Time
Part of the Series: How We Got to Now

PBS
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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…
Electromagnetic Waves
Explore the remarkable insight of physicist James Clerk Maxwell in the 1860s that changing electric fields give rise to magnetic fields in the same way that changing magnetic fields produce electric fields. Together, these changing fields result in electromagnetic waves, one component of which is visible light.
Playing with Entropy
Sharpen your understanding of entropy by examining different macroscopic systems and asking, which has higher entropy and which has lower entropy? Also evaluate James Clerk Maxwell's famous thought experiment about a demon who seemingly defies the principle that entropy always increases.
How We Got to Now - with Steven Johnson
PBS
Join best-selling author Steven Johnson to discover extraordinary stories behind six remarkable ideas that made modern life possible, the unsung heroes who brought them about and the unexpected and bizarre consequences each of these innovations triggered.
Clean
Part of the Series: How We Got to Now
Dirty water has killed more humans than all the wars of history combined, but in the last 150 years, a series of radical ideas, extraordinary innovations and unsung heroes have changed our world. Johnson plunges into a sewer to understand what made a maverick engineer decide to lift the city…
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…
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…
Cold
Part of the Series: How We Got to Now
Only in the last 200 years have humans learned how to make things cold. Johnson explains how ice entrepreneur Frederic Tudor made ice delivery the second biggest export business in the U.S. and visits the place where Clarence Birdseye, the father of the frozen food industry, experienced his eureka moment.…
What Time Is It?
Do we really know what time it is? CERN physicist Professor Brian Cox (and author of E=mc2) unlocks the secrets of time in this entertaining and informative program. His journey starts with the Sun, our historical dictator of time, but Brian discovers that the world doesn't always spin like clockwork…
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.
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 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.