In the first three chapters of this book, we have outlined a theory of “technophysio evolution” and examined various techniques for estimating key elements of this theory, including the calculation of food supplies and the heights and weights of past generations. We have applied these insights to a range of practical problems, including the distribution of calories in Britain and France in the eighteenth and nineteenth centuries, the relationship between food availability and physical size, and the contributions made by changes in diet and physique to economic growth. We have also emphasized the critical importance of other factors, such as the quality of the sanitary environment, in helping to explain changes in the health of western populations over the last three hundred years.

This chapter examines the history of health and mortality in Britain in the light of these ideas. Section 4.1 examines changes in the heights and weights of both adults and children in Britain over the last three hundred years, and Section 4.2 summarizes the main trends in the history of mortality. Section 4.3 presents new evidence on the availability of food in Britain and Section 4.4 explores some of the implications of these findings for the history of work intensity. Section 4.5 considers a range of other factors which have been associated with improvements in health, including the roles of sanitary intervention and medical provision.

We are taller, heavier, healthier, and longer lived than our ancestors; our bodies are sturdier, less susceptible to disease in early life and slower to wear out. These changes have occurred in all parts of the world and are continuing to occur, though not all at once and not all at the same speed; they have accumulated, generation by generation, and together constitute a major and surprising change to the human body. It is not as dramatic as the evolutionary changes that produced Homo sapiens, but it has occurred far more rapidly, within the course of about twelve to fifteen generations, from the beginning of the eighteenth century.

This book has described, and sought to explain, these changes and their causes and consequences. Its central thesis is that, over those generations, the process, which we have called technophysio evolution, has allowed many – although certainly not yet all – parts of the world to escape from hunger and disease. This has allowed men, women, and children to live longer and healthier lives and, at the same time, to enhance their own productivity and thus to contribute to the ability of our societies and economies to produce more material goods, to enjoy more leisure, and to nurture healthier and more productive succeeding generations. The process of industrialization that began in western Europe in the eighteenth century has allowed human beings to reach further toward the achievement of the potential which resides in every human body; economic growth and human ingenuity have given us the technological potential to underpin that achievement and to ensure that it continues.

In Chapter 2, we developed techniques for the estimation of the supply of calories as well as techniques for choosing among competing estimates of caloric consumption. We also described the way in which data on height and weight (or BMI) can be used to assess long-term changes in the risk of dying at specific ages for men and women.

In this chapter, we apply these and related techniques to evaluate and propose resolutions to issues under debate by economic and social historians concerned with long-term trends in health, longevity, and other aspects of human welfare. Section 3.1 considers the nature of the samples contained in the Davies and Eden surveys. Section 3.2 evaluates the biases that arise from using cross-sectional data in estimating longitudinal income elasticities of demand for food. Section 3.3 assesses French productivity in food production during the eighteenth century. Section 3.4 examines the nature of European famines. Section 3.5 deals with the effect of variations in body size on the balance between population and the food supply. Section 3.6 evaluates the impact of improved nutrition on the growth of labor productivity. Section 3.7 discusses the impact of thermodynamic and physiological factors on long-term economic growth.

The previous chapter examined long-term changes in health and welfare in England and Wales since circa 1700 and explored their implications for the future. The present chapter extends these ideas to different parts of continental Europe. Section 5.1 looks at long-term trends in height, weight, and (where possible) BMI, and Section 5.2 examines the main changes in mortality rates. Section 5.3 presents a brief overview of the main changes in environmental and nutritional conditions in different parts of Europe since the beginning of the eighteenth century, and Section 5.4 examines the relationship between improvements in health and economic growth. The final section assesses the implications of these changes for health in later life and future trends in life expectancy.

The anthropometric history of continental Europe

Although anthropometric historians have recently begun to examine very long-run changes in human stature from skeletal remains (Steckel 2003), we possess very little information about the heights of Europeans who were measured while they were still alive before the early eighteenth century. However, Komlos (2003) has unearthed data relating to the heights of French army recruits who were born during the second half of the seventeenth century. These men were extremely short by modern European standards and the height data confirm the traditional view that the seventeenth century was a time of considerable and sustained hardship in many parts of Europe (de Vries 1976). However, some of Komlos's other results are rather more surprising.

In the preceding chapters, we have explored how the health indicators of British and European populations – stature, mortality, and morbidity – have evolved over the past three centuries. That evolution was a process of adapting to the long-term changes in social, environmental, and nutritional factors that have been substantially influenced by technological progress in various fields. One lesson from the preceding chapters is that the experience of technophysio evolution varied across countries, mainly depending on country-specific trends of diet and environment. During the course of the past three centuries, America shared the experiences of urbanization, industrialization, and agricultural progress with Britain and European countries, but there existed differences as well as similarities. America uniquely observed dynamic demographic, social, and economic features such as mass migration from Europe, slavery, the opening up of unexploited land, overcoming the variety of disease environments, and so on. America also developed institutions and technologies that are peculiar to those unique experiences. Thus, the study of American experiences and its comparison with British and European experiences deepens our understanding of technophysio evolution.

Since the eighteenth century, changes in human development in Europe have been tied, to a large extent, to the acceleration of economic change. As we have already seen, economic historians now believe that the pace of economic change during the eighteenth century was somewhat slower than an earlier generation of historians may have supposed but the cumulative effect of the changes which did occur was nevertheless dramatic.

This book is based on the belief that changes in the size, shape, and capability of the human body since the beginning of the eighteenth century both reflect and illuminate economic and demographic change over those three centuries. Such change has been immense. To take the United Kingdom as an example, its population has risen from under 5.5 million in 1700 to over 61 million today; life expectation at birth has risen from about 38 years to 75 years for males and 80 years for women; and gross domestic product per capita has risen in real terms from £1,643 to £20,790 (Wrigley 2004). Despite the setback in the world economy at the time of writing, similar changes have occurred in every other country of the developed world and are now occurring in almost every country in the world as a whole. At the same time, humans have become much taller and heavier and now experience lives which are much healthier, as well as longer, than ever before in human history.

It is only recently that the full potential of linking these apparently disparate aspects of the human experience has been realized by historians, economists, human biologists, and demographers. It was a matter of common observation during the eighteenth and nineteenth centuries that men and women from the richer groups within a given society – be it France, Britain, Belgium, or the United States – tended to be taller and heavier than those from poorer backgrounds, to suffer less from chronic and debilitating diseases, to live as long or longer and to be capable of harder and more sustained work.

This first complement is devoted to a completely classical approach of lightmatter interaction which was proposed by Lorentz at the end of the nineteenth century, before the advent of quantum mechanics, but after the discovery of the electron. Lorentz' phenomenological model is based on the experimental fact that atoms have well-defined and sharp absorption lines: he assumed that atoms behaved like harmonic oscillators, in which the electrons are bound to the atomic nucleus by a restoring force which varies linearly with its displacement (from its equilibrium point close to a nucleus), and makes them oscillate at a given frequency ω0 equal to the experimentally determined absorption frequency.

Within the frame of this model, we first calculate the electromagnetic field radiated by an oscillating electron. We show that in the absence of an externally applied force the free oscillations of the electron are damped, because electromagnetic energy is radiated at the expense of mechanical energy. We then study the characteristics of the radiation that is emitted when the oscillations are forced by the application of an external oscillatory electromagnetic field of angular frequency ω. We characterize the different regimes of this scattering of the incident electromagnetic wave and finally determine the polarization induced in the atomic medium by the incident electromagnetic wave.

The Lorentz model can be considered as a lowest order approximation to a description of the light–matter interaction, a better approximation being the semi-classical treatment presented in Chapter 2, and the rigorous treatment being the completely quantum mechanical model presented in Chapter 6.

The difference between laser light and the light emitted by an incoherent source can only be fully appreciated with reference to certain notions of energetic photometry, which are spelt out in the first part of this complement. It will be shown in Section 3C.2 how the laws of photometry drastically reduce the energy density that can be obtained from a conventional incoherent source (such as a heated filament, or a discharge lamp) in comparison with a laser source (Section 3C.3). Far from being merely circumstantial, these laws for classical sources are of a fundamental kind that can be deduced from the basic principles of thermodynamics. Another way to relate these properties of light to the fundamental principles of physics is to examine them in the context provided by the statistical physics of photons, as will be discussed in Sections 3C.4 and 3C.5.

Conservation of radiance for an incoherent source

Étendue and radiance

An incoherent source comprises a large number of independent, elementary emitters, emitting electromagnetic waves with a random distribution of uncorrelated phases. It emits light in every direction. A light beam produced by this source can be decomposed into elementary pencils of light. Since the light is incoherent, the total power carried by the beam is the sum of the powers carried by the elementary pencils.

An elementary pencil is defined by the element dS of the source from which it originates, and a second surface element dS′, as shown in Figure 3C.1.

Introduction

In 1961, just a few months after Maiman invented the ruby laser, Franken focused the pulses emitted from such a laser, of wavelength 694 nm, on a quartz plate, and examined the spectrum of the light transmitted using a simple prism (see Figure 7.1). He thus discovered that ultra-violet light of wavelength 347 nm was emerging from the quartz plate. Clearly, as it propagated through the quartz, the light of frequency ω had generated the second harmonic, of frequency 2ω.

It thus transpires that in optics, as in any other part of physics, a system subjected to a strong enough sinusoidal excitation will leave the linear response regime. Nonlinearities cause harmonics of the excitation frequency to appear.

But what intensity is needed before nonlinear effects will appear? One might think that a natural scale would be the electric field of the nucleus at the location of an atomic electron. In the case of the hydrogen atom in its ground state, this field is about, or 3 × 1011 V.m−1. (Here e is the charge of the electron, and a0 the Bohr radius, of the order of 5 × 10−11 m). In fact, experiment shows that, in the transparency zone of a dielectric material like quartz, a field of just 107 V.m−1 (corresponding to a light intensity of 2.5 kW.cm−2) is sufficient for nonlinear effects to appear perturbatively.

One-photon sources are important elements in quantum optics. The archetypal example is an atom raised to an excited state at time t = 0, then de-exciting with emission of a single photon. The development of this kind of source depends on progress with experimental techniques, e.g. the possibility of isolating a single atom, molecule or quantum well. In this complement, we present the formalism for describing the corresponding radiation, and use it to discuss some spectacular experiments which bring out properties quite incompatible with a classical description of the electromagnetic field. We begin in Section 5B.2 by describing the anti-correlation between detections on either side of a semi-reflecting mirror, establishing the quantitative difference with a classical field. Section 5B.3 discusses a quantum optical effect that was only demonstrated at the beginning of the twenty-first century, namely the quantum coalescence of two one-photon wave packets on a semi-reflecting mirror, which occurs even when the two photons were emitted by independent atoms. An analogous effect, the Hong–Hou–Mandel effect, is discussed in Chapter 7. These effects exemplify quantum interference involving two photons. Finally, Section 5B.4 is concerned with quantum calculations involving quasi-classical states. As we now know, this leads to results that are identical to the predictions of semi-classical theory.

In this complement we discuss several examples of optical phenomena in media where the refractive index depends nonlinearly on the intensity, known as optical Kerr media. This nonlinear effect exists in all materials, even isotropic ones, like glass or fused silica, but it is particularly marked in certain physical systems to be exemplified in Section 7B.1. After investigating the propagation of light through such media in Section 7B.2, we shall discuss three applications of the optical Kerr effect (which can be studied in any order). We begin by describing a bistable optical system, when this nonlinear medium is inserted in a Fabry–Perot cavity (Section 7B.3). We then study phase conjugate mirrors and examine their potential applications in adaptive optics (Section 7B.4). Finally, we discuss certain effects occurring during the propagation of an isolated wave, bounded either transversely or temporally, in a Kerr medium, and describe self-focusing effects (Section 7B.5) and self-phase-modulation effects (Section 7B.6). In particular, we shall show that nonlinear effects and dispersion effects can compensate to produce stable structures known as solitons, which maintain their shape during propagation.

Examples of third-order nonlinearities

Nonlinear response of two-level atoms

We begin by studying a simple case of a nonlinear interaction, namely a two-level quantum system under the effects of a plane wave.