## Summary

Julian Barbour is an independent theoretical physicist who has worked on fundamental issues in physics for over thirty-five years. Although having opted out of an academic career, Barbour is a recognized expert in theories dealing with time and inertia. He is the author of the widely praised*Absolute or Relative Motion?: Volume I*, *The Discovery of Dynamics* (1989).

The ultimate goal of science is to find the common laws that govern the phenomena that are observed in the universe so that this knowledge can be used to make predictions and practical applications. Pursuing this goal in physics, Barbour has continued to ask himself a very basic question: What do space, time, and motion mean at the most fundamental level? In *The End of Time*, Barbour’s primary objective is to develop a new way of looking at the universe in which time does not exist at all and motion itself is portrayed as a pure illusion. This makes some of Barbour’s arguments rather difficult to follow, but overall, the book is fascinating, challenging reading. Since time is such a fundamental aspect of existence and Barbour’s line of reasoning is so counterintuitive, the reader is motivated to continue turning the page. The basic physics for understanding the book is duly explained, and knowledge of calculational techniques is not required.

Although *The End of Time* casts doubt on Albert Einstein’s greatest contribution to modern physics—the space-time continuum—it points to a solution of the fundamental problem in physics: the uniting of quantum mechanics with gravity into a “grand unified theory.” Barbour separates the contents of his book into five major parts. Part 1 introduces a new way to think about intervals of time. By using his notion of “time capsules,” Barbour outlines the basic concepts for developing a theory of a static, timeless universe. In part 2, he documents the role of absolute space and time in classical physics as it was developed by Sir Isaac Newton. In the Newtonian framework, space and time exist at a more fundamental level of importance than the physical objects themselves. However, Barbour’s contention is that the objects should be the fundamental level of focus. Consequently, in place of a Newtonian space-time reference system, Barbour chooses a new frame of reference, an arena he calls “Platonia,” to view the physics of objects. Platonia is the space that contains all the possible qualitative shapes that can be generated by the particles of the universe. Barbour argues that the classical physics of objects can be assessed in Platonia without the use of time.

Moving on to Einstein’s special and general theories of relativity in part 3, Barbour illustrates that they, too, can be completely understood in the framework of Platonia. By using his idea of time capsules, Barbour describes Einstein’s four-dimensional space-time reference frame in Platonia in an even more complete manner than he does Newtonian space-time. In Platonia, the deeper structure of the general theory of relativity is revealed without any dependence on the existence of time. Along the way in part 3, Barbour outlines many of the historical events that have continued to obscure the deeper structure of general relativity.

In part 4, Barbour considers the problem of combining the general theory of relativity and quantum mechanics into one unified quantum theory of the universe known as quantum cosmology. Since quantum mechanics should apply to macroscopic objects as well as atoms and molecules, a quantum mechanical formulation of the universe should be possible. In the Platonia reference frame, Barbour is able to shed light on the main differences between classical and quantum mechanics. Finally, in part 5, Barbour addresses the question of how to interpret history in a universe where time does not exist.

As Barbour points out, time appears to be an inevitable part of life and a fundamental part of the universe; but few people, including Einstein, have given much thought about what time really means. Reviewing the history of science, Barbour indicates that two distinct concepts of time have evolved. The first is Newton’s picture of absolute time and space in which objects exist in an immense space with time flowing forward like an invisible river. In the development of physics, Newton’s idea of time has been slightly modified on two occasions. The first modification was Einstein’s fusion of space and time together into a four-dimensional space-time framework. In Einstein’s special theory of relativity, space-time coordinates can be viewed in different ways so that the simultaneity of events is no longer absolute but dependent on the state of motion. This, however, is only a mathematical formulation of space and time that does not contain any new insight into what time itself really means.

The second modification to Newton’s concept of time developed from quantum mechanics. One interpretation of Werner Heisenberg’s uncertainty principle implies that an interval of time is not absolute or infinitely small but depends on the energy content of the physical objects being observed. Consequently, the fact that...

(The entire section is 2112 words.)