“It turns out that our solar system is, in fact, chaotic,” he says.Researchers investigating the long-term motions of the planets have made some surprising discoveries One of the first bodies they scrutinised was Mars. This, explains Murray, is the kind of behaviour that shows up with the Sun and panets over many millions of years. Sometimes it seems to stand still; a few seconds later it is spinning around like Roger Daltrey’s microphone at a Who concert. When he gives them a bigger push, however, the interplay between the two rods takes over.
The upper one still swings to and fro, but the lower rod is now wildly out of control. For a demonstration, Murray turns to a simple apparatus clamped to his bookshelf. A metal rod swings from a hinge; at the bottom end, a second rod is attached via another hinge – a kind of double-jointed pendulum. When he gives the rods a small push, they swing back and forth. Over millions of years, however, those predictions start to fail.”Even a simple system can have complicated behaviour,” says Norm Murray of the Canadian Institute for Theoretical Astrophysics (CITA) in Toronto. (You can think of these computer programs as the digital equivalent of the old-style brass orreries, used as educational tools in the 18th century, in which a system of gears drove a tiny model of the solar system.) At first, the forecasts of these computer programs is indeed accurate. By feeding the positions and speeds of each object into the computer, and simulating the gravitational force acting between them, they can model systems like our solar system over thousands or even millions of years.
The case of the solar system would appear to be simpler: after all, the only force acting over these vast distances is gravity and, as Newton showed, the strength of that force is easily worked out for any pair of massive bodies. But when more than two bodies are involved, the picture gets messier.
To tackle what the physicists call the “many-body problem”, they’ve turned to computers. In its place, a more uncertain solar system has emerged – one in which long-term predictions are impossible. According to chaos theory, even the slightest uncertainty in initial conditions can have far-reaching effects on the future state of a physical system.
No matter how well we know the starting positions, the end result may be impossible to predict. The classic example is in weather forecasting, where the flap of a butterfly’s wing in Brazil, it is said, can result in a storm in China. With the advent of chaos theory, however, this traditional view has start-ed to change. Using sophisticated computer programs to simulate the motions of the Sun and planets, astronomers have shattered the idea of the “clockwork universe” that Isaac Newton envisioned. FEW THINGS in our universe seem as predictable as the planets.