Pale Blue Dot: A Vision of the Human Future in Space - Carl Sagan, Ann Druyan (1997)

Chapter 20. DARKNESS

Far away, hidden from the eyes of daylight, there are watchers in the skies.


As children, we fear the dark. Anything might be out. there. The unknown troubles us.

Ironically, it is our fate to live in the dark. This unexpected finding of science is only about three centuries old. Head out from the Earth in any direction you choose, and—after an initial flash of blue and a longer wait while the Sun fades—you are surrounded by blackness, punctuated only here and there by the faint and distant stars.

Even after we are grown, the darkness retains its power to frighten us. And so there are those who say we should not inquire too closely into who else might be living in that darkness. Better not to know, they say.

There are 400 billion stars in the Milky Way Galaxy. Of this immense multitude, could it be that our humdrum Sun is the only one with an inhabited planet? Maybe. Maybe the origin of life or intelligence is exceedingly improbable. Or maybe civilizations arise all the time, but wipe themselves out as soon as they are able.

Or, here and there, peppered across space, orbiting other suns, maybe there are worlds something like our own, on which other beings gaze up and wonder as we do about who else lives in the dark. Could the Milky Way be rippling with life and intelligence—worlds calling out to worlds—while we on Earth are alive at the critical moment when we first decide to listen?

Our species has discovered a way to communicate through the dark, to transcend immense distances. No means of communication is faster or cheaper or reaches out farther. It’s called radio.

After billions of years of biological evolution—on their planet and ours—an alien civilization cannot be in technological lockstep with us. There have been humans for more than twenty thousand centuries, but we’ve had radio only for about one century. If alien civilizations are behind us, they’re likely to be too far behind to have radio. And if they’re ahead of us, they’re likely to be far ahead of us. Think of the technical advances on our world over just the last few centuries. What is for us technologically difficult or impossible, what might seem to us like magic, might for them be trivially easy. They might use other, very advanced means to communicate with their peers, but they would know about radio as an approach to newly emerging civilizations. Even with no more than our level of technology at the transmitting and receiving ends, we could communicate today across much of the Galaxy. They should be able to do much better.

If they exist.

But our fear of the dark rebels. The idea of alien beings troubles us. We conjure up objections:

“It’s too expensive.” But, in its fullest modern technological expression, it costs less than one attack helicopter a year.

“We’ll never understand what they’re saying.” But, because the message is transmitted by radio, we and they must have radio physics, radio astronomy, and radio technology in common. The laws of Nature are the same everywhere; so science itself provides a means and language of communication even between very different kinds of beings—provided they both have science. Figuring out the message, if we’re fortunate enough to receive one, may be much easier than acquiring it.

“It would be demoralizing to learn that our science is primitive.” But by the standards of the next few centuries, at least some of our present science will be considered primitive, extraterrestrials or no extraterrestrials. (So will some of our present politics, ethics, economics, and religion.) To go beyond present science is one of the chief goals of science. Serious students are not commonly plunged into fits of despair on turning the pages of a textbook and discovering that some further topic is known to the author but not yet to the student. Usually the students struggle a little, acquire the new knowledge, and, following an ancient human tradition, continue to turn the pages.

“All through history advanced civilizations have ruined civilizations just slightly more backward.” Certainly. But malevolent aliens, should they exist, will not discover our existence from the fact that we listen. The search programs only receive; they do not send.*

THE DEBATE IS, for the moment, moot. We are now, on an unprecedented scale, listening for radio signals from possible other civilizations in the depths of space. Alive today is the first generation of scientists to interrogate the darkness. Conceivably it might also be the last generation before contact is made—and this the last moment before we discover that someone in the darkness is calling out to us.

This quest is called the Search for Extraterrestrial Intelligence (SETI). Let me describe how far we’ve come.

The first SETI program was carried out by Frank Drake at the National Radio Astronomy Observatory in Greenbank, West Virginia, in 1960. He listened to two nearby Sun-like stars for two weeks at one particular frequency. (“Nearby” is a relative term: The nearest was 12 light-years—70 trillion miles—away.)

Almost at the moment Drake pointed the radio telescope and turned the system on, he picked up a very strong signal. Was it a message from alien beings? Then it went away. If the signal disappears, you can’t scrutinize it. You can’t see if, because of the Earth’s rotation, it moves with the sky. If it’s not repeatable, you’ve learned almost nothing from it—it might be terrestrial radio interference, or a failure of your amplifier or detector … or an alien signal. Unrepeatable data, no matter how illustrious the scientist reporting them, are not worth much.

Weeks later, the signal was detected again. It turned out to be a military aircraft broadcasting on an unauthorized frequency. Drake reported negative results. But in science a negative result is not at all the same thing as a failure. His great achievement was to show that modern technology is fully able to listen for signals from hypothetical civilizations on the planets of other stars.

Since then there’ve been a number of attempts, often on time borrowed from other radio telescope observing programs, and almost never for longer than a few months. There’ve been some more false alarms, at Ohio State, in Arecibo, Puerto Rico, in France, Russia, and elsewhere, but nothing that could pass muster with the world scientific community.

Meanwhile, the technology for detection has been getting cheaper; the sensitivity keeps improving; the scientific respectability of SETI has continued to grow; and even NASA and Congress have become a little less afraid to support it. Diverse, complementary search strategies are possible and necessary. It was clear years ago that if the trend continued, the technology for a comprehensive SETI effort would eventually fall within the reach even of private organizations (or wealthy individuals); and sooner or later, the government would be willing to support a major program. After 30 years of work, for some of us it’s been later rather than sooner. But at last the time has come.

THE PLANETARY SOCIETY—a nonprofit membership organization that Bruce Murray, then the Director of JPL, and I founded in 1980—is devoted to planetary exploration and the search for extraterrestrial life. Paul Horowitz, a physicist at Harvard University, had made a number of important innovations for SETI and was eager to try them out. If we could find the money to get him started, we thought we could continue to support the program by donations from our members.

In 1983 Ann Druyan and I suggested to the filmmaker Steven Spielberg that this was an ideal project for him to support. Breaking with Hollywood tradition, he had in two wildly successful movies conveyed the idea that extraterrestrial beings might not be hostile and dangerous. Spielberg agreed. With his initial support through The Planetary Society, Project META began.

META is an acronym for “Megachannel ExtraTerrestrial Assay.” The single frequency of Drake’s first system grew to 8.4 million. But each channel, each “station,” we tune to has an exceptionally narrow frequency range. There are no known processes out among the stars and galaxies that can generate such sharp radio “lines.” If we pick up anything falling into so narrow a channel, it must, we think, be a token of intelligence and technology.

What’s more, the Earth turns—which means that any distant radio source will have a sizable apparent motion, like the rising and setting of the stars. Just as the steady tone of a car’s horn dips as it drives by, so any authentic extraterrestrial radio source will exhibit a steady drift in frequency due to the Earth’s rotation. In contrast, any source of radio interference at the Earth’s surface will be rotating at the same speed as the META receiver. META’s listening frequencies are continuously changed to compensate for the Earth’s rotation, so that any narrow-band signals from the sky will always appear in a single channel. But any radio interference down here on Earth will give itself away by racing through adjacent channels.

The META radio telescope at Harvard, Massachusetts, is 26 meters (84 feet) in diameter. Each day, as the Earth rotates the telescope beneath the sky, a swath of stars narrower than the full moon is swept out and examined. Next day, it’s an adjacent swath. Over a year, all of the northern sky and part of the southern is observed. An identical system, also sponsored by The Planetary Society, is in operation just outside Buenos Aires, Argentina, to examine the southern sky. So together the two META systems have been exploring the entire sky.

The radio telescope, gravitationally glued to the spinning Earth, looks at any given star for about two minutes. Then it’s on to the next. 8.4 million channels sounds like a lot, but remember, each channel is very narrow. All of them together constitute only a few parts in 100,000 of the available radio spectrum. So we have to park our 8.4 million channels somewhere in the radio spectrum for each year of observation, near some frequency that an alien civilization, knowing nothing about us, might nevertheless conclude we’re listening to.

Hydrogen is by far the most abundant kind of atom in the Universe. It’s distributed in clouds and as diffuse gas throughout interstellar space. When it acquires energy, it releases some of it by giving off radio waves at a precise frequency of 1420.405751768 megahertz. (One hertz means the crest and trough of a wave arriving at your detection instrument each second. So 1420 megahertz means 1.420 billion waves entering your detector every second. Since the wavelength of light is just the speed of light divided by the frequency of the wave, 1420 megahertz corresponds to a wavelength of 21 centimeters.) Radio astronomers anywhere in the Galaxy will be studying the Universe at 1420 megahertz and can anticipate that other radio astronomers, no matter how different they may look, will do the same.

It’s as if someone told you that there’s only one station on your home radio set’s frequency band, but that no one knows its frequency. Oh yes, one other thing: Your set’s frequency dial, with its thin marker you adjust by turning a knob, happens to reach from the Earth to the Moon. To search systematically through this vast radio spectrum, patiently turning the knob, is going to be very time-consuming. Your problem is to set the dial correctly from the beginning, to choose the right frequency. If you can correctly guess what frequencies that extraterrestrials are broadcasting to us on—the “magic” frequencies—then you can save yourself much time and trouble. These are the sorts of reasons that we first listened, as Drake did, at frequencies near 1420 megahertz, the hydrogen “magic” frequency.

Horowitz and I have published detailed results from five years of full-time searching with Project META and two years of follow-up. We can’t report that we found a signal from alien beings. But we did find something puzzling, something that for me in quiet moments, every now and then, raises goose bumps:

Of course, there’s a background level of radio noise from Earth—radio and television stations, aircraft, portable telephones, nearby and more distant spacecraft. Also, as with all radio receivers, the longer you wait, the more likely it is that there’ll be some random fluctuation in the electronics so strong that it generates a spurious signal. So we ignore anything that isn’t much louder than the background.

Any strong narrow-band signal that remains in a single channel we take very seriously. As it logs in the data, META automatically tells the human operators to pay attention to certain signals. Over five years we made some 60 trillion observations at various frequencies, while examining the entire accessible sky. A few dozen signals survive the culling. These are subjected to further scrutiny, and almost all of them are rejected—for example, because an error has been found by fault-detection microprocessors that examine the signal-detection microprocessors.

What’s left—the strongest candidate signals after three surveys of the sky—are 11 “events.” They satisfy all but one of our criteria for a genuine alien signal. But the one failed criterion is supremely important: Verifiability. We’ve never been able to find any of them again. We look back at that part of the sky three minutes later and there’s nothing there. We look again the following day: nothing. Examine it a year later, or seven years later, and still there’s nothing.

It seems unlikely that every signal we get from alien civilizations would turn itself off a couple of minutes after we begin listening, and never repeat. (How would they know we’re paying attention?) But, just possibly, this is the effect of twinkling. Stars twinkle because parcels of turbulent air are moving across the line of sight between the star and us. Sometimes these air parcels act as a lens and cause the light rays from a given star to converge a little, making it momentarily brighter. Similarly, astronomical radio sources may also twinkle—owing to clouds of electrically charged (or “ionized”) gas in the great near-vacuum between the stars. We observe this routinely with pulsars.

Imagine a radio signal that’s a little below the strength that we could otherwise detect on Earth. Occasionally the signal will by chance be temporarily focused, amplified, and brought within the detectability range of our radio telescopes. The interesting thing is that the lifetimes of such brightening, predicted from the physics of the interstellar gas, are a few minutes—and the chance of reacquiring the signal is small. We should really be pointing steadily at these coordinates in the sky, watching them for months.

Despite the fact that none of these signals repeats, there’s an additional fact about them that, every time I think about it, sends a chill down my spine: 8 of the 11 best candidate signals lie in or near the plane of the Milky Way Galaxy. The five strongest are in the constellations Cassiopeia, Monoceros, Hydra, and two in Sagittarius—in the approximate direction of the center of the Galaxy. The Milky Way is a flat, wheel-like collection of gas and dust and stars. Its flatness is why we see it as a band of diffuse light across the night sky. That’s where almost all the stars in our galaxy are. If our candidate signals really were radio interference from Earth or some undetected glitch in the detection electronics, we shouldn’t see them preferentially when we’re pointing at the Milky Way.

But maybe we had an especially unlucky and misleading run of statistics. The probability that this correlation with the galactic plane is due merely to chance is less than half a percent. Imagine a wall-size map of the sky, ranging from the North Star at the top to the fainter stars toward which the Earth’s south pole points at the bottom. Snaking across this wall map are the irregular boundaries of the Milky Way. Now suppose that you were blindfolded and asked to throw five darts at random at the map (with much of the southern sky, inaccessible from Massachusetts, declared off limits). You’d have to throw the set of five darts more than 200 times before, by accident, you got them to fall as closely within the precincts of the Milky Way as the five strongest META signals did. Without repeatable signals, though, there’s no way we can conclude that we’ve actually found extraterrestrial intelligence.

Or maybe the events we’ve found are caused by some new kind of astrophysical phenomenon, something that nobody has thought of yet, by which not civilizations, but stars or gas clouds (or something) that do lie in the plane of the Milky Way emit strong signals in bafflingly narrow frequency bands.

Let’s permit ourselves, though, a moment of extravagant speculation. Let’s imagine that all our surviving events are in fact due to radio beacons of other civilizations. Then we can estimate—from how little time we’ve spent watching each piece of sky—how many such transmitters there are in the entire Milky Way. The answer is something approaching a million. If randomly strewn through space, the nearest of them would be a few hundred light years away, too far for them to have picked up our own TV or radar signals yet. They would not know for another few centuries that a technical civilization has emerged on Earth. The Galaxy would be pulsing with life and intelligence, but—unless they’re busily exploring huge numbers of obscure star systems—wholly oblivious of what has been happening down here lately. A few centuries from now, after they do hear from us, things might get very interesting. Fortunately, we’d have many generations to prepare.

If, on the other hand, none of our candidate signals is an authentic alien radio beacon, then we’re forced to the conclusion that very few civilizations are broadcasting, maybe none, at least at our magic frequencies and strongly enough for us to hear:

Consider a civilization like our own, but which dedicated all its available power (about 10 trillion watts) to broadcasting a beacon signal at one of our magic frequencies and to all directions in space. The META results would then imply that there are no such civilizations out to 25 light-years—a volume that encompasses perhaps a dozen Sun-like stars. This is not a very stringent limit. If, in contrast, that civilization were broadcasting directly at our position in space, using an antenna no more advanced than the Arecibo Observatory, then if META has found nothing, it follows that there are no such civilizations anywhere in the Milky Way Galaxy—out of 400 billion stars, not one. But even assuming they would want to, how would they know to transmit in our direction?

Now consider, at the opposite technological extreme, a very advanced civilization omnidirectionally and extravagantly broadcasting at a power level 10 trillion times greater (1026 watts, the entire energy output of a star like the Sun). Then, if the META results are negative, we can conclude not only that there are no such civilizations in the Milky Way, but none out to 70 million light-years—none in M31, the nearest galaxy like our own, none in M33, or the Fornax system, or M81, or the Whirlpool Nebula, or Centaurus A, or the Virgo cluster of galaxies, or the nearest Seyfert galaxies; none among any of the hundred trillion stars in thousands of nearby galaxies. Stake through its heart or not, the geocentric conceit stirs again.

Of course, it might be a token not of intelligence but of stupidity to pour so much energy into interstellar (and intergalactic) communication. Perhaps they have good reasons not to hail all comers. Or perhaps they don’t care about civilizations as backward as we are. But still—not one civilization in a hundred trillion stars broadcasting with such power on such a frequency? If the META results are negative, we have set an instructive limit—but whether on the abundance of very advanced civilizations or their communications strategy we have no way of knowing. Even if META has found nothing, a broad middle range remains open—of abundant civilizations, more advanced than we and broadcasting omnidirectionally at magic frequencies. We would not have heard from them yet.

ON OCTOBER 12, 1992—auspiciously or otherwise the 500th anniversary of the “discovery” of America by Christopher Columbus—NASA turned on its new SETI program. At a radio telescope in the Mojave Desert, a search was initiated intended to cover the entire sky systematically—like META, making no guesses about which stars are more likely, but greatly expanding the frequency coverage. At the Arecibo Observatory, an even more sensitive NASA study began that concentrated on promising nearby star systems. When fully operational, the NASA searches would have been able to detect much fainter signals than META, and look for kinds of signals that META could not.

The META experience reveals a thicket of background static and radio interference. Quick reobservation and confirmation of the signal—especially at other, independent radio telescopes—is the key to being sure. Horowitz and I gave NASA scientists the coordinates of our fleeting and enigmatic events. Perhaps they would be able to confirm and clarify our results. The NASA program was also developing new technology, stimulating ideas, and exciting schoolchildren. In the eyes of many it was well worth the $10 million a year being spent on it. But almost exactly a year after authorizing it, Congress pulled the plug on NASA’s SETI program. It cost too much, they said. The post—Cold War U.S. defense budget is some 30,000 times larger.

The chief argument of the principal opponent of the NASA SETI program—Senator Richard Bryan of Nevada—was this [from the Congressional Record for September 22, 1993]:

So far, the NASA SETI Program has found nothing. In fact, all the decades of SETI research have found no confirmable signs of extraterrestrial life.

Even with the current NASA version of SETI, I do not think many of its scientists would be willing to guarantee that we are likely to see any tangible results in the [foreseeable] future …

Scientific research rarely, if ever, offers guarantees of success—and I understand that—and the full benefits of such research are often unknown until very late in the process. And I accept that, as well.

In the case of SETI, however, the chances of success are so remote, and the likely benefits of the program are so limited, that there is little justification for 12 million taxpayer dollars to be expended for this program.

But how, before we have found extraterrestrial intelligence, can we “guarantee” that we will find it? How, on the other hand, can we know that the chances of success are “remote”? And if we find extraterrestrial intelligence, are the benefits really likely to be “so limited”? As in all great exploratory ventures, we do not know what we will find and we don’t know the probability of finding it. If we did, we would not have to look.

SETI is one of those search programs irritating to those who want well-defined cost/benefit ratios. Whether ETI can be found; how long it would take to find it; and what it would cost to do so are all unknown. The benefits might be enormous, but we can’t really be sure of that either. It would of course be foolish to spend a major fraction of the national treasure on such ventures, but I wonder if civilizations cannot be calibrated by whether they pay some attention to trying to solve the great problems.

Despite these setbacks, a dedicated band of scientists and engineers, centered at the SETI Institute in Palo Alto, California, has decided to go ahead, government or no government. NASA has given them permission to use the equipment already paid for; captains of the electronics industry have donated a few million dollars; at least one appropriate radio telescope is available; and the initial stages of this grandest of all SETI programs is on track. If it can demonstrate that a useful sky survey is possible without being swamped by background noise—and especially if, as is very likely from the META experience, there are unexplained candidate signals—perhaps Congress will change its mind once more and fund the project.

Meanwhile, Paul Horowitz has come up with a new program—different from META, different from what NASA was doing—called BETA. BETA stands for “Billion-channel ExtraTerrestrial Assay.” It combines narrow-band sensitivity, wide frequency coverage, and a clever way to verify signals as they’re detected. If The Planetary Society can find the additional support, this system—much cheaper than the former NASA program—should be on the air soon.

WOULD I LIKE TO BELIEVE that with META we’ve detected transmissions from other civilizations out there in the dark, sprinkled through the vast Milky Way Galaxy? You bet. After decades of wondering and studying this problem, of course I would. To me, such a discovery would be thrilling. It would change everything. We would be hearing from other beings, independently evolved over billions of years, viewing the Universe perhaps very differently, probably much smarter, certainly not human. How much do they know that we don’t?

For me, no signals, no one calling out to us is a depressing prospect. “Complete silence,” said Jean-Jacques Rousseau in a different context, “induces melancholy; it is an image of death.” But I’m with Henry David Thoreau: “Why should I feel lonely? Is not our planet in the Milky Way?”

The realization that such beings exist and that, as the evolutionary process requires, they must be very different from us, would have a striking implication: Whatever differences divide us down here on Earth are trivial compared to the differences between any of us and any of them. Maybe it’s a long shot, but the discovery of extraterrestrial intelligence might play a role in unifying our squabbling and divided planet. It would be the last of the Great Demotions, a rite of passage for our species and a transforming event in the ancient quest to discover our place in the Universe.

In our fascination with SETI, we might be tempted, even without good evidence, to succumb to belief; but this would be self-indulgent and foolish. We must surrender our skepticism only in the face of rock-solid evidence. Science demands a tolerance for ambiguity. Where we are ignorant, we withhold belief. Whatever annoyance the uncertainty engenders serves a higher purpose: It drives us to accumulate better data. This attitude is the difference between science and so much else. Science offers little in the way of cheap thrills. The standards of evidence are strict. But when followed they allow us to see far, illuminating even a great darkness.

*Surprisingly many people, including New York Times editorialists, are concerned that once extraterrestrials know where we are, they will come here and eat us. Put aside the profound biological differences that must exist between the hypothetical aliens and ourselves; imagine that we constitute an interstellar gastronomic delicacy. Why transport large numbers of us to alien restaurants? The freightage is enormous. Wouldn’t it be better just to steal a few humans, sequence our amino acids or whatever else is the source of our delectability, and then just synthesize the identical food product from scratch?