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.
