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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 150.
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Rated 5 out of 5 by from Good attempt to explain a difficult-to-explain sub This is a very difficult to explain subject. Professor Carroll try to explain this difficult and still actively researched subject in a laypeople’s language without using mathematical equations. I enjoyed hearing things like entropy of black holes, first time in my life. I recommend this course to those curious about modern physics, but do not have a background to read the research papers.
Date published: 2024-06-17
Rated 5 out of 5 by from Clear focus, great sequence of talks He distils the mystery of time to the arrow of time and explains how that arises from the best physical theories we have. Putting those theories together leads to some unsolved mysteries - principally related to the low entropy beginning of the universe and the maximum entropy future of thermal equilibrium leading in to ideas as yet unproven about how this thermal equilibrium trap can be avoided and why the low entropy beginning may not be so unlikely. Its all very well explained by Professor Carroll who is an excellent communicator and essentially no mathematics is involved (occasional formula is stated).
Date published: 2024-03-21
Rated 5 out of 5 by from Well worth it One's inability to fully comprehend a subject is not a basis for a negative course review. As such, I give this course 5 stars even as I continue, after a number of Great Courses and much effort on my part, to fully get my head around many of the concepts of quantum physics. I can parrot the words, and I would score well on an exam, but my putrid little brain is firmly stuck in Newtonian physics. Prof Carroll does an excellent job. This course goes beyond physics to include topics in neuroscience, in which Prof Carroll does not hold formal credentials but nevertheless does a respectable and accurate job. The latter is a field in which I am very well read (though the subspecialty of my formal training and professional career is in another branch of medical science) and can vouch for the accuracy of what Prof Carroll reports as I have read the primary literature.
Date published: 2024-02-10
Rated 3 out of 5 by from Too much arm waving and repetition. Science without mathematics is not very interesting. Prof. Carroll makes the point that entropy and time are intimately related, but these are quantitative measures. To understand their conceptual utility, one must see how they are related quantitatively. This course would have been more useful if it explained how to use entropy in physical process calculations, e.g. ideal gas expansion, chemical reactions, the Arrhenius equation, steam engines, etc. Only simple algebra is required. The material presented could have been adequately conveyed in six lectures, not twenty-four. It was not until lecture #20, “Black Hope Entropy,” that Carroll got to something interesting, but I had to locate a proper technical paper (“Introductory Lectures on Black Hole Thermodynamics” by Ted Jacobson, Institute for Theoretical Physics, University of Utrecht) before my curiosity was satisfied. HWF, Mesa AZ
Date published: 2023-11-30
Rated 5 out of 5 by from Wonderful series Thoroughly enjoyed and very educational. I’m shocked by some of the negative comments, especially those who said he didn’t cover time. That’s what this entire lecture covers.
Date published: 2023-08-14
Rated 5 out of 5 by from Suddenly stars make sense No, I don’t understand quantum physics or string theory but at least I can discern their shadows in the multiverse. Too absorbing for me to play solitaire while I watched. Carrol’s explanations are easy to follow through dense matter from dark matter to gravity while describing the arrow of time.
Date published: 2023-06-28
Rated 5 out of 5 by from simplified very difficult. Concepts so much information and the history storyline done well will likely watch again
Date published: 2023-06-19
Rated 5 out of 5 by from Comprehensible, yet mind-bending Professor Carroll’s clear, energetic, balanced, and occasionally humorous presentation style very effectively communicates the material in his “Mysteries of Modern Physics: Time” course, which is a challenge considering the difficulty of using 2-dimensional diagrams or 3-dimensional models to visually represent theoretical multi-dimensional universes. His explanations of mathematical and subatomic perspectives of time are made more interesting and comprehensible by descriptions of macro-world implications and analogies. Professor Carroll honestly indicates the current limits of human scientific understanding and acknowledges the possible limitations on what is ultimately knowable or provable by human brains (for a more abstract philosophical debate about the nature of knowledge and perception, I suppose one could always turn to Plato’s various dialogues, i.e. “Theaetetus”, etc.). Any attentive listener will surely finish this course with an enhanced understanding of the concept of time, and also enjoy the fun of having a heightened sense of questioning and wonder about whether time as one traditionally intuits or thinks of it, whether cyclical or linear in nature, even actually exists at all (hint: entropy and the past may have more to do with it than one may typically believe).
Date published: 2023-06-10
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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

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.


Johns Hopkins University

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.

By This Professor

The Many Hidden Worlds of Quantum Mechanics
The Higgs Boson and Beyond
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