The story of Sadi Carnot is captivating - the young military engineer with no hope of advancement on account of a notorious father, who established wonderfully general results by pure thought. A genius who died before anyone understood his work. Almost as remarkable is that of brewer James Joule whose hobby was experimental physics, and for almost a decade presented demonstrations that heat is a form of energy to the British Association, only to met with disbelief. Fortunately, three very young professors did take Joule's point and the science of `thermodynamics' was born. It revolutionised chemistry and facilitated the rapid growth of chemical industries in the second half on the 19th century.

The fundamental laws of physics are almost all time reversible: if something changes from A to B, the laws permit B to change to A. Yet our everyday experience is of irreversible events: spilt milk will never jump back into the jug; a bullet will never fly back into the muzzle of the gun; an apartment block shattered by a missile will never pull back together and send the missile back to the truck that launched it. Entropy explains why in practice these processes go in only one direction. In principle they could go backwards, but the probability that they will do so is absurdly small.

In everyday life things settle down once they have reached the state that maximises their entropy given their energy: tea goes cold; things slid on a table come to rest; ripples in a tub of water fade away. What keeps life and the Universe going is a remarkable property of gravity: a system bound by gravity can always achieve higher entropy on a fixed energy budget. This principle drives the evolution of the Sun and of our Galaxy.

I hope intelligent readers of all ages and occupations will gain an understanding of a sublimely beautiful and powerful idea. I hope they will start  to look at the world around them from an entropic perspective, and to wonder how a mathematical measure of ignorance can so often seem an intrinsic property of matter. I hope they will enjoy reading about the remarkable men who uncovered the existence of entropy and gain insight into how science develops: through steps of varied length, sometimes backwards, always amid doubt and debate.  I hope the chapters on quantum physics and black holes will give insight into how the scientific community continues to stumble forward on the research frontier.

Our civilisation is now totally dependent on advanced technologies, which have at their base concepts such as energy and entropy that are very poorly understood by the great majority of people. This is a shame because these are wonderful ideas that enrich your appreciation of what's going on around you. There's poetry in them as well as power.