“Fashion, Faith, and Fantasy” seems an odd title for a book about the fundamental physics of the universe by one of the most eminent researchers in the field. But, as the author describes in mathematical detail (which some readers may find forbidding), these all-too-human characteristics play a part in what researchers may present to the public as a dispassionate, entirely rational, search for truth, unsullied by such enthusiasms. While researchers in fundamental physics are rarely blinded to experimental evidence by fashion, faith, and fantasy, their choice of areas to explore, willingness to pursue intellectual topics far from any mooring in experiment, tendency to indulge in flights of theoretical fancy (for which there is no direct evidence whatsoever and which may not be possible to test, even in principle) do, the author contends, affect the direction of research, to its detriment.
To illustrate the power of fashion, Penrose discusses string theory, which has occupied the attentions of theorists for four decades and been described by some of its practitioners as “the only game in town”. (This is a piñata which has been whacked by others, including Peter Woit in Not Even Wrong [June 2006] and Lee Smolin in The Trouble with Physics [September 2006].) Unlike other critiques, which concentrate mostly on the failure of string theory to produce a single testable prediction, and the failure of experimentalists to find any evidence supporting its claims (for example, the existence of supersymmetric particles), Penrose concentrates on what he argues is a mathematical flaw in the foundations of string theory, which those pursuing it sweep under the rug, assuming that when a final theory is formulated (when?), its solution will be evident. Central to Penrose's argument is that string theories are formulated in a space with more dimensions than the three we perceive ourselves to inhabit. Depending upon the version of string theory, it may invoke 10, 11, or 26 dimensions. Why don't we observe these extra dimensions? Well, the string theorists argue that they're all rolled up into a size so tiny that none of our experiments can detect any of their effects. To which Penrose responds, “not so fast”: these extra dimensions, however many, will vastly increase the functional freedom of the theory and lead to a mathematical instability which will cause the theory to blow up much like the ultraviolet catastrophe which was a key motivation for the creation of the original version of quantum theory. String theorists put forward arguments why quantum effects may similarly avoid the catastrophe Penrose describes, but he dismisses them as no more than arm waving. If you want to understand the functional freedom argument in detail, you're just going to have to read the book. Explaining it here would require a ten kiloword review, so I shall not attempt it.
As an example of faith, Penrose cites quantum mechanics (and its extension, compatible with special relativity, quantum field theory), and in particular the notion that the theory applies to all interactions in the universe (excepting gravitation), regardless of scale. Quantum mechanics is a towering achievement of twentieth century physics, and no theory has been tested in so many ways over so many years, without the discovery of the slightest discrepancy between its predictions and experimental results. But all of these tests have been in the world of the very small: from subatomic particles to molecules of modest size. Quantum theory, however, prescribes no limit on the scale of systems to which it is applicable. Taking it to its logical limit, we arrive at apparent absurdities such as Schrödinger's cat, which is both alive and dead until somebody opens the box and looks inside. This then leads to further speculations such as the many-worlds interpretation, where the universe splits every time a quantum event happens, with every possible outcome occurring in a multitude of parallel universes.
Penrose suggests we take a deep breath, step back, and look at what's going on in quantum mechanics at the mathematical level. We have two very different processes: one, which he calls U, is the linear evolution of the wave function “when nobody's looking”. The other is R, the reduction of the wave function into one of a number of discrete states when a measurement is made. What's a measurement? Well, there's another ten thousand papers to read. The author suggests that extrapolating a theory of the very small (only tested on tiny objects under very special conditions) to cats, human observers, planets, and the universe, is an unwarranted leap of faith. Sure, quantum mechanics makes exquisitely precise predictions confirmed by experiment, but why should we assume it is correct when applied to domains which are dozens of orders of magnitude larger and more complicated? It's not physics, but faith.
Finally we come to cosmology: the origin of the universe we inhabit, and in particular the theory of the big bang and cosmic inflation, which Penrose considers an example of fantasy. Again, he turns to the mathematical underpinnings of the theory. Why is there an arrow of time? Why, if all of the laws of microscopic physics are reversible in time, can we so easily detect when a film of some real-world process (for example, scrambling an egg) is run backward? He argues (with mathematical rigour I shall gloss over here) that this is due to the extraordinarily improbable state in which our universe began at the time of the big bang. While the cosmic background radiation appears to be thermalised and thus in a state of very high entropy, the smoothness of the radiation (uniformity of temperature, which corresponds to a uniform distribution of mass-energy) is, when gravity is taken into account, a state of very low entropy which is the starting point that explains the arrow of time we observe.
When the first precision measurements of the background radiation were made, several deep mysteries became immediately apparent. How could regions which, given their observed separation on the sky and the finite speed of light, have arrived at such a uniform temperature? Why was the global curvature of the universe so close to flat? (If you run time backward, this appeared to require a fine-tuning of mind-boggling precision in the early universe.) And finally, why weren't there primordial magnetic monopoles everywhere? The most commonly accepted view is that these problems are resolved by cosmic inflation: a process which occurred just after the moment of creation and before what we usually call the big bang, which expanded the universe by a breathtaking factor and, by that expansion, smoothed out any irregularities in the initial state of the universe and yielded the uniformity we observe wherever we look. Again: “not so fast.”
As Penrose describes, inflation (which he finds dubious due to the lack of a plausible theory of what caused it and resulted in the state we observe today) explains what we observe in the cosmic background radiation, but it does nothing to solve the mystery of why the distribution of mass-energy in the universe was so uniform or, in other words, why the gravitational degrees of freedom in the universe were not excited. He then goes on to examine what he argues are even more fantastic theories including an infinite number of parallel universes, forever beyond our ability to observe.
In a final chapter, Penrose presents his own speculations on how fashion, faith, and fantasy might be replaced by physics: theories which, although they may be completely wrong, can at least be tested in the foreseeable future and discarded if they disagree with experiment or investigated further if not excluded by the results. He suggests that a small effort investigating twistor theory might be a prudent hedge against the fashionable pursuit of string theory, that experimental tests of objective reduction of the wave function due to gravitational effects be investigated as an alternative to the faith that quantum mechanics applies at all scales, and that his conformal cyclic cosmology might provide clues to the special conditions at the big bang which the fantasy of inflation theory cannot. (Penrose's cosmological theory is discussed in detail in Cycles of Time [October 2011]). Eleven mathematical appendices provide an introduction to concepts used in the main text which may be unfamiliar to some readers.
A special treat is the author's hand-drawn illustrations. In addition to being a mathematician, physicist, and master of scientific explanation and the English language, he is an inspired artist.
The Kindle edition is excellent, with the table of contents, notes, cross-references, and index linked just as they should be.