Time
Part of the Series: How We Got to Now

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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…
Einstein to the Rescue
In 1905 a young Swiss patent clerk named Albert Einstein resolved the crisis that flowed from the Michelson-Morley result. When Einstein discarded the ether concept and asserted that the principle of relativity holds for all of physics, mechanics as well as electromagnetism, he was making a simple claim with almost…
Heaven and Earth, Place and Motion
Understanding motion is the key to understanding space and time. Is there a "natural" state of motion? Learn why the ancients gave different answers to this question, and how Copernicus, Kepler, and Galileo laid the foundation for a new approach.
Uncommon Sense - Stretching Time
Why does the simple statement of relativity - that the laws of physics are the same for all observers in uniform motion - lead directly to absurd-seeming situations that violate our commonsense notions of space and time?
Faster than Light? Past, Future, and Elsewhere
Relativity implies that the time order of events can be different in different reference frames. Does this wreak havoc with cause and effect? Finally, why is it that nothing can go faster than light?
Toward a Theory of Everything
Why can't we answer questions about what happened before the Big Bang, or what goes on at the center of a black hole? Can we manage the formidable task of combining quantum physics with general relativity? Physics may well be the most important subject in the universe, a theoretical realm…
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.
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.
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…
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…
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.