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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 4.2 out of 5 by 138.
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Rated 5 out of 5 by from I really enjoyed the course. There were many aspects that were enlightening.
Date published: 2022-06-15
Rated 5 out of 5 by from Is The Passage Of Time Real Or Just In Our Minds? Professor Carroll does an excellent job of examining the human experience of time passage and the underlying physical world that, unlike us, may not experience any difference between past, present, and future. This is an excellent course for people with some basic physics in their background. As an engineer, it was particularly fascinated by Dr. Carroll's explanation that an engineer, Sadi Carnot, was the first person to bring forward the concept of entropy and how that law of energy has been a key player in scientists' quest to understand time.
Date published: 2022-03-24
Rated 5 out of 5 by from Wonderfully mind-boggling Fantastic lectures, easy to listen to, and incredibly mentally-stimulating. Personally a big fan of Sean Caroll's style of teaching, and look forward to investing in more Great Courses that he has presented.
Date published: 2022-03-20
Rated 5 out of 5 by from Nicely done Very impressive and thought provoking course about time, but more accurately a course about cosmology. I think Dr. Carroll did a very good job of taking what I assume to be a field with blisteringly difficult mathematics and converting into something that is digestible. I also thought his book "From Eternity to Here" was also very well done. Between the two, I think I've gotten about as much understanding of cosmology as is possible for a person like me.
Date published: 2022-01-29
Rated 5 out of 5 by from Truly a great course. What a brilliant man. How anyone could give Professor Carroll less than 5 stars is one of the greater mysteries of our time and probably says a lot more about the reviewers than the lectures. Dr Carroll conveys and discuses technical and difficult subjects without getting into the math, which is quite a talent and an art in itself. This is truly a very great course -- very well delivered.
Date published: 2021-12-26
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
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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? 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.

About

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.

INSTITUTION

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

The Higgs Boson and Beyond
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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