Time rules our lives. From the rising and setting of the sun to the cycles of nature, the thought processes in our brains, and the biorhythms in our day, nothing so pervades our existence and yet is so difficult to explain. Time seems to be woven into the very fabric of the universe. But why?
A movie of a person diving into a pool has an obvious arrow of time. When the movie is played backward, everyone recognizes that it shows an event that would never occur in the real world.
But zoom in on any part of this scene at the atomic scale and the movie can be run backward or forward and be indistinguishable. Either way, the particle interactions are consistent with the laws of physics.
Why does one movie have an arrow of time moving in only one direction and the other does not? Surprisingly, the search for an answer leads through some of the most pioneering fields of physics, including thermodynamics, relativity, quantum theory, and cosmology.
The key concept is called “entropy,” which is related to the second law of thermodynamics, considered by many scientists to be the most secure law in all of physics. The second law has even been compared to Shakespeare’s plays in its importance to the education of a culturally informed person.