★This is the first volume in the series which traces the development of human thought and experiments on “the infinitely small” from the early Greek school of philosophy to the late 1960s. The focus is mostly on the beginning of the twentieth century when discoveries revolutionized the way matter was interpreted. At first, physicists experimented with methodologies based on natural radioactivity; later they exploited cosmic rays, and from 1950s they built accelerators that delivered particles in a controlled manner.

★In Volume II, we will talk about accelerators and detectors. After the 1960s, more and more versatile accelerators were built with higher energy and higher intensity of acceleration. More sophisticated techniques were exploited to control particles and arrive at collisions, which was called head-on collision in the automotive field. In addition, new detectors replaced bubble chambers within a decade. New technology was called electronics, and that is why we will talk about electronic devices and how information is collected and channeled to a computer. It will also be necessary to understand how a particle behaves when it passes through a detector and how the electrical signal that reveals its passage is formed. To replace the information that can be extracted from a particle passing through a bubble chamber, various detectors must be assembled efficiently.

★In Volume III, we will see how, guided by enlightenment thought, pieces of the puzzle will form a simple description of matter and the forces that hold it together. Two ingredients are needed to obtain and consolidate knowledge: experiments and theory. Both are equally important; without the skills and means to obtain the information about how nature behaves, there is no way to synthesize it into a theory. Conversely, without the skills to see inside the results of experiments and to interpret what nature reveals to us, there is no complete achievement. Throughout the three volumes, an attempt is made to present both experiments and theory despite in the experiment-oriented tendency.

Humanity's curiosity has been a powerful driving force to the development of knowledge. Among the most important and enduring questions developed through the ages are those concerning the infinitely small and the infinitely large.

Children always ask: if I keep cutting, for example, a cookie, that is, I cut it in two, then I take one of the two pieces and cut it again and so on, what will I find in the end (other than hurting my fingers)? After I cut it in two for one billion times? Children also ask: what is beyond the stars? Where does it all end? Where does the Universe end? These simple questions have plagued the human mind since time immemorial.

This book traces in a simple way the achievements of human thought in answering the question of the infinite small, namely: what can be found by going deeper and deeper into matter, or, to be more precise, “what is the structure of matter?” From the ancient Greeks' view of the atom, we arrive at the models of subatomic particles that physicists create today, all with great attention to the experimental dimension of the discipline.

Flavio P. Marchetto is a member of the National Institute of Nuclear Physics, Turin section, at which, he has worked for several decades as a researcher in subatomic physics. His research has been mainly in the field of fundamental physics with experiments based at CERN in Geneva and FermiLab in Chicago.

Over the past two decades, he has developed detectors for medical physics, especially for radiation and hadron therapy.

A science popularizer and lecturer, he teaches at the University of the Third Age in Turin.

Introduction 7

Chapter 1
From the ancient Greeks to the eighteenth century 12
1.1 Greek philosophers and matter 12
1.2 Galileo and the experimental method 14
1.3 Eighteenth century: atomic theory 15
1.4 Beginnings of chemistry 18
Dalton's theory 23
1.5 Summary of the period between the eighteenth century and the end of the nineteenth century 34
Summary 36

Chapter 2
The revolution of the early twentieth century 37
2.1 Introduction 37
2.2 Thomson's model of the atom 38
2.3 Rutherford experiment 40
How much space does an atom occupy? 41
2.4 Interpretation of Rutherford's experiment 46
Some considerations on Rutherford's experiment 51
2.5 The electric charge in the atom 57
2.6 Summary of the years 1900 to 1920 60
Summary 65

Chapter 3
Special relativity and quantum mechanics 66
3.1 Introduction 66
3.2 Restricted relativity 67
3.3 Quantum mechanics 79
Summary 87

Chapter 4
The discovery of the neutron and a look at the atom 88
4.1 Introduction 88
4.2 The discovery of the neutron 90
4.3 A recipe for building atoms 98
Summary 100

Chapter 5
Bohr's atom 101
5.1 Introduction 101
5.2 The atom according to Bohr 102
Summary 109

Chapter 6
First discoveries with cosmic rays 110
6.1 Introduction 110
6.2 First detectors: the fog chamber 112
6.3 Discovery of the positron 118
6.4 Discovery of the muon 121
6.5 Discovery of the pion 123
Summary 129

Chapter 7
The hypothesis of the existence of the neutrino and its discovery 130
7.1 Introduction 130
7.2 Law of conservation of energy 131
7.3 Calculation of the energy of a particle 133
An example of calculation 136
7.4 Law of conservation of momentum 140
7.5 Assumptions about the existence of the neutrino 146
7.6 The discovery of the neutrino 152
Summary 162

Chapter 8
Detectors and accelerator experiments: the beginnings 163
8.1 Introduction 163
8.2 Accelerator: how does it work? 164
8.3 Detectors 172
The stereoscopic view 184
8.4 How was a bubble chamber experiment performed? 186
8.5 Beam extraction and photographs 189
Summary 193

Chapter 9
The discovery of particles up to the 1960s 194
9.1 Introduction 194
9.2 How momentum and energy are measured 195
9.3 Method for the discovery of long-lived particles 197
9.4 Method for the discovery of short-lived particles 205
Summary 209

Chapter 10
The determination of some properties of particles 210
10.1 Introduction 210
10.2 What is meant by the lifetime of a particle? 210
10.3 Measurement of the average lifetime of long-lived particles 214
10.4 Measurement of the average lifetime of short-lived particles 219
10.5 Determination of some of the quantum numbers 223
10.6 Matter and antimatter 226
Summary 230

Chapter 11
Late 1960s: a general summary 231
11.1 Classification of particles 231
11.2 Production of particles 233
11.3 Particle decays 235
11.4 Organization of experiments 238
Summary 243

Chapter 12
Forces in nature 244
12.1 Introduction 244
12.2 Types of forces 247
12.3 Summary of the characteristics of forces 257
Summary 262

Introduction to forthcoming volumes 263
Appendix A
Symbols used in the book 266

Appendix B
Units of Measurement 268
B.1 Units of measurement of energy 270
B.2 Gauss and tesla 272
B.3 Prefixes of units of measurement 273

Appendix C
Electromagnetic radiation 276
C.1 Wave-corpuscle duality 280
C.2 Frequency and wavelength 282
C.3 The characteristics of the photon 285

Appendix D
Distribution of a magnitude 287

Appendix E
Electric potential and magnetic dipole 290
E.1 Electric potential 290
E.2 Magnetic dipole 294

Chronology 298

Glossary 303