The methodology of physics is now being applied to the social sciences. Social cohesion, which assures social stability and continuity, is both observable and measurable. It may be observed in events that repeat: in test events such as the destruction of the mosque in Ayodhya, India (1992), and of the twin towers of the World Trade Center in Manhattan (2001); in catastrophes such as the Great Fire of London (1666), the earthquakes and fires of San Francisco (1906) and Tokyo (1923); in the riots of rejection in Lawrence, Mass., U.S.A. (1984), and in Aigues-Mortes, France (1893); in the protest riots in Brixton, U.K. (1981); and in resistance to foreign occupation, as in France (1940). Social cohesion can be measured through the reactions of a given society in the aftermath of a shock: for example, in the number of Hindu temples that were burned down or mosques that were destroyed following the first two test events listed above. By borrowing the methods of physics, social scientists have been able to make predictions in their own field. Bertrand Roehner is a member of the Laboratory of Theoretical Physics at Pierre et Marie Curie-University of Paris VII. He is the author of Un siècle de commerce du blé en France (Economica), Theory of Markets (Springer), Application of Physics in Economic Modelling, Pattern and Repertoire in History (Harvard University Press) and Separatism and Integration (Rowman and Littlefield).