I certainly recommend this book to any layperson that enjoys reading accounts of both the history of physics and the state of progress in modern physics. Greene is an excellent writer and he discusses and teaches often difficult concepts in an accessible way. On reflection, my only criticism of the book is really an issue with the subject of string theory, rather than with Greene’s descriptions.
For me, the book breaks down into two parts. The first 2/3 is an account of historical developments in physics using an excellent organizing scheme. Greene sets out two key questions. First, are space and time fundamental or do they simply arise as descriptions of relations among other fundamental entities? Second, how do we account for the unidirectional flow of time (“the arrow of time”) which we experience?
With these questions in mind, Greene reviews classical physics, Einstein’s relativity, quantum mechanics, and recent cosmological theories.
In Newton’s classical mechanics, space and time are fixed and absolute backdrops for matter and motion. (Leibniz was a contemporary that advocated the “relational” view that space and time emerged from relations between objects -- a view expressed in the late 19th century by Mach.) Einstein overthrew Newton with his special and general theories of relativity, where space and time are not independent and absolute, but are dynamically integrated with each other and with matter/energy. Greene explains carefully how Einstein’s framework was not fully relational, however. In quantum mechanics (the scientific method which accurately explains the behavior of sub-atomic entities), there is no absolute location in space, and the notions of space and time as usually conceived are bypassed by the phenomenon of entangled particles which instantaneously act on each other at a distance.
The best part of this first section of the book for me was the review of cosmological theories, particularly the inflationary hypothesis. After describing the second law of thermodynamics (the only part of traditional science which has an explicit arrow of time), Greene examines theories of the history of our universe for a possible explanation of both the flow of time we experience as well as the geometry of observed space. I understood the attraction of the inflationary scenario much better after reading the book.
The second part of the book is a discussion of progress in the attempt to reconcile relativity and quantum mechanics in a unified framework. Most of the discussion is on string theory (and its development into M-theory), which is both Greene’s specialty as well as the major (but not only) unification theory being developed.
String theory’s major exciting starting point was its promise to explain all of the fundamental particles and forces (including gravity) in a framework of one-dimensional units called strings. To laypeople, it is usually put forth that different vibrations in the strings describe all of the fundamental entities. (For me, it has always seemed like a good idea to try to create a model which replaces the dimensionless mathematical point of traditional physical theory with a basic unit which has more structure.) However, the working out of string theory was accompanied by difficulties. First, there were several versions, not just one. More importantly (in terms of the nature of space and time), in its initial formulations string theory required an absolute backdrop of space-time, thus in a way reverting to a pre-relativity stance. Also, to work, there had to exist many more dimensions of space – a total of ten space-time dimensions. (I should note that, to his credit, Greene does a good job throughout presenting criticisms and possible shortcomings of string theory.)
In recent years, Greene tells us, it was discovered that several different versions of string theory really were one theory after all. This overarching theory (which now featured an eleventh space-time dimension) also introduced new structures beyond one-dimensional strings. These 2, 3 and higher dimensional entities became known as branes, and the theory as M-theory (“M” possibly standing for membrane, but maybe several other things as well). Given that the mathematics in which the theory is described is so far beyond the typical reader, Greene describes the theory effectively and defends it against the common criticism that its details are not provable – he outlines experiments which could make key features testable in the not-too-distant future.
Greene finishes by trying to recover the concepts of space and time (as we know them) by postulating that they emerge from a more complex foundational reality described by M-theory. I should also note that in the book he discusses a number of interesting topics that are somewhat off the track of his core narrative, such as time-travel and wormholes, and the holographic principle. As always, the descriptions are interesting and reader-friendly.
Despite the fact that I doubt string theory can be described any better than Greene does it, the second part of the book is less compelling than the first. Part of the reason is simply the benefit of hindsight which enables the author to organize and present an effective narrative of the physics of the past, in contrast to describing the messier developments of a work-in-progress. However, in reflecting on Greene’s account, I think there’s more to it than that. I’m struck by the fact that many of the historical examples of progress in physics featured brilliant conceptual advances which built a framework for the resulting theory, while this is less clearly the case for M-theory. The paradigm case is general relativity, where Einstein had the insight that gravitation is equivalent to acceleration, and then he found a pre-existing mathematical framework in which to formulate the specifics of the theory. In contrast, my impression is that the small army of mathematically gifted M-theory modelers steer a course somewhat un-tethered to guiding concepts, and then attempt later to go back and fix things up. For example, Greene describes current attempts to draw connections to cosmological theories like inflation and to address conceptual shortcomings like space-time background-dependency.
It may be that a new key conceptual insight will be needed to guide the advance of modern physics.