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Introducing Time traces the history of time from Augustine's suggestion that there is no time, to the flowing time of Newton, the conventional time of Poincaré, the static time of Einstein, and then back, full circle, to the idea that there is no time in quantum gravity. What is time? The 5th-century philosopher St. Augustine famously said that he knew what time was, so long as no one asked him. Is time a fourth dimension similar to space or does it flow in some sense? And if it flows, does it make sense to say how fast? Does the future exist? Is time travel possible? Why does time seem to pass in only one direction? These questions and others are among the deepest and most subtle that one can ask, but Introducing Time presents them—many for the first time—in an easily accessible, lucid and engaging manner, wittily illustrated by Ralph Edney. What is Time? All Kinds of Clocks Biological Clocks Psychological Time Is Time Merely in the Head? Clocks and Time How Long is an Interval of Time? The Most Reliable Clocks The Atomic Clock Absolute, True and Mathematical Time True Time Opponents of Newtonian Time: Relationalism A Scenario of Time without Change Can Relationalism replace Absolute Time? Conventionalism A Universe Out of Sync? The Nature of Time: Relative and Non-Relative Tenseless and Tensed Theories of Time Tensed Time Tenseless Time Representing Dimensions The Fourth or Time Dimension Diagrams of Space and Time Picture of a “Tenseless” Life The Now and the Here The Problem of Motion and Change McTaggart’s Argument Avoiding McTaggart’s Trap How Fast Does Time Flow? Galilean Relativity Frames of Reference Einstein’s Relativity Simultaneity is Relative to the Observer The Spacetime Event Lightcones Time and Observer Dependency Relativity and Tenses Does Logic Allow Time Travel? The Logic of Impossibility The Book That No One Wrote The Causal Loop A Logical Contradiction of Time Travel Logical Contradictions Cannot Occur Personal Time Dying at a Time Earlier Than Your Birth Future Compatibility Can We Change the Past? Can We Affect the Past? Two Sorts of Time Travel Stories Does Physics Allow Time Travel? Moving Clocks Run Slower Small Savings in Time General Relativity and Four-dimensional Curvature Why We Don’t Need a Fifth Dimension Spacetime Curvature General Relativity and Time Travel Gödel’s Rotating Universe Spacetime in a Rotating Universe The Effect of Spacetime Curvature Taub-NUT-Misner Spacetime Gödel’s Complete Spacetime Travel Is Gödelian Time Travel Possible? Gödel Against Tenses Another Problem for the Tensed Theory Was Gödel Wrong? Cosmic String Theory Wormholes in Spacetime Wormholes May Not Allow Travel Exotic Possibilities For Time Möbius Twist in Space Möbius Twist in Time Branching Time Does Space “Run Out”? Finite Space Without Limit Geroch’s Theorem Eternal Recurrence Travelling to Big Bang A Philosophical Objection Closed and Open Time To Sum Up, So Far … The Direction of Time “Irreversible” Processes Time Reversal Invariance Seeing in Terms of Particles The Science of Heat Spontaneous Processes The Law of Entropy The Problem of Newtonian Particles, Again Statistical Mechanics A Statistical Asymmetry Is Reversal Probable? The Most Probable State of Entropy The Loschmidt Paradox In What Direction Does Entropy Increase? The Universe’s Statistical Development The Boundary Conditions of the Universe An Unlikely Hypothesis Why Does Entropy Really Increase? The “Temporal Double Standard” A Reversal of Time’s Arrow Communication with the Time Reversed? Time-reversed Communication Quantum Gravity: The End of Time? The Wheeler-DeWitt Equation Rounding-up of Positions The Perfect or “Master” Clock The Inexistence of Time A Better-known Mystery