical except in the most optimistic science-fiction scenarios. To reconnoiter all single F-, G-, and K-type stars within merely 1000 light-years of Earth would require about a million probes. Given the number of days in a year, one probe would then need to be launched every day for ~3000 years. Aside from these formidable logistics problems, the cost of such a program would be staggering.

In a minimal way, our civilization has already launched several such probes, although they lack the technical sophistication of the robots just noted. Figure 8.28 is a reproduction of a plaque mounted aboard the American Pioneer 10 spacecraft launched in the mid-1970s. Similar information was also included aboard two Voyager spacecraft launched a few years later. After visiting the outer planets, these probes are now on their way out of our Solar System, but they have no specific destination thereafter.

FIGURE 8.28 — Key features of this message onboard the Pioneer 10 spacecraft include: a scale drawing of the craft itself, as well as a man and a woman (right center); a schematic diagram of a hydrogen atom undergoing an energy change (top left); a starburst pattern sketching the directions and frequencies of various pulsars to estimate when the craft was launched (middle left); and a depiction of our Solar System, showing that the spacecraft departed the third planet from the Sun and passed the fifth planet on its journey (bottom). (Carl Sagan)

Even if Pioneer and Voyager do someday encounter a distant star system housing an alien civilization, these machines are incapable of reporting that news back to Earth. Their plaques are essential a "message in a bottle." Should advanced civilizations intercept one of these probes, they nonetheless should be able to unravel most of its contents using the universal language of "mathematiceese." Figure 8.28 includes binary-coded markings from which sizes, distances, and times can be derived, thus revealing from where and when these spaceprobes were originally launched. The aliens would then know that we are here (or were when the probes were sent), although we would remain unaware of the aliens' existence. Perhaps not such a good thing, as noted one paragraph hence.

All things considered, methods that rely on space travel, either crewed or uncrewed, don’t seem to hold much promise in contacting extraterrestrial intelligent life. One can always argue that future technological breakthroughs may someday make such projects more favorable. The microelectronic revolution now under way, especially its rapid advancement of smart machines, might eventually make unmanned probes more feasible and attractive. But given what we know now, long-distance space travel seems logistically and economically unlikely.

Aside from these practical problems, some scientists argue that it's not a smart idea to signal extraterrestrials actively. As noted earlier in this FUTURE EPOCH, our recent emergence as a technologically competent civilization implies that we’re now among the dumbest technological intelligences in the Galaxy. Any other civilization either that we discover, or that discovers us, will almost surely be more advanced than us. Consequently, a healthy degree of skepticism is warranted. After all, if you were lost in a jungle populated by unknown natives, you would be wise not to yell, scream, or send up smoke signals to let them know of your presence. It's a lot safer to quietly explore your surroundings for a while, to listen to the sound of the drums, and to get a feeling for their intentions.

Some anthropologists have even speculated about the behavior of advanced galactic civilizations. If extraterrestrials behave even remotely like human civilizations on Earth, then the most advanced aliens might naturally try to dominate all others. "Smarter" species have often taken advantage of others throughout the history of life on Earth. Even so, the aggressiveness of Earthlings may not in any way apply to extraterrestrials. And in any case, the vast distances separating galactic civilizations would probably prohibit a civilization on one planet from physically dominating or enslaving a less advanced civilization on some other planet. In fact, physical contact among different cosmic civilizations may not be biologically desirable; what is healthy for one life form might be a disease to another.

Still, unsettling queries linger: Is the competitive aspect of neo-Darwinism a common feature of life everywhere in the cosmos? Are the principles of anthropology as universal as those of physics and chemistry? What might be the attitude of galactic aliens toward humankind on Earth?

Radio Communication A third technique is much cheaper than either of the two signaling methods noted above and has many advantages. It's designed to make contact with extraterrestrials using only electromagnetic radiation, in particular it doesn’t utilize any hardware traveling through space. This technique is economically feasible, can be undertaken with existing equipment, and doesn’t reveal our presence should some extraterrestrials be hostile.

The third method uses radiation as the fastest known means of transferring information from one place to another. Since light and other types of high-frequency (x-ray, gamma-ray) radiation are heavily scattered while moving through dusty interstellar space, long-wavelength radio radiation seems the best way to accurately transfer information in the plane of the Galaxy where civilizations are likely to be located. Accordingly, radio telescopes on Earth can be used to listen passively for radio signals emitted by someone else. No radiation would be actively transmitted by our civilization toward distant star systems. Best of all, engineers have already invented the equipment needed to detect such alien radio signals. Some preliminary searches are now underway, thus far without success.

Radio searches of this type are not without problems, however. The foremost problem is that this method assumes that extraterrestrials are in fact broadcasting radio signals for one reason or another. If they aren’t, our search technique of merely listening will fail. Another problem concerns the need to distinguish radio signals artificially generated by aliens from signals naturally emitted by interstellar gas clouds; interference of many kinds could be a major impediment. Yet more problems include the direction to aim our radio telescopes (where?), the frequency at which to tune our receivers (how?), and the time of day to listen (when?).

Such a search could be undertaken by following either of two strategies. One strategy attempts to eavesdrop on the radio radiation normally leaking from some planet while its civilization goes about its daily business. A sample of what might be expected can be gained by reversing the problem and examining the appearance of Earth from afar. Figure 8.29 shows the pattern of radio signals unintentionally leaked into space by our own civilization. From the viewpoint of some distant observer, the spinning Earth emits a bright flash of radio radiation every few hours. The flashes result from the periodic rising and setting of hundreds of radio stations and television transmitters. Because the great majority of these transmitters are clustered in eastern United States and western Europe, and because they emit their radiation parallel to the ground where people live, a distant observer would detect blasts of radiation leaking from Earth as our planet rotates each day. This radiation races out into space as a growing sphere of radio, television, and other electromagnetic signals (such as radar). It’s been doing so since the invention of these technologies several decades ago. In fact, Earth is now a more intense radio source than the Sun—thanks to our technological civilization.

FIGURE 8.29 — Some radio and television transmitters broadcast their energy wastefully from Earth's surface, causing electromagnetic radiation to leak into interstellar space. Because the great majority of transmitters are clustered in the eastern United States and western Europe, a distant observer would detect blasts of radiation from Earth as our planet rotates each day. (Prentice Hall)

If any advanced civilizations reside within ~60 light-years of Earth, we have already broadcast our presence to them. Whether or not any extraterrestrials have received the message, we don’t yet know. But if they have, they might well have concluded that we aren’t so intelligent, given the content of our radio and television programs.

This eavesdropping strategy resembles the spaceprobe technique discussed earlier, although here the "bugging" of other planetary systems can be accomplished from Earth using large radio telescopes. However, such telescopes now in operation are likely sensitive enough to eavesdrop on only a handful of the nearest star systemsXX—assuming a level of technological expertise similar to our own. Remember, the strength of radiation decreases with the square of the distance, meaning that such signals diminish in intensity rapidly as they move across large swarths of space.

We do currently have the engineering ability to build more sensitive equipment that could intercept the hodgepodge of radio, radar, and television signals wastefully leaked into space by more distant civilizations like our own. Figure 8.30 shows the major features of a huge array of radio telescopes capable of eavesdropping on Earth-like civilizations within ~3000 light-years of us. Called Project Cyclops, this vast machine hasn't yet been built—and it may never be since the price tag for such a device is ~$20 billion (1995 dollars). Its construction is currently improbable, given all the social, bureaucratic, and militaristic demands for taxpayers' money. Keep in mind, though, that this cost of this powerful device is roughly equivalent to the price tag of one American aircraft carrier or a fleet of Russian warheads. It's simply a matter of where our society wants to place its priorities.

FIGURE 8.30 — This is a design sketch for a gigantic array of 1000 interconnected radio telescopes, called Project Cyclops. Such a device could be used to eavesdrop on civilizations much like our own, provided they are within ~1000 light-years of Earth. (NASA)

Another radio-based strategy might be used to search effectively for extraterrestrial intelligent life. However, this one relies on a single, key assumption, the validity of which is totally unknown. The assumption is that at least one advanced civilization (well beyond our own technical prowess) is beaming strong radio signals toward many star systems in the Galaxy—our Sun among them—actively hoping to contact us. Such a radio beacon, set up by an alien broadcaster specifically to attract our attention, might be detectable with radio telescopes already built on Earth. But there’s a catch here too, as other concerns hinder this passive, optimistic search strategy. To give but one example, in what direction should we listen? Even if we limit our targets to Sun-like stars, ~500 of them exist within only ~300 light-years of Earth. Any one of them could have a planetary civilization that is transmitting toward us. To be thorough, we would need to sample every such star system. And the number of candidates ramp up dramatically when searching larger, more distant volumes—a needle in a haystack comes to mind.

The basic issue in estimating the number of extraterrestrial civilizations we could contact isn’t the extent to which a society develops technology. Rather, the real issue is the degree to which a society generates exploratory curiosity and maintains it for long periods of time. A two-way interstellar dialogue requires not only technological competence, but also sufficient motivation on both ends—ours and theirs. How motivated is a society to communicate with other societies beyond its own home planet? How universal is curiosity? How persistent is it? Would the residents of every inhabited planet have a deep desire to know if others exist in the Universe?

Humans on Earth do seem to have a genuine curiosity about the Universe. Since consciousness dawned, humankind has wondered about who we are and where we came from. Many people have also thought about extraterrestrial life beyond Earth. But how long does this inherent curiosity last? Does technologically competent intelligent life seek new knowledge indefinitely?

Curiosity may wax and wane over the ages, as it has on our planet throughout recorded history. The Greeks and Romans of antiquity were curious, the barbarians of the Middle Ages apparently less so; now in post-Renaissance times, we are again highly curious people. Perhaps other advanced civilizations reside in the depths of space, but their societies have reached the stage of mental and physical stagnation broached earlier in this FUTURE EPOCH. Maybe their curiosity has lapsed, or is even gone forever. We repeat our earlier query: If curiosity dies, does intelligence die also?

Distressingly, if the average technological lifetime is only 1000 years, then, as noted above, the average distance separating galactic civilizations would be ~3000 light-years. Hence, within our local realm of several thousand Sun-like stars, we would expect only one technological civilization. Since we are such a civilization, we would then be that one—presumably the only one in this part of the Galaxy. To have any real hope of success, then, this search strategy must be directed toward many additional candidate stars far beyond 1000 light-years. Over such vast distances, a two-way dialogue will not exactly be a snappy conversation. A simple reply to an initial "Hello" may take thousands of years.

As if the problem of where to search weren’t enough, there’s another dilemma. At what frequency should we listen for alien beacons? The electromagnetic spectrum is enormous; the radio domain alone covers many orders of magnitude in wavelength. To hope to detect a signal at some random radio frequency is indeed like searching for a needle in a haystack. The technique would seem doomed to failure, unless we have some prior information concerning the likely frequencies at which aliens might transmit.

Fortunately, some basic arguments suggest that civilizations will probably communicate at wavelengths around 20 cm. The basic building blocks of the Universe, namely hydrogen (H) atoms, naturally radiate near 21-cm wavelength. And, one of the simplest molecules, hydroxyl (OH), radiates near 18-cm wavelength. Given that these two substances form water (H₂O) and that water is likely to be a common interaction medium for life anywhere, some researchers have proposed that the interval 18 – 21 cm is the best part of the spectrum for civilizations to transmit or to listen. Called the "water hole," this radio interval might serve as an oasis where all advanced galactic civilizations conduct their electromagnetic business.

These water-hole frequencies comprise only an educated guess, but they’re supported by other arguments as well. In particular, the wavelength domain from 18 to 21 cm is precisely that part of the entire electromagnetic spectrum—from radio waves all the way across to gamma rays—for which the galactic static from stars and interstellar clouds is minimized. Furthermore, the atmospheres of typical planets, such as our own, are also expected to interfere least at these wavelengths.

Figure 8.31 shows the water hole's position in the radio spectrum. This figure also plots the amount of natural emission from our Galaxy and from Earth's atmosphere, showing how the water hole is positioned within the quietest part of the spectrum. Thus the water hole seems like a good choice for the frequency of a galactic beacon, although we can’t be sure of this reasoning until contact is actually achieved. Perhaps some other transmitting frequency is better for reasons unknown to us at this time.

FIGURE 8.31 — The "water hole" is bounded by the natural emission frequencies of the hydrogen (H) atom at 21-cm wavelength and the hydroxyl (OH) molecule at 18-cm. The blue curve sums the natural emissions of our Galaxy and of a planet's atmosphere (in this case Earth's). This sum is minimized near the water hole frequencies, providing a quiet "electromagnetic oasis." (SETI Institute)

A few passive radio searches are now in progress at frequencies in and around the water hole, yet so far nothing resembling alien signals has been detected. But that's not surprising, given the small efforts made to date. Attempts to detect extraterrestrials have been so brief in relation to the task at hand that they resemble a "Columbus" who, while searching for a new route to the Indies, ventured about a kilometer off the coast of Spain. To maximize the chance for success, a dedicated effort is needed—one perhaps requiring hundreds of years of searching on a continuous basis. To have much hope of making contact, we will probably need to monitor millions of stars.

Many uncertainties plague this sort of search strategy, the foremost being the desire of any civilization to transmit. From what we know, transmitting is boring, expensive, and potentially dangerous. Accordingly, perhaps everyone is just listening. If so, then the prospects for contact are dim; to create a dialogue, somebody's got to start talking.

Other uncertainties concern "language." Will galactic civilizations be able to find a common language? This puzzle includes not only the difficulty of matching the frequencies of transmission and reception, but also the potential problem of ever being able to understand or appreciate the content of an interstellar message. This may be especially troublesome if the transmitting civilization is much more advanced than ourselves. Even a few centuries more progress would likely give them technological talents hardly imagined here on Earth; just think of the advances we’ve made in the 20^th century, indeed in only the past decade or two. Extraterrestrials might use methods of searching and means of contact entirely foreign to us, making it nearly impossible to detect their signals or decipher their code. They may be as uncommunicative with us as we are with, for example, dolphins or even ants. All such civilizations may be doomed to loneliness for as long as each survives.

On the other hand, if advanced aliens are eager to contact neophyte civilizations in our Galaxy, they will realize that less sophisticated means must be used. They would want to make the task as easy as possible for us and other emergent societies elsewhere, thereby sending signals that inexperienced civilizations like ourselves could detect and decipher. Their transmitting language would probably be built around mathematics, since counting should be universal: 2 + 2 ought to equal 4 everywhere. Figure 8.32 exemplifies how mathematics can be used to send messages through space. In this case, no words are needed, just a picture.

FIGURE 8.32 — In this hypothetical transmission (a), a message is buried within a long series of digital information (dots and dashes, or ones and zeroes). The total number of digits is 725, which is a product of two whole integers 25 and 29. When arranged in a geometrical pattern having 25 rows and 29 columns (b), the message is unrecognizable. But, when arranged in 29 rows and 25 columns (c), the message is clear. (Lola Chaisson)

All of the many uncertainties noted above, especially when combined, cause some researchers to be pessimistic about contacting aliens. They argue that a search for extraterrestrials is unreasonable and unwarranted. They claim that the assumptions needed to estimate the prospects for extraterrestrial intelligence contain too many unknowns. The search strategy itself adds additional unknowns. They conclude that any expenditure of time, effort, or money for such a search is unsupported by the meager evidence at hand.

By contrast, proponents argue that we have good reasons to suspect that extraterrestrials exist somewhere in space, given the diverse products of our evolution on Earth. They admit that we have only a minute chance of making contact in the near future. But they argue that now is the time to test the theory that advanced civilizations inhabit the Galaxy. To fail to try, is to commit the cardinal sin of pre-Renaissance workers—thinking or believing without experimentally testing. Failure to try might prematurely terminate humankind's natural exploratory drive. Our longevity as a civilization may be shortened for the very reason that we didn't undertake the challenge to search.

One thing is worth remembering: The space surrounding all of us could be, right now, inundated with radio signals from alien civilizations. If we only knew the proper direction and frequency, we might be able to make one of the most startling discoveries of all time. The result would not only likely provide whole new opportunities to know and understand the cosmic evolution of energy, matter, and life in the Universe, but it might also be the key to our civilization’s long-term survival along the arrow of time.