The Zeitgeist, a word of German origin which literally means the spirit of the time, refers to a set of attitudes, beliefs and ideas that, at one point in time, are accepted uncritically because they appear to be shared by almost everybody. The best way to convince oneself that well-accepted ideas are in fact a product of their time is to look back at past centuries. Two examples from eighteenth-century England may illustrate this point. (i) At that time it was held as self-evident that in order to become an English “citizen” a foreigner had to embrace the Anglican faith. (ii) The fact that trading slaves, provided they were not Christian, was considered a legitimate business for decent Christian merchants is attested by many documents from religious pamphlets to court rulings. Once we have accepted the notion that most of our beliefs are shaped by the society in which we live, we would like to understand how these views, beliefs and convictions are formed. In fact, this question has far reaching consequences in various fields, from marketing to political campaigns to speculation frenzies. In recent times the issue of consensus formation has attracted the attention of econophysicists and several models have been proposed, e.g. Behera and Schweitzer (2003), Quentin and Bouchaud (2005), Stauffer (2003). One of the main difficulties in the study of consensus formation is that we do not know how to distinguish between the action of small and big players. What we mean by these expressions can best be explained through an analogy with stock markets.

The emergence of two new fields, the science of chaos and the science of networks, has changed the way in which we look at physical and social systems. From the first we learned that simple (in the sense of having few degrees of freedom) physical systems can undergo chaotic motions and display intricate trajectories. The double pendulum is one of the simplest systems of this kind. Initiated by Benoît Mandelbrot (1975) and Robert May (1976), the science of chaotic systems and fractals has already produced substantial achievements. This book relies only occasionally on the analysis of chaos; in contrast it relies heavily on the ideas of network science. Although it can be traced back to system theory which flourished in the 1960s and 1970s, network science really emerged in the late 1990s through the works of people such as Albert-László Barabási (2002), Sergei Maslov (2002), Steven Strogatz (2003) or Duncan Watts (2003). It has been instrumental in convincing us that what really matters in a system is its nodes, its links and their respective weights. Seen in this perspective, the real nature of the system, whether of physical, biological or social nature, is of little relevance.

But looking at physical and social systems in an abstract, purely structural way takes away much of their substance. The real challenge is to do real physics and real sociology in the framework of network theory. This is what we call “bridging the gap”.

The statistical investigation of the phenomenon of suicide began in the mid nineteenth century when data about suicide rates became available in all industrialized countries. Emile Durkheim's book about suicide which appeared in 1897 was a landmark in this research but there had been many earlier studies. The contribution of Durkheim stands out because he proposed a social interpretation of suicide rather than one based on individual, psychological factors. He maintained that one of the main factors of suicide is social isolation. According to this view it is the paucity of links and interactions with other people which leads to suicide. This perspective is similar to the interpretation of apoptosis according to which cell death occurs when the cells no longer receive the “stay alive” message from their neighbors (see the chapter on apoptosis).

Yet, to this day, the phenomenon of suicide remains fairly mysterious. Common sense seems to be misleading rather than helpful as illustrated by the two following observations. (i) Contrary to the assumption or expectation of many people, the monthly maximum of suicide rates does not occur in fall or in wintertime but in May or June. (ii) Even events as dramatic as the attack of September 11, 2001 have no visible impact on the number of suicides. This observation, which will be explained in more detail in a subsequent chapter, holds at monthly as well as at daily level.

The central theme of this book is the analysis of interactions in social systems. As a first (and perhaps easier) step we will study the consequences of a lack of interaction between social agents. For instance, what happens when there are no interactions between landowners and tenants, holding companies and employees, governments and subjects? These questions might seem fairly difficult, but fortunately we have a good starting point because we can rely on the results of a famous experiment performed by Stanley Milgram in the 1960s.

First of all, to introduce the issue of absentee ownership, we describe landownership in the Philippines and Japan as seen by General Douglas MacArthur.

Land reform in Japan under General MacArthur

After General MacArthur retook Luzon in March 1945, some of his officers suggested that he send a punitive expedition against the Huks who were waging a guerrilla war in Central Luzon to dispossess the landlords. He refused and justified his position in the following way (Manchester 1978, p. 420).

Equilibrium phenomena are simpler to analyze than time-dependent phenomena. The fact that non-linear time-dependent systems may display perplexing behaviors is illustrated in Fig. 4.1. It does not come as a surprise, therefore, that the equilibrium requirement is an essential assumption in statistical mechanics. For time-dependent systems the notions of statistical ensemble, temperature, entropy as well as several other fundamental concepts are no longer clearly defined and many standard results of statistical thermodynamics, such as the equipartition law, are no longer satisfied. In a general way, whereas equilibrium statistical mechanics provides a systematic and unified theoretical framework, non-equilibrium statistical mechanics is rather a collection of loosely connected techniques and equations.

It is certainly important to know whether or not a social system can be considered as being in equilibrium. What criteria do we have in this respect? This is the first question that we consider. For a system to remain in equilibrium there must be an equilibrium restoring mechanism, therefore one is naturally led to examine these mechanisms. This is done in the first part of this chapter. In the second part we investigate metastable systems. From a common sense perspective a diamond would seem to be a perfect example of a system in equilibrium; the fact that diamonds are in fact metastable shows that common sense can be fairly misleading. By discussing several examples of metastable systems in physics and in chemistry we try to develop a qualitative understanding of metastability; in particular we emphasize the role of facilitator played by “seeds” or “precursors” of the new organization.

In the previous chapter we studied the effect of weakened marital bonds on suicide. Of course, the family unit is only one of the entities to which an individual belongs. In the language of physical interactions, family bonds are short-range ties whereas links with society at large can be seen as long-range ties. Intermediate between short and long are medium-range ties with relatives, friends, neighbors, colleagues, etc. Typically, these links have a duration of a few years, whereas family ties have a duration of several decades.

A quick estimate of the strength of long-range ties can be obtained through the extreme value approach. It leads us to analyze the impact on suicides of major events such as the attack on Pearl Harbor and September 11, 2001. We will see that even such dramatic events have no visible effect on suicide rates. In the subsequent sections we consider various situations of social isolation such as illness, imprisonment or emigration. In each case the excess-suicides with respect to the general population can be taken as a proxy of the social ties which are missing or severed.

Effect of major historical events on suicide

First we examine the effect of the attack of September 11; then we discuss more briefly the consequences of the attack on Pearl Harbor.

September 11, 2001

In accordance with the extreme value approach we consider the number of suicides in the place where one may expect the effect to be greatest, namely in New York City. The series in Table 11.1a gives the monthly numbers of suicides in the three years 2000–2.

In the previous chapters we pointed out that there are basically two ways to win the battle against noise. The first and by far the most effective solution is to find situations where the effect that one wishes to investigate is sufficiently strong to give a high signal to noise ratio. For instance, the effect on suicide of a male–female imbalance will be stronger in a population whose sex ratio is 20 than in one whose sex ratio is 1.2. This is what we called the extreme value technique and it played a considerable role in previous chapters. That approach is particularly useful in the first stage of a study when one wishes to identify an effect and to get a rough idea of its magnitude. In a second stage the objective is to perform the most accurate measurements of the phenomenon under consideration. In this case one resorts to the second method of reducing the influence of the background noise, namely increasing the number N of events; this means that one will collect and analyze the largest possible number of (good quality) data.

To reach these goals it is essential to have access to a great variety of large databases. Needless to say, the Internet marks a revolution in this respect. Let us illustrate this statement by two examples drawn from previous chapters.

The purpose of this book is to explore the similarities as well as the differences between natural and social phenomena. While several chapters are devoted to the second objective, the main message of the book is to show that the experimental methodology which has been used so successfully in the natural sciences can also be applied to social phenomena.

Basically, one can distinguish two main approaches in the social sciences, the anthropomorphic perspective and the system theory approach. Throughout this book we try the second of these options. In order to illustrate them, let us consider the phenomenon of suicide. In the anthropomorphic perspective one would try to establish connections between suicide and psychiatric disorders. In the system theory approach one tries to formulate the problem of suicide in a more general framework in which the interactions between the elements composing the system play the key role. This approach was pioneered by the sociologist Emile Durkheim at the end of the nineteenth century. He was able to show that there is a clear relationship between the likelihood of suicide and the strength of the bonds which link an individual to the rest of the society. The main challenge (and prerequisite) in the application of the network science approach is to be able to estimate the strength of the interactions between the elements of the system. This is also the main stumbling-block because both system theory and network science are mathematical frameworks which provide useful modeling tools but do not tell us how to investigate complex, interconnected networks and how to measure the interactions between their elements.

Historical review

1924

The history of the study of single molecules in physics starts with the well-known oil-drop experiments of R. Milliken, in which the charge-to-mass ratio of a single electron was determined. It may be assumed that the history of studying single molecules in biology started in 1950s when researchers were able to detect the presence of massive molecules such as DNA using conventional electron microscopes in high vacuum. The advent of the scanning tunnelling microscope in the 1980s made it possible to image individual molecules and atoms on the surface. However, the observation of individual molecules within solids or in solution remained an extremely difficult and unsolved problem.

1961

B. Rotman was the first to use fluorescence detection for single-molecule studies in solution. Using a fluorogenic substrate, he measured the presence of a single β-D-galactosidase molecule by detecting the fluorescent product molecules accumulated in a microdroplet through enzymatic amplification. He did not achieve single-molecule sensitivity but clearly demonstrated the great potential of fluorescence detection. In 1976 T. Hirschfeld reported the use of fluorescence microscopy to detect single antibody molecules tagged with 80–100 fluorescein molecules under evanescent-wave excitation.

1989

W. Moerner's group first used a laser to see single, small organic molecules trapped inside a transparent host crystal. In 1990 M. Orrit's group showed that very high sensitivity at the single-molecule level can be achieved by probing ‘guest’ molecules in solid hosts at cryogenic temperatures.

Protein structure and function

ESI and MALDI have become increasingly useful for the mass spectrometric analysis of proteins. The two ionisation techniques have been exploited to study protein folding, to characterise non-covalent protein complexes, to map protein function and for many other applications (Comment B2.1).

Comment B2.1: Mass spectrometry and X-ray crystallography

Mass determination

ESI and MALDI have made the mass analysis of proteins a routine procedure.

Historical review

Light microscope

Mid-fifteenth century

The simple one-lens microscope was available at this time, with low-power magnifiers being used for the examination of insects. From these early beginnings, glass lenses of increased power were developed. A. Leeuwenhoek (1632– 1723) produced remarkable microscopes. One of them had a resolving power of 1.35 μm, which was enough for basic cytology. About the same time the basic idea for the compound microscope, in which two or more lenses are arranged in such way to form an enlarged image of an object, occurred independently to several people in the Netherlands (H. Jansen, his son Zacharias and H. Lippershey). Subsequently, compound microscopes became widely available.

1830

J. Lister published the first work in which the construction and design of achromatic microscope lenses was placed on theoretical bases. He showed how two separate achromatic lenses could be combined to act as a single lens so that the image rendered would be completely free from chromatic aberration and from distortion caused by curvature of the lenses. This discovery opened the way to the construction of high-power microscope lenses over the next 50 years. The first achromatic microscope lenses appeared in the Netherlands at about the end of the eighteenth century.

The latter part of the nineteenth century

E. Abbe was the outstanding figure in development of the microscope. Abbe placed the theory of microscopic image formation on a sound basis by emphasising the importance of the light diffracted by the object in resolving its fine details.

Historical review and introduction to biological problems

Optical activity is the property of certain materials to change the angle of polarization of a beam of light that is shone through them as a function of its wavelength (optical rotation dispersion, ORD), or to absorb light differently according to its wavelength and polarization (circular dichroism, CD, from the Greek related to two colours).

1812–1838

Jean-Baptiste Biot and Auguste Fresnel independently made careful studies of optical activity. Biot'spolarimeter (an optical instrument used to measure the angle of rotation of polarised light) used sunlight, plane-polarised by reflection off glass, as a light source. Biot defined [α] the angle of specific rotation of the axis of plane-polarised light by an optically active solution in terms of the concentration in grams per cubic centimetre and the optical path in decimetres, a definition still in use. In 1824, Fresnel predicted that helical structures would be optically active.

1848

Louis Pasteur observed that optically inactive crystals of sodium ammonium tartrate were in fact mixtures of two classes. When separated, both classes turned out to be optically active with respective specific rotation angles of equal value but opposite in sign. The molecules making up the crystals were themselves in one of two asymmetric forms, enantiomorphs (from the Greek for opposite shapes), each a mirror image of the other. Since a two-dimensional molecule and its mirror image can be made to coincide and considered to be identical, the implication was that molecules are three-dimensional (Comment E4.1).