What Is Time?

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Why Time Is a Mystery
Begin your study of the physics of time with these questions: What is a clock? What does it mean to say that "time passes"? What is the "arrow of time"? Then look at the concept of entropy and how it holds the key to the one-way direction of time in…
Time and Relativity
According to Einstein's special theory of relativity, there is no such thing as a moment in time spread throughout the universe. Instead, time is one of four dimensions in spacetime. Learn how this "relative" view of time is usefully diagramed with light cones, representing the past and future.
Approaches to the Arrow of Time
Use what you have learned in the course to investigate a range of different possibilities that explain the origin of time in the universe. Professor Carroll closes by presenting one of his favorite theories and noting how much remains to be done before conclusively solving the mystery of time.
The Higgs Boson and Beyond Course
The discovery of the Higgs boson is a triumph of modern physics. The hunt for the Higgs was the subject of wide media attention due to the cost of the project, the complexity of the experiment, and the importance of its result. And, when it was announced with great fanfare…
Evolution of the Universe
Follow the history of the universe from just after the big bang to the far future, when the universe will consist of virtually empty space at maximum entropy. Learn what is well founded and what is less certain about this picture of a universe winding down.
Big Bang Cosmology
Episode 6 of Redefining Reality
Investigate the underlying reality that governs the universe. Is the universe eternally the same? Or is it changing and unstable? In modern times, this debate culminated in the contest between the steady state theory and the big bang model. Hear how unexpected events led to a spectacular solution.
Mysteries of Modern Physics: Time Series
Time seems to be woven into the very fabric of the universe. But why? In 24 riveting half-hour episodes, Mysteries of Modern Physics: Time shows how a feature of the world that we all experience connects us to the instant of the formation of the universe--and possibly to a multiverse…
Complexity and Life
Discover that Maxwell's demon from episode 10 provides the key to understanding how complexity and life can exist in a universe in which entropy is increasing. Consider how life is not only compatible with, but is an outgrowth of, the second law of thermodynamics and the arrow of time.
Falling Freely
Use concepts from the previous episode to analyze motion when an object is under constant acceleration due to gravity. In principle, the initial conditions in such cases allow the position of the object to be determined for any time in the future, which is the idea behind Isaac Newton's clockwork…
Reversibility and the Laws of Physics
Isaac Newton's laws of physics are fully reversible; particles can move forward or backward in time without any inconsistency. But this is not our experience in the world, where the arrow of time is fundamentally connected to irreversible processes and the increase in entropy.
Time Travel
Use a simple analogy to understand how a time machine might work. Unlike movie scenarios featuring dematerializing and rematerializing, a real time machine would be a spaceship that moves through all the intervening points between two locations in spacetime. Also explore paradoxes of time travel.
The Perception of Time
Turn to the way humans perceive time, which can vary greatly from clock time. In particular, focus on experiments that shed light on our time sense. For example, tests show that even though we think we perceive the present moment, we actually live 80 milliseconds in the past.