Keeping Time

Keeping Time
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Keeping Still
What's the safest angle to lean a ladder against a wall to keep the ladder from slipping and falling? This is a problem in static equilibrium, which is the state in which no net force or torque (rotational force) is acting. Explore this condition and develop tools for determining whether…
What Is Time?
Approach time from a philosophical perspective. "Presentism" holds that the past and future are not real; only the present moment is real. However, the laws of physics appear to support "eternalism"--the view that all of the moments in the history of the universe are equally real.
Time in Quantum Mechanics
Quantum mechanics is the most precise theory ever invented, yet it leads to startling interpretations of the nature of reality. Probe a quantum state called the collapse of the wave function that may underlie the arrow of time. Are the indications that it shows irreversibility real or only illusory?
Time
Part of the Series: How We Got to Now
The world today is obsessed by time. Johnson boards a submarine to discover what a lack of natural light means for a sailor's working day and visits Heathrow, the world's busiest airport, to try to get timings right at air traffic control. The story of getting a grip on time…
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…
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’s Arrow
Embark on the quest that will occupy the rest of the course: Why is there an arrow of time? Explore how memory and aging orient us in time. Then look at irreversible processes, such as an egg breaking or ice melting. These capture the essence of the one-way direction of…
Time Reversal in Particle Physics
Explore advances in physics since Newton's time that reveal exceptions to the rule that interactions between moving particles are fully reversible. Could irreversible reactions between elementary particles explain the arrow of time? Weigh the evidence for and against this view.
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.
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.
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.
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.