Mysteries of Modern Physics: Time
Sean Carroll is the Homewood Professor of Natural Philosophy at Johns Hopkins University and both a member of the Fractal Faculty and an External Professor at the Santa Fe Institute. He received his PhD in Astrophysics from Harvard University. He is the author of several books, including Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime, and the host of the weekly Mindscape podcast. He has been awarded prizes and fellowships by the National Science Foundation, NASA, and the Guggenheim Foundation, among others.
01: 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 our universe.
02: What Is Time?
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.
03: 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 of a second.
04: 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 time.
05: The Second Law of Thermodynamics
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.
06: 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.
07: Time Reversal in Particle Physics
Explore advances in physics since Newton's time that reveal exceptions to the rule that interactions between moving particles are fully reversible. Could irreversible reactions between elementary particles explain the arrow of time? Weigh the evidence for and against this view.
08: Time in Quantum Mechanics
Quantum mechanics is the most precise theory ever invented, yet it leads to startling interpretations of the nature of reality. Probe a quantum state called the collapse of the wave function that may underlie the arrow of time. Are the indications that it shows irreversibility real or only illusory?
09: Entropy and Counting
After establishing in previous lectures that the arrow of time must be due to entropy, begin a deep exploration of this phenomenon. In the 1870s, physicist Ludwig Boltzmann proposed a definition of entropy that explains why it increases toward the future. Analyze this idea in detail.
10: 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.
11: The Past Hypothesis
Boltzmann explains why entropy will be larger in the future, but he doesn't show why it was smaller in the past. Learn that physics can't account for this difference except by assuming that the universe started in a state of very low entropy. This assumption is called the past hypothesis.
12: Memory, Causality, and Action
Can physics shed light on human aspects of the arrow of time such as memory, cause and effect, and free will? Learn that everyday features of experience that you take for granted trace back to the low entropy state of the universe at the big bang, 13.7 billion years ago.
13: Boltzmann Brains
One possible explanation for order in the universe is that it is a random fluctuation from a disordered state. Could the entire universe be one such fluctuation, now in the process of returning to disorder? Investigate a scenario called "Boltzmann brains" that suggests not.
14: Complexity and Life
Discover that Maxwell's demon from lecture 10 provides the key to understanding how complexity and life can exist in a universe in which entropy is increasing. Consider how life is not only compatible with, but is an outgrowth of, the second law of thermodynamics and the arrow of time.
15: The Perception of Time
Turn to the way humans perceive time, which can vary greatly from clock time. In particular, focus on experiments that shed light on our time sense. For example, tests show that even though we think we perceive the present moment, we actually live 80 milliseconds in the past.
16: Memory and Consciousness
Remembering the past and projecting into the future are crucial for human consciousness, as shown by cases where these faculties are impaired. Investigate what happens in the brain when we remember, exploring different kinds of memory and the phenomena of false memories and false forgetting.
17: 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.
18: Curved Spacetime and Black Holes
By developing a general theory of relativity incorporating gravity, Einstein launched a revolution in our understanding of the universe. Trace how his idea that gravity results from the warping of spacetime led to the discovery of black holes and the big bang.
19: Time Travel
Use a simple analogy to understand how a time machine might work. Unlike movie scenarios featuring dematerializing and rematerializing, a real time machine would be a spaceship that moves through all the intervening points between two locations in spacetime. Also explore paradoxes of time travel.
20: Black Hole Entropy
Stephen Hawking showed that black holes emit radiation and therefore have entropy. Since the entropy in the universe today is overwhelmingly in the form of black holes and there were no black holes in the early universe, entropy must have been much lower in the deep past.
21: Evolution of the Universe
Follow the history of the universe from just after the big bang to the far future, when the universe will consist of virtually empty space at maximum entropy. Learn what is well founded and what is less certain about this picture of a universe winding down.
22: The Big Bang
Explore three different ways of thinking about the big bang-as the actual beginning of the universe; as a "bounce" from a symmetric version of the universe on the other side of the big bang; and as a region that underwent inflationary expansion in a much larger multiverse.
23: The Multiverse
Dig deeper into the possibility that the big bang originated in a multiverse, which provides a plausible explanation for why entropy was low at the big bang, giving rise to the arrow of time. But is this theory and the related idea of an anthropic principle legitimate science or science fiction?
24: 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.