Mysteries of Modern Physics: Time

Rated 5 out of 5 by from Great Lectures!!! Sean Carroll is one of my favorite professors. All his lectures and courses are great. He explains the mysterious, complex, and abstract concept of the arrow of time by presenting how entropy was lower in the past, all the way back to the Big Bang. The course is fascinating and challenging. A course like this makes the Great Course Plus subscription worthwhile. Thank you very much!!
Date published: 2020-09-02
Rated 5 out of 5 by from Changes Your Perception of Reality Keeps it accessible enough and exciting enough for lay people
Date published: 2020-08-12
Rated 2 out of 5 by from Too much emphasis on particle physics This is the one course I regret purchasing. The first few lectures were excellent, but then he immediately beings discussing particle physics, and I had no background in that area. You would definitely need that as prerequisite. He lost me at that point, and I quit.
Date published: 2020-07-26
Rated 5 out of 5 by from Brilliant lecture! Professor Sean Carroll offer a highly relevant, easy to understood perspective on Time. High quality course (lecture, video, audio) as usual. Thatks The Great Courses, I enjoyed every second!
Date published: 2020-06-26
Rated 1 out of 5 by from The title is misleading. Three basic things are wrong with this course. First, Professor Carrol makes the assumption that his students are dumb and uneducated. Second, he repeats himself over and over. Finally, he brags that he is skilled in physics and has a difficult time reducing his vocabulary to the level of students. If this had been the First Great Course I purchased, I would never have purchased another.
Date published: 2020-03-24
Rated 5 out of 5 by from Time, Entropy and Relativity Professor Carroll is a superb lecturer. Probably the best in all The Great Courses. I'd give him a ten, if I could. I usually don't read reviews, before watching these courses, so I don't prejudge anything. I was surprised at some of the low ratings people gave this course. There is only so much you can say about Time. I have read and watched many science programs about Time. I thought I knew quite a bit about the subject. However, this course is divided into three main sections: Time, Entropy and Relativity, each eight lectures in length. Each of these main divisions was explained with many new (to me) ideas. As an example, I knew about the arrow of time, which is related to Entropy, but, I found out there is much more to the story than I had ever heard, before. I thought the course was well organized and the subject matter was thorough. The presentation kept me engaged. If there is any criticism at all, it's that a lot of emphasis is paid to Entropy and how it relates to the past, present and future. Entropy is presented with every angle imaginable. By the way, not to give away the plot, but, there is still a lot about Entropy that is still not fully known. Professor Carroll goes into these knowns and unknowns about Entropy in detail. This is a course about Time. It does, however, go into all aspects of Time. I was taught that Time is not a basic thing in Physics. Its units are contrived. The physical world doesn't use time like we, as humans, do. So, Time can only be discussed in generalities. This course, and Professor Carroll, does a good job in describing what we call Time. I highly recommend this course to anyone who appreciates physics and is interested in all aspects of Time.
Date published: 2020-03-14
Rated 1 out of 5 by from I bought this for a grandson in college. He is delights with it. I am considering The 30 Greatest Orchestral Works for myself. You seem to have courses for everyone. I still enjoy those I already have.
Date published: 2020-03-06
Rated 5 out of 5 by from Very interesting but challenging This course was very interesting. It tackled the fascinating subject of the arrow of time. I am a trained experimental physicist but found some of the theoretical aspects and ideas challenging at times but overall I am pleased I listened to the whole course. Professor Carroll is an excellent lecturer and he led me through a difficult, but interesting, subject. Without his valuable guidance I would have given up early in the course.
Date published: 2019-12-20
  • y_2020, m_9, d_27, h_15
  • bvseo_bulk, prod_bvrr, vn_bulk_3.0.12
  • cp_1, bvpage1
  • co_hasreviews, tv_11, tr_115
  • loc_en_CA, sid_1257, prod, sort_[SortEntry(order=SUBMISSION_TIME, direction=DESCENDING)]
  • clientName_teachco
  • bvseo_sdk, p_sdk, 3.2.1
  • CLOUD, getReviews, 3.54ms
Mysteries of Modern Physics: Time
Course Trailer
Why Time Is a Mystery
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.

33 min
What Is Time?
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.

30 min
Keeping Time
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.

31 min
Time's Arrow
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.

29 min
The Second Law of Thermodynamics
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.

31 min
Reversibility and the Laws of Physics
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.

30 min
Time Reversal in Particle Physics
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.

31 min
Time in Quantum Mechanics
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?

31 min
Entropy and Counting
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.

31 min
Playing with Entropy
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.

32 min
The Past Hypothesis
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.

29 min
Memory, Causality, and Action
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.

30 min
Boltzmann Brains
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.

31 min
Complexity and Life
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.

31 min
The Perception 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.

32 min
Memory and Consciousness
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.

31 min
Time and Relativity
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.

31 min
Curved Spacetime and Black Holes
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.

30 min
Time Travel
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.

31 min
Black Hole Entropy
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.

30 min
Evolution of the Universe
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.

31 min
The Big Bang
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.

30 min
The 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?

31 min
Approaches to the Arrow of Time
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.

32 min
Sean Carroll

We need to push on our understanding of cosmology, particle physics, gravity, not to mention how complexity and entropy evolve through time, and eventually you'll be able to really understand what our theories predict.


Harvard University


California Institute of Technology

About Sean Carroll

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 Technology and at the Institute for Theoretical Physics at the University of California, Santa Barbara. Professor Carroll is the author of Spacetime and Geometry: An Introduction to General Relativity, published in 2003. He has taught more than 200 scientific seminars and colloquia and given more than 50 educational and popular talks. In addition, he has written for numerous publications including Nature, New Scientist, The American Scientist, and Physics Today. Professor Carroll has received research grants from NASA, the U.S. Department of Energy, and the National Science Foundation, as well as fellowships from the Sloan and Packard foundations. He has been the Malmstrom Lecturer at Hamline University, the Resnick Lecturer at Rensselaer Polytechnic Institute, and a National Science Foundation Distinguished Lecturer. While at MIT, Carroll won the Graduate Student Council Teaching Award for his course on general relativity. In 2006 he received the Arts and Sciences Alumni Medallion from Villanova University.

Also By This Professor