Introduction

The phrase “infrared emission lines,” like so many other topics covered at this meeting, is extremely broad. Therefore I will begin by defining and limiting the scope of this review. I will discuss only lines that arise from gas-phase atoms, ions, and molecules, and will not include spectral features produced by interstellar dust. In this review, “infrared” will mean the spectral region 1–200 μm, which corresponds to certain types of astronomical detectors; shorter wavelengths will be called “far red” (with apologies to Don Osterbrock), and longer wavelengths, “submillimeter.” Another boundary condition is that I will restrict myself to low densities, n ≤ 108 cm–3. Apart from these constraints, I will attempt to be as general as possible, though with no pretentions to completeness. This review is organized by physical properties and methods of analysis, rather than by class of astronomical object.

Introduction

Molecules have been detected in a broad range of astronomical objects—the atmospheres of stars, diffuse, translucent and dense interstellar clouds, photon-dominated regions (PDRs), circumstellar shells, HII regions, planetary nebulae, stellar outflows, stellar winds, jets, Herbig-Haro objects, novae, supernova remnants and the eject a of Supernova 1987a. Their presence is a controlling force in the determination of the thermal balance, the ionization structure, the dynamics and the evolution of the entities in which they reside. Molecules provide unique diagnostic probes of the physical nature of their environment, yielding information on the densities, the temperatures, the magnetic fields, the velocities, the isotopic composition, the radiation fields, the masses and the ages.

In dense molecular clouds where star formation takes place, molecules have been detected in remarkable diversity. Over twenty of them have been detected in external galaxies. Many additional species have been discovered in circumstellar shells.

I will not attempt to survey the myriad ways in which molecules serve as diagnostic probes. I will instead make an arbitrary personal selection, beginning not with emission lines but with the absorption lines of CN which provided the first measurement of the temperature of the cosmic blackbody background radiation field.

Absorption by CN

Absorption by the CN molecule has been measured towards many stars. Several lines have been observed showing that CN is present in its low-lying rotational levels with rotational quantum numbers j = 0, 1 and 2.

Introduction

Let me preface this paper to say how honoured I am to have been given this opportunity to pay tribute simultaneously to two of the principal sources of scientific inspiration of my career. In my attempts over the past twenty years to understand and to analyse the optical and UV spectra of shock-excited plasmas, Don's books (1974, 1988) have been invaluable to both myself and to my students. In Australia we used to refer to the Physics of Gaseous Nebulae somewhat irreverently as “the new testament” to distinguish it from the earlier work by Aller! The famous diagnostic diagrams of Baldwin, Phillips & Terlevich (1981), of Veilleux & Osterbrock (1987), and of Osterbrock, Tran & Veilleux (1992) provide both an inspiration, and a powerful means of distinguishing between various excitation mechanisms.

Introduction

In 1973 Drake and Sagan proposed a SETI frequency standard of V0 ∼ 56 GHz tied to the observed cosmic microwave background hv0 = kT0, where T0 is the current temperature of the cosmic microwave background. They noted that a transmitting civilization in a distant galaxy will, however, have transmitted its signal to the Earth at an earlier cosmological epoch when T was larger than is measured today, tending to increase the ‘natural’ frequency, but that the cosmological Doppler effect will tend to decrease the frequency. Not knowing of their work, I proposed this same frequency standard (hv0 = kT0: Gott, 1982) in the first edition of this book. I had noticed that the two effects mentioned above in fact cancel each other out exactly (which was a new result), so that this frequency standard was indeed universal. If a transmitting civilization is at a redshift z, it will observe a microwave background temperature of T1 = T0 (1 + z) and will emit signals at a frequency of hve = kT1 = kT0 (1 + z), but because of the cosmological Doppler shift we will observe these transmitted photons at a frequency hv0 = hve (1 + z)–1 so that we observe hv0 = kT0 (1 + z) (1 + z)–1 = kT0 ∼ 56 GHz independent of the redshift of the emitting civilization.

The purpose of a second edition of Where Are They? is to enlarge upon and update issues that were debated in 1978 at a two-day Symposium of the same name. As might be expected, comparison of the present book with the first edition shows that relatively little has changed in the field of interstellar travel and colonization – there are not many interstellar travelers among us. By way of contrast, because the world is full of biologists and astronomers, there have been many new experiments and insights in these fields. We are especially pleased that three distinguished biologists – Drs Diamond, Joyce and Mayr – have contributed new chapters to the present volume. Typically, biologists appear to be less sanguine about the likelihood of abundant intelligent life in the universe than are engineers and physicists.

At the time of publication of the first edition, the only things that humans knew of with certainty that orbited stars other than the Sun were other stars. Then, in 1983, NASA's IRAS satellite discovered that many nearby stars similar to our Sun emit infrared (heat) radiation well in excess of that expected from their visible surfaces. In a chapter in the first edition entitled ‘Searches for electromagnetic signals from extraterrestrial beings’ I discussed the possibility that IRAS might discover a so-called ‘Dyson Sphere’.

In one school of thought it is customary to begin discussions of galactic life by appeal to Drake's equation and then to proceed to a detailed examination of the numerical magnitude of one or more of the string of factors whose values have to be estimated. An example of this procedure is furnished by Michael H. Hart's analysis (Chapter 22), in which he concentrates on the probability that 600 or more nucleotides might line up in the right order; then he proposes that one of the factors may be very much less than 10–30. Of course, 10–30 is already very small, and, if included as a factor in almost any expression having to do with the physical universe, will cut the product down to negligible size. In this application the conclusion is that the number of technological civilizations independently arising in a galaxy is very much less than 1. Well, this may be an excess of zeal, and many of those addicted to the use of Drake's equation would, in similar circumstances, have arranged for the product to emerge with an order of magnitude around unity because, after all, a calculation condemns itself if it seriously contradicts the possibility of the one technological civilization we know about, namely our own.

But is Drake's equation correct? It seems that it suffers from oversimplification - surely at least one plus sign ought to be there.

The next time you're outdoors on a clear night and away from city lights, look up at the sky and get a sense of its myriads of stars. Train your binoculars on the Milky Way and appreciate how many more stars escaped your naked eye. Then look at a photograph of the Andromeda nebula as seen through a powerful telescope to realize the enormous number of stars that escaped your binoculars as well. When all those numbers have sunk in, you're ready to ask: How many civilizations of intelligent beings like ourselves must be out there, looking back at us? How long before we are in communication with them, before we visit them or before we are visited?

Many scientists have tried to calculate the odds. Their efforts have spawned a whole new field of science termed exobiology – the sole scientific field whose subject matter has not yet been shown to exist. Since a summary of the calculations fills seven pages of the Encyclopaedia Britannica, what more could we learn by further speculation? I'll suggest, nevertheless, that woodpeckers offer a fresh perspective.

Exobiologists find the numbers in their subject matter encouraging. Billions of galaxies each have billions of stars. Many stars probably have one or more planets, and many of those planets probably have an environment suitable for life. Where suitable conditions exist, life will probably evolve eventually.

Introduction

The possibility that life, primitive or advanced, might exist in other parts of the universe has occupied the thoughts of scientists and laymen for thousands of years. One of the earliest was the statement by the ancient Greek philosopher Metrodorus of Chios around 400 b.c., who wrote in his book On Nature that: ‘It is unnatural in a large field to have only one shaft of wheat, and in the infinite Universe only one living world.’

In a.d. 1690 the famous Dutch physicist Christian Huygens wrote in his book Cosmotheoros that: ‘Barren planets, deprived of living creatures that speak most eloquently of their Divine Architect, are unreasonable, wasteful and uncharacteristic of God, who has a purpose for everything.’

In the nineteenth century, several proposals were made by different distinguished scientists. The most famous was mathematician Carl Friedrich Gauss, who proposed to establish contacts with advanced civilizations on other planets of our solar system, by planting a rectangular triangle with wheat in Siberia, with squares of pine trees at its three sides, to show that the Earth has intelligent beings that know the Pythagorean Theorem. None of these proposals, however, was implemented.

The modern era of the Search for Extra-Terrestrial Intelligence (SETI) started in 1959 with a paper to Nature by Cocconi and Morrison, which was followed soon after in the spring of 1960 by the first radio search by Frank Drake (Project OZMA), using the then new 85 foot radio telescope at the National Radio Astronomy Observatory in West Virginia.

Interstellar Travel and Extraterrestrial Intelligence

The success of several proposals to search for extraterrestrial intelligence (ETI) in the Galaxy (Cocconi & Morrison, 1959; Oliver & Billingham, 1971; Michaud, 1979) requires the existence of a large number of technologically competent cultures over a long period of time. For example, to expect to find one ETI within 1000 light-years in a perfectly efficient search would require about a million ETI in the Galaxy, each signalling for a million years. (Or it would require 108 ETI signalling 104 years, or 104 ETI signalling 108 years, etc.) Many people have asked why some of these ETI should not have taken advantage of their prolonged technological capability to find a method for interstellar travel and settlement of nearby stellar systems (see, e.g., Hart, 1975; Jones, 1976; Winterberg, 1979). If the initial problem of interstellar travel and settlement were solved, then it should become progressively easier for daughter settlements to eventually continue the process until every available stellar system in the Galaxy (including possibly our own) were inhabited.

The chances of this happening have been discussed extensively, often with minimal thought given to the physical requirements for interstellar settlement. In particular, it has been argued that interstellar settlement is either impossible (see, e.g., Purcell, 1960; Marx, 1973) or absurdly expensive (e.g. requiring trillions of man-years of effort to amass the nuclear fuel needed).

Earth is unique in this solar system – it is the only planet that seems to support life. Its hospitable ecosphere stands in stark contrast to the empty, lifeless landscapes of the Moon, Mars, Venus, Mercury and other worlds probed by our spacecraft. Recent arguments suggest that while planets may be common in the universe, habitable worlds may not be. Internally, a candidate planet's proper composition, tectonic dynamics and very narrow but extremely long-period thermal stability may be rare. External biosphere-destroying natural processes, from interstellar dust clouds to sterilizing radiation sources, may periodically rid vast regions of a galaxy of planet-bound life forms.

Such a severe limitation on life-supporting worlds significantly impacts discussions of the Search for Extraterrestrial Intelligence at both ends of the question, the search for causes and the search for consequences. On the former issue, it seriously reduces the stage on which the formation and evolution of life may take its chances against the odds: few candidate planets means even fewer ultimate successes. At the other end of the question, it suggests strategies for searching for the few successful technologies that do evolve, by identifying potential technological activities they would choose to engage in, activities that may have very long range of detectability. The significance to human searches for ETI may be profound.

The issues of ‘probability of intelligence arising’ are dealt with in other chapters. My purpose is to address the question of final consequences.

Where are they? Enrico Fermi is reputed to have asked this question at the dawn of the atomic age. He must have been wondering why, having discovered and tamed nuclear energy sources, advanced extraterrestrials were not in evidence here on Earth or out in the skies.

During the 1960s and early 1970s, Fermi's question was largely forgotten or ignored. Advances in radioastronomy, the American and Soviet space programs, the blossoming of the study of molecular biology and progress in laboratory simulations of prebiological organic chemistry all contributed, in their own way, to a euphoric belief among many scientists that life in the cosmos is commonplace and might even be discovered soon. At a more popular level, numerous reports of close encounters of the second and third kind, lavishly bankrolled science fiction movies and enormously popular books on ancient astronauts all served to promote the idea that They are out there and will soon be, or already have been, here.

The past few years have seen the introduction of new and sobering input into this picture. The US program of planetary exploration, while highly successful from a technological and scientific standpoint, has failed to produce even a hint of an extraterrestrial biology. Although the search for simple nonterrestrial life in our solar system cannot be considered complete, the prospects for eventual success do not look good. In addition, searches for evidence of advanced technology, either in deep space or in the solar system, have been discouraging.

Introduction

Cocconi and Morrison (1959) closed their seminal paper on SETI with a statement that still well characterizes our current situation: ‘The probability of success is difficult to estimate, but if we never search the chance of success is zero.’ This chapter is a brief summary of how and why NASA has shaped the High Resolution Microwave Survey (HRMS), which it inaugurated on 12 October 1992. Some of the alternative search strategies that were considered are also noted, since these may well form the basis for the next generation of searches, should the HRMS fail to detect a signal.

Although this endeavor is often referred to as SETI (the Search for Extraterrestrial Intelligence), as it is implemented today, and into the foreseeable future, individual search projects are actually seeking evidence of extraterrestrial technology. Thus for scientists and engineers engaged in this exploration, a species' ability to technologically modify its local environment in ways that can be detected over interstellar distances has become a pragmatic substitute for the overly complex and convoluted definitions of ‘intelligence’ offered by researchers in other fields. Far in the future lies the promise of being able to detect indirect, but compelling, evidence of life itself on a distant planet. The coexistence of highly reactive gases (such as methane and oxygen) in the atmosphere of a planet, orbiting at an appropriate distance from its host star (so that liquid surface water might be possible) would suggest a continuous biological source at the base of that atmosphere.

Introduction

Here in a nutshell is the entire concept of chemical evolution. What the experimentalist does is to try to recreate Darwin's warm little pond and to see whether those reactions that preceded the emergence of life can be retraced in the laboratory. Such ideas lay fallow for a long period of time until the Russian biochemist Alexander Oparin, in a dissertation published in Russia in 1924, contended that there was no fundamental difference between a living organism and lifeless matter and that the complex combinations, manifestations and properties so characteristic of life must have arisen in the process of the evolution of matter (Oparin, 1924). In 1928, Haldane had similar ideas. He described the formation of a primordial broth by the action of ultraviolet light on the Earth's primitive atmosphere (Haldane, 1929). The Oparin-Haldane hypothesis is the basis of the scientific study of the origin of life.

Primitive Earth's Atmosphere

The composition of the primitive atmosphere is of paramount importance for the synthesis of organic material. The primary Earth's atmosphere was probably formed from the gravitational capture of gases from the solar nebula (Rasool, 1972); however, it was rapidly lost during the early evolution of the Sun.

Introduction

All of the life that is known, all organisms that exist on Earth today or are known to have existed on Earth in the past, are of the same life form: a life form based on DNA and protein. It does not necessarily have to be that way. Why not have two competing life forms on this planet? Why not have biology as we know it and some other biology that occupies its own distinct niche? Yet that is not how evolution has played out. From microbes living on the surface of antarctic ice to tube worms lying near the deep-sea hydrothermal vents, all known organisms on this planet are of the same biology.

Looking at the single known biology on Earth, it is clear that this biology could not have simply sprung forth from the primordial soup. The biological system that is the basis for all known life is far too complicated to have arisen spontaneously. This brings us to the notion that something else, something simpler, must have preceded life based on DNA and protein. One suggestion that has gained considerable acceptance over the past decade is that DNA and protein-based life was preceded by RNA-based life in a period referred to as the ‘RNA world’.

Even an RNA-based life form would have been fairly complicated – not as complicated as our own DNA-and protein-based life form – but far too complicated, according to prevailing scientific thinking, to have arisen spontaneously from the primordial soup.

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.