The Search for Extraterrestrial Intelligence

The first class of objects a human infant learns to recognize out of the welter of images that bombard its newborn brain is human faces. As the infant grows, it quickly decodes the language of facial expressions so essential to participation in human society. So deep is this urge to pick the human face out from the background that we see faces in random patterns everywhere — the face of Jesus in a field of snow, the face of an old man on a rocky outcropping in New Hampshire, or a humanoid face on Mars. We are a gregarious race, and such face-finding is a symptom of our hunger for companionship.

We have always sought for others like ourselves in the great beyond. In the dim days when that region was defined as "over the next hill", before fire or flint or steel, even before language itself, that hunger was in us. True, we did not always make the search with welcoming thoughts in mind; concerns for survival in a hostile world have also shaped us. But the hunger for community is a part of humanity. It may be the essence of humanity. Finding human qualities in nature, or ascribing them to it, satisfies that hunger as well as giving us a measure of control, real or imagined, over impersonal, sometimes inimical forces.

Once we imagined the natural forces of storm or flood as animated by personalities that could be angered by misdeeds or propitiated by offerings. Later, as we learned to grow crops and live in cities, we peopled the skies and seas and mountaintops with gods of human form and foibles, weaving them into myths that let us make sense of the vagaries of man and nature. The myths gave rise to religions, some of which survive today. When we invented glass lenses and put them into telescopes, we began to understand the true scope of the universe. Science began. We learned that the heavens above are not Heaven, perfect and immutable, but a physical realm like Earth, subject to the same chaotic and sometimes catastrophic events.

Now, with our modern, sophisticated outlook, we define both "the great beyond" and "others like ourselves" much more broadly than our ancestors, or our grandparents, would have. We behold a universe immensely greater than they knew. And we can imagine it harboring inhabitants far stranger. Yet we look outward with the same mixture of apprehension and anticipation. Our growing understanding of science has given us powers our ancestors would have deemed godlike. Yet we still need myths and religions for meaning. And, crowded as we are on this planet, we still seek new companions elsewhere. Those drives run deep. Grafted onto them is our genuine delight in the intellectual quest to solve the ancient riddle. It is a compelling combination.

There are trends in our scientific understanding of how the universe works. One leads us to expect that life is common in that immensity, and that intelligence, if rare, is not unique to this planet. Another fosters the belief that the more intelligent life becomes, the more advantages it sees in cooperation. At this moment in our history, such expectations are better labeled faiths than the kind of expectation that says a pebble held on your palm will drop to the ground if you turn your hand over.

Three facts, quite recently learned, buttress these beliefs. One is that organic materials exist throughout space. The second is that life, once established, can adapt to very harsh environments. And the third is that planets are common around other nearby suns. The origin of life is still a mystery, and science can say nothing definite about how it began or how common it might be. Nevertheless, the more we learn, the more likely seems the universe we imagined from the time of the first telescopes — a universe that teems with life, some of it intelligent, and some of that star-faring.

Countervailing trends, and contrary arguments, do exist. The chief one is The Great Silence, also known at The Fermi Paradox. Simply put, this is the argument that if, elsewhere in this galaxy, a star-traveling species arose even as recently as 10 or 20 million years ago, we would be seeing indisputable signs of its presence by now.

The question of whether or not our species is the only intelligence in the universe is a momentous one, and has been hotly debated for centuries at least. So what have we been doing to answer it?

It was Frank Drake, then at the National Radio Astronomy Observatory in Green Bank, West Virginia, who first put into practice the idea of listening for transmissions from civilizations on planets orbiting other suns than ours. Drake's pioneering search, called Project Ozma, used the 300-foot dish at Green Bank to listen on the 1420-MHz band for artificial signals from Tau Ceti and Epsilon Eridani, two nearby sunlike stars. It lasted several months in the spring of 1960, for a total of 400 observing hours, and produced no results.

The idea itself was then fairly new. Of course, once we invented radio ourselves, it was easy to realize that others might do the same; and the concept of communication across space followed naturally from that. Indeed, early in the Twentieth Century, some had listened for signals from Mars 1, when that nearby planet was thought to be inhabited. But the idea of listening to radio signals from the stars was recent, for this reason: Most of the energy being radiated into space comes from stars, and no stars are cool enough to radiate primarily at radio wavelengths.

To put it more technically, stars behave like the ideal black body. The intensity of radiation from a black body is proportional to the square of the frequency. Radio frequencies are a million times smaller than light frequencies, so the radio output of any star would be a million million times fainter than its light output. Thus, went the thinking of astronomers in the 1930s, it was a waste of time to try observing stars at radio wavelengths. And the thinking of those astronomers determined what equipment went into their observatories.

A serendipitous discovery made by a man named Karl Guthe Jansky, an employee of the Bell Telephone Laboratories, changed that thinking. Jansky had a degree in physics, but knew nothing about the astrophysical principle that, according to the professionals, forbade stars from producing detectable radio emissions. Bell Labs wanted to use "short waves" (10-20 meters) for transatlantic telephone circuits, and Jansky was given the assignment of tracking down sources of random noise that might interfere with these conversations.

After spending several months using a movable antenna he built to record signals coming from all directions, Jansky identified three types of static:

He spent over a year tracking the source of the third type of static. It rose and fell once a day, and at first Jansky thought he was seeing radiation from the Sun. But after a few months, the Sun was nowhere near where his antenna was pointing when it picked up the strongest signal. Eventually, he figured out that the radiation was coming from the center of our Milky Way galaxy, in the constellation Sagittarius.

The discovery was widely publicized, appearing in the New York Times of May 5, 1933. Jansky wanted to investigate in more detail, but Bell Labs assigned him to another project and he did no more radio astronomy. It fell to two other men to take the next steps. One was Grote Reber, who singlehandedly built a radio telescope in his back yard in 1937 and did the first systematic survey of radio waves from the sky. The second was John Kraus (1910-2004), who, after World War II, started a radio astronomy observatory at Ohio State University.

Some Later Developments

The scientific 3 basis of SETI was a 1959 paper by Giuseppe Cocconi and Philip Morrison. One crucial element was the choice of a frequency band on which to listen for artificial signals. The reasoning goes this way: Hydrogen is abundant throughout the universe. Oxygen is less common, but still plentiful enough that these elements, combined into the "hydroxyl radical", generate a substantial portion of that radio background. It happens that the hydroxyl radical radiates on a frequency of 1640 MHz, while ionized hydrogen broadcasts at 1420 MHz. It also happens that these signals penetrate Earth's ionosphere fairly easily, and that our galaxy is relatively quiet in this part of the radio spectrum. Thus it is a natural choice for any life forms like us to use if they wish to transmit signals likely to be detected by other intelligences. It also has a symbolic importance, because ionized hydrogen and the hydroxyl radical combine to form water, one of the bases of life as we know it. This radio band has come to be known as the Water Hole.

A variety of searches, using increasingly capable equipment, have taken place since Drake's pioneering effort — with equal lack of success. There have been some tantalizing hints, but none lasted long enough to be confirmed.

Neither have we found any other verifiable indications of extraterrestrial intelligence. No alien artifacts have been dug up on Earth. No derelict alien spaceships have been seen, by telescopes or radar or robotic probes or naked eyes. Nor, despite the deluge of UFO reports of all stripes, is there any real evidence for visitations or abductions by little green men — or by aliens of any shape or color.

Critics will say that this absence of evidence demonstrates the futility of the search. They argue that no extraterrestrial intelligence has been detected because none exists. This is the essence of the Fermi Paradox, dogmatically raised to the status of a natural law. The argument is not totally baseless, but I maintain that it is far too early to render such a verdict. If we cannot think of reasons why other civilizations have not reached us, in person or via radio signals, that does not mean such reasons cannot exist.

1 Attempts to discover life on the other planets of the inner solar system started well before the advent of radio communication. Many schemes depended on making some sort of signal on Earth, to which the aliens would respond. Examples included cutting miles-wide geometrical patterns into forests and planting the cleared areas with wheat for color contrast. Most of the interest centered on Mars, because it was thought to be older than Earth and hence its inhabitants would be farther advanced. Indeed, when Madame Guzman, around 1900, deposited 100,000 francs in a Paris bank to establish the Prix Guzman for the first successful interplanetary communication, she specifically excluded Mars because communicating with Martians was deemed too easy.

2 I suspect Jansky made this distinction between thunderstorms because of sferics. Pulses of lightning generate a wide range of radio frequencies, heard as brief crackles when the storm is nearby. But the radio burst from a distant lightning flash is channeled and filtered by the Earth's magnetic field, sounding in a receiver like a brief glissando — a whistle of descending tone.

3 It has been argued that SETI is not science, because it is not falsifiable. That is, one can never say that there is no extraterrestrial civilization, for bigger dishes and better equipment could always push the limits of detectability a little farther into the universe. This is true — and trivial. For one can falsify the proposition for any particular star system, and this is enough to justify a modest level of effort.

To Open The Sky

The Search for Extraterrestrial Intelligence

Some Later Developments