What is the peculiarity of the physicist and chemist point of view on matter ? Why do they refuge in mysterious laboratories, which the common run of people cannot access ? Moreover, their explanations of the behaviour of everyday matter are often obscure... Why is physics so difficult to understand ? And what is then the use of writing (and, yet worse, reading !) another popularisation book ?

One reason is that physics can help understanding not only the infinitely big or infinitely small worlds, which are as fascinating as abstract : it can also explain many facets of our everyday world. But I hope to do more in this book ! First by showing how the scientific understanding of our world was progressively built and eventually considered matter as a machine obeying the strict rules of mathematics. Next, and this is the heart of the book, by showing what happens behind the scenes, the way the physicists perfect their theories, their vision of the world. After all, physics does not come from heavens : ordinary men and women build it everyday ! After this guided tour, I hope that the reader will understand better what is the peculiarity of the physicist's vision of matter, why physicists use atoms to understand it and why they need laboratories.

Let's take an egg white, which is liquid, and mix it to air by whisking : who could have guessed that the combination of two fluids produces quite a solid mousse ? Certainly not the physicists and their atoms ! Watch a leaking tap : a small drop forms and grows, until it detaches from the rest of the liquid. How can one explain this behaviour ? And why another liquid like honey is much more reluctant to detach, producing those runs which greatly complicate our breakfast ?

These are not the only puzzling behaviours of matter. Why is a solid solid ? Because its atoms are tough ? It is not that simple... How can one explain that some hammers are more resistant (and more expensive !) than others ? Solids often do not deserve their name : why can they be deformed, broken, and why do they behave so differently in that respect ? We are afraid of breaking a glass but no one cares to drop a key... How does glue manage to stick ? We will see that physicists have found simple explanations for all these phenomena.

Ice melts and the obtained water evaporates when one further heats : we all learnt at school the classic trilogy solid-liquid-gas. But how can one explain these changes ? What is heat ? How can one understand that other solids, like butter or chocolate, melt when they are heated but burn or turn black afterwards ? What about the classic trilogy ? Where can one classify purée, flour or toothpaste, all these familiar substances which do not flow like liquids but are not really solids either ? Who said that matter, even inert, was stupid, predictable, uninteresting ?

Most of the preceding questions are as old as mankind. People have always been faced to matter, and have therefore sought to understand the reasons of these puzzling behaviours. What kind of explanations were proposed ? There exists, in the West, two main traditions. First, the "vitalist" vision, which relies on the living world and has amply dominated the societies until now. Alchemy represents its last appearance within the scientific community, and we will see that it does not deserve the scorn that it usually gets. The second main tradition considers instead that matter is essentially inert, life being due to a happy chance or to a divine intervention. It is the atomistic vision, which is the only one accepted today among the scientists. It defeated the "vitalist" one with difficulty for reasons we will understand. Indeed, the scientific community only accepts these tiny particles since the beginning of the 20th century. Why did scientists prefer the atomistic tradition ? This review of the historical visions of matter, even if inevitably incomplete and simplistic, should allow us to understand the specificity of the physicist's approach.

In what consists this specificity ? For physicists, the properties of all the materials can (and shall !) be explained by the combination of the constituent atoms. For example, they say that a solid does not break because its atoms hold together well. I will give concrete examples of how the atomistic vision of matter helps explaining many puzzling properties of matter. Still : isn't it unbelievable that less than a hundred kinds of atoms suffice to explain the world ? How did physicists manage in practice to find out these atoms ? What kind of tricks did they use ? Also very important : to what extent can these atoms help the non-scientists in understanding their everyday world ? For we shall see that the scientific interpretation of matter is far from neutral, and that it can only be relevant and efficient within our technological world. Atoms are as useless as cars for Amazon Indians...

This book is divided in three parts that can be read independently, depending on the reader's mood and interest. The first part corresponds to the reductionist approach and starts with everyday matter to reach the atoms. It begins (chapter 1) by a description of some of the amazing behaviours of matter. I also present the personality of several materials as seen by artists and the complex networks in which our stuff is inserted : do we always realize that the entire world is brought into play to allow for our daily coffee cup ? In the second chapter, I introduce some historical elements about the approaches that mankind used to understand matter. The present explanations are introduced in some detail in the second part of the book. We travel along the same path as in the first part but in the inverse direction : from perfectly known atoms, we "reconstruct" our everyday matter to explain its general properties (chapter 3) and also the strange behaviours pointed out in the first chapter (chapter 4).

The third part of the book starts from the daily work of physicists to focus on a more critical examination of what they mean by "understanding matter". This concerns first their experimental practices. In chapter 5, I try to understand why laboratories are so important for physics. We shall see that physicists need them to simplify, to "digest" matter and render it understandable by their theories. Then, theoreticians make use of a few intellectual "tools" which eventually become glasses through which they see the world. Those tools are presented in chapter 6, which is therefore slightly technical and can be tackled rapidly in a first reading. It should be interesting for students who will understand the meaning of several concepts that are often introduced without much justification in the classroom (for example, the "free" electrons, which are actually strongly interacting with their surroundings). I hope that this study throws light on the peculiar way physicists understand matter, and also on their successes, their a priori and their failures. The last chapter discusses them and the specific ideology that the physicist's approach implies : it can be read independently from the rest of the book.

Is this unusual point of view on physics helpful to understand the scientific approach of matter ? Let the reader decide !