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

Roger Penrose is not only a great physicist, but a great communicator of complicated scientific issues that embrace mathematics, physics, artificial intelligence and cosmology. At age 85, he has published a remarkable tour de force that depicts the state of modern physics.

The title of his book suggests that he proposes to question whether many of the fashionable ideas in physics, generally accepted as true, are all they are cracked up to be. Chief among those are string theory, superstring theory, cosmic inflation, and quantum field theory. After examining them in some detail he finds them wanting. He is convinced that today's version of quantum mechanics is provisional, waiting for a new formulation that will resolve some of its problems, in particular the difficult problem integrating the effects of gravitation into quantum effects.

This is a very technical book, written primarily for physicists, cosmologists and others who have at least an undergraduate grounding in those fields. The lay reader may find the technical level challenging. However, a simplified version of the book would not have worked because the arguments that Penrose raises are subtle, requiring a detailed knowledge of the field.

Of many fashionable theories, Penrose first takes aim at string and superstring theory. String theory became popular because of its inherent elegance. Its original version purported to show that all elementary particles can be explained as different modalities of vibration of elementary strings. A wonderful unifying theory! Unfortunately, by the early 1970s, it became clear that in order for the theory to explain observable quantum rules, it required a world of 25 dimensions. In the 1990s, a later development called M-Theory pared down the number of dimensions to eleven. Penrose examines in some detail the problems that arise as a result of the extra dimensions. Chief among them is "functional freedom" of the strings that rapidly becomes infinite. Regarding supersymmetry, a development of string theory, Penrose takes the position that the theory is not falsifiable. There are so many types and versions of string theory, that if one version is disproved, there remain an infinite number of others. There is no experiment, performed or to be performed, that could put the nail in the coffin of string theory.

When writing on modern cosmology Penrose raises significant issues that many cosmologists still tend to sweep under the carpet. Chief among those is the low entropy of the Big Bang. The present day cosmic microwave background is at thermal equilibrium, but at the time of the Big Bang, the entropy was vanishingly small. More, it had highly restricted degrees of gravitational freedom. How did such an unlikely event arise? Cyclical models of the universe offer no answers as a universe that implodes on itself, creates a high entropy black hole, not the low entropy situation of the Big Bang. The most popular theory to explain the evenness of the cosmic microwave background is inflation --- the notion that the early universe underwent a rapid expansion as a result of a (as yet unidentified) "inflation field". Penrose doubts whether this took place. Given that the initial singularity was extremely uniform, with restricted degrees of freedom, its expansion would have resulted in a uniform cosmic microwave background, without the need for inflation.

Penrose looks also at the Anthropic Principle. He takes seriously the fine tuning of many physical constants, that we live in a special universe that appears designed for the development of stars, galaxies and eventually life. However, he feels that the Anthropic Principle does little to explain the low entropy of the Big Bang --- unless the latter arose purely by chance. An explanation via multiple universes, some of which contain life but most of which are dead, he regards as a cop-out. As with superstring theory, he regards the multiverse hypothesis as non-falsifiable.

The fine tuning of physical constants, and the low entropy of the Big Bang, could all be explained, according to Penrose, by his Conformal Cyclical Cosmology. While he proposed the idea over ten years ago, its mathematical details are yet to be worked out, so the theory is mostly conceptual. In this model, we live in a series of universes. The present universe is only one of many Aeons. It will continue to expand at an accelerating rate. Galaxies will eventually collapse to form supermassive black holes that will themselves evaporate via Hawking Radiation. Even the Higgs Bosons will decay. All matter will dissolve to form a sea of energy. This sea of energy will be the seed for the next Aeon, the next Big Bang. However, information from one Aeon can be transferred to the next via evaporating black holes. Thus, physical constants can be passed on down the generations of universes. He points to observed anomalies in the cosmic microwave background that could have resulted from collisions between black holes in the previous Aeon. However, for most cosmologists those observations remain controversial.

In summary, this book offers exciting insights into the boundaries of modern physics and cosmology --- a look at some of the greatest controversies, what is unknown, and also what may remain unknowable. The writer also makes a compelling case for why many popular ideas in physics today will eventually have to be abandoned; why quantum mechanics will most likely be superseded by a new, more embracing theory. He offers a few suggestions for ways out of many contradictions in physics, but he also admits that he does not know how or where the next developments will occur.