The classical methods of analysis, such as outlined in the previous section on Newton and differential equations, have their limitations. For example, differential equations describing the motion of the solar system do not admit solutions by power series. Ultimately, this is because the dynamics of the solar system is too complicated to be captured by such simple, well-behaved objects as power series. One of the most important modern theoretical developments has been the qualitative theory of differential equations, otherwise known as dynamical systems theory, which seeks to establish general properties of solutions from general principles without writing down any explicit solutions at all. Dynamical systems theory combines local analytic information, collected in small “neighbourhoods” around points of special interest, with global geometric and topological properties of the shape and structure of the manifold in which all the possible solutions, or paths, reside—the qualitative aspect of the theory. (A manifold, also known as the state space or phase space, is the multidimensional analog of a curved surface.) This approach is especially powerful when employed in conjunction with numerical methods, which use computers to approximate the solution.

The qualitative theory of differential equations was the brainchild of the French mathematician Henri Poincaré at the end of the 19th century. A major stimulus to the development of dynamical systems theory was a prize offered in 1885 by King Oscar II of Sweden and Norway for a solution to the problem of determining the stability of the solar system. The problem was stated essentially as follows: Will the planets of the solar system continue forever in much the same arrangement as they do at present? Or could something dramatic happen, such as a planet being flung out of the solar system entirely or colliding with the Sun? Mathematicians already knew that considerable difficulties arise in answering any such questions as soon as the number of bodies involved exceeds two. For two bodies moving under Newtonian gravitation, it is possible to solve the differential equation and deduce an exact formula for their motion: they move in ellipses about their mutual centre of gravity. Newton carried out this calculation when he showed that the inverse square law of gravitation explains Kepler’s discovery that planetary orbits are elliptical. The motion of three bodies proved less tractable—indeed, nobody could solve the “three-body problem”—and here was Oscar asking for the solution to a ten-body problem (or something like a thirty-body problem if one includes the satellites of the planets and a many-thousand-body problem if one includes asteroids).

Undaunted, Poincaré set up a general framework for the problem, but, in order to make serious progress, he was forced to specialize to three bodies and to assume that one of them has negligible mass in comparison with the other two. This approach is known as the “restricted” three-body problem, and his work on it won Poincaré the prize.

Ironically, the prizewinning memoir contained a serious mistake, and Poincaré’s biggest discovery in the area came when he hastened to put the error right (costing him more in printing expenses than the value of the prize). It turned out that even the restricted three-body problem was still too difficult to be solved. What Poincaré did manage to understand, though, was why it is so hard to solve. By ingenious geometric arguments, he showed that planetary orbits in the restricted three-body problem are too complicated to be describable by any explicit formula. He did so by introducing a novel idea, now called a Poincaré section. Suppose one knows some solution path and wants to find out how nearby solution paths behave. Imagine a surface that slices through the known path. Nearby paths will also cross this surface and may eventually return to it. By studying how this “point of first return” behaves, information is gained about these nearby solution paths. (See the illustration of a Poincaré section.)

Today the term chaos is used to refer to Poincaré’s discovery. Sporadically during the 1930s and ’40s and with increasing frequency in the 1960s, mathematicians and scientists began to notice that simple differential equations can sometimes possess extremely complex solutions. The American mathematician Stephen Smale, continuing to develop Poincaré’s insights on qualitative properties of differential equations, proved that in some cases the behaviour of the solutions is effectively random. Even when there is no hint of randomness in the equations, there can be genuine elements of randomness in the solutions. The Russian school of dynamicists under Andrey Kolmogorov and Vladimir Arnold developed similar ideas at much the same time.

These discoveries challenged the classical view of determinism, the idea of a “clockwork universe” that merely works out the consequences of fixed laws of nature, starting from given initial conditions. By the end of the 20th century, Poincaré’s discovery of chaos had grown into a major discipline within mathematics, connecting with many areas of applied science. Chaos was found not just in the motion of the planets but in weather, disease epidemics, ecology, fluid flow, electrochemistry, acoustics, even quantum mechanics. The most important feature of the new viewpoint on dynamics—popularly known as chaos theory but really just a subdiscipline of dynamical systems theory—is not the realization that many processes are unpredictable. Rather, it is the development of a whole series of novel techniques for extracting useful information from apparently random behaviour. Chaos theory has led to the discovery of new and more efficient ways to send space probes to the Moon or to distant comets, new kinds of solid-state lasers, new ways to forecast weather and estimate the accuracy of such forecasts, and new designs for heart pacemakers. It has even been turned into a quality-control technique for the wire- and spring-making industries.