### Fashion, Faith and Fantasy in the New Physics of the Universe

*By Roger Penrose*

**Princeton University Press**

Also available at the CERN bookshop

The well-known mathematician and theoretical physicist Roger Penrose has produced another popular book, in which he gives a critical overview of contemporary fundamental physics. The main theme is that modern theoretical physics is afflicted by an overdose of fashion, faith and fantasy, which supposedly has led recent research astray.

There are three major parts of the book to which these three f-words relate, corresponding one-to-one with some of the most popular research areas in fundamental physics. The first part, labelled “fashion”, deals with string theory. “Faith” refers to the general belief in the correctness of quantum mechanics, while “fantasy” is the verdict for certain scenarios of modern cosmology.

The book starts with an overview of particle physics as a motivation for string theory and quickly focuses on its alleged shortcomings, most notably extra dimensions. Well-known criticisms, for instance linked to the multitude of solutions (“landscape of vacua”) of string theory or the postulate of supersymmetry, follow in due course. This material is mostly routine, but there are also previously unheard of concerns such as the notion of “too much functional freedom” or doubts about the decoupling of heavy string states (supposedly excitable, for example from the orbital kinetic energy of Earth).

Next the book turns to quantum mechanics and gives an enjoyable introduction to some of the key notions, such as superposition, spin, measurement and entanglement. The author emphasises, with great clarity, some subtle points such as how to understand the quantum mechanical superposition of space–times. In doing so, he raises some concerns and argues – quite unconventionally – that, to resolve them, it is necessary to modify quantum mechanics. In particular he asks that the postulate of linearity should be re-assessed in the presence of gravity.

The fantasy section gives an exposition of the key ideas of cosmology, in particular of all sorts of scenarios of inflation, big bang, cyclic universes and multiverses. This is all very rewarding to read, and particularly brilliant is the presentation of cosmological aspects of entropy, the second law of thermodynamics and the arrow of time. I consider this third section as the highlight of the book. The author does not hide his suspicion that many of these scenarios should not be trusted and dismisses them as crazy – while saying, as if with a twinkle in the eye: not crazy enough!

There is a brief, additional, final section that has a more personal and historical touch, and which tries to make a case for Penrose’s own pet theory: twistor theory. One cannot but feel that some of his resentment against string theory stems from a perceived under-appreciation of twistor theory. In particular, the author admits that his aversion to string theory comes almost entirely from its purported extra dimensions, whereas twistors work primarily in four dimensions.

This touches upon a weak point of the book: the author argues entirely from the direction of classical geometry, and so shares a fixation with extra dimensions in string theory with many other critics. What Penrose misses, however, is that these provide an elegant way to represent certain internal degrees of freedom (needed matter fields). But this is by no means generic – on the contrary, most string backgrounds are non-geometric. For example, some are better described by a bunch of Ising models with no identifiable classical geometry at all, so the agony of how to come to grips with such “compactified” dimensions turns into a non-issue. The point is that due to quantum dualities, there is, in general, no unambiguous objective reality of string “compactification” spaces, and criticism that does not take this “stringy quantum geometry” properly into account is moot.

Somewhat similar in spirit is the criticism of quantum mechanics, which according to Penrose should be modified due to an alleged incompatibility with gravity. Today most researchers would take the opposite point of view and consider quantum mechanics as fundamental, while gravity is a derived, emergent phenomenon. This viewpoint is strongly supported by the gauge-gravity duality and its recent offspring in terms of space–time geometry arising via quantum entanglement.

All-in-all, this book excels by covering a huge range of concepts from particle physics to quantum mechanics to cosmology, presented in a beautifully clear and coherent way (spiced up with many drawings), by an independent and truly deep-thinking master of the field. It also sports a considerable number of formulae and uses mathematical concepts (like complex analysis) that a general audience would probably find difficult to deal with; there are a number of helpful appendices for non-experts, though.

Thus, *Fashion, Faith and Fantasy in the New Physics of the Universe* seems to be suitable for both physics students and experienced physicists alike, and I believe that either group will profit from reading it, if taken with a pinch of salt. This is because the author criticises contemporary fundamental theories through his personal view as a classical relativist, and in doing so falls short when taking certain modern viewpoints into account.

• *Wolfgang Lerche, CERN*

### Ripples in spacetime

*By Govert Schilling*

**The Belknap Press of Harvard University Press**

In February 2016 the LIGO and Virgo collaborations announced the first detection of gravitational waves from the collision of two black holes. It was a splendid result for a quest that started about five decades ago with the design and construction of small prototypes of laser interferometers. Since this first discovery, at least five other binary black-hole mergers have been found and gravitational waves from two colliding neutron stars have also been detected. Gravitational-wave science is now booming, literally, and will continue to do so for a long time. The upcoming observational progress in this field will impact the development of astrophysics, cosmology and, perhaps, particle physics.

Govert Schilling is an award-winning science journalist with a special interest in astronomy and space science. In this book, he guides the reader through the development of gravitational-wave astronomy, from its very origin deep in the early days of general relativity up to the first LIGO discovery. He does so, not only by delving into the key moments of this wonderful piece of history, but also by explaining the main physical and engineering ideas that made it possible.

Moreover, Schilling does a very good job discussing the scientific context in which these events and ideas arose. Far from being a mere collection of events, the book offers the reader a journey that goes beyond its title, exploring and connecting topics such as the cosmic-microwave background and its polarisation, radioastronomy and pulsars, supernovae, primordial inflation, gamma-ray bursts and even dark energy. In addition, the last few chapters of the book discuss the science that may come next, when new interferometers will join LIGO and Virgo in this adventure, observing the sky from Earth (e.g. KAGRA) and space (LISA).

The book clearly aims to target a non-specialist readership and will surely be enjoyed by people lacking a prior knowledge of astrophysics, gravitational waves or cosmology. However, this does not mean that readers more well-versed in these topics will find the book uninspiring. Schilling addresses the reader in a direct, entertaining, almost colloquial manner, managing to explain complex concepts in a few paragraphs while keeping the science sound. Besides, the book gives an interesting (and sometimes surprising) glimpse into the lives, aspirations and mutual interactions of the scientific pioneers in the field of gravitational waves.

If an objection had to be found, it would be that in the first chapter the author belittles general relativity by introducing it as “the theory behind [the movie] *Interstellar*”. If this scares you, read on and fear nothing. As always happens, science outshines fiction, and the rest of the book proves why this is so.

• *Guillermo Ballesteros, Instituto de Física Teórica UAM-CSIC, Madrid, Spain*

### Books received

### Natural Complexity: A Modeling Handbook

*By Paul Charbonneau*

**Princeton University Press**

This book aims to introduce readers to the study of complex systems with the help of simple computational models. After showing how difficult it is to define complexity, the author explains that complex systems are an idealisation of naturally occurring phenomena in which the macroscopic structures and patterns generated are not directly controlled by processes at the macroscopic level but arise instead from dynamical interactions at the microscopic level. This kind of behaviour characterises a range of natural phenomena, from avalanches to earthquakes, solar flares, epidemics and ant colonies.

In each chapter the author introduces a simple computer-based model for one such complex phenomenon. As the author himself states, such simplified models wouldn’t be able to reliably foresee the development of a real natural phenomenon, thus they are to be taken as complementary to conventional approaches for studying such systems.

Meant for undergraduate students, the book does not require previous experience in programming and each computational model is accompanied by Python code and full explanations. Nevertheless, students are expected to learn how to modify the code to tackle the problems included at the end of each chapter. Three appendices provide a review of Python programming, probability density functions and other useful mathematical tools.

### Introduction to Accelerator Dynamics

*By Stephen Peggs and Todd Satogata*

**Cambridge University Press**

This concise book provides an overview of accelerator physics, a field that has grown rapidly since its inception and is progressing in many directions. Particle accelerators are becoming more and more sophisticated and rely on diverse technologies, depending on their application.

With a pedagogical approach, the book presents both the physics of particle acceleration, collision and beam dynamics, and the engineering aspects and technologies that lay behind the effective construction and operation of these complex machines. After a few introductory theoretical chapters, the authors delve into the different components and types of accelerators: RF cavities, magnets, linear accelerators, etc. Throughout, they also show the connections between accelerator technology and the parallel development of computational capability.

This text is aimed at university students at graduate or late undergraduate level, as well as accelerator users and operators. An introduction to the field, rather than an exhaustive treatment of accelerator physics, the book is conceived to be self-contained (to a certain extent) and to provide a strong starting point for more advanced studies on the topic. The volume is completed by a selection of exercises at the end of each chapter and an appendix with important formulae for accelerator design.

### Data Analysis Techniques for Physical Scientists

*By Claude A Pruneau*

**Cambridge University Press**

Also available at the CERN bookshop

Since the analysis of data from physics experiments is mainly based on statistics, all experimental physicists have to study this discipline at some point in their career. It is common, however, for students not to learn it in a specific advanced university course but in bits and pieces during their studies and subsequent career.

This textbook aims to present all of the basic statistics tools required for data analysis, not only in particle physics but also astronomy and any other area of the physical sciences. It is targeted towards graduate students and young scientists and, since it is not intended as a text for mathematicians or statisticians, detailed proofs of many of the theorems and results presented are left out.

After a philosophical introduction on the scientific method, the text is presented in three parts. In the first, the foundational concepts and methods of probability and statistics are provided, considering both the frequentist and Bayesian interpretations. The second part deals with the basic and most commonly used advanced techniques for measuring particle-production cross-sections, correlation functions and particle identification. Much attention is also given to the notions of statistical and systematic errors, as well as the methods used to unfold or correct data for the instrumental effects associated with measurements. Finally, in the third section, introductory techniques in Monte Carlo simulations are discussed, focusing on their application to experimental data interpretation.