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Mysteries of Modern Physics: Time

Tackle one of the greatest problems in all of science-the nature of time itself-in this groundbreaking course by one of the foremost researchers in this field.
Mysteries of Modern Physics: Time is rated 4.1 out of 5 by 134.
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Rated 4 out of 5 by from Timely Dr Carroll's lecture content and style are great...he speaks clearly and directly of a complex and sometimes paradoxical subject. I was surprised at the direction of the topic...but not disappointed. I learned a great deal, especially that refresher course in the second law of thermodynamics. It seems that time is just an aspect of entropy... Just an aspect? Well, kinda, sorta. I've always considered time as a human construct...something we invented to , well, pass the time. Is time really just seconds, hours and millennia? As it turns out, maybe. Dr Carroll delves into the physics of entropy as it pertains to our concepts of time. It's an eye opening adventure, guaranteed to make you think over time. Recommended...when on sale and with a timely coupon
Date published: 2021-08-13
Rated 5 out of 5 by from Question about entropy Sean Carroll gives the example of 2000 gas particles in two boxes. There is only 1 possibility that 2000 particles are in 1 box. So the entropy is 0. When 1999 particles are in 1 box and 1 particle is in the second box, then there are 2000 possibilities and entropy is 3.3. And so on. The entropy is always increasing because from a macrostate point of view it doesn’t matter which particles are in the second box. But from a microstate point of view it does! The possibility that there are 1999 particles in 1 box and particle number 736 is in the second box is also 1. So the entropy is also zero. The particles in the microstate are not all the same, only for us they are. So from the microstate point of view entropy is always zero. Also for the demon of Laplace entropy does not exist because every moment is completely determined by its information. So isn’t entropy just an illusion? Like Sean says, it all depends on our definition of course graining. And if so, then it’s pointless to ask why entropy at the moment of the Big Bang was so low. It is always low, namely zero. It never changes. Can anyone reply on this please? I emailed to Sean Carroll himself but he answered that he doesn't have time to discuss over email. I would like to know if I'm wrong.
Date published: 2021-06-20
Rated 3 out of 5 by from Good extrapolation, but not entirely scientific I am a big fan of TGC courses. Being a physics aficionado, I liked Prof Carroll's take on entropy and that is the best part of the series. However some theories are a stretch of imagination, like unverified existence of multiverse. That is the part that just goes out-of-bounds and I felt like mythology rather than based on any observation. Mathematics presented in course is bare minimum and if you are at an advanced level of STEM, there isn't much to gain.
Date published: 2021-06-04
Rated 5 out of 5 by from Entropy explored After watching "Examining the Big Questions of Time" taught by Laura Helmuth, I got a recommendation to watch "Mysteries of Modern Physics: Time" taught by Sean Carroll. Although there is some overlap between the two courses, Carroll's course is quite distinct. For Carroll, the main questions are: Why was the Universe in a low entropy state when it began 13.7 billion years ago? What is the arrow of time and why does the Universe have it? What does entropy have to do with the arrow of time? Why do we remember the past and not the future? And: Did the Universe really begin at the big bang? These are all good questions and Carroll does his best to explore them in as many ways as possible. For most of the questions raised above, Carroll says words to the effect of We just don't know. Such is the state of contemporary cosmology. Carroll does an excellent job of defining the issues inherent in the questions mentioned above. He is a gifted explainer and he delves more deeply into cosmology than do many of the other Teaching Company professors whose courses I've watched--with the possible exception of Helmuth. Carroll's course was recorded in 2012 and, while it touches on string theory, it does not include any discussion of loop quantum gravity. Helmuth's course, recorded in 2020 I think, does include this discussion. Helmuth's course is a 12-lesson "variety pack" discussion of topics addressing various aspects of time; Carroll's course is more of a sustained discussion with each lecture leading to the next. If you are interested in Boltzmann and the science of entropy, Carroll's course is the course for you. Carroll's course is intellectually rigorous without being overly mathematical. He only presents three or four equations over the span of 24 lectures. Carroll himself is an excellent presenter and is very easy to listen to. I recommend this course.
Date published: 2021-04-23
Rated 5 out of 5 by from I'm now a big fan of Sean Carroll I really appreciate his approach to this course. I think the "digressions" (if you want to call them that) that he makes into philosophy, history, etc., helped my brain start to slowly wrap itself around this slippery topic. I especially liked Lecture 9 as a foundation and then Lecture 14 where he expounds more on complexity (which he hints at earlier). My least favorites were probably 15 and 16, concerning the perception of time and memory and consciousness, but even with these I felt like he was laying a soft foundation that was relevant elsewhere. I think his approach is great and is certainly far superior to some of what I find on YouTube--and not even because the YouTube videos are bad in any way. I just feel like this is designed to let you really dig deep, where sometimes YouTube feels too compressed to really let you sink into the material.
Date published: 2021-04-22
Rated 5 out of 5 by from Difficult subject skilfully presented Excellent Course - I'm glad to see this here for sale an excellent course with lots of technical content and presented in a skilled way.
Date published: 2021-04-14
Rated 5 out of 5 by from A great and easily accessible lecture series Let me begin by saying I do not have a significant physics background, though did take a 101 course many years ago in college. I found this course to be a fun and thought provoking approach to time and rough overview of the concepts/physics behind time. I listed to this series essentially entirely via audio and did not have any issues with understanding the material. Toward the end 1/3 of the lecture, things did feel a little slow, but overall I really enjoyed the material and the professor. Highly recommended.
Date published: 2021-03-24
Rated 1 out of 5 by from Dishonest Science Not sure this is honest science. Making assumptions about the age of the universe. Making formulas with constants can only prove a) that evolution with its randomness cannot be true, for it it was then science would not be possible because everything would continually change randomly. b) Cannot constants be used to prove what you believe by faith anyway. True science must start with an hypothesis which then can be validated by constant testing and physical sight. To say that the earth is 13.5 billion years old cannot be verified by true science. In the words of God Almighty creator of heaven and earth found in the book of Job: 'where you there' you were not!!!
Date published: 2021-01-19
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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? In 24 riveting half-hour lectures, 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.


Sean Carroll
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.


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 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.

By This Professor

Mysteries of Modern Physics: Time
The Higgs Boson and Beyond
Mysteries of Modern Physics: Time


Why Time Is a Mystery

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.

33 min
What Is Time?

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.

30 min
Keeping Time

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.

31 min
Time's Arrow

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.

29 min
The Second Law of Thermodynamics

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.

31 min
Reversibility and the Laws of Physics

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.

30 min
Time Reversal in Particle Physics

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.

31 min
Time in Quantum Mechanics

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?

31 min
Entropy and Counting

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.

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