Photo credit: Stone.
Philip Ball is a science writer and journalist based in London. This article from Oct 2003 was based around his book Critical Mass: How One Thing Leads To Another
"It may be", said US sociologist George Lundberg in 1939, "that the next great developments in the social sciences will come not from professed social scientists, but from people trained in other fields."
Take a look at any issue of a physical-sciences journal in the past five years and you will see one such field staking its claim vigorously. Physics is muscling its way into social science. Not content with explaining the behaviour of atoms and electrons, semiconductors, sand and space-time, physicists are now setting out to understand the behaviour of people.
From theories of pedestrian movement and traffic flow to voting processes, economic markets and war, researchers are striving towards a physics of society. Phase transitions, critical points and scaling laws are just some of the phenomena from physics that turn up when people are treated as interacting particles.
Lundberg would have approved. He was part of a tradition that sought to establish a scientific grounding for sociology that would make it every bit as quantitative and deterministic as the natural sciences. The title of Lundberg's 1947 book - Can Science Save Us? - says it all. This positivistic approach to social science can be traced to the French philosopher Auguste Comte (1798-1857), who called for a "social physics" that could claim its place alongside celestial, terrestrial, mechanical and chemical physics.
But the impulse to identify natural laws of society is, in fact, much older. Plato may have been the first to hint at it, and the Roman writer Cicero in the second century BC believed in laws that transcended the customs and particularities of individual nations and which would apply to societies everywhere at all times.
The physicists today who seek rules that govern traffic or market economies have inherited this tradition - whether they know it or not. Implicit in their models and equations is the assumption that despite the quirks and caprice of individual human nature, there are emergent universal properties and laws that describe these complex systems.
It is no coincidence that this echoes the notion of universality in statistical physics. Phenomena that appear at first to be unconnected, such as magnetism and the phase changes of liquids and gases, share some identical features. This universal behaviour pays no heed to whether, say, the fluid is argon or carbon dioxide. All that matters are broad-brush characteristics such as whether the system is one-, two- or three-dimensional and whether its component elements interact via long- or short-range forces.
Universality says that sometimes the details do not matter.
The basic idea is simple: we replace the atoms of conventional statistical mechanics by people. Of course, while atoms interact via well defined forces of attraction and repulsion, people are seldom so straightforward. But in some situations human interactions do not amount to very much more than this basic concept. For example, by avoiding collisions and not encroaching on one another's "personal space", we act just as though there was a repulsive force between us.
Modern physical models of social phenomena are not really imposing some deterministic tyranny on human actions. Rather, they are simply acknowledging that in reality our choices are often extremely limited. However much we treasure a belief in free will, social norms and conventions exist partly to reduce the need to make choices in the first place. People within a culture dress similarly, eat the same kinds of food and use the same words. We do not question whether drivers have free will simply because they predictably follow one another down the motorway at more or less the same speed. And in an election we do not exercise our free will by voting for our grandmother - we vote for one of the handful of names on the ballot sheet. Statistical physics does not prescribe which way our mental "compass needle" points. It merely asserts that the choice of orientations is limited, and that this choice is typically influenced by our neighbours.
A physics of society cannot tell us how things should be, but it can hopefully elucidate the consequences of particular choices and policies. Physicists would be right to be wary of constructing a "utopia theory", but historian Richard Olson explains the role social physics could serve: "One way of expressing the relationship between physical and moral laws... is to say that social systems are 'softly' deterministic. Left alone, they will inevitably develop along certain lines; but the possibility of changing those lines by conscious and intentional intervention does exist. The whole point of a 'social science', then, is to explore the opportunities for and likely consequences of intentional moral action. Without the science, morality is blind; but without the morality, science is useless, pointless, and paralytic."
In comparison with the moral questions, the physics seems to be the easy part. As economics Nobel laureate Herbert Simon puts it: "We know that going to the Moon was a simple task indeed, compared with some others we have set for ourselves, such as creating a humane society or a peaceful world."