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If elementary life forms are common throughout the cosmos, could intelligent beings have evolved elsewhere, and are they seeking us out? A father-and-son-team of scientists--both with research background in astronomy and physics--gives us the most up-to-date scientific answers about extraterrestrial civilizations and our attempts to find them. If they exist, why haven't we been able to make contact? Could they be reluctant or unable to make themselves known? If aliens visited us before recorded history, are we now overdue for another visit? Even if we discount most UFO sightings as erroneous, how do we explain that more than four million Americans claim they have been abducted by aliens? Is there a case to be made for a future scientific study of UFOs?

In answering these questions Aliens takes us to the very cutting edge of science in language requiring no specialized knowledge. As the authors explain how the "Drake Equation" became cosmology's most powerful tool in the quest to unlock the secret of life on other planets, they pull together the strands from all relevant scientific evidence to give us a fascinating and balanced report of what state-of-the-art research has uncovered. Here is a unique merging of current astronomical knowledge with thought-provoking philosophical reflection about what is surely the greatest scientific quest of all time.

Andrew J.H. Clark is a physicist and philosopher. Dr. David H. Clark, PH.D., his father and author of The Cosmos from Space, led the space astronomy research team at the Rutherford Appelton Laboratory in Oxford. They both live in Oxford, England.

Published by Fromm International; 0880642335; $25.00US; Sept. 99

Copyright © 1999 Andrew J.H. Clark and David H. Clark

One of the monumental achievements of twentieth-century science has been to gain an excellent understanding of how the universe was formed in a "big bang" some 15 billion years ago--and what its ultimate fate might be. The big bang is envisaged to be the epoch of creation. The early universe was composed of 75 percent hydrogen and 25 percent helium, plus radiation. As the nascent universe expanded, galaxies were formed, and within the galaxies the first generations of stars and planets were born. Although the current theories for the formation of galaxies leave some unanswered questions, the general picture is understood. Scientists can now describe the birth, life, and death of stars and planetary systems with some confidence. It is in the stars (giant globes of gas at extreme temperatures) where the elements that make up everything of familiar experience (including the carbon, nitrogen, and oxygen that combine in the building blocks of life-forms) are forged. Hence, ancient mythologies that described humans as being "the children of the stars" contained a semblance of truth! It is a sobering thought that all the elements in our bodies, other than hydrogen, have been processed through the stars.

Humans through the millennia have viewed the heavens with wonder and with awe, sensing the vastness of space, the power of the creation, and perhaps even something of their own origins as they looked out into a clear night sky. Until the past half-century, however, they could have had no real appreciation of the true enormity of the cosmos, its cataclysmic origin, or their own close relationship to the stars. Our ancestors even wondered whether they might not be alone in the universe. Metrodorus, a fourth-century-B.C. philosopher, had no doubts: "To consider the Earth the only populated world in infinite space is as absurd as to assert that in an entire field sown with millet only one grain will grow." However, opinion on the possibility of other populated worlds was divided, and in recent centuries the prospect caused anguish within evolving religious doctrine. Giordano Bruno was burned alive at the stake in 1600 by the Roman Inquisition for his cosmological views, including his belief that many populated worlds existed. One of the more powerfully expressed opinions was that of Thomas Paine in his Age of Reason (1793), where he stated:

Modern scientific understanding, and the liberalization of traditional religious dogma, now allow the two beliefs to be reconciled.

The scope of present-day astronomical research extends from the origin of the universe, beyond its present turbulent state, to speculation about its ultimate fate. It extends from the Earth's nearest, and comparatively well-understood planetary and stellar neighbors, to bizarre and enigmatic objects at the extremities of the observable universe. It now actively embraces the prospect of ETI, and diligently seeks it out. SETI has secured a legitimate place in modern astronomical research, and uses major research facilities to look for signals from extraterrestrials. Only a few of the world's largest radio telescopes have not searched for ETI at some time.

Because of the vast distances involved in the cosmos, it is no longer appropriate to use familiar units of distance such as the mile or kilometer. Instead astronomers use the light-year, the distance a pulse of light travels in 1 year. Since the speed of light is a staggering 300,000 kilometers each second, a light-year is a considerable distance--some 10 trillion kilometers. (Billion is the term used for 1,000 million, and trillion is the term used for a million million.) At 300,000 kilometers per second, the light from the Sun takes about 8 minutes to reach the Earth. The Solar System is about 12 light-hours in diameter. The nearest star to the Sun is almost 5 light-years away. The extreme distances to the stars present particular difficulties for SETI, as we will discover.

To describe the vast distances in the cosmos and the long time intervals involved in the evolution of the universe, scientists find it convenient to use a system of "powers of ten." Remember how in the story Alice's Adventures in Wonderland, Alice grew larger when she ate the cake? Let us imagine that one bite of cake meant increasing in size ten times--and each successive bite would mean a further tenfold increase in size. After the first bite, a 1-meter-high Alice would be 10 meters tall; after two bites, she would be 100 meters tall; and after three bites, she would be 1,000 meters high. By the fourth bite her head would be at the cruising altitude of a jumbo jet, and by the sixth bite she would be well on the way to the Moon. This process demonstrates powers of ten; the second imaginary bite is ten to the power of two (100 meters), written as 10² meters, and the fourth imaginary bite is ten to the power of four (10,000 meters), written 10⁴ meters, and so forth. To get to the distances of the stars requires sixteen imaginary bites of cake, and to get to the extremities of the cosmos would require twenty-five imaginary bites of cake (that is, we are at distances of 10²⁵ meters). The number 10²⁵ is very large--it is the number one followed by twenty-five zeros (10,000,000,000,000,000,000,000,000 meters). A light-year is equivalent to 10¹⁶ meters. The value 10³ is often summarized by the prefix kilo (thus, a kilometer is 10³ meters); 10⁶ by the prefix mega; 10⁹, by giga; and 10¹², by tera. A million is 10⁶; a billion is 10⁹; and a trillion is 10¹². Astronomy has to deal with lots of very large numbers, and the use of powers of ten certainly saves on ink and paper in writing out endless strings of zeros. But despite its convenience, powers of ten can disguise the enormous distances and time spans we need to deal with in discussing the search for ETI. Think big--and if it helps to think big, try to imagine the long string of zeros.

Stars are not uniformly scattered throughout the cosmos, but accumulate in vast conglomerates called galaxies, containing many billions of stars. Galaxies themselves tend to accumulate in clusters. Our Sun is just one of an estimated 400 billion (4 x 10¹¹) stars within our Galaxy (the capital letter signifying by convention the galaxy the Solar System lies in, rather than any old galaxy). Our Galaxy is called the Milky Way (from the appearance of the nebulous band of neighboring stars stretching across the night sky). The Milky Way Galaxy is discus- shaped, a full 100,000 (10⁵) light-years across at its widest, with our Sun occupying a rather insignificant location closer to its periphery than its heart. The bright stars of the Milky Way lie in intertwined spiral arms. Such spiral formations are a common species of galaxy, although many galaxies adopt a more amorphous elliptical shape or an irregular shape. A tenuous interstellar medium lies between the stars; space is not quite a perfect vacuum, although it comes close to it (three grains of sand in Yankee Stadium are more closely packed than atoms in the interstellar medium).

If our place within the Milky Way Galaxy seems insignificant, then the place of the Milky Way within the universe seems even more so. The observable universe is believed to contain at least 10 billion (10¹⁰) galaxies, clustering together in the thousands but still spaced from one another by millions of light-years. The Milky Way lies within a cluster of galaxies called the Local Group.

If the vast distances in the cosmos are difficult to assimilate, further adjustments to our terrestrial scale of thinking are required if we are to appreciate the mass scales involved in the universe. Scientists choose to measure the mass of objects of common experience in terms of a convenient standard, the kilogram. (There are about 2.2 pounds in a kilogram.) Thus, for example, an adult male may have a mass of about 80 kilograms. The mass of planet Earth is 6 trillion, trillion (10²⁴) kilograms! The Sun is some 300,000 times more massive than the Earth. The mass of the Milky Way is probably at least 500 billion (5 x 10¹¹) times that of the Sun! And the mass of the universe? Well, it is certainly greater (perhaps very much greater) than a billion, trillion (10²¹) solar masses! Just how much matter there is in the universe remains somewhat uncertain, and will determine its ultimate destiny.

No less of a challenge to the human imagination are the time scales involved in describing astronomical phenomena. Earth-bound events are conveniently measured in terms of the sidereal year, the time it takes the Earth to complete one orbit about the Sun, measured relative to the fixed stars. The Sun and other stars orbit around the center of the Milky Way, like a gigantic Catherine wheel. At the Sun's distance from the center, the stars take over 200 million years to complete one revolution. The Sun is believed to be some 5 billion years old, and will survive for a similar period. The universe itself is thought to be some 15 billion years old. Time is not a problem when it comes to thinking about the eons needed for the processing of elements in the stars, to enable the eventual formation of planetary systems, and then for life to develop. Time is something the universe has had plenty of, and has plenty more to come. There has certainly been no shortage of time for any ETI to evolve. After all, the path of evolution on Earth led from primitive sea creatures to humans in approximately 500 million years. Just try to imagine what several billion years of evolution to ETI might produce! When it comes to the question of whether extraterrestrial life-forms could evolve to levels, of intelligence and gain technological capabilities far beyond our own, time is not an issue. The universe has provided time aplenty.

In thinking about the cosmos, one needs to get used to very big numbers for times, sizes, and distances!

When looking out into the cosmos through a telescope, because of the finite speed of light, we are seeing not only deep into space, but also back in time. Thus, the nearby stars are viewed as they were several years ago. More distant stars within the Milky Way are seen as they were thousands of years ago when the light now reaching the Earth commenced its cosmic journey. The nearby galaxies appear as they were millions of years ago, and the more distant galaxies as they were hundreds or even thousands of millions of years ago. Many of the objects we observe do not still exist at this instant, at least in the form we presently see them. Thus, the history of the universe is laid out for Earth-bound heaven gazers to contemplate. The telescope represents a form of time machine in which we can study stars and galaxies at various stages of their evolution: nascent stars procreated from giant clouds of interstellar gas and dust, young stars, old stars, dying stars, and dead stars--young galaxies, interacting galaxies, and galaxies being torn apart. The universe reveals itself as a spectacle of unfolding drama, as stars and star systems are born and die, often violently.

On the universal scale, planet Earth must be considered to be no more than a mere speck of cosmic sand. Although pre-Renaissance theology placed the Earth and its peoples at the center of God's creation, we must now accept a more humble place in the grand scheme of things. The Milky Way Galaxy is not special; the Sun is not special; the Earth is not special; humans are not a special life-form.

Is it really conceivable that a cosmos of such vastness could have produced its single intelligent life-form on a planetary system of little consequence, circling a star of common type, on the outskirts of a very ordinary galaxy, within a cluster of galaxies of no special character? The sheer ordinariness of planet Earth challenges any assumption that humans could possibly be unique as an intelligent life-form.

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