Seasons change, time passes. What would we do without it? Much as we might like to, we cannot rewind or fast forward the tape of our lives. The past is behind us, and the future is yet to come. There is a powerful sense that time flows, and does so at a steady pace, and we assume that it will do this throughout the Universe. The Hands of TimeTime goes only one way; it seems to be asymmetric, or lopsided. This lopsidedness affects many objects and events that we experience in the physical world. Physicists like to call this asymmetry in time 'the arrow of time', meaning a sort of cosmic signpost pointing in a single direction.
When people first looked round for some means of measuring time, they thought in terms of some material that flowed, just as time itself flowed (or so they thought). Popular options were water and sand, each of which could be persuaded to flow at a more or less steady rate. However, clocks using such materials were never very accurate – there is nothing that can flow evenly enough to measure time accurately. The breakthrough came when people stopped thinking in terms of flow and instead turned to systems with a so-called periodic or repetitive motion. Using a periodic system to measure time simply involves counting the number of oscillations, or cycles. Christiaan Huygens (1629-95) first devised a method to use a swinging pendulum to drive the hands of a clock. Pendulum clocks are not reliable enough to meet today's high standards of timekeeping. But our most advanced clocks still depend on counting oscillations to keep time.
The physics of Newton and of Einstein work fine for everyday timescales such as we find on the Earth and the vast timescales of the Universe respectively. But once physicists started to think about what happens inside an atom, they realised they needed a whole new physics to explain it. Understanding what goes on at this level has led to an explosion of new technologies, These include most modern electronics and, of course, the atomic clocks that today provide the world standard of accuracy in timekeeping. Now people are predicting that in the future they will be able to exploit the odd behaviour of the quantum world to create computer processors that operate at light speed.
On 11 August 1999, everyone knew that a total eclipse of the Sun was going to take place and exactly where you had to be if you wanted to experience it. It had been predicted by astronomers for decades. It is thanks to the work of Isaac Newton that the movements of the Sun, Moon and Earth are known so predictably. In fact for a long time after Newton, people thought that everything in the Universe was just as predictable. But the Second Law of Thermodynamics put a bit of a spanner in the works of the clockwork universe. Everything, the Second Law insists, must gradually become more disordered. Anything that actively increases order, such as spending half an hour tidying the house, has the inevitable consequence that disorder will increase elsewhere in the Universe. You generate heat by working, as does the power station generating electricity for the vacuum cleaner. This ever-increasing disorder, or entropy, is inescapable.
The idea of travelling forward into the future or back into the past has always fascinated science fiction writers. The 'grandfather paradox' is the argument many people use to suggest that time travel is impossible. What if you went back in time and prevented your grandfather from meeting your grandmother so that your mother was never born? Then you would never have been born . . . and so on. Until very recently such arguments led most scientists to believe that time travel could never exist outside science fiction. But amazingly, some interpretations of the weirdness of the quantum world now suggest that time travel is possible - at least in theory.