Mysteries of Modern Physics: Time

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 our universe.

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.

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 of a second.

Embark on the quest to understand time. 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.

Trace the history of the second law of thermodynamics, considered by many physicists to be the one law of physics most likely to survive unaltered for the next thousand years. The second law says that entropy - the degree of disorder in a closed system - only increases or stays the same.

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.