by Ben Bova
Radio Frequencies
THE WATER HOLE
Cocconi and Morrison suggested using radio to search for alien civilizations for three main reasons:
1. The enormous distances between stars make interstellar travel by spacecraft seem unlikely. Even traveling at the speed of light, which is far beyond our technological capabilities (and may always be), it would take a ship 4.3 years to reach Alpha Centauri, the nearest star to us, and 30,000 years to reach the heart of the Milky Way galaxy. Radio signals, however, travel at the speed of light naturally. It is much easier to transmit information across the galaxy than to send explorers.
2. Some radio waves can penetrate the clouds of gas and dust that obscure much of the Milky Way from optical telescopes and can also penetrate our own atmosphere, which blocks many other forms of radiation (such as X rays and gamma rays) from Earth’s surface.
3. The rapid development of electronics after World War II that had led to the new field of radio astronomy produced radio telescopes capable of receiving intelligent signals as well as the natural radio-frequency emissions from the stars.
Where to look? The two Cornell physicists suggested “the water hole.”
The Milky Way is pervaded with a thin gas of neutral (un-ionized) hydrogen. These ubiquitous hydrogen atoms naturally emit a radio signal at 1,420 megahertz, which equals a wavelength of 21 centimeters. (For comparison, visible light ranges in wavelength from 350 to 750 nanometers [billionths of a meter]. Visible light wavelengths are in the range of 100 million million hertz [1014 Hz].)
Many radio frequencies are absorbed by the Milky Way’s gas clouds or our own planet’s atmosphere, but there is a region—a “window”—between the 1,420-MHz “song of hydrogen” and the 1,662-MHz emission of the hydroxyl molecule that comes through loud and clear. The hydroxyl molecule is two-thirds of a water molecule, OH as compared to H2O. Hence the window was quickly dubbed “the water hole.”
PROJECT OZMA
Even before Cocconi and Morrison’s paper was published, Frank Drake arrived at the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia, with ideas of his own. A graduate of Cornell and Harvard, Drake had become intrigued with the possibilities of locating alien civilizations ever since, as a student at Harvard, he had detected a strong radio signal while observing the Pleiades star cluster. The signal turned out to be from a terrestrial source, but that sudden thrill of thinking that maybe, just maybe, he had found an extraterrestrial civilization was a turning point in Drake’s life.
Few astronomers admitted even to thinking about alien intelligence in those days of the 1950s, but Drake was fortunate enough to hear a lecture by Otto Struve while he was at Cornell. Struve was an astronomer of impeccable international reputation and the son of a world-famous astronomer, as well. Among his many interests in astronomy was the way that stars spin; Struve believed that slow-spinning stars must be accompanied by planetary systems that have absorbed the star’s angular momentum, as the planets of our solar system have absorbed the Sun’s angular momentum. At that time there was no evidence of planets orbiting other stars (that was not to come until 1995), but Struve maintained that stellar spin rates showed that other stars must have planets accompanying them. In those days, hardly any astronomers avowed an interest in whether other stars were accompanied by planets.
Struve became the first head of NRAO, and Drake joined his staff in West Virginia, several months before the publication of Cocconi and Morrison’s paper. Somewhat hesitantly, Drake suggested using the new 26-meter radio telescope to search for possible alien signals. His reasoning was very similar to what Cocconi and Morrison would write. After all, they were looking at the same problem and had the same tools at hand. Struve enthusiastically agreed with Drake, but, because the search for alien life was still a touchy subject in academia and carried a large “giggle factor” among the politicians who decided on NRAO’s funding, Drake’s search was started in secret, “piggybacked” on a legitimate study of the 21-centimeter radiation.
Drake calculated that his equipment could detect radio signals similar to those being broadcast around Earth out to a distance of little more than ten light-years. He did not expect to find alien messages deliberately beamed toward us; he was hoping to detect the background chatter of a global civilization’s radio and television broadcasts. He picked the two nearest solar-type stars to study: Epsilon Eridani and Tau Ceti, 10.7 and 11.3 light-years distant, respectively. He dubbed his “bootlegged” program Ozma, after the queen of L. Frank Baum’s fictional land of Oz, which, in Drake’s words, is “a place very far away, difficult to reach, and populated by exotic beings.”
The very first day (radio telescopes can operate in daylight), their first look at Epsilon Eridani produced a sharp signal that electrified Drake and his colleagues. But it turned out to be a false alarm: Something nearby had put out an electrical pulse that the radio telescope picked up. The search was not going to be that easy. Far from it.
Then the Cocconi-Morrison paper came out and suddenly the idea of searching for extraterrestrial intelligence became a hot topic. Here were two highly respected physicists saying it was not only possible to search, but desirable.
Struve, with his enormous reputation behind him, “went public” with Drake’s work in a lecture he gave at MIT in November 1959. He wanted to make certain that his NRAO got credit for conducting the first radio search.
THE DRAKE EQUATION
Struve went one step further. He organized a small, informal conference at Green Bank to bring together the top thinkers in a variety of scientific fields to discuss future strategies for searching with radio telescopes. Eleven researchers met at Green Bank, including Cocconi, Morrison, and biochemist Melvin Calvin, pioneer of the concept of chemical evolution as a precursor to the development of life (Chapter 9).
In an effort to focus the group’s discussions, Drake tried to summarize the problem of locating alien civilizations in a single mathematical formula:
N = R*fpneflfifcL
This is now known as the Drake equation, and it has served as a focus for SETI for more than four decades. The equation is an attempt to estimate how many extraterrestrial civilizations we might be able to detect with radio telescopes.
N—The number of communicating civilizations we may expect to exist in the Milky Way galaxy. The symbols on the right side of the equation represent the factors that determine what that number might be.
R*—How many stars are there in the Milky Way galaxy? Estimates range from 100 billion to several hundred billion.
fp—How many of these stars have planets? At the time Drake wrote his equation, no stars were known to have planets, although most astronomers assumed that at least some do. Since 1995, more than 100 planets have been detected orbiting stars other than the Sun, but no one can yet say with any confidence what fraction of the Milky Way’s stars harbor planetary systems. Note that this factor assumes that intelligent extraterrestrials must exist on planets; probably a good assumption but possibly too conservative.
ne—How many of these planets have environments suitable for life? As we have seen, our ideas on suitable environments have expanded greatly in the past ten years to include the extremophiles. We do not really know how many bizarre (to us) environmental niches alien organisms might exist in. Moreover, this factor overlooks the possibility that life may exist on the moons of planets, such as Jupiter’s Galilean satellites.
fl—On how many of these planets did life actually arise? A suitable environment for life does not necessarily mean that life emerged on that world. (Or does it?)
fi—On how many of the planets that bear life has intelligent life developed? It took almost the entire history of Earth before intelligence arose here; if Earth’s 4.6 billion years were condensed down to twenty-four hours, the advent of intelligent human beings happened a fraction of a second before midnight. If our own history is any guide, it takes billions of years for a planet to develop an intelligent specie
s.
fc—How many intelligent species will communicate? Up until a century ago, Earth bore abundant life and at least one intelligent species, yet could not communicate because radio had not yet been invented. On the other hand, it is possible that some intelligent species will have no interest in communicating, even if they have the capability to do so. They won’t all be descended from curious, chattering apes.
L—How long might an intelligent, communicative society last? When Drake first wrote his equation, the Cold War threatened nuclear holocaust. Archaeologists have found abundant evidence of civilizations that have collapsed and perished. Today the human race faces the perils of global warming and widespread pollution in addition to the continuing threat of nuclear devastation. Or Earth might be struck by an asteroid such as the one that wiped out the dinosaurs 65 million years ago. There is no guarantee that our civilization or our species will last indefinitely.
Put all these factors together and you have an estimate of the number of civilizations that may be “out there,” able and willing to communicate over interstellar distances. The trouble is, all the factors on the right side of the equation are unknowns except the number of stars in the Milky Way (and that is arguable). So the Drake equation is more of a guide to thinking than a means of producing a hard and fast number.
Estimates of the number of communicating civilizations have ranged from one to millions, depending on whether the person making the estimate was a pessimist or an optimist. Sagan, for example, came up in 1966 with a value for N of about 1 million, one of the more optimistic guesstimates. As we will soon see, others have concluded that the number is one: There are no other intelligent civilizations in the Milky Way galaxy. Such widely diverging results are a sign that no one knows enough as yet to answer the basic question.
Freeman Dyson of the Institute for Advanced Study at Princeton (and no stranger to unorthodox ideas himself) wrote: “I reject as worthless all attempts to calculate . . . the frequency of occurrence of intelligent life-forms in the universe. Our ignorance of the chemical processes by which life arose on earth makes such calculations meaningless.”
Perhaps so, but the Drake equation has served as a focal point for planning the search for extraterrestrial intelligence.
A MESSAGE FROM THE STARS
Drake also pondered how we might recognize a deliberate signal from an intelligent extraterrestrial civilization out of the random background noise of the stars. And, once we recognized it, would it be possible to decode it?
Most SETI investigators believe that an intelligent signal will stand out clearly from the background noise. To be recognized as artificial, it will probably be repeated over and over again and will most likely be based on mathematics, which can be considered a universal language.
Thinking about these problems, Drake produced an “alien” message and sent it to a group of scientist friends in 1961. The message was a string of 551 “ones” and “zeroes,” representing the kind of digital signal that a radio telescope might receive. Drake asked his colleagues, Suppose you had received this message. Could you decipher it?
Fig 18-1. This was the message sent by Dr. Frank Drake to his friends.
Fig 18-2. The 551 characters of the code message are arranged in a rectangle that is 19 units across and 29 units long. All the “ones” are represented by a dark space while all the “zeroes” are left blank.
Fig 18-3. Below is the correct decoding of the “message from the stars.” Dr. Drake’s message shows that there is a chance to understand a race from another star system, once we make radio contact
The recipients quickly recognized that the message was in a binary code, the sort of language that computers use. By arranging the 551 units in a rectangle of 19 by 29 units and putting a dark space wherever there was a “one” while leaving the “zeroes” blank, the scientists came up with a crude pictogram that told them the following information:
The senders have two arms, two legs, and one head.
Their solar system has nine planets.
They come from the fourth planet, which has a population of 7 billion.
There are 3,000 of their people on the third planet and 11 on the second. This shows they have interplanetary space flight, with a good-sized colony on the third planet and a research team on the second.
Their body chemistry is based on carbon and oxygen, the same as ours.
Their bodies are 31 units tall. If we assume that the unit of measurement is the wavelength on which the message was received, which was given as 6 centimeters, then they are 186 centimeters (about 6 feet) tall.
While no single recipient of the message deciphered the entire pictograph, among the group of them they got the whole story. Drake believed that his “message” showed that significant information could be relayed even through a simple communications method. Critics have pointed out that the sender of the message and the receivers all shared the same graphic, linguistic, and mathematical conventions, something that probably would not be true of alien intelligences.
Still, Drake’s “alien message” gave hope that an intelligent signal from an extraterrestrial civilization might be deciphered—if we ever received one.
OZMA BECOMES SETI
The Green Bank Conference made the search for alien intelligence reputable. Scientists of the caliber of Struve, Morrison, Calvin (who won the Nobel Prize in 1961 for his work on photosynthesis), et al. put an imprimatur of respectability on a subject that had been scoffed at or ignored altogether up until then by most of academia, the government, and the public at large.
A twenty-six-year-old assistant professor of astronomy at Harvard’s Smithsonian Observatory, Carl Sagan, felt liberated by the Green Bank Conference, saying years later that “we had finally penetrated the ridicule barrier.” He went on to become a leader in the quest for life on other worlds.
Ozma cost $2,000 in equipment and spent a total of 200 hours over several months scanning Tau Ceti and Epsilon Eridani at 7,200 individual radio frequencies, grouped around the 1,420-MHz (21-cm) hydrogen line. No intelligent signals were discovered.
Over the next four decades, more than a dozen radio searches have probed the stars with ever more sophisticated equipment. NASA became involved and turned the world’s largest radio telescope at Arecibo in Puerto Rico, and the Jet Propulsion Laboratory’s Deep Space Network array of radio dishes at Goldstone, California, to the search for intelligence.
Aside from one “Wow!” signal that was picked up by an array at Ohio State University in 1977—a strong radio pulse that was never repeated and therefore could not be identified—no signs of intelligent communications have been found.
LGM SIGNALS
However, there was a brief flurry of intense excitement in 1967. For a few weeks radio astronomers thought they just might have picked up signals from “little green men”: intelligent extraterrestrials.
Jocelyn Bell was interested in astronomy from childhood. Born in Belfast, her father had been the architect of the Armagh Observatory in Northern Ireland, and the astronomers she met there encouraged her to study the stars. In 1967, as a twenty-four-year-old graduate student at Cambridge University, Bell was assigned to work at the Mullard Radio Astronomy Laboratory. The Mullard radio telescope was not a dish antenna, but an array of 2,048 dipole antennas spread over an 18,000-square-meter area; it looked somewhat like a large field of metal clothesline poles.
Among Bell’s menial tasks was monitoring the long scrolls of paper on which the radio telescope’s incoming signals were transcribed. One afternoon she was startled to find that the radio telescope had picked up pulses precisely spaced 1.333730113 seconds apart. Each individual pulse was 10 to 20 milliseconds long and as accurately timed as the most precise atomic clock on Earth.
Nothing like this had ever been detected before. For several weeks Bell, her faculty mentor Antony Hewish, and the other members of the staff searched for a cause for the pulses. Their first thoughts were that it was a glitch in the instrument
ation or some terrestrial source of interference. Then they thought it might have been an artificial satellite orbiting the Earth. It was none of those.
Eventually they began to consider what they called “the LGM hypothesis”: They thought that they just might have discovered signals from intelligent aliens, “little green men.”
This raised an intriguing question: How do you announce such a discovery? Hewish decided that they had to exhaust all other possibilities before they told the world they had found an intelligent extraterrestrial civilization. Within a few months they found three more such pulsating radio sources among the stars.
It turned out that what they had discovered was even weirder than “little green men,” in a way. They had detected the first known pulsars, a type of collapsed star that sends out a beam of visible light and radio energy like a rapidly spinning lighthouse. A pulsar is the debris that remains after an aged star explodes into a supernova. No more than a few kilometers across, a pulsar still contains as much material as the Sun, crushed down to such a density that all its protons and electrons are squeezed together to form neutrons.
Thomas Gold correctly deduced the pulsars’ nature, and his explanation for the pulsars was borne out in 1969 when a pulsar was discovered at the heart of the Crab Nebula, the remnants of a supernova whose light was first seen on Earth in the year 1054.
No LGMs, but a new type of astronomical phenomenon. In 1974, Hewish received the Nobel Prize for the discovery of the pulsars. Bell did not. Today she is Jocelyn Bell Burnell. After several prestigious positions at Mullard and the Royal Observatory at Edinburgh, she was appointed professor of physics at the Open University.
