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 133.
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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
Rated 3 out of 5 by from i think about time and heres what i come up with... The Phenomena of Time And Tempus Industria(the study of time as energy For centuries scientists have wandered over time: what is time, how old is time, can we reverse stop or fast forward time, or time travel? If you could go back in time what would you do? You may go back to the Texas revolution and watch the battle of the alamo, or you might go back and see the formation of the solar system, or you might prefer just to go back and stop yourself from cheating on that test. It does not matter what you would like to see or do if you could go back in time, all that matters is that your time traveling. What exactly is time travel? Well it is a little complicated. So before we understand time travel we must understand what time is. Not many people think about what time is. Some suggest it is some sort of exotic matter, Some say it is everything, some say it is nothing ,some say it's too complicated for people to understand it. If you are asking me what is time it depends on how you put it. If you are talking about stopping reversing or traveling in time you are talking about different subjects. If you're wondering about reversing time then you are wandering about reversing energy. So what do i mean by energy. Energy is all forms of motion, for example if i toss a ball at 10 miles per hour it has kinetic energy or pe. If I have a plate and it's 100 degrees Fahrenheit then it has heat energy(heat energy is the motion of individual atoms and molecules and vibrations whereas kinetic energy is the motion of a grouping of atoms or and molecules. Sound is the oblations or waves of expanding and contracting configurations of atoms and molecules, for example if I push this atom at another it will bomb it and the atom it hit will go flying away and the one that hit it will stop. Or light, light is a electromagnetic wave. It acers when you move an electron or change the electron level by shooting light at it. There are many other kinds of energy as well. But these are the most important. So, what does all this mean? It means that if you take all the energy in the universe and reverse it you have reversed time. but you would need to reverse the motions of everything in the universe at the exactly same time. It would be the exact same as reversing time. So if we fell in a hole and then we reversed energy we would come right back out from where we fell in? Yes we would. You see when you fall in a hole you don't bounce back up becuase you give off a little light heat and mostly sound in the case of falling in a hole. all the energy you gave off whould come right back to you and shoot you back to were you fell from. but it whould be physicly imposible to do such a thing because it would take an infinite amount of precision to exactly reverse the motions of the sound light and heat(heat as in the motions of all the atoms and molecules) energy, we just don't have that sort of technology. But you can still see this just film yourself dropping a ball and then run the video backwards, you will see all the energy come back to the ball then it will bounce back to your hand. So if we drop a glass, it falls, then shatters, if we reverse energy it will reform? Yes, it will. Because when you drop it the ionic bonds give way. So, if you reverse energy with exact precision the glass will reform back into a cup and the ionic bonds will reform as well. So, what does it mean to stop time? to stop time, you would have to take all the energy away from the universe. So now let's try to take all the energy away from the universe. What happened? You might have guessed, we stopped time. But what about gravity, does gravity and all the other forces just stop working if you take all the energy away from the universe? Yes, they do stop. Why do they? Because gravity is really a change in energy. Isack Newton says that if an object is far away from the center of gravity that it has high gravitational potential energy and low gravitational kinetic energy if it's close vice versa. But really we don't need gravitational potential energy to save the first law of thermodynamics(energy nor be created nor be destroyed. We can just use gravitational time dilation. So, if an object is close to the center of gravity it experiences strong gravitational time dilation time dilation(in other words all other forms of energy go down so the object can accelerate towards the center of gravity. If it's far away it does not experience much gravitational time dilation. So why does an object experience time dilation if it's stationary by a massive object? Because there is a constant force being exerted between the two objects, therefore all other forms of energy decrees in equal ways creating the illusion of time dilation, but it's really energy levels that are changing not time itself slowing down. So, is time travel possible? No, why? Say you could somehow accelerate yourself to the speed of light. Then you go a little faster, what happens? Well as you've heard it's impossible to go faster than the speed of light(186,000 mph speed limit ok. Why? Because of time dilation. Now this is a little different than gravitational time dilation, this is kinetic time dilation. Einstein says that the faster you go the stronger time dilation gets or the less time you experience. This means that as you go faster and faster, that all your other energy levels go down. So, if you travel at the speed of light you do not experience any time or all your other energy levels are at zero, so why can't you go faster? Because your energy levels won't go past zero, it's like 0 degrees kelvin, you can't get colder than 0 degrees kelvin because there are no motions in the atoms at this point, or they can't move slower than not moving.
Date published: 2020-11-14
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
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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


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