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Explores the concept of time travel. Views of astrophysicist Dr. J. Richard Gott, author of "Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time"; Focus of a small group of physicists on ways to travel through time; Reference to Einstein's theory of gravity and space-time; Excerpt from Dr. Stephen W. Hawking's book, "The Universe in a Nutshell"; Significance of discovering a new insight into time; Issues raised by time travel; Comments of physicists; Discussion of the so-called arrow of time problem; Research on time machines; Why Albert Einstein's theory of relativity is incompatible with quantum mechanics; A brief history of time travel; Conclusion by two scientists that the laws of physics may allow you to use wormholes to travel in time; Topic of so-called phantom energy; Overview of the research. INSET: A Trip Forward in Time. Your Travel Agent: Einstein.

Discusses the development of the theory of relativity in the 19th century. Details of how Albert Einstein was challenged by the mysteries of light and motion, came to the conclusion that there is no absolute time, and discovered that moving clocks would not run at the same speed; Views of Harvard professor and science historian, Dr. Peter Galison, about the impact of industrialization and the standardization of time on the development of relativity theory; Mention of Galison's book, 'Einstein's Clocks, Poincaré's Maps: Empires of Time'; Background on Galison who grew up in New York City; Discussion of trading zones, creoles, or common convergence areas, and how clock synchronization functioned as a trading zone for Einstein; Role of mathematician, physicist, and philosopher Henri Poincaré in helping invent the chaos theory; The use of ether to explain the world; Idea that Einstein and Poincaré had different visions for the future of physics.

Reports that some theorists say Einstein's theory of relativity needs revising to fit extreme conditions that prevailed during the Big Bang, when nature was presumably ruled by a single unified force. How Einstein's deceptively simple notion underlies all of modern physics and technology; Comparison of Einstein's equation with a proposed new version called doubly special relativity; Concept of marrying Einstein's general theory of relativity to quantum mechanics; How doubts about relativity have been caused partly by a mystery regarding cosmic rays. INSET: Interpreting the Cosmic Rays.

Discusses physicists' efforts to unify Einstein's general theory of relativity and the principles of quantum mechanics in a cohesive explanation of all the particles and forces in the universe. Comparison of tenets of relativity theory, quantum mechanics and loop quantum gravity; Quantification of space-time and evidence of graininess; The Planck scale and superstrings.