Mysteries of Modern Physics: Time

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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?
Consider these contrasting views of time:

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.

But that’s only the beginning, since the quest for the ultimate theory of time draws on such exciting ideas as black holes, cosmic inflation, and dark energy, before closing in on a momentous question that until recently was considered unanswerable: What happened before the big bang?

In 24 riveting half-hour lectures, Mysteries of Modern Physics: Time takes you on a mind-expanding journey through the past, present, and future, guided by Professor Sean Carroll, noted author and Senior Research Associate in Physics at the California Institute of Technology.

Designed for nonscientists as well as those with a background in physics, 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 that is unimaginably larger and more varied than the known cosmos.

Clues to the Origin of Time

Break an egg. Melt an ice cube. Mix coffee and cream. Each starts with an ordered state and ends with one that is much more disorderly. Each is an example of an increase in entropy, which is a measure of the degree of disorder in a closed system. The entropy of the universe was lower in the past; it will be higher in the future. Increasing entropy defines the arrow of time, implying that at the beginning of the universe entropy must have been extraordinarily low. This course seeks to understand why.

Professor Carroll begins like a detective by gathering the facts. What do we know about time, what characterizes it, and how do we measure it? Then he combs the universe for clues, from the contrasting views on time of Isaac Newton and Albert Einstein, to Rudolf Clausius’s invention of the concept of entropy and Ludwig Boltzmann’s brilliant insight about why entropy increases and therefore why time proceeds from past to future.

In the time that has passed since you started reading this, the entropy of the universe has increased. The future of a few moments ago is now the present. “What is time?” asked Saint Augustine 1,600 years ago. “If no one asks me, I know. But if I wish to explain it to someone who asks, I know not.” With Mysteries of Modern Physics: Time, you will be much closer to an answer.

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1
Why Time Is a Mystery
2
What Is Time?
3
Keeping Time
4
Time’s Arrow
5
The Second Law of Thermodynamics
6
Reversibility and the Laws of Physics
7
Time Reversal in Particle Physics
8
Time in Quantum Mechanics
9
Entropy and Counting
10
Playing with Entropy
11
The Past Hypothesis
12
Memory, Causality, and Action
13
Boltzmann Brains
14
Complexity and Life
15
The Perception of Time
16
Memory and Consciousness
17
Time and Relativity
18
Curved Spacetime and Black Holes
19
Time Travel
20
Black Hole Entropy
21
Evolution of the Universe
22
The Big Bang
23
The Multiverse
24
Approaches to the Arrow of Time