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.

The many measurements of the spectrum of the CBR discussed in the preceding chapter are consistent with a Planck spectrum with T0 = 2.73 ± 0.02 K over a wavelength range 0.1 cm ≲ λ≲75 cm. Only the submillimeter observations reviewed in Section 4.8.4 provided any evidence for a significant deviation from a thermal spectrum, and these appear to be erroneous. What conclusions may we draw from the essentially thermal spectrum of the CBR? This chapter provides some answers to that question. It is presented as an introduction to, not an exhaustive treatment of, the processes that determine the CBR spectrum. There are a number of reviews, which treat these topics in more detail, such as Danese and De Zotti (1977), Sunyaev and Zel'dovich (1980) and Bond (1988).

We begin by noting the conditions under which we would expect an exactly thermal spectrum to have been produced early in the Hot Big Bang, then consider a number of physical processes that could have distorted an initially thermal spectrum. We also consider the possibility that one or more additional ‘cosmic’ backgrounds may be present, adding to the CBR at wavelengths below about 1 mm. Finally, we investigate the constraints that the spectral measurements of Chapter 4 place on these processes.

A characteristic feature of the CBR, noted at the time of its discovery by Penzias and Wilson (1965), is its approximate isotropy (see Appendix A). Approximately equal intensity in all directions is expected if the radiation is a relic of the Hot Big Bang. On the other hand, there are a variety of mechanisms that can induce small amplitude variations in intensity, or anisotropies, into an initially uniform CBR; some of these were outlined in Chapter 2 and will be discussed in detail in Chapter 8.

Careful measurements of the angular distribution of the CBR have therefore been pursued both to confirm the cosmic, Hot Big Bang, origin of the CBR and to search for small amplitude anisotropies imprinted in it. In this chapter we deal with observations of the angular distribution of the CBR on the largest angular scales, θ ≳ 10°, and in particular with the dipole and quadrupole moments of the CBR. The value of about 10° for the boundary between ‘large’ scale anisotropies (discussed in this chapter) and smaller scale anisotropies (Chapter 7) is obviously rather artificial. When we turn in Chapter 8 to the implications of the measurements of and upper limits on CBR anisotropies, we will be drawing on the results of both Chapters 6 and 7.

One of the basic problems of cosmology is the singularity characteristic of the familiar cosmological solutions of Einstein's field equations. Also puzzling is the presence of matter in excess over antimatter in the universe, for baryons and leptons are thought to be conserved. Thus, in the framework of conventional theory we cannot understand the origin of matter or of the universe. We can distinguish three main attempts to deal with these problems.

1. The assumption of continuous creation (Bondi and Gold 1948; Hoyle 1948), which avoids the singularity by postulating a universe expanding for all time and a continuous but slow creation of new matter in the universe.

2. The assumption (Wheeler 1964) that the creation of new matter is intimately related to the existence of the singularity, and that the resolution of both paradoxes may be found in a proper quantum mechanical treatment of Einstein's field equations.

3. The assumption that the singularity results from a mathematical over-idealization, the requirement of strict isotropy or uniformity, and that it would not occur in the real world (Wheeler 1958; Lifshitz and Khalatnikov 1963).

If this third premise is accepted tentatively as a working hypothesis, it carries with it a possible resolution of the second paradox, for the matter we see about us now may represent the same baryon content of the previous expansion of a closed universe, oscillating for all time.

The science that treats the properties and evolution of the Universe as a whole is cosmology. Among the sciences, it is unique in having only a single object of study – there are no other Universes for us to use as controls, nor can we readily run the whole experiment over again. As a consequence, much of the effort in modern cosmology has been to determine the best mathematical description, or ‘model’, of the Universe we inhabit. As we shall see, that task is not yet complete, despite the rapid advances of the past few decades. The range of possible models is presented later in this chapter. First, though, we need to look at the observational bases of modern cosmology, a set of astronomical observations which have established the Hot Big Bang theory and restricted the range of models we need to consider.

Astronomical constituents of the Universe

Since cosmology is the study of the Universe as a whole and as a single system, it is only indirectly concerned with subsystems within the Universe. Here, I will mention only two: galaxies and clusters of galaxies. The galaxies are assemblies of 108–1012 stars; many galaxies also contain appreciable amounts of interstellar gas and dust.

The very first astronomical signal at radio wavelengths, by happy coincidence, was also detected at the Bell Telephone Laboratories; in 1932, Karl Jansky detected at 15 m wavelength radio emission, which he correctly identified as coming from the Galactic plane. Astronomers paid little attention. Observational radio astronomy did not really come into its own until after World War II (see, e.g., Hey, 1973, and Sullivan, 1984). It is now recognized as a powerful adjunct to optical astronomy, particularly in the study of low density cosmic matter and of energetic objects and phenomena such as quasars and the collimated jets seen in radio galaxies. We will look very briefly at radio sources later in this chapter, but most of it will be devoted to the tools and techniques of observational radio astronomy. Chapter 3 is designed to introduce the more specialized radio astronomical techniques used in studying the CBR; it is not intended to be a complete introduction to radio astronomy. For further details, readers may want to consult one or more of the following texts: Kraus (1986); Rohlfs (1986); and Christiansen and Högbom (1985). Interferometry is very fully treated by Thompson, Moran and Swenson (1986), and radio sources by Pacholczyk (1970) and Verschuur and Kellermann (1988), among others. The treatment of radio astronomy in this book is closest to the work of Rohlfs.

Humankind has made stories about the origin of the world since prehistoric times. These creation stories often have a grand beauty and are sometimes richly detailed. It is only in the present century that such myths and images have been supplanted by a well-established scientific description of the origin of the world. ‘World’ is now understood to mean the Universe as a whole, not just the Earth or the solar system, and the modern picture of its origin and evolution is the Hot Big Bang model. This book describes one crucial piece of astronomical evidence supporting the Big Bang model, namely the cosmic microwave background radiation, heat radiation left over from a hot and dense phase early in the history of the Universe.

The cosmic background radiation (CBR) was discovered, by accident as it happens, a quarter of a century ago. Within a few years, the basic properties of the radiation had been established. Those properties, especially the thermal 3 K spectrum and the very uniform distribution of the CBR across the sky, have convinced virtually all astrophysicists that the radiation is a relic of the Hot Big Bang, and that it comes to us from a very early time in the history of the Universe. It thus provides information about the early history of the Universe obtainable in no other way.