The exposition of Chapter 10 would be incomplete if we could not extend the framework to matter also, at least to the matter of the standard model. This is straightforward for gauge field matter, however for fermionic and Higgs matter one must first develop a background-independent mathematical framework [443]. We will discuss the essential steps in the next section and then outline the quantisation of the matter parts of the total Hamiltonian constraint in the section after that, see [441] for details.

We should point out that these representations are geared towards a background-independent formulation. The matter Hamiltonian operator of the standard model in a background spacetime is not carried by these representations. They make sense only if we couple quantum gravity. Also, while we did not treat supersymmetric matter explicitly, the following exposition reveals that it is straightforward to extend the formalism to Rarita–Schwinger fields. We will follow closely [441, 443].

Before we start we comment on a frequently stated criticism: as we will see there is no obstacle in finding background-independent kinematical representations of standard matter quantum field theories and these support the matter contributions to the Hamiltonian constraint. Thus, it seems as if in LQG there is no restriction on the matter content of the world. However, that is a premature conclusion: the associated Master Constraint of geometry and matter could have zero in its spectrum depending on the type of matter coupled. Indeed, the reason why the spectrum of the Master Constraint could not contain zero is due to normal or factor ordering effects which are finite but similar in nature to the infinite vacuum energies of background-dependent quantum field theories.

Outline and historical overview

In the first part of this book we have derived a canonical connection formulation of classical General Relativity. We have defined precisely what one means by the canonical quantisation of a field theory with constraints and have emphasised the importance of n-form fields for a background-independent quantisation of generally covariant theories. In this part we will systematically carry out the canonical quantisation programme step by step and almost complete it. In more detail we will show that:

1. There exists a mathematically rigorous and, under natural physical assumptions, unique kinematical platform from which constraint quantisation is launched.

2. There exists at least one, consistent, well-defined quantisation of the Wheeler–DeWitt constraint operator whose action is explicitly known.

3. A corresponding physical inner product is known to exist.

4. There is a concrete proposal for constructing Dirac observables and physical Hamiltonians.

What is left to do is to check whether this solution of the quantisation problem has the correct semiclassical limit (semiclassical states at the kinematical level are already under control, however, not yet on the space of solutions to the constraints) and to construct quantisations of the classical formula for complete Dirac observables explicitly. This will involve, besides the improvement of the already available semiclassical techniques, the development of appropriate approximation schemes because the exact theory is too complicated to be solvable explicitly. After these steps have been completed one is ready to make physical predictions from the theory. The task will then be to identify quantum gravity effects, which lie in the realm of today's experimental precision, and to falsify the theory.

“Astrobiology” was originally defined as “the consideration of life in the Universe elsewhere than on Earth” (Lafleur, 1941). But as the field has advanced, we have learned to place no artificial barrier between the study of life on Earth and life that may exist elsewhere in the Cosmos. Astrobiology today is “the study of the living Universe” (NAI, 2004), be it here or elsewhere. It would be foolish to narrow the definition, for the approaches we take in searching for extraterrestrial life are strongly informed by our understanding of life on Earth, and our understanding of the origin and evolution of terrestrial life is informed both by the study of other planetary environments and by Earth's environment within the Solar System and Galaxy. As Carl Sagan (1974) remarked decades ago, we are able for the first time in human history to assess life on Earth “in a cosmic context.” The assessment is still nascent and inchoate, but as the chapters in this book illustrate, the floodgates have opened and our knowledge is expanding quickly now. We will soon know much more.

Besides “astrobiology,” the study of life in the Universe has also been called “cosmobiology” (Bernal, 1952), “exobiology” (Lederberg, 1960), and “bioastronomy” (IAU, 2004) (see Sections 2.3.1 and 2.4 for discussion). Under its exobiological label, the entire field was famously criticized by the biologist George Gaylord Simpson (1964), “in view of the fact that this ‘science’ has yet to demonstrate that its subject matter exists!” If astrobiology meant only the study of extraterrestrial life itself, Simpson's criticism would still have weight, four decades later.

Introduction

The process of metabolism, in which cells carry out biochemical reactions, is a hallmark of all living organisms. Catabolic reactions generate energy for the organism while anabolic reactions are used for the synthesis of cell material. Metabolic pathways in today's living organisms have been evolving for more than 3.5 Gyr. In fact, since metabolism would have been necessary even for the earliest organisms, its evolution cannot be separated from the origin of life. Contemporary metabolic pathways are presumed to be much more elaborate and sophisticated than those that first evolved. Indeed, metabolism today is extraordinarily rich and diverse, ranging from the use of various inorganic chemicals such as hydrogen or sulfur for nutrients and energy, to several forms of photosynthesis, to the metabolism of hundreds of organic compounds. It is impossible for us, at least at this time, to know which pathways originated first and how they evolved. Nonetheless, because metabolism is essential to life, understanding how metabolism evolved is of considerable importance. Furthermore, we have good grounds to speculate on which of life's diverse metabolisms evolved earliest and which could only have come later. Microorganisms, most likely resembling present day Archaea and Bacteria, were the first organisms, so it is their metabolism that is of relevance. Indeed, all basic metabolic pathways on Earth today can be traced to microorganisms.

The goal of this chapter is to describe, insofar as possible, the evolution of metabolism. Although there are several principles that guide our considerations, two are predominant.

Problems with the record

Astrobiology has only a single successful experiment in planetary life available to investigate: that on the Earth. Hence, the history of terrestrial life must act as the archetype, albeit an ever more contingent and unique one, for astrobiological models of the appearance and radiation of life anywhere in the Universe. Indeed, it could be argued that all habitable planets would have had similar environmental constraints and pathways of physical and chemical development, so the process of biological initiation elsewhere should be broadly reminiscent of Earth's experience of the phenomenon. If so, astrobiology is saved from the challenges of studying things far away, but is instead faced with the difficulties of examining events here long ago.

Unfortunately, and perhaps surprisingly, the origin and early evolutionary history of terrestrial life is poorly known, as is the corresponding record of environmental conditions on the early Earth. There are many reasons for this. Firstly, like all old things, ancient rocks are rare (Fig. 12.1). Almost all potential information about the first half of Earth's history is contained in geological materials. But rocks of such great antiquity have mostly been hidden or destroyed by geological processes like burial, erosion, or subduction back into the mantle via plate-tectonic recycling of crust along ocean trenches. Even ejection into space by catastrophic meteorite impacts, of which there were plenty during the heavy bombardment that occurred over the first billion years of Earth history (Chapter 3), is a viable mechanism for destruction of the earliest crust.

Over half a century of collective study has not diminished the fascination of searching for a consistent theory of quantum gravity. I first encountered the subject in 1969 when, as a young researcher, I spent a year in Trieste working with Abdus Salam who, for a while, was very interested in the subject. In those days, the technical approaches adopted for quantum gravity depended very much on the background of the researcher: those, like myself, from a theoretical particle-physics background used perturbative quantum field theory; those whose background was in general relativity tended to use relatively elementary quantum theory, but taking full account of the background general relativity (which the other scheme did not).

The perturbative quantum field theory schemes foundered on intractable ultraviolet divergences and gave way to super-gravity – the super-symmetric extension of standard general relativity. In spite of initial optimism, this approach succumbed to the same disease and was eventually replaced by the far more ambitious superstring theories. Superstring theory is now the dominant quantum gravity programe in terms of the number of personnel involved and the number of published papers, per year, per unit researcher.

However, notwithstanding my early training as a quantum field theorist, I quickly became fascinated by the “canonical quantization”, or “quantum geometry,” schemes favored by those coming from general relativity. The early attempts for quantizing the metric variables were rather nave, and took on various forms according to how the intrinsic constraints of classical general relativity are handled. In the most popular approach, the constraints are imposed on the state vectors and give rise to the famous Wheeler–DeWitt equation arguably one of the most elegant equations in theoretical physics, and certainly one of the most mathematically ill-defined.

Introduction

The appeal of astrobiology lies in two primary features, the first being the important questions astrobiologists ask: “Are we alone? How did life begin? How will it all end?” The second attractor is the opportunity to work across many disciplines. Astrobiology challenges us to draw from many intellectual resources in the attempt to answer these questions – biology, chemistry, physics, astronomy, geology, and engineering are all required. Life detection, in particular, requires a strong interdisciplinary approach. In this chapter we focus on life detection within the context of Solar System exploration; techniques for detecting planets and possible associated life beyond the Solar System are discussed in Chapters 21 and 26. Within our Solar System life detection efforts have been and still are primarily focused on Mars (Chapter 18), and so we will use Mars exploration as a model for discussion, though our approach is applicable to any potential habitat for life.

The success of a life detection mission to another planet should not be focused solely upon whether or not live organisms are detected, but rather it must be able to correctly classify observations as evidence of (a) life, (b) non-life, (c) once-alive-but-now-dead, or (d) made-by-life (biogenic). It is essential that the scientific community agree on which measurements should be made, and then how to interpret those measurements.

Communication across the many disciplines involved in astrobiology is fraught with difficulty on many levels, including even the seemingly simple matter of units and usage of terms and abbreviations. When first introduced in any chapter, unusual units often not known to those outside the field are defined. In this appendix we give conventions and conversions for various units used throughout the book.

Astrobiologists hope to understand the origin, history, extent, and future of life in the Universe. This is a huge task, considering that two of the terms in this mission statement are difficult to define. The definition of “life” is itself worth a chapter in this book (Chapter 5). Here in this chapter, we must not only concern ourselves with a definition of life, but also with the concept of “understanding.” What does it mean to say that we “understand life”?

Any attempt to understand life soon engages organic chemistry. Biology today is increasingly focused on the molecular scale. Indeed, it is difficult to find a biologist today who is not attempting to put a molecular structure on the phenomenon that they are studying, so much so that biology can be (provocatively!) viewed as the subfield of chemistry dealing with chemical systems capable of Darwinian evolution.

Some illustrations make this point. The human genome is nothing more (and nothing less) than a collection of chemical structures, recording how carbon, oxygen, nitrogen, hydrogen, and phosphorus atoms are bonded in the natural products directly responsible for heritance. Molecular evolution uses organic chemistry to describe the Darwinian evolution of species, the process that drives biology. Neurobiologists are attempting to describe the inventory of molecules, including messenger RNA, that allow neurons to learn and remember.

Nowhere is this more evident than in the segment of astrobiology that investigates the origin of life.

Earth is not the only body in the Solar System that is habitable. Life as we know it requires liquid water and free energy gradients, both of which probably also exist on Mars and Europa, although liquid water on those bodies is restricted to the subsurface. Earth is, however, the only planet in the Solar System that has liquid water at its surface. Similar planets may exist around other stars (Chapter 21) and would be of profound interest for two reasons. First, biology on such planets might resemble life on Earth. Second, the biosphere on such planets would interact with the planet's atmosphere and could modify it in a way that may be detectable remotely. Today, life may be thriving on Mars or Europa but its discovery will require subsurface exploration. In contrast, we might be able to tell whether a distant Earth-like planet is inhabited by measuring the spectrum of its atmosphere.

Thus, from an astrobiological standpoint, one of the most fundamental characteristics of a planet is its surface temperature Ts. If Ts is not within the range in which liquid water can exist, remotely detectable life will probably not exist there. Consequently, the first part of this chapter is concerned with planetary surface temperatures. The constraint on temperature is not as obvious as 0 < Ts < 100 °C.

Introduction

The search for extraterrestrial life is intimately linked with our understanding of the distributions, activities, and physiologies of Earth-life. This is not to say that only Earth-life could exist on other planets and moons but it is important to know the extent of environmental conditions that can support terrestrial organisms as a first-order set of criteria for the identification of potential extraterrestrial habitats. Even though the life forms may have different biochemistries and in fact may have had different origins, the limits of life on Earth may help define the potential for habitability elsewhere. It is also likely that many of the limits of Earth-life could extend out of the bounds of extreme conditions found on modern-day Earth. This is the case for the bacterium Deinococcus radiodurans that can tolerate levels of radiation beyond those found naturally on Earth, and also for the apparent tolerance by Escherichia coli to hydrostatic pressures that exceed by more than ten times the pressures in the deepest ocean trenches (Cox and Battista, 2005; Sharma et al., 2002).

Since Earth is the only planet that unequivocally supports modern, living ecosystems, it is logical to first look for life elsewhere that resembles Earth-life. Earth-life requires either light or a chemical energy source, and other nutrients including nitrogen, phosphorus, sulfur, iron, and a large number of elements in trace concentration; 70 elements in all are either required or are targets of interaction by various species of Earth-life (Wackett et al., 2004).