What Is Time?

What Is Time?
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Keeping Time
How do we measure the passage of time? Discover that practical concerns have driven the search for more and more accurate clocks. In the 18th century, the problem of determining longitude was solved with a timepiece of unprecedented accuracy. Today's GPS navigation units rely on clocks accurate to a billionth…
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?
Humble Physics: What We Don't Know
Having covered the remarkable discoveries in physics, turn to the great gap in our current knowledge, namely the nature of the dark matter and dark energy that constitute more than 95% of the universe. Close with a look at other mysteries that physicists are now working to solve.
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.
Time and Space
Einstein's special theory of relativity upends traditional notions of space and time. Solve the simple formulas that show the reality of time dilation and length contraction. Conclude by examining the twins paradox, discovering why one twin who travels to a star and then returns ages more slowly than the twin…
Space-Time and Mass-Energy
In relativity theory, contrary to popular views, reality is what's not relative: that is, what doesn't depend on one's frame of reference. See how space and time constitute one such pair, merging into a four-dimensional space-time. Mass and energy similarly join, related by Einstein's famous E = mc2.
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.