There are both scientific and social reasons for wanting to go to the stars. On the scientific side, astronomy and planetary science (and very likely the biological sciences also) would benefit tremendously. Just consider the advantages of taking thermometers, magnetometers, mass spectrometers, gravimeters, seismometers, microscopes, and all the other paraphernalia of experimental science, to objects that today can only be observed telescopically across light-years of empty space. On the human side, it would seem that the total number of people who ultimately receive a chance of life, and the survival time of our species itself, would increase enormously if colonization of even a small part of the Galaxy were to prove possible. As pointed out by Shepherd (1952), ‘humanity dispersed over many worlds would appear to be more secure than humanity crowded on one single planet’. At the very least, the resulting cultural diversity would provide an exciting alternative to Fukuyama's (1989) dire predictions for the ‘end of history’ (a point discussed in more detail by Crawford, 1993a).

In this chapter we review some of the propulsion methods that might make it possible to travel interstellar distances on a timescale of decades (i.e. velocities ≥ 0.1c). The concepts discussed are necessarily selective, and the reader who wishes to dig deeper is referred to the extensive bibliography of interstellar travel and communication compiled by Mallove et al. (1980) and updated by Paprotny et al. (1984, 1986, 1987).

Are there intelligent beings elsewhere in our Galaxy? This is the question which astronomers are most frequently asked by laymen. The question is not a foolish one; indeed, it is perhaps the most significant of all questions in astronomy. In investigating the problem, we must therefore do our best to include all relevant observational data.

Because of their training, most scientists have a tendency to disregard all information which is not the result of measurements. This is, in most matters, a sensible precaution against the intrusion of metaphysical arguments. In the present matter, however, that policy has caused many of us to disregard a clearly empirical fact of great importance, to wit: There are no intelligent beings from outer space on Earth now. (There may have been visitors in the past, but none of them have remained to settle or colonize here.) Since frequent reference will be made to the foregoing piece of data, in what follows we shall refer to it as ‘Fact A’.

Fact A, like all facts, requires an explanation. Once this is recognized, an argument is suggested which indicates an answer to our original question. If, the argument goes, there were intelligent beings elsewhere in our Galaxy, then they would eventually have achieved space travel, and would have explored and colonized the Galaxy, as we have explored and colonized the Earth. However, (Fact A), they are not here; therefore they do not exist.

Introduction

An important corollary of the question ‘Where are they?’ is the question ‘Could they have gotten here yet?’ If we imagine a spacefaring civilization arisen a billion years ago and a thousand parsecs from Earth, what are the odds that the descendants of that civilization would have established settlements in the solar system before now? The answer, I believe, is that, if such a civilization had arisen and if interstellar travel is practical at a small percentage of light speed, it is virtually certain that the solar system would have been settled by non-natives long ago. Unless we discover that interstellar travel is impractical, I conclude that we are probably alone in the Galaxy.

We know nothing of any extraterrestrial civilization. If we assume that some have existed, it is also reasonable to assume that at least some would be as inquisitive and as eager for adventure as humanity (Hart, 1975; Jones, 1985). It would take but one such species to fill the Galaxy.

Humanity has a history of expansion into available areas on Earth. If we examine our past, we can estimate how long it might be before humanity would expand throughout the Galaxy.

How Probable Is It That Life Exists Somewhere Else in the Universe?

What is the chance of success in the search for extraterrestrial intelligence? The answer to this question depends on a series of probabilities. My methodology consists in asking a series of questions which narrow down the probability of success.

Even most skeptics of the SETI project will answer the above question affirmatively. Molecules that are necessary for the origin of life, such as amino acids and nucleic acids, have been identified in cosmic dust, together with other macromolecules, so that it would seem quite conceivable that life could originate elsewhere in the universe. Some of the modern scenarios of the origin of life start out with even simpler molecules, which makes an independent origin of life even more probable. Such an independent origin of life, however, would presumably result in living entities that are drastically different from life on Earth.

Where Can One Expect To Find Such Life?

Obviously only on planets. Even though we have up to now secure knowledge only of the nine planets of our solar system, there is no reason to doubt that in all the galaxies there must be millions if not billions of planets. The exact figure, e.g. for our own Galaxy, can only be guessed.

According to several estimates, up to 0.5% of all stars could have a planet similar to our Earth, but on the average about four billion years older than Earth, because our Sun is not an old star and star formation was most productive in the early times. Regarding the origin and evolution of life, our own case is at present the only instance of life we know of. Are we permitted to generalize this single case? Can we do statistics with n = 1? The laws of statistics say that n = 1 yields an estimate for the average, but none for the mean error (which would need at least n = 2). This means that assuming us to be average has the highest probability of being right, but we do not have any indication of how wrong this may be. Leaving statistics and arguing by analogy, we may add that most things in nature do not scatter over too large a range, up to a few powers of ten, mostly. Thus, the best we can do is to assume that we are average, but to allow for a wide (but not infinite) error of this assumption. If we now generalize our own case, then life in our Galaxy would have started on about one billion planets several billion years ago. And, arguing by extrapolation, we should expect this life to have developed meanwhile extremely far beyond our own present state.

Many members of the general public, and some academic scientists as well, maintain that at least some UFO sightings result from the activities of extraterrestrial visitors. Recent polls show that approximately 57% of the public believes that UFOs are ‘something real’ as opposed to ‘just people's imagination’. The figure rises to 70% belief for those who are less than 30 years old (Gallup, 1978), and have thus lived their entire lives in the age of television. UFO belief is not found predominantly only among the uneducated. A 1979 poll of its readers by Industrial Research and Development magazine shows that 61% believe that UFOs ‘probably or definitely exist’, a figure that rises to over 80% for those applied scientists and engineers under age 26. ‘Outer space’ is the most widely held explanation of their origin.

It is obviously true that even if the reality of UFOs were somehow to be fully established, it would not prove the reality of extraterrestrial visitors. UFOs could possibly be, for example, some poorly understood atmospheric phenomenon, or the result of some secret terrestrial technology, or even a life form or natural phenomenon which lies totally beyond the scope of present-day science. But in the public mind, the subject of UFOs is inextricably linked with the idea of extraterrestrial intelligent life, and since ETI is the subject matter of this book, I shall henceforth adopt the popular usage of terms, and examine UFO reports in the context of the evidence they purport to contain concerning extraterrestrial visitors.

Introduction

It is a widely held view that life will arise spontaneously on the surface of any planet that provides a suitable physical and chemical environment. This belief is saved from tautology by the generously broad definitions of ‘suitable’ that abound in discussions of the origin of life. Indeed it is almost sufficient to require only that liquid water occur on the planet's surface, for then it follows that the atmospheric pressure and ambient temperature will be in ranges that promote a rich variety of organic reactions.

On the ancient Earth, as today, the simultaneous presence of the three states of matter along terrestrial shorelines provided reaction sites and macroscopic transport for most of the planet's chemicals. The temperature was low enough to confer a substantial lifetime on thermodynamically improbable molecules formed in sunlit waters, yet high enough to give speed to the processes of chemistry, and to the evolution of life. The importance of speed in chemistry and evolution is emphasized by the reflection that a cooler planet than ours, where reaction rates were one-fourth as great, would see its sun burn away from the main sequence of stars before it witnessed intelligent life.

This standard scenario of life's origin has been strengthened greatly by the outcome of laboratory experiments in geochemistry (Fox & Dose, 1972). They show that the assumed primitive molecules of our planet's early atmosphere, if supplied with free energy, could form sugars, amino acids, purines, pyrimidines and other life-related organic substances.

There is no lack of propulsion systems available to any creatures which possess some technical competence and a desire to travel around in the galaxy. The following is an incomplete list of propulsion systems which have been suggested and studied by members of our own species.

Group A: Systems which are certainly feasible but are limited to mission velocities of the order of 10–2c.

1. Nuclear-electric. Uses a fission reactor as energy source, and ion-beam or magnetohydrodynamic plasma jet for propulsion. One can imagine a ‘minimal starship’ using nuclear-electric propulsion, with 10–6 g acceleration, a mass of 5 kg/kW (electric) for reactor and radiator, and a mission duration of 104 yr for voyages of the order of 10 pc.

2. Old-fashioned Orion nuclear pulse propulsion, using full-sized fission or fusion bombs. This is also limited to velocities of the order of 10-2c but can have acceleration of the order of 1g, giving it much better performance in local maneuvers (see Dyson, 1968; Martin & Bond, 1979).

Group B: Systems which are probably feasible but require very demanding new technology. These systems should be capable of mission velocities of the order of 0.5c, and mission durations of a few decades for distances of a few parsecs.

1. 3. Laser-driven sails (see Norem, 1969).

2. 4. Microwave-driven sails (see Forward, 1985).

3. 5. Pellet-stream propulsion (see Singer, 1980).

4. 6. Direct electromagnetic launch (see Clarke, 1950).

Introduction

We can make only a brief presentation here of our ideas on the extent of life in the universe. A much more detailed account is given in our book Life Beyond Earth: The Intelligent Earthling's Guide to Life in the Universe (Feinberg & Shapiro, 1980).

In discussions concerning intelligent life elsewhere, the assumption is often made that such life will develop only in circumstances resembling those on Earth. Estimates are then given of the number of Earth-like planets in our Galaxy, as suitable locations in which life might arise. Such estimates may vary, according to the pessimism or optimism of the observer, from the very few (Hart, 1979) to a billion or more (von Hoerner, 1978). Only those planets are considered habitable which fall into a limited zone around each star. In that zone, liquid water can be present on the surface, and carbon compounds will be abundant. If this view were correct, even in the most optimistic form, then life would be a rare phenomenon, confined to only an insignificant fraction of the material in the universe. From an extreme pessimist's viewpoint, as expressed elsewhere in this book, life may have originated only on the planet Earth. The idea of the specialness of the Earth is of course an old one, and has been expressed many times in theology.

We represent a very different point of view: that the generation of life is an innate property of matter.

Introduction

One of the most controversial aspects of the problem of life in the universe is the value of N, the number of technological civilizations that exist in an average spiral galaxy such as the Milky Way. N has been debated at various meetings (e.g. Papagiannis, 1980) and extreme values between 1010 and 10–24 have been suggested. One of the strongest arguments in favor of small N is the so-called ‘Fermi paradox’: If N is a large number, then why are extraterrestrials not physically present in our solar system (see, e.g., Hart & Zuckerman, 1982, hereafter HZ)? Various arguments have been advanced to explain this paradox and yet allow a large value of N. For example, Drake (see Papagiannis, 1980, p. 27) has contended that it is not cost-effective to travel between the stars using rocket ships and, therefore, even if N is a large number, the extraterrestrials will choose to stay home. The question of cost-effectiveness is a debatable one, in any event (e.g. Singer in HZ, p. 46).

The purpose of this chapter is to point out that, if N is large, then, for a wide class of reasonable scenarios, extensive rocket travel between the stars seems not only likely but inevitable, quite independent of considerations of cost-effectiveness, speed of colonization waves, etc. The basic reason is that large N implies that L, the lifetime of an average technological civilization, must be very long, at least millions of years.

Introduction

We do not know the total set of conditions necessary for the development of sentient life, but it is a pretty safe bet that chemically based life, at least, requires both a wide range of chemical elements and a good deal of time. Other chapters in this volume attempt the difficult task of estimating how many planets might have had long-lived, stable supplies of water, carbon dioxide, etc., and temperate climates. This chapter addresses the much simpler issue of the numbers, ages and locations in our Galaxy of stars with adequate supplies of heavy elements to make terrestrial planets, in principle, possible.

Carbon, oxygen, phosphorus and the other substances needed by terrestrial living creatures have not been here since the beginning of the universe. Rather, they are the products of a long series of nuclear reactions that occur in the centers of (mostly massive) stars and that have timescales of millions to billions of years (Burbidge et al., 1957; Trimble, 1991). In some of the very oldest stars in our Milky Way Galaxy, only one atom in 100 000 is not hydrogen or helium (Edmunds & Terlevich, 1992; Spite, 1992). Enrichment is a continuous process. Even as you read this, massive stars like Betelgeuse and Antares are synthesizing new heavy elements out of H and He, and supernovae like 1987A are spewing out the products to be raw materials for future generations of stars and planets.

Quasars, which can be a thousand times brighter than an ordinary galaxy, are the most distant objects observable in the Universe. How quasars produce the luminosity of 1013 suns in a volume the size of the solar system continues to be a major question in astronomy. Distant quasars are very rare objects whose study has been blocked by their scarcity. Recent technical advances, however, have opened new paths for their discovery. Forty quasars with redshifts greater than 4 have been found since 1986. Redshift 4 corresponds to a light travel time of more than 10 billion years. As a result, we are now able to probe the epoch shortly after the Big Bang when quasars may have first formed and to study the universe when it was less than a tenth its present age.

Quasars were one of the main discoveries thirty years ago that revolutionized astronomy. While they and the black holes thought to occur in their centers have become household words today, quasars are as enigmatic in many ways as they were when first discovered. Whatever their nature, they offer us views of the Universe never before seen, especially at distances far beyond what astronomers of the previous generation expected to see. In this chapter I wish to review briefly their history, how extraordinary their properties are, and how they serve as probes of the Universe to nearly as far as the visible horizon.

This book originated as a symposium at the American Association for the Advancement of Science annual meeting in San Francisco in 1989. The topic, The Farthest Things in the Universe, suggested itself to me as the most interesting and significant topic that people could hear about. An earlier AAAS Symposium had led to a book, The Redshift Controversy, that was still in use, and we hope that this volume will prove itself of similarly lasting interest.

Two of the original speakers, Hyron Spinrad of the University of California at Berkeley, and Patrick Osmer, then of the National Optical Astronomy Observatories, revised their pieces to bring them up-to-date for inclusion in this book. Further, Ed Cheng of the COBE Science Team and NASA's Goddard Space Flight Center agreed to write a new piece for inclusion in the book. We appreciate his taking time during the period of his duties as Chief Scientist for the Hubble Space Telescope's repair mission to complete his piece. During the interval from the time of the symposium to the present, the Cosmic Background Explorer spacecraft was launched and has had its tremendous successes in showing that the Universe has a blackbody spectrum and in finding ripples in space that may be the seeds from which galaxy-formation began. Thus this book appears at an optimum time.

The technical ability of astronomers to obtain images and spectra of very faint galaxies has improved greatly over the last decade. Since galaxies are vast collections of gas and stars, they must physically evolve with time. We should be able to directly observe the time-evolution of galaxies by studying very distant systems; the look-back internal corresponding to the mostdistant galaxies known in 1992 now approaches 15 billion years (80% of the total expansion age of the Universe)!

The line spectra of these faint galaxies are invaluable for redshift determination and physical study. The realization that Ly α (121.6 nm), formed in neutral hydrogen gas, is a strong emission line in most active galaxies and perhaps normal star-forming galaxies also, has helped us measure much larger redshifts in 1987–92 than was previously possible. Recall that this wavelength is in the ultraviolet; it can be observed only by satellites. But when galaxies are very far away, their Doppler effect shifts this spectral line into the region of the spectrum that we can observe with large telescopes on Earth. The largest redshifts for radio galaxies now approach z=3.8. Differing selection effects control which galaxies can be seen/isolated that far away. At least some red galaxies must form at redshift zf>5 (where the subscript f stands for the epoch of star formation).