Professor Allen Tough
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Intelligent Life in the Universe:
What Role Will It Play in Our Future?

by Allen Tough, Ph.D., University of Toronto
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How important is extraterrestrial intelligence? How diverse? Is it helpful, benign, or hostile? What are its capacities? How should we respond? What will be the long-term consequences of contact? Can life and intelligence survive forever?

Here are my thoughts on those questions.


In recent years, scientists and the general public have realized that intelligent life may well be found throughout the universe. We are probably not the only civilization in our galaxy; it may even contain dozens or hundreds of civilizations scattered among its 400,000,000,000 stars. If we receive a richly detailed message from one of these civilizations or have some other form of contact with it, the effects on our civilization could be pervasive and profound.

The search for extraterrestrial intelligence (SETI) has now become reasonably mainstream within the scientific enterprise (Angelo, 1985; McDonough, 1987). Radio astronomy's efforts to detect a signal or message from another civilization are increasing rapidly. Cosmology may be shifting toward emphasizing life throughout the universe, not just stars and sterile dust (Dick, 1988). Gallup polls find that 50% of adults believe there is intelligent life beyond the earth.

Sometime in the future of human civilization, contact or interaction with intelligent life from somewhere else in our galaxy will probably occur. It might occur next year, for instance, or 100 years from now. Our rapidly increasing efforts make contact particularly likely within the next 20 or 30 years. Few events in the entire sweep of human history would be as significant and far-reaching, affecting our deepest beliefs about the nature of the universe, our place in it, and what lies ahead for human civilization. Seeking contact and preparing for successful interaction should be one of the top priorities on our civilization's current agenda.

The Importance of Intelligent Life in the Universe

The study of the physical universe is very important. At the same time, many people who study cosmic evolution and cosmology are realizing that the evolution of life and intelligence throughout that physical universe is at least as important. Steven Dick (1988) has outlined a biophysical cosmology that emphasizes life and intelligence as key components of the universe, thus redefining our place and significance in that universe. Chaisson (1987), Harrison (1981), and Sagan (1980) have also emphasized the importance of intelligent life throughout the universe.

This is not a new idea, of course. The extraterrestrial life debate over the last 2400 years has been documented by Crowe (1986) and Dick (1982). My own interest was sparked in 1963 when I heard a lecture by pioneer Harlow Shapley and read his book The View from a Distant Star (1963).

Many astronomers, biologists, philosophers, and others now believe that the existence of diverse life throughout the universe is a supreme value. That is, in the entire universe, nothing is of greater value, importance, or significance than advanced civilizations and intelligent species?including our own, of course. If asked, "What thing or idea is more important or valuable than diverse life throughout the universe, including human civilization?" many people would reply, "Nothing; human and other intelligent life is the most important thing in the universe."

Perhaps a similar answer will be given by human beings 100 or 1000 years from now, especially if interaction with advanced extraterrestrials has occurred by then. Advanced extraterrestrials themselves might also give a similar answer.

It is important to note that there is no conflict between a belief in a divine or supernatural God and a belief that advanced life is the most important thing in the universe. God might well have created and nourished a diversity of life throughout the universe. Indeed, God's own supreme value might well be this diversity of flourishing life. Let me emphasize that human civilization, at present and in the future, is a significant part of all life in the universe. For us, long-continuing human life is a supreme value of ultimate importance. Because we do not yet, as far as we know, have contact with extraterrestrial life, our top priority at present must be our own civilization. At the same time, however, we should continue and enlarge our present efforts to make successful contact with intelligent life from some other part of this galaxy.

The fundamental importance of intelligent life in the universe can be confirmed by some of the thought-provoking mental exercises in chapter 2. For instance, imagine that you are in some distant galaxy, viewing our entire universe over many eons. From this perspective, what is of supreme importance? To me, it seems most important that humankind and most other advanced species throughout the universe continue to survive and flourish and develop.

Perhaps some sort of grand project is underway to spread highly positive life (marked by love, compassion, cooperation, wisdom, intelligence, knowledge, harmony, and effectiveness) throughout the universe. We cannot contribute much at present to the flourishing of extraterrestrial species but we can choose a flourishing future for our own human species as one of our fundamental priorities. As Carl Sagan concluded in his Cosmos television series, "Our obligation to survive and flourish is owed not just to ourselves but also to that Cosmos, ancient and vast, from which we spring."


How widespread and how diverse is the intelligent life that has evolved in various places in our galaxy?

An enormous amount of scientific literature has been written about these two questions. We are thinking here of naturally evolved (not divine or supernatural) species that have reached at least our level of intelligence, insight, knowledge, and culture. Among scientists who have studied the question, the general consensus is that many intelligent species have developed throughout our galaxy at one time or another, and some of them may still be alive today.

Here is a summary of the four main reasons supporting the likelihood that at least a few intelligent species have evolved in our galaxy somewhere beyond our solar system.

1. The number of stars in our Milky Way galaxy (about 400,000,000,000) is so large that it is almost beyond our imagination. A fair number of these stars probably have planets that are potentially hospitable for the development of life.

2. The incredible variety of life crowded together here on earth, ranging from microbes and moss to trees and people, suggests a pervasive natural tendency for life to spring up and spread. Similar tendencies and processes presumably pervade the universe, as do similar chemical and biological principles. The laws of nature are universal and nature is generally uniform. There is no reason to suppose that our planet is the only place suitable for life in the entire universe. Given the right conditions, life will develop on any suitable planets or their satellites. Therefore, life has probably arisen in various places in our galaxy.

3. Rudimentary communication, social organization, tools, and intelligence have arisen independently in several species on earth. At least one of these accomplishments can be found among chimpanzees, gorillas, dolphins, whales, dogs, cats, and horses, for example. It does not seem farfetched, then, to assume that such characteristics have arisen over time on other planets.

4. An extraordinarily advanced species in another galaxy might have figured out a way to cross the gap between their galaxy and ours. Although this seems impossible to us within our present understanding of physics, that understanding might someday be modified or transcended.

Several patterns of evolution, dispersion, and organization of advanced civilizations are possible in our galaxy. Perhaps a single civilization long ago produced several settlements that then evolved independently and became widely dispersed in the galaxy. Perhaps many species have independently developed intelligent civilizations on various planets and are still alive, either communicating and interacting with others or not. Perhaps a single civilization or federation has conquered or even eliminated all other civilizations except ours. Perhaps most intelligent species have voluntarily merged their cultures or genes, thus becoming in effect a single culture or species with internal diversity. We have to be careful not to assume that we know which of these possibilities is the actual case: none are so implausible, absurd, or illogical that they should be crossed off the list of possibilities.

We must also keep in mind that some of the intelligent life in our galaxy may be deeply alien to us. Their thinking patterns, knowledge, emotions, bodies, perception, social organization, communication, and norms may be even stranger than our strangest science fiction images. Some intelligent beings in our universe could turn out to be silicon-based entities or supercomputers.

It may not be pure fiction to imagine intelligent life evolving even further, as it does in 2010 by Arthur C. Clarke (1982). One species in that novel, beginning as flesh and blood, eventually learned to transfer their brains and then their thoughts into shiny new homes of metal and plastic. Then they learned "to store knowledge in the structure of space itself, and to preserve their thoughts for eternity in frozen lattices of light. They could become creatures of radiation, free at last from the tyranny of matter. Into pure energy, therefore, they presently transformed themselves. They could rove at will among the stars and sink like a subtle mist through the very interstices of space" (p. 308).

Helpful, Benign, or Hostile?

If at least one form of advanced life somewhere in the universe is aware of us, what will their aims and behavior be toward us? Will they be helpful, benign, or hostile?

In order to wrestle with this question, I used a combination of four methods (Tough, 1986). (1) One method was an extensive search of potentially relevant literature, located through abstracts in three fields (astronomy and astrophysics, aerospace, and physics) and through nine other bibliographic tools. (2) As any writer does, I spent many days thinking about the various issues and possibilities. (3) When alone in my home, I held two lengthy tape-recorded mock meetings in which a variety of advanced extraterrestrials expressed their views on how to relate to fledgling civilizations, particularly humankind. The purpose of these two mental exercises was to empathically generate ideas about extraterrestrial aims and behavior. Although I spontaneously created the various voices and views myself in a normal state of consciousness, a surprising diversity of views arose on some of the issues because each view triggered another view. In 1988 I briefly repeated this exercise twice. (4) In two classes at the University of Toronto, I conducted a tape-recorded meeting in which all the people present played the role of advanced extraterrestrials. A printed agenda focused the discussion on several major questions about helping the fledgling human civilization.

I concluded that most or all of the advanced civilizations in the universe avoid harming fledgling civilizations. The cardinal principle guiding behavior toward all other civilizations is probably this: avoid unnecessary harm and interference. Do not hurt any other civilization, nor hinder their development. If another civilization is clearly about to break the cardinal rule (through a powerful attack or through spreading a plague, for instance), and if this poses a definite and immediate threat to an advanced species, then it is permissible to intervene powerfully and even harmfully in order to prevent this. Under any other circumstances, however, an advanced civilization will probably not interfere harmfully in the development of another civilization.

There are several reasons for concluding that advanced beings are helpful or at least benign, and are unlikely to harm fledgling civilizations such as ours. Here are the main reasons:

1. They still recall their own early history, including their primitive stages, their dark periods, and their follies; therefore, they may feel sympathetic toward our foibles.

2. Anyone bent on capturing our planet would have done so long ago, before we despoiled it so much.

3. Any hostile civilization with advanced technology would have programmed its robot Replicator probes to eliminate any potential civilization long before reaching the stage at which it could attack the Replicator; that is, long before our present stage (O'Neill, 198l, p. 265).

4. Advanced civilizations are probably letting us develop freely, without interference, in order to maximize the amount of information they gain; if they interfere and control us, they will learn less (Kuiper and Morris, 1977). Their greatest gain from us may be sociological and anthropological knowledge about our culture and civilization.

5. Intelligent life forms that are destructively aggressive and irresponsible will usually eliminate themselves or revert back to primitive conditions before they achieve interstellar communication or travel (Harrison, 198l). If a ruthlessly hostile species manages to avoid these usual consequences of natural selection, and then prepares for interstellar communication or travel, it may well be terminated by more advanced beings in the galaxy. "How this is done is a matter of more than academic interest to the human race in the next few centuries," adds Harrison, wryly (pp. 399-400).

How much help will they give us? Will they bother transmitting an encyclopedic radio message to us, for example, or send detailed information to us by means of a spacecraft controlled by computers and robots or by live beings?

Some advanced civilizations may decide to send us no help or information at all. Others may adopt a low-budget approach. Particularly generous and altruistic civilizations may do a great deal to foster a rich diversity of good, wise, intelligent, compassionate, harmonious life throughout their region of the universe. Even the most generous civilizations, though, will not put this goal ahead of the survival and development of their own culture. Indeed, if humanity blunders, deteriorates, and even becomes extinct, no other civilization will mourn this as the worst possible tragedy. They might be about as upset as humanity would be if all whales became extinct or if an earthquake sent Toronto to the bottom of Lake Ontario.

Age and Capacities

Any other civilizations in our galaxy are probably much older than human civilization.

Two factors support this assumption. First, the vast majority of stars in our galaxy are much older than our sun, many of them millions of years older. It follows, then, that any civilizations on planets revolving around those stars likely arose much earlier than our own civilization did. Second, it seems quite possible that some civilizations survive for a million years or even longer. If the civilizations in our galaxy range in age from a few thousand years up to a million years old, then we are one of the youngest: by most definitions, human civilization is not much more than 10,000 years old. Indeed, thinking of ourselves for a moment as a species rather than as a civilization, we realize that several species on earth are 300,000,000 years older than we are (Calder, 1983).

Because other civilizations in our galaxy are thousands of years older than human civilization, they have probably advanced in certain ways beyond our present level of development. Some civilizations presumably fail to survive once they discover nuclear weapons or other means of extinction, but surely others learn to cope successfully with this problem and then survive for a very long time. Some of them may be 100,000 years or even millions of years more advanced than we are.

We cannot at present be certain about the particular capacities of highly advanced civilizations in our galaxy. We can, however, make some thoughtful guesses based on established human knowledge (such as history and futures studies) combined with intelligent speculation.

Our own progress has been very dramatic in several areas of life over the past 10,000 years. If we survive another 10,000 years, it is highly likely that we will again make dramatic progress in several areas. When we turn our attention to other civilizations that are 10,000 or even a million years older than we are, there can be little doubt that some of them will be far beyond us in their biological, mental, technological, communication, or travel capacities. Also, because they originated in bodies, physical environments, and social environments that are vastly different from ours, their patterns of perceiving, thinking, and relating may be vastly different from ours.

It is highly likely, therefore, that many of the capacities in the following list have already been developed by one advanced civilization or another in our galaxy. It is unlikely that any one civilization will have all of the listed capacities: it is quite probable, though, that each of these capacities (with one or two exceptions) exists somewhere in our galaxy. We ourselves will presumably possess many of these capacities if we continue to develop for another 10,000 years.

Such a list may strike us as unbelievable when we first read it. Would a human being 10,000 years ago, though, have reacted any differently to a list of our present capacities? Electricity, airplanes, astronauts, moon-walks, telescopes, selective breeding, television, microbes, hospitals, DNA, computers, universities, skyscrapers, nuclear weapons, and many other aspects of today's world would have been dismissed 10,000 years ago as ridiculous or impossible. That was the time when the Ice Age ended, humanity's main crops became domesticated, and the world's first town arose. Pigs, cattle, and horses had not yet been tamed 10,000 years ago. Weaving, wagon wheels, and writing had not yet been invented. The Bronze Age and Iron Age had not yet begun. Stone buildings, philosophy, and science still lay in the future (Calder, 1983). No wonder the people of that time could not have anticipated today's capacities. For us, in turn, the actual capacities of a civilization 10,000 years beyond us will probably make my list seem unimaginative. Their role in our galaxy could be magnificent and pervasive.

What We Can Do

Information or advice from a highly advanced civilization, or even gaining knowledge about its characteristics, could be extraordinarily beneficial to humanity. A great increase in our efforts to achieve successful contact or interaction would be a very wise investment for us.

At an International Astronomical Union colloquium in Hungary, I spelled out 20 possible strategies that are serious candidates for next steps in such efforts (Tough, 1988). Three assessments were provided for each strategy: (1) the likelihood of success if it is given adequate funding and effort; (2) the magnitude of benefits to human civilization if it is successful; and (3) the likely payoff from greatly increased efforts and resources. Here I will simply summarize the strategies that emerged as particularly high priority. They can be grouped into three clusters.

The first cluster emphasizes strengthening the whole field of study devoted to life in the universe. This field is usually called bioastronomy, exobiology, or the search for extraterrestrial intelligence (SETI). One very useful direction is to develop this entire field, its ideas, and their dissemination. Today this field may be one of the most important scientific enterprises anywhere in our civilization in the sense that its potential benefits, impact, and significance are so great. Although the bioastronomy field has made great progress in the past 15 years, it still lacks its own academic journal, its own annual international meeting, and its own multi-discipline association. Another specific need is to study the likely capacities, aims, guiding principles, projects, help, and methods of advanced extraterrestrials. (We have to figure out what they are like and what they are doing before we can detect their messages. Each search strategy is based on major assumptions, rarely examined in sufficient depth, about extraterrestrial technology and psychology.) Yet another specific need is to prepare for contact or interaction that will be successful and beneficial. In particular, simulate possible scenarios, study possible consequences, implement arrangements for immediate dissemination of anomalous data to other SETI scientists, seek international agreement for activities (including possible replies) following the detection of a signal, establish an international team to implement this agreement, and prepare to handle negative possibilities (alien bandits, hostile warriors, deadly probes, a threatening message). People interested in futures studies could contribute greatly to these various strategies.

A second major cluster of strategies is to search for richly detailed messages that have been transmitted from far beyond the solar system. Such messages may be reaching earth right now. Detecting such a message is certainly a high priority. Certain civilizations, at least at one stage of their development, may have broadcast advice, knowledge, techniques, values, ethics, principles of social and political organization, religious beliefs, even instructions for building something. Consequently, our society should continue our efforts to check various possibilities within the electromagnetic spectrum.

The third cluster of strategies focuses on searching the solar system and our own planet. There are four specific strategies. (a) It is quite possible that an intelligent civilization has chosen to send some sort of automated probe to the solar system. The simplest type of probe or sonde (beyond a fast flyby that is unable to stop) would park or cruise in the solar system and send data back to its home civilization. In addition, a more complex probe might be programmed to release a significant detailed message to any beings who trigger it by approaching it, by directing certain radio waves or laser beams at it, or by achieving an advanced state of technology. Creative efforts to detect any type of probe are clearly high priority. (b) It is also important to search the solar system for signs of intelligent beings (not just machines) and their activities. Such beings presumably would have come here in a spacecraft, perhaps with a propulsion system that we have not yet discovered. We could search the asteroid belt and other parts of the solar system for signs of a space station, space colony, parked spacecraft, mining operation, materials processing plant, or some other ongoing astroengineering project. Waste heat could be a sign, for instance, and infrared data could yield valuable clues. (c) During the past few decades, many people have claimed that they have seen an extraterrestrial spacecraft and even its occupants. Upon investigation, most of these experiences turn out to be the result of misperception, an inability to distinguish between fantasy experiences and reality, or even a hoax. There is a chance, however, that a few of the past or future reports will turn out to be valid. Some additional effort should therefore be made to study any promising cases or avenues. (d) We should try to create and develop arrangements to obtain any potentially useful data from military, security, intelligence, and other government agencies in various countries. Such data could include any inexplicable anomalies that might indicate extraterrestrial surveillance or messages, for instance.

The Long-Term Consequences of Contact

An encyclopedic radio message, interaction with the intelligent computer in an automated probe, or some other form of contact with another civilization could be a highly significant event in our future. It would probably have an extraordinary and central impact on humanity's subsequent development. Whether this impact occurs next year or several hundred years from now, it will likely affect our civilization profoundly at the time and for several centuries afterwards. Indeed, as we look ahead at the long-term future of human civilization, we realize that one of the highest-impact events of all time will probably be contact with another civilization.

What will the specific effects of extraterrestrial messages, interaction, or intervention turn out to be? This is a common theme in science fiction. Unfortunately, few nonfiction writers treat the question with much thought or depth. There are stimulating and useful exceptions, however, such as Angelo (1985), Michaud (1977), Prytz (1985), Regis (1985), Thatcher (1978), and hopefully Tough (1990).

Let us note the most likely long-term consequence if three common hopes and assumptions in the bioastronomy field turn out to be correct. First, let us assume that sometime in the next few decades, our first contact with any extraterrestrial civilization is a detailed radio message that covers a wide range of topics, something like an encyclopedia. Second, let us assume that the message is not based on much knowledge about us; it is not geared specifically to human civilization. Although this assumption may well be wrong (Tough 1986), it is a common assumption today in the bioastronomy field. Third, we eventually decode the message and make the translation freely available to everyone.

Four sorts of long-term consequences are particularly likely to result: (1) practical information; (2) new insights about certain major questions; (3) a transformation in our view of ourselves and our place in the universe; (4) participation in a joint galactic project. Let us examine each of these in more detail.

1. We might well receive practical information and advice that helps our human civilization to survive and flourish. Possible examples include technology, transportation, a new form of energy, a new way of producing food or nourishing ourselves, the importance of halting population growth, more effective governance and social organization, fresh views on values and ethics, inspiration to shift direction dramatically in order to achieve a reasonably positive future. The message might also bring home to people the importance of eliminating warfare or at least eliminating weapons of extraordinary destruction. Viewing ourselves from an extraterrestrial perspective might be very useful in alleviating our civilization's problems: it "might help us to transcend our cultural conceits and political divisions and think constructively about our own global civilization" (Finney, 1986, p. 9).

Such deep-seated changes will no doubt produce enormous disruption, at least for a short time. We might suffer from massive culture shock and temporarily feel inferior or lose our confidence in our own civilization. Disruption could also occur in the sciences, in business and industry if we learn about new processes and products, in the legal system if we move toward cosmic or universal laws, and in the armed forces and their suppliers if we eliminate the threat of war. Such disruption will probably be tolerable and short-lived. It is best regarded as simply the major cost we have to pay for incorporating new knowledge and possibilities.

2. We might gain new insights, understanding, and knowledge about major questions that go far beyond ordinary practical day-to-day matters. Topics in an encyclopedic message could include astrophysics, the origin and evolution of the universe, religious questions, the meaning and purpose of life. The message could include detailed information about the sending civilization, which might be deeply alien to us, and about its philosophies and beliefs. Similar information could be provided about several other civilizations throughout our galaxy, too.

We might even receive a body of knowledge accumulated over the past billion years through contributions by dozens of alien civilizations throughout the galaxy. "Included in this vast body of knowledge, something we might call the `galactic heritage,' could be the entire natural and social histories of numerous species and planets. Also included, perhaps, would be extensive astrophysical data that extend back countless millennia, providing accurate insights into the origin and destiny of the Universe" (Angelo, 1985, p. 23).

What sorts of consequences will contact have for our religious ideas and institutions? A historical survey found that several religions have already incorporated the idea of extraterrestrial life (Crowe, 1986). Although some preachers may denounce an extraterrestrial message as the work of the devil or the Anti-Christ, others will surely embrace it as further evidence of God's infinite greatness. Both religion and philosophy may be beneficially stimulated by a message from an advanced civilization.

3. A richly detailed message from an alien civilization might transform our view of ourselves and our place in the universe?even our ultimate destination. We might gain a much deeper sense of ourselves as part of intelligent life and evolving culture throughout the universe.

Michaud (1977, p. 20) pointed out that "contact would be immensely broadening and deprovincializing. It would be a quantum jump in our awareness of things outside ourselves. It would change our criteria of what matters. We would have to think in interstellar, even galactic frames of reference.... We would leave the era of Earth history, and enter an era of cosmic history. By implying cosmic future, contact might suggest a more hopeful view of the universe and our fate."

4. We might eventually play a role in some joint galactic project in art, science, philosophy, or philanthropy. Such a project might aim to solve fundamental mysteries of the universe, help other civilizations develop and flourish, or spread harmonious intelligent life throughout the galaxy. We could participate in this project through two-way radio messages, despite the length of time required for round-trip communication.

Angelo (1985, p. 23) has noted that contact "might lead to the development of branches of art and science that simply cannot be undertaken by just one planetary civilization but rather require joint, multiple-civilization participation across interstellar distances.... Perhaps the very survival and salvation of the human race depends on finding ourselves cast in a larger cosmic role?a role far greater in significance than any human can now imagine."

It is also possible that our culture will be overwhelmed by an advanced alien culture. However, all terrestrial examples of contact between two cultures have involved physical contact rather than radio messages. Also, terrestrial contact has usually involved territorial expansion by the stronger culture. "If contact has occurred without aggression, the lesser culture has often survived and even prospered" (Angelo, 1985, p. 27). We might well adopt portions of the alien culture but avoid being completely overwhelmed by it.

Can Life and Intelligence Survive Forever?

If we look far enough into the future, we can imagine the end of our physical universe, at least in any form that would support human life as we know it. We can imagine the universe expanding forever, becoming colder and colder, until finally it is completely frozen, quiet, and barren. Alternatively we can imagine it collapsing inwardly, becoming hotter and hotter, ending in a "Big Crunch."

Can life and intelligence somehow survive if the physical universe meets either of these fates? Intelligent life will have countless billions of years to advance and change before there is any need to adjust to the various stages of the "end" of the physical universe. By that time, at least in some parts of the universe, intelligent life may have progressed so much that it can figure out how to avoid the extermination of all life, knowledge, intelligence, and wisdom. Indeed, Freeman Dyson (1979, 1988) and Michael Michaud (1982) have already explored some possible ways of achieving this. For instance, a huge, cooperative, galactic or intergalactic project may find some method of altering the physical universe (or one portion of it) in some powerful and massive way that will enable life to continue. A second possibility is that life itself will change and adapt in ways that will permit it to continue forever. If it is true "that life is organization rather than substance, then it makes sense to imagine life detached from flesh and blood and embodied in networks of superconducting circuitry or in interstellar dust clouds" (Dyson, 1988, p. 107). Alternatively, some way may be found to break out of this universe into another one, either existing parallel to it or arising subsequent to it. Perhaps the best of our knowledge, intelligence, consciousness, or life can be transferred to this other universe.

As the result of the 137 equations presented in his 1979 paper, Dyson concluded in his 1988 book (p. 117) that science provides a solid foundation for a philosophy of hope. "I have found a universe growing without limit in richness and complexity, a universe of life surviving forever and making itself known to its neighbors across unimaginable gulfs of space and time."


Angelo, Joseph A., Jr. 1985. The extraterrestrial encyclopedia: Our search for life in outer space. New York: Facts on File.

Calder, Nigel. 1983. Timescale: An atlas of the fourth dimension. New York: Viking.

Chaisson, Eric. 1987. The life era: Cosmic selection and conscious evolution. New York: Atlantic Monthly Press.

Clarke, Arthur C. 1982. 2010: Odyssey Two. New York: Ballantine.

Crowe, Michael. 1986. The extraterrestrial life debate, 1750-1900: The idea of a plurality of worlds from Kant to Lowell. Cambridge: Cambridge University Press.

Dick, Steven J. 1982. Plurality of worlds: The origins of the extraterrestrial life debate from Democritus to Kant. Cambridge: Cambridge University Press.

Dick, Steven J. 1988. The biophysical cosmology: The place of SETI in the history of science. Paper presented at the Planetary Society SETI conference, Toronto, October. These ideas have now been expanded into a comprehensive 1996 book: The biological universe: The twentieth-century extraterrestrial life debate and the limits of science. Cambridge: Cambridge University Press.

Dyson, Freeman J. 1979. Time without end: Physics and biology in an open universe. Reviews of Modern Physics 51 (3): 447-460.

Dyson, Freeman J. 1988. Infinite in all directions. New York: Harper & Row.

Finney, Ben. 1986. The impact of contact. Preprint of paper read at the International Astronautical Congress, Innsbruck, October.

Harrison, Edward. 1981. Cosmology: The science of the universe. Cambridge: Cambridge University Press.

Kuiper, T. B. H., and M. Morris. 1977. Searching for extraterrestrial civilizations. Science 196 (4290; May 6): 616-621.

McDonough, Thomas R. 1987. The search for extraterrestrial life: Listening for life in the cosmos. New York: Wiley.

Michaud, M. A. G. 1977. The consequences of contact. AIAA Student Journal, Winter 1977-1978: 18-23.

Michaud, M. A. G. 1982. Towards a grand strategy for the species. Earth Oriented Applied Space Technology 2: 213-219.

O'Neill, Gerard K. 1981. 2081: A hopeful view of the human future. New York: Simon & Schuster.

Prytz, John. 1985. CETI: What are the benefits? Journal of the British Interplanetary Society 38: 524-526.

Regis, Edward, Jr. 1985. SETI debunked. In Extraterrestrials: Science and alien intelligence, ed. Edward Regis, Jr., 231-244. Cambridge: Cambridge University Press.

Ridpath, Ian. 1987. Messages from the stars: communication and contact with extraterrestrial life. New York: Harper & Row.

Sagan, Carl. 1980. Cosmos. New York: Random House.

Shapley, Harlow. 1963. The view from a distant star: Man's future in the universe. New York: Basic.

Thatcher, M. 1978. Cosmic culture shock. Human Behavior 7 (10): 18-22.

Tough, Allen. 1986. What role will extraterrestrials play in humanity's future? Interstellar Studies: Journal of the British Interplanetary Society 39: 491-498.

Tough, Allen. 1988. The next steps: 20 possibilities. In Bioastronomy? The next steps: Proceedings of the 99th Colloquium of the International Astronomical Union held in Balaton, Hungary, June 22-27, 1987, ed. George Marx, 397-404. Dordrecht: Kluwer.

Tough, Allen. 1990. A critical examination of factors that might encourage secrecy. Acta Astronautica 21 (2), 97-102.

Copyright © 1991 Allen Tough. All rights reserved. This is a slightly revised version of chapter 7 from the book Crucial Questions About the Future by Allen Tough. In the U.S.A., the book was published in 1991 by University Press of America (Lanham, Maryland); phone 1-800-462-6420 to order a copy (or order from your usual source for books). In other countries, the United Kingdom version of the book (published in 1995 by Adamantine Press Limited) can be ordered from Central Books, 99 Wallis Rd, London E9 5LN; telephone 181 986 4854 or fax 181 533 5821, or order from your usual source for books.

The train of thought from this chapter eventually led on to Allen Tough's 2000 paper about five promising strategies for achieving contact.

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