Introduction

Hervé ZWIRN1,2

1Institut d'Histoire et de Philosophie des Sciences et des Techniques, Paris, France

2École Normale Supérieure de Paris-Saclay, Centre Borelli, France

In recent years, contemporary physics has made spectacular progress in both infinitely large and infinitely small realms. The aim of this book is to share with the reader some of these discoveries, presented from a generalist and sometimes philosophical angle. The aim is not to present things in a technical way, which is only accessible to specialists, but rather to adapt the level so that the chapters are comprehensible, at least for the most part, by most readers with scientific knowledge at the first-year university level.

The chosen style of presentation is comparable to that of high-caliber popular magazines such as Scientific American (Pour la Science). I would like to thank the authors who have contributed to this volume and have made the educational effort to present their discipline in a way that is accessible to as many readers as possible. However, not all of the chapters in this book are equal in terms of ease of reading; some require slightly more effort from readers than others. This is mainly due to the diversity of the scientific fields covered.

Some chapters present recent observations that shake up what we thought we knew, such as how the universe was formed and evolved. Other chapters look at scientific questions that are currently unresolved. Today's leading scientists are currently so hard pressed to solve them that they are torn between whether answers can be found within the framework of current theories or whether, on the contrary, it will be necessary to completely revolutionize our way of thinking in order to do so.

Understandably, the effort required to understand each subject will have to be adapted to its intrinsic difficulty.

I would particularly like to thank Nathalie Palanque-Delabrouille for her careful proofreading and correction of the chapters in the cosmology section.

The subjects covered are quantum physics and cosmology, which make up the two main subjects of this book. Of course, the aim is not to be exhaustive - that would take more than one volume. Instead, we will focus on certain aspects of these disciplines that have recently undergone major advances or that are simply of particular interest. Many topics will not be covered here due to insufficient space, but readers who have been captivated by the stimulating chapters in this book will be able to continue their journey through the many sources available, both in specialized books and on the Internet.

In 2025, we celebrated the centenary of the discovery of quantum mechanics. Quantum mechanics describes the behavior of atoms and elementary particles and is one of the most fertile and best corroborated physical theories. First and foremost, it is fertile because it explains many phenomena that classical physics has failed to understand, such as the stability of matter, the color of bodies, black-body radiation, the photoelectric effect, the behavior of semiconductors, superfluidity and superconductivity. Without it, many of our everyday objects would never have been invented, starting with our smartphones and computers. Second, it is fertile because it is the basis of quantum field theory, which is the foundation on which the standard model of particle physics rests today. This model's success no longer needs to be demonstrated, including the prediction of the existence of the W and Z intermediate bosons (mediators of the electroweak interaction) and the Higgs boson (involved in the mechanism that gives particles their mass), all of which were discovered at CERN. This is corroborated by the fact that, since its creation, it has been subjected to a staggering number of tests in all kinds of circumstances and has never been contradicted by any experiment.

Paradoxically, however, despite this success, quantum mechanics is still subject to fierce debate in regard to understanding what it tells us about the world. Physicists all agree on how to use its equations to predict the outcome of performed experiments but disagree on the ontological significance of formalism. What does quantum mechanics tell us about reality? Unlike classical physics, which can be understood directly as describing the world as it is, quantum mechanics poses serious conceptual problems when we attempt to approach it in the same way. Of course, quantum mechanics puts forward many strange properties, such as indeterminism, the superposition of states and the incompatibility of certain properties. However, the most serious problems are the so-called measurement problem, for which no consensual solution has yet been found. The various proposals for solving this problem result in what are known as "interpretations", which often give incompatible views of the world. The problem is that, to date, there has been no experimental evidence to enable us to settle between them.

To set the scene and enable the reader to situate the debates in question, Hervé Zwirn's overview chapter (Chapter 1) briefly presents the problem of measurement, the reasons for these debates and the main interpretations to which they have given rise.

Thomas Ryckman analyzes the position of Niels Bohr, who, along with Werner Heisenberg, is considered the main author of the so-called "Copenhagen interpretation". This interpretation, which for a long time has remained the overwhelming majority view among physicists and is still highly influential today, is not monolithic. The positions of Heisenberg and Bohr differ in many subtle respects. However, beyond these divergences, Bohr's thought itself has often been described as obscure, and it is not easy to obtain a clear idea of what Bohr thought about reality. This is the subject of Chapter 2.

Lev Vaidman described the multiple worlds interpretation (MWI), which was first invented by Hugh Everett in 1957. This interpretation, which was later popularized by DeWitt and Graham in 1973, has been widely mentioned for its extraordinary character, which has led some to describe it as science fiction. Indeed, the concept of parallel worlds inevitably brings to mind films of this nature. However, as it may seem, this way of solving the problem of measurement is not absurd and is supported by many advocates. Nevertheless, there are several ways of understanding Everett's position which are not compatible with each other. Lev Vaidman describes his personal version here (Chapter 3) after recalling the basic elements common to all versions.

Jeffrey Barrett focuses on how to make sense of the concept of probability in Everett's interpretation. Indeed, one of the main criticisms of this interpretation is that it is difficult to reconcile with the usual probabilistic approach to quantum mechanics. Since all possible events occur during a measurement, giving rise to as many distinct worlds as there are possible outcomes, it seems that the notion of probability disappears. Various attempts to solve this problem have been proposed but have failed to win the support of physicists. Jeffrey Barrett (Chapter 4) analyzes some of these attempts in detail and arrives at a conclusion in favor of one of them.

Pragmatism is one possible approach to the problems of interpretation; this was originally coined by Charles Peirce, John Dewey and William James. When applied to quantum physics, the wave function that is supposed to describe the state of a system in quantum mechanics does not actually have this role. More generally, for pragmatists, a scientific theory does not describe the world as it is but is intended to guide an agent in estimating the probabilities of a given outcome occurring in the course of the agent's experiments. In Chapter 5, Richard Healey presents his pragmatist position and how it solves the measurement problem and avoids introducing non-locality into Einstein-Podolski-Rosen-type experiments.

It seems that if we stick to the standard formalism of quantum mechanics without modifying it, it is difficult to solve the problem of measurement without making the observer play a central role. This is the approach taken by Hervé Zwirn, who, in Chapter 6, presents convivial solipsism (ConSol). This interpretation, which he has developed, considers that no modification of the system's physical state takes place during a measurement. In this view, a measurement is essentially an act of perception by an observer. This way of looking at measurement solves the problem posed in the traditional framework and provides a number of solutions to other problems, such as non-locality.

ConSol is part of a large family of interpretations that David Schmid, Yile Ying and Matthew Leifer describe as "Copenhagenish", in that they are related to, but distinct from, the Copenhagen interpretation. These interpretations have in common that they consider the result an observer obtains from a measurement to be unique. They also consider that quantum mechanics applies to all physical systems on any scale, quantum mechanics is a complete theory that cannot be supplemented by hidden variables and the state vector of a system does not directly represent its physical state. In Chapter 7, the authors compare different interpretations of this type and analyze how they explain Wigner's friend experiment (a brief description of which can be found in Chapter 1).

At the other end of the dimensional spectrum, in the infinitely large realm, the profound...