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Where Is Everybody?

Written by Ben Bova

Is there intelligent life in the universe?

The standard comeback to that question is: Is there intelligent life on Earth?

Well, what do we mean by “intelligent”? How do we define intelligence?

For the purposes of this article, we can define intelligence this way: An intelligent species is one in which any member of the species has the potential to acquire the experiences that all the other members of the species have had.

Communication, in this view, is the essence of intelligence. Many animal species of Earth can communicate some of their experience to other members of the species. Birds, dogs, bears, apes, dolphins all do a certain amount of communication, especially in training their young to survive.

But all their communications are face-to-face. Chimps cannot communicate to their young the experiences that their grandsires had. It’s all here-and-now, watch-me-and-learn.

Humans can read Homer and learn the theorems of Pythagoras. Humans can study the stars and pass on their knowledge from one generation to the next.

Human speech, and its offspring writing, are central to this view of intelligence. Thus, when we search for other intelligent species among the stars, we look for someone we can communicate with.

And we’ve found nothing. Nobody. Zip. Nil. Nada.

For nearly half a century astronomers have been searching the skies with radio telescopes, seeking signals from an intelligent civilization. In vain.

Of course, nobody expected the search for extraterrestrial intelligence (SETI) to be easy. Look at what the searchers are faced with.

The Milky Way galaxy consists of at least a hundred billion stars. Even if you limit SETI to stars that are like the Sun—yellow G-type stars—there are billions of them.

If you’re trying to pick up radio signals from another civilization, which frequency do you tune to? There are millions upon millions of possible radio frequencies that alien broadcasters might be using. How do you find a frequency on which they might be signaling? SETI investigators must pick a target star, sift through as many frequencies as possible, and then go on to the next candidate star and go through the same procedure, time and again. Much of the work in radio SETI has been devoted to developing electronic equipment that can sift through millions of frequencies quickly, so that the radio telescope may be moved from one star to another in a reasonable time.

Is radio the right medium to use for searching? We’ve been using radio because it was the first technology we had available for interstellar communication when we began to think about looking for other intelligent species. It’s a sort of historical accident, really. Maybe other intelligent species have found other means of communication, and they don’t use radio at all.

Some SETI researchers are shifting to optical searches, assuming that alien civilizations might use lasers for chit-chat over interstellar distances.

In 1959 SETI pioneer Frank Drake tried to summarize the problem of locating alien civilizations in a single mathematical formula:

N = R*f_pn_ef_lf_if_cL

This is now known as the Drake equation, and has served as a focus for SETI for nearly five decades. The equation is an attempt to estimate how many extraterrestrial civilizations we might be able to detect with radio telescopes.

N is 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.

f_p—How many of these stars have planets? At the time Drake wrote his equation, no stars except the sun were known to have planets, although most astronomers assumed that at least some do. Since 1995 more than 300 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.

n_e—How many of these planets have environments suitable for life? Our ideas on suitable environments have expanded greatly in the past couple of decades thanks to the discovery of organisms dubbed extremophiles, microscopic critters that live happily in extremes of heat, cold, pressure, lack of oxygen, lack of sunlight, and other outré conditions that would kill you and me.

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 or Saturn’s aptly-named Titan.

f_l—On how many of these planets did life actually arise? A suitable environment for life does not necessarily mean that life did emerge on that world. (Or does it?)

f_i—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 24 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 species.

f_c—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. Archeologists have found abundant evidence of civilizations that have collapsed and perished. Today the human race faces the perils of terrorism, 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. Carl Sagan, for example, came up in 1966 with a value for N of about one million, one of the more optimistic guesstimates. 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.

In 1950, at lunch with fellow scientists at the Los Alamos National Laboratory, Nobel laureate Enrico Fermi asked a simple question: Where is everybody?

If there are intelligent extraterrestrial civilizations out there, why haven’t we seen evidence of them? UFO enthusiasts, of course, insist that we have, but most serious investigators want more hard evidence than stories of UFOs and alien abductions.

The obvious answer to Fermi’s question was that, if intelligent ETs exist, the distances between their worlds and ours are so vast that the only hope we have of finding evidence for them lies in searches for signals from them. Hence SETI.

But by 1975, after some 15 years of radio searches, two scientists published papers that proposed something close to heresy, as far as the SETI researchers were concerned. American Michael Hart and David Viewing of Britain independently came to the conclusion that the reason no intelligent signals have been found is that there are no intelligent creatures out there. Planets like Earth are so rare, they argued, that we should not expect to find a similar world, or intelligent life.

In 1980 Tulane University mathematician Frank Tipler claimed that the universe is only capable of hosting one intelligent species, and we are it. Tipler is not a hidebound conservative, nor a chalkdust-dry academic. His work has included ideas on how to build time machines. Yet, he concluded that there are no other intelligent species in the entire universe.

Tipler made a practical argument for the nonexistence of extraterrestrials. The Milky Way galaxy is at least twice as old as our solar system. There are billions of stars that have existed for billions of years longer than we have. If intelligence has arisen on even a few of these ancient stars, those alien civilizations would be far older and far more knowledgeable than we. Their technological capabilities would immensely exceed our own.

Such a civilization would be able to colonize the entire galaxy, Tipler suggested. It need not send its own people into space, it could send self-replicating machines that move from one star to another, colonizing any planets they find, then using those planets’ natural resources to build more copies of themselves and move on to the next stars. In effect, such machines would be like a virus spreading from star to star, planet to planet. If their spacecraft could achieve velocities of only 10 percent of the speed of light (something that we should be able to do before this century is out), they could spread exponentially across the entire Milky Way in less than a million years, a small fraction of the Milky Way’s multi-billion-year existence.

The fact that Earth has not been visited by these mechanical representatives of a superior civilization, Tipler concluded, is proof that no such civilizations exist.

Howls of protest greeted Tipler’s pessimistic argument. The SETI optimists pointed out that his conclusions rest on enormous assumptions. For example: Could such ancient civilizations exist? The oldest stars are metal-poor, they might not have contained the proper elements for life, or for a technologically sophisticated civilization, to arise around them. Or, if such very old and wise civilizations do exist, perhaps they would have no interest in colonizing the galaxy.

While there may be many, many civilizations among the stars, some of them may be so much older (or younger) than we are that communicating with them would be impossible.

What are the chances of finding an intelligent civilization at our level of development?

Consider the history of the Earth. Let our planet's 4.5 billion years be represented by the height of the Empire State Building. At that scale, the human race's few million years of existence can be represented by a one-foot ruler standing upright at the top of the building. The thickness of a dime placed atop the ruler represents the entire span of our civilization—some ten thousand years. On top of that dime, glue a postage stamp. Its thickness is equivalent to the length of time since humankind has developed modern science: nearly 400 years.

The Milky Way may be brimming with intelligent species, but how many of them are in the same phase of development—and technology—as we are? How many are within the thickness of that postage stamp? They are the ones we will be most likely to communicate with.

Older, more developed species might not deign to talk to us. They might regard us as no better than apes, or insects. Perhaps they will want to study us, as human anthropologists study the primate apes. But to make such studies valid, they would have to avoid letting us know they are watching us.

On the other hand, younger species will not yet have invented technologies capable of interstellar communication.

To make contact with an extraterrestrial civilization, we will have to locate one that is close to our own level of development, somewhere between Tarzan and the angels.

As they say in The X Files, “The truth is out there.”

We’ll look more deeply into this next time.

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