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Mysteries of Modern Physics: Time

Mysteries of Modern Physics: Time

Professor Sean Carroll, Ph.D.
California Institute of Technology

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Mysteries of Modern Physics: Time

Course No. 1257
Professor Sean Carroll, Ph.D.
California Institute of Technology
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Course No. 1257
  • Audio or Video?
  • You should buy audio if you would enjoy the convenience of experiencing this course while driving, exercising, etc. While the video does contain visual elements, the professor presents the material in an engaging and clear manner, so the visuals are not necessary to understand the concepts. Additionally, the audio audience may refer to the accompanying course guidebook for names, works, and examples that are cited throughout the course.
  • You should buy video if you prefer learning visually and wish to take advantage of the visual elements featured in this course. The video version is well illustrated and features nearly 400 illustrations, portraits, diagrams, and graphs. Some of the course's most memorable visuals include an illustration of how wave functions show a quantum object's state is actually just a superposition of where it is, a graphic of entropy states used to measure a system's uncertainty, and a diagram of the universe's expansion over time. There are also portraits of noted figures like Albert Einstein and Ludwig Boltzmann, and illustrations that bring to life concepts and thought experiments including time's arrow and Maxwell's demon.
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Course Overview

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. The second law has even been compared to Shakespeare’s plays in its importance to the education of a culturally informed person.

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.

While focusing on physics, Professor Carroll also examines philosophical views on time, how we perceive and misperceive time, the workings of memory, and serious proposals for time travel, as well as imaginative ways that time has been disrupted in fiction.

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.

You explore Boltzmann’s statistical explanation for the nature of time, and you see how, carried to its logical conclusion, it leads to a bizarre scenario called Boltzmann brains. You look at another curious thought experiment, called Maxwell’s demon, which helps explain the presence of order and life in a universe of relentlessly increasing disorder.

In the course of these inquiries, you consider time from many perspectives, including these:

  • A dimension with a difference: Time is the fourth dimension. But unlike the three dimensions that constitute space, time can’t be explored randomly from point to point. You just experience it sequentially second after second. This continuous flow from past to future is the arrow of time.
  • The view from “nowhen”: The present moment seems real in a way that the past and future do not. But to better understand why time and the universe are the way they are, it’s useful to view all moments—past, present, and future—as equally real. This is the view from “nowhen.”
  • Quantum time: Some phenomena at the quantum scale are not reversible with respect to time—unlike all other processes in fundamental physics. Could these events be the origin of the arrow of time? Could they explain why we remember the past but not the future?

You also investigate the past hypothesis, which assumes that atomic theory and fundamental physics cannot account for the difference between the past and the future by themselves. Instead, the arrow of time can only be explained by the initial conditions that gave birth to the universe itself. Which brings you to the big bang, one of the major focuses of this course.

Time to Get This Course

Your time-traveling adventures also include excursions into fiction and film, which Professor Carroll engages with characteristic enthusiasm and wit. While storytellers are seldom concerned with getting the physics right, it’s instructive how they usually get it very wrong:

  • Stopping time: Stories that stop time as the hero moves through a stationary world fail to consider that no one could function in such an environment. Air would be as immovable as a brick wall. Light and sound would stop. No plot would be possible!
  • Time going backward: A character who experiences the arrow of time in reverse faces grave difficulties relating to another character going through time the normal way. They would be like travelers on the highway going in opposite directions.
  • Time travel: Fictional time travelers typically dematerialize and then rematerialize at a different point in time. But real time travel, if it were possible, could not skip over the intervening part of spacetime. Real time travel would be a journey through spacetime.

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. You are at a different point in spacetime, even if you haven’t moved from your chair. “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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24 lectures
 |  30 minutes each
Year Released: 2012
  • 1
    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. x
  • 2
    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. x
  • 3
    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. x
  • 4
    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. x
  • 5
    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. x
  • 6
    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. x
  • 7
    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. x
  • 8
    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? x
  • 9
    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. x
  • 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. x
  • 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. x
  • 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. x
  • 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. x
  • 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. x
  • 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. x
  • 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. x
  • 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. x
  • 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. x
  • 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. x
  • 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. x
  • 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. x
  • 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. x
  • 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? x
  • 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. x

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Your professor

Sean Carroll

About Your Professor

Sean Carroll, Ph.D.
California Institute of Technology
Professor Sean Carroll is a Senior Research Associate in Physics at the California Institute of Technology. He earned his undergraduate degree from Villanova University and his Ph.D. in Astrophysics from Harvard in 1993. Before arriving at Caltech, Professor Carroll taught in the Physics Department and the Enrico Fermi Institute at the University of Chicago, and did postdoctoral research at the Massachusetts Institute of...
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Rated 4.2 out of 5 by 63 reviewers.
Rated 5 out of 5 by great course Really fantastic course, especially worth listening to his theory on the the 2d law of thermodynamics and the origin of life on earth. Blink and you'll miss it. It's a little repetitive but complicated ideas so nice to have the reinforcement. September 26, 2015
Rated 3 out of 5 by The Arrow of Time Not Well Spent This is a case of an inherently interesting subject delivered in a mind numbingly monotonous and uninteresting manner. Professor Carroll is highly qualified and fully engaged but he seems to have missed his target audience. Perhaps if I were smarter I would have been carried away, but in the event I was just anesthetized into a comfortable sleep, which was not without value. I made my way through 15 of the lectures but couldn't hang in there any longer. July 29, 2015
Rated 4 out of 5 by Wow ! Food for the brain. The key word in the title is "Mysteries". If you are looking for a simple explanation of "time" this course will not give it to you. What it will give you is a lot of intriguing hypotheses, theories, and concepts. I can see some validity in some of the lower ratings given to this course. However, Professor Carroll provides a through discussion of time from the history of calendars (including leap seconds , leap months, leap years), to the thoughts of ancient philosophers, to concepts presented by the deepest thinkers of the modern world. I think that completeness, even though some of it is rather simple, enhances the course. You do not have to be told the Earth rotates around the Sun about every 365 days; however, those comments lead into something else to help make a point. If you have no background in college physics/math/thermodynamics this course may be a challenge. However, I still recommend the course because Professor Carroll makes every attempt to put advanced physics into terms none physicists can understand. My one complaint is the Guidebook does not have a glossary of important terms and no biographical sketches of the people mentioned in the lectures. July 3, 2015
Rated 3 out of 5 by Unanswered questions I gave the course a higher value because it got me interested and fascinated by cosmology. I gave the course lower marks because I found that Professor Carroll made many conclusions unsupported by the information he presented. He talked about the universe being created from nothing and then went on to explain what that meant in effect claiming what amounted to gibberish. Early on when he first discussed the meaning of entropy, he pointed out that it increased as the number of particles in a given volume increase. In a much later lesson he claims that an empty universe would have maximum entropy. He claims that most of the entropy of the universe is in black holes. My question unanswered in the course is, if the matter in a black whole is being attracted towards a singularity why wouldn't the entropy be decreasing? I would not recommend the course because the questions raised for me were unanswered by the presentation. June 14, 2015
  • 2015-11-29 T10:48:29.339-06:00
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