Introduction

The question “What is life?” is foundational to biology and especially important to astrobiologists who may one day encounter utterly alien life. But how should one approach this question? One widely adopted strategy among scientists is to try to define ‘life.’ This chapter critically evaluates this strategy. Drawing from insights gained by philosophical investigations into the nature of logic and language, we argue that it is unlikely to succeed. We propose a different strategy, which may prove more fruitful in searches for extraterrestrial life.

We begin in Section 5.2 by reviewing the history of attempts to define ‘life,’ and their utility in searches for extraterrestrial life. As will become apparent, these definitions typically face serious counterexamples, and may generate as many problems as they solve.

To explain why attempts to define ‘life’ are fraught with so many difficulties, we must first develop the necessary philosophical background. Therefore, in Sections 5.3 and 5.4 we discuss the general nature of definition and of so-called theoretical identity statements. Section 5.5 then applies the material developed in these sections to the project of defining ‘life.’ We argue that the idea that one can answer the question “What is life?” by defining ‘life’ is mistaken, resting upon confusions about the nature of definition and its capacity to answer fundamental questions about natural categories (Cleland and Chyba, 2002).

To answer the question “What is life?” we require not a definition but a general theory of the nature of living systems.

Introduction

Ten times farther from the Sun than the Earth is, shrouded in an orange haze, preserved at temperatures near 100 K, Saturn's moon Titan seems an unlikely astrobiological target. In fact, its extremes suggest images of death rather than life.

Yet this planet-sized moon possesses a dense atmosphere of nitrogen and methane, which, over time and with the action of ultraviolet radiation, may have generated 1016 tons of hydrocarbons and nitriles – the constituents of life – that were then deposited on its surface in liquid and solid form. Within these vast organic deposits, at those times when water ice might liquefy because of volcanism or of impact heating, some of the organic chemical steps leading toward the origin of life might be replicated and then preserved for study on the surface of Titan.

Titan's dense atmosphere has easily won it the high status we reserve for terrestrial planets with atmospheres (Venus, Earth, and Mars) although it is consigned to the outer Solar System. It possesses methane-driven meteorology, and its large size for an ice-rock world likely means a wealth of tectonic activity in its interior. A variety of observed surface landforms produced, apparently caused by erosional processes driven by winds and surface liquid hydrocarbons. In short, other than the likely absence of extant life because of the extreme cold, Titan exhibits a broad range of atmospheric and geologic processes that rival in complexity those of Mars and perhaps of Earth.

Exploring knowledge space for microbes

Astrobiology and biology more generally are integrating new visions of biodiversity with evolutionary and ecological processes. The body of knowledge about hundreds of thousands of microbial species is huge, and involves data on ontogenetic transitions and intraspecific variation; encompasses scales of biology ranging from molecular variation in a particular kind of cell to the role of individuals in complex ecosystems; and accommodates the biology of individuals whose identity and role change as a function of time and place, as well as in response to biotic and abiotic interactions. The knowledge may be either digital or in traditional media, such as often found in libraries, museums, and herbaria. Researchers need solutions that lead to a comprehensive and evolving “knowledge space” (which includes information and its interpretation). The solutions will include tools to empower experts to transfer knowledge from traditional to contemporary media, as well as allow them to integrate old with new. An approach that is universal, inclusive, scalable, and flexible can evolve into a comprehensive Encyclopedia of Life (Wilson, 2003).

The absence of websites offering comprehensive treatments of microbial diversity is a serious impediment for students and investigators who are in need of morphological, physiological, and lifestyle information about microbes. In fact, this is a problem not only for microbial life but for all life. There are no robust standards for indexing phenotypic and biodiversity data, it is difficult to parse and recompile existing relevant data resources.

Introduction

Astrobiology is a discipline that is best enjoyed in the field. What follows is a series of short descriptions by University of Washington students and faculty of selected astrobiological destinations that our planet offers. We cannot hope to provide a comprehensive list – with more space and time we might have included the Burgess shale of Canada; the Atacama desert of Chile; the Cretacious/Tertiary boundary at Gubbio, Italy; Louis Pasteur's home and lab in Paris; Witwatersrand mine in South Africa; the channelled scabland of eastern Washington state, to name but a few. Nevertheless, the ten selected locales have played primary roles in determining how we have come to view the phenomenon of life, and how we have placed constraints on its potential occurrence both on our own planet and elsewhere.

From boiling microbial ponds in Yellowstone to frozen wastes of Greenland harboring Earth's oldest sedimentary rocks, a lifetime of exploration awaits you.

Technology, not intelligence

SETI (Search for Extra Terrestrial Intelligence) can be defined as the branch of astrobiology looking for inhabited worlds by taking advantage of the deliberate technological actions of extraterrestrial organisms. This definition usually draws a chuckle during public lectures, but it underscores why this chapter is somewhat different from the preceding ones. As in other parts of astrobiology, one must consider the diversity of physical environments in the cosmos, and the limitations imposed by them. But with SETI one must also consider modifications to the environment that are not just the byproduct of life, but the result of deliberate actions by intelligent organisms intended to achieve some result.

For millennia people have speculated about the existence of other habitable worlds, and their inhabitants (Chapter 1), but the rules of the game underwent a profound change in the second half of the twentieth century. The publication of the initial scientific paper on SETI (Cocconi and Morrison, 1959) and Drake's (1961) first radio search (Project Ozma, described in Section 1.9) turned speculation into an observational science. No longer were priests and philosophers the sole respondents to the “Are we alone?” question; scientists and engineers could work on finding an answer empirically. Following the first flurry of observing programs in the US and the Soviet Union (Chapter 2), the acronym SETI became the accepted name for this new exploratory activity. But, in fact, SETI is a misnomer because there is no known way to detect intelligence directly across interstellar distances.

In this chapter we give a systematic description of which steps the method of canonical quantisation consists of. The basic idea, due to Dirac, is that one quantises the unconstrained phase space, resulting in a kinematical Hilbert space and then imposes the vanishing of the constraints as operator equations on physical states. The motivation behind this ‘quantisation before constraining’ is that in the opposite procedure one would need to know the full set of Dirac observables. This may not only be practically hard even classically as in the case of interacting field theories such as GR but, even if the full set of Dirac observables could be found, it could be very hard to find representations of their corresponding Poisson algebra, see, for example, [189, 205, 260] and the previous chapter.

Thus, Dirac quantisation is a way to enter the quantum regime even if the underlying classical system is too complicated in order to find all its gauge invariants. While it will be even harder to find all the quantum Dirac observables, the real advantage is that (1) in a concrete physical situation we only need a few of these invariants rather than all of them and (2) starting from the kinematical representation of non-observable quantities we automatically arrive at an induced representation of the invariants which can be expressed in terms of the non-observables.

The canonical approach is ideally suited to constructing background metric-independent representations of the canonical commutation relations as is needed, for example, in quantum gravity. Dirac's original work was subsequently refined by many authors, see, for example, [16, 17, 266–278]. In what follows we present a modern account.

People have wondered for centuries whether we are alone or share this Universe with extraterrestrial beings. Questions about the origin of life and the possible existence of extraterrestrial life have deep roots in the history of both science and culture (Chapters 1 and 2; Dick 1997, 1998). In ancient times, the debate centered mainly on our uniqueness versus the plurality of worlds. Following the advances of the Copernican era, scientists gradually accepted notions about the plurality of solar systems and recognized the large-scale nature of the Universe. Today, cosmic evolution, from the Big Bang to the evolution of intelligence, has become a working hypothesis for astrobiology, one that combines characteristics of both biological and physical universes into a “biophysical cosmology.” This new world view recognizes the enormity of the Universe and hypothesizes that life is one of its basic and essential properties, a cosmic imperative rather than an accidental or incidental property found only on Earth. The current key questions in astrobiology are whether biological laws reign throughout the Universe, whether Darwinian natural selection is a universal phenomenon rather than simply a terrestrial one, and consequently whether there may be other biologies, histories, cultures, religions, and philosophies beyond Earth. In short, is the ultimate outcome of cosmic evolution merely planets, stars, and galaxies – or life, mind, and intelligence? The answers to these questions raise a multitude of issues in the realms of both science and society.

Introduction to cellular metabolism

While it may be impossible to derive a satisfactory definition of life in a global or astrobiological context, we can circumvent such metaphysical questions if we define life not by what it is, but rather by what it does. Every function of life requires metabolism, the coupled chemical processes that generate and exploit biochemical energy. This chapter discusses what can be deduced from examination of extant organisms about the origin and early history of life. First, however, in this section we lay out the basic principles of cellular metabolism.

It is constructive to divide metabolism into functional categories and then consider the various biochemical details. The primary distinction between metabolic functions is whether they generate biologically useful energy or, instead, use this energy. Processes that use energy from the environment for the production of biological energy are referred to as catabolic metabolic processes. Anabolic metabolic processes, on the other hand, use stored biological energy to do biosynthesis, i.e., to synthesize the required building blocks for cellular structure. In any organism, the coordination (or regulation) of anabolic and catabolic processes is the essence of cellular metabolism (Fig. 8.1). At the core of metabolism lies the flow of carbon within an organism – its chemical pathways govern all essential cellular function and are the basis of what is called intermediary metabolism.

For biological energy conversion and structural biosynthesis, it is fundamental that life exploits chemical gradients and thermodynamic potentials that naturally exist in the external environment.

As an application of the concepts of Chapters 19, 21 and in order to see their interplay in a concrete physical application, we sketch the main ideas of geometric quantisation. This will also provide the necessary background material for the treatment of quantum black holes in LQG.

Geometric quantisation concerns the quantisation of an arbitrary symplectic manifold (M, ω) using only natural symplectic structures during the quantisation process. It consists of three steps: (1) prequantisation, (2) polarisation and (3) quantisation. In the first step one is able to quantise every function on phase space in a natural representation, provided that a certain topological condition, Weil's integrality criterion, is satisfied. The famous Groenwald–van Hove theorem is evaded because that representation is highly reducible. In order to obtain an irreducible representation one has to invoke the polarisation step which selects a subspace of the Hilbert space. The final step then consists of finding the induced subrepresentation of the operators.

The strength of geometric quantisation is that it applies to the case when M is not a cotangent bundle, for example, when M is compact. Its weakness is that only a limited number of functions on phase space survive the final quantisation step because they are supposed to preserve the subrepresentation. This is in particular a problem for Hamiltonians and/or constraints which are polynomials of high degree in the momenta, which is why one can usually apply geometric quantisation in its strict form (i.e., without introducing factor ordering ambiguities) only on the reduced phase space constructed in Section 19.3.

Introduction

The fossil record conjures up images of dinosaurs, trilobites, and extinct humans. This reflects not only our anthropocentric (or better, metazoan-centric) bias, but also a visual bias. Starting about 542 Ma, macroscopic remains of animals, traces, shells, and later bones and plants, become quite evident in the fossil record, hence the geological term Phanerozoic, which literally means the eon of “visible life.” These 542 Myr of the fossil record demonstrate that changes have occurred in organisms and have provided compelling evidence for the theory of evolution. This chapter addresses the history of life in the two youngest eons of geological time, the Proterozoic and the Phanerozoic, which span the last 2500 Myr. It is a record of contrasts. Whereas the Proterozoic was mainly a microbial world, the Phanerozoic is a world of macroscopic animals and plants. The transition from the Proterozoic to the Phanerozoic marks what may be one of the most significant evolutionary events in the history of life, when many of the modern animal phyla evolved. Highlights of this 2500 Myr record include the rise to dominance of cyanobacteria in shallow marine environments, the early evolution and diversification of eukaryotes, the first appearance of multicellular algae, the appearance of animals and their subsequent rapid diversification (the Cambrian explosion), the first land plants and their subsequent diversification, and the appearance and dominance of intelligent life (humans).

Quantum gauge fixing

Applications of Loop Quantum Gravity to Particle Physics (see A. [637, 638] in the canonical framework where scalar, electromagnetic and fermionic-free matter propagation on fluctuating quantum spacetimes is studied, B. [773,774, 836–839] in the 4D spin foam framework where graviton propagators are studied and C. [724, 840, 841] in the 3D spin foam framework where the relation with Feynman diagrams is studied) and Quantum Cosmology (see [834, 835] for the full theory where the homogeneous sector has been studied; for homogeneous minisuperspace models see the next section) have just begun. This important research area is so far little explored because ideally one would need to have sufficient control over the physical Hilbert space. Since this is not yet the case, one must think about approximation schemes in order to make progress. As a possible starting point or approximation scheme one could use the kinematical, semiclassical framework developed in Chapter 11: namely, if we use states which are peaked on points in the phase space that solve the constraints, then the expectation value of the constraints in these states is either exactly or close to zero and the fluctuations are small in a suitable sense. Hence, while the states are not physical states, even the norm of the constraints on those states is small. They are therefore approximately physical states.

Notice that kinematical semiclassical states are labelled by a point on the constraint surface contained in some gauge orbit but not by the gauge orbit itself. In other words, we must choose a classical gauge fixing, that is, a section of the bundle whose total space is the constraint surface, whose fibres are the gauge orbits and whose base space is the reduced phase space.