From the 1999 lecture programme:
When the Royal Institution was founded two hundred years ago, people thought that the Universe ran like a reliable, well-oiled clock. As a result of the pioneering discoveries of Sir Isaac Newton, the Universe was viewed as having a regular, almost predictable structure. Only a few inconsistencies seemed to remain between the predictions of Newton's theory and the observed motion of the planets.
However, one hundred years later Einstein astounded the world with his theory of relativity showing how space and time are in fact linked and that Newton's theory is, at best, just a working approximation to the truth. Einstein's work showed that time is relative as opposed to being absolute, and that nothing can travel faster than the speed of light. The cosy picture of a clockwork Universe and of universal time immediately fell apart.
However, Einstein could not answer the question as to why time only moves forwards. Nor could he establish whether time has a beginning and an end. To have any hope of answering such questions, we need to deepen our understanding of the role of time in the Universe.
A simple, yet fundamental law of physics tells us that the Universe and its contents will becomes increasingly disordered as time moves on. On the other hand, structures appear in the Universe which seem to possess some kind of order, even though they arose out of apparent chaos.
So is there a more fundamental principle at work in the Universe? If the basic laws of physics are simple, why is the Universe so complex?
BBC / Royal Institution