Mysteries of Modern Physics: Time

by Sean Carroll

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? Consider these contrasting views of time: A movie of a person diving into a pool has an obvious arrow of time. When the movie is played backward, everyone recognizes that it shows an event that would never occur in the real world. But zoom in on any part of this scene at the atomic scale and the movie can be run backward or forward and be indistinguishable. Either way, the particle interactions are consistent with the laws of physics. Why does one movie have an arrow of time moving in only one direction and the other does not? Surprisingly, the search for an answer leads through some of the most pioneering fields of physics, including thermodynamics, relativity, quantum theory, and cosmology. The key concept is called "entropy," which is related to the second law of thermodynamics, considered by many scientists to be the most secure law in all of physics. But that's only the beginning, since the quest for the ultimate theory of time draws on such exciting ideas as black holes, cosmic inflation, and dark energy, before closing in on a momentous question that until recently was considered unanswerable: What happened before the big bang? In 24 riveting half-hour lectures, Mysteries of Modern Physics: Time takes you on a mind-expanding journey through the past, present, and future, guided by Professor Sean Carroll, noted author and Senior Research Associate in Physics at the California Institute of Technology.

Genres: Science, Physics, Nonfiction, Audiobook, Philosophy, Metaphysics

12 pages, Audio CD

First published January 1, 2012

About the author

Sean Carroll is a physicist and philosopher at Johns Hopkins University. He received his Ph.D. from Harvard in 1993. His research focuses on spacetime, quantum mechanics, complexity, and emergence. His book The Particle at the End of the Universe won the prestigious Winton Prize for Science Books in 2013. Carroll lives in Baltimore with his wife, writer Jennifer Ouellette.

Reviews

[Clif Hostetler · Rating 4 out of 5]

This is a collection of 24 lectures that carry a heavy dose of thermodynamics, relativity, quantum theory, and cosmology. I expose myself to this sort of cutting edge science at least once per year in the hopes that a little bit of the knowledge will stick with me. When I first started the lectures I thought perhaps the relativity of time in relation to gravity, acceleration, and travel speed were mysteries to be understood. Well, those concepts were certainly mentioned, but that stuff is elementary. The question that kept getting asked throughout the lectures was what caused the low entropy state of the universe in the past. In many ways the mystery being explored in this lectures isn't time, but rather the change in entropy.

All fundamental laws of physics allow time to be either positive or negative. There is no fundamental reason why time can not go backward. But we live in a material world that behaves in one direction only. The direction is sometimes called "the arrow of time," but really it's the moving from lower to higher entropy. The whole universe has been fueled by the change in entropy for the last 13 billion years. The final lecture reviews three possible explanations as to why the universe began with low entropy at the big bang. The lecture admits none of them are very satisfying, and I agree. One solution (not one of the three) is that God made it that way.

I was glad to learn that time is still considered to be continuous (i.e. no quantum time). Gravity and space haven't yet been successfully fit into quantum theory, but it hasn't been for lack of trying. Gravity is spooky. It exerts force all the way across the universe and even distorts time and the path of photons (i.e. light). I can't decide which is more spooky, low entropy or gravity.

In case you don't know the meaning of moving from low to high entropy, it's what happened to Humpty Dumpty.

The following is a list of the lecture titles. They provide a more complete description of the lecture contents. My comments above are flash-back impressions which don't do justice to the overall scope of the lectures.

1. Why Time is a Mystery
2. What is Time?
3. Keeping time
4. Time’s Arrow
5. The Second Law of Thermodynamics
6. Reversibility and the Laws of Physics
7. Time Reversal in Particle Physics
8. Time in Quantum Mechanics
9. Entropy and Counting
10. Playing with Entropy
11. The Past Hypothesis
12. Memory, Causality, and Action
13. Boltzmann Brains
14. Complexity and Life
15. The Perception of Time
16. Memory and Consciousness
17. Time and Relativity
18. Curved Spacetime and Black Holes
19. Time Travel
20. Black Hole Entropy
21. Evolution of the Universe
22. The Big Bang
23. The Multiverse
24. Approaches to the Arrow of Time

[Menglong Youk · Rating 5 out of 5]

4.5/5 stars

We all know what time is: 6 o'clock, tomorrow, three weeks ago, etc. But what is it really? How did it come to existence? Newton's laws work both forward and backward, but why does time always go forward?

I found this course more intense than I had anticipated. It is divided into three parts: what time is, entropy, and quantum physics and cosmology. The first part is relatively easy to digest, but the next two-thirds are quite intimidating.

Sean Caroll guides viewers into entropy deeper than I've ever been before, and this is exiting. We learn that entropy increases because there are more configurations of organizing things when it becomes chaos than when it is in order. What's more, the amount of energy alone is not enough to make it useful. Imagine a universe which is 600K everywhere. Sure, it has lots of energy, but it is useless since there is no different temperatures in space in order to generate work. On the other hand, imagine a box filled with a finite amount of energy, but one side of the box contains more energy than the other. The energy must become equilibrium, which in turn there will be a flow of energy, and this creates useful work.

In the second part, the second law of thermodynamics, the presenter also discusses Laplace's demon, Boltzmann demon, Boltzmann's brain, etc., all of which I could gather limited information, making my attempt to explain them disastrous; so I will skip that part.

In the third part, Sean Caroll talks about Relativity and the concepts of modern cosmology. He admits that our explanations on 'time' and what caused the Big Bang are not satisfying yet, and we need better theories and observations in order to understand the nature of time and our universe. I agree with this entirely because some of what was discussed in this section is hypotheses needed to be confirmed by experiments and observations. What sucks is that many scientists are skeptical whether some of their hypotheses will ever be proven, let alone in the near future.


Perhaps one statement that shocked me the most is that black hole is about time, not space *insert a mind-blowing gif* because the timeline happening inside the black hole does not align with the time outside the event horizon, making it a separate universe, if I understand correctly.

This is a tough course I've ever taken in quite awhile, and there are many fascinating and scratching-head moments, but they are all worth the time.

[Jagdish Tripathy · Rating 4 out of 5]

I listened to these lectures on Audible.

From Classical Newtonian physics to the more modern theories of quantum mechanics, time does not have a direction. Time is just another dimension we should be able to travel along - given enough information, we should be able to reconstruct the past; something conjectured by Laplace way back in the 19th century.

But a direction of time is something we are keenly aware of. We have memories of the past not the future and we (appear to?) make choices of abt the future not the past. So what accounts for our directional experience with time?

The stars of the show are Boltzmann, entropy and the Big Bang. We perceive time as having a direction because entropy has a direction - one can show with statistics and classical mechanics that entropy of a system will always go up. Boltzmann is the genius who came up with this idea and a measure of entropy. Maybe this can account for why time has a direction...why eggs can be poached, but poached things can’t be egged.

BUT...one may reach a current state from both high and low entropy states. Both low and high entropy states are equally likely in the past. We know for a fact that the entropy of the world has been continuously rising. What accounts for that? The Big Bang - an extremely low entropy state. What accounts for the low entropy state during Big Bang? We don’t know.

The direction of time continues to be a mystery. Which is cool - there is a lot of sexy physics waiting to be done.

Sean Carroll is a phenomenal narrator. He is very very good at giving you the history of thought connecting Newton’s physics to quantum mechanics today. He even takes you to Black Holes, how Black Holes are Big Bangs in reverse, multi-verses etc. There is a lot and I will be getting back to these lectures again.

Where I felt these lectures were less satisfying were the more speculative parts on the later lectures where Carroll discussed theories that might explain some of the remaining mysteries of time. These parts lacked the authoritativeness of the earlier sections.

[Kathy Nealen · Rating 3 out of 5]

Good course on the dimension of time from the perspective of physics. Later lectures in the 24 lecture series were difficult for me to comprehend. I will pursue additional study n this area to improve my understanding. I think some more animated graphics would be helpful to illustrate the ideas.

[Biblioworm · Rating 3 out of 5]

Автор - гений.
12 часов говорить ни о чем.
Физики в лекциях практически ноль, исключительно философия.

С другой стороны он в супер-популярной форме изложил бесспорные концепции.
Правда. подавляющее большинство - из школьного курса физики.
Или в США физику в школе не изучают вообще?

[Remo · Rating 4 out of 5]

Interesantísimo libro que promete abordar la física del tiempo (del tiempo que se mide en segundos, no del tiempo que hace en Benidorm) pero que acaba dándonos una repaso gigantesco por la cosmología, la termodinámica y la física de partículas, a un nivel de concepto (sin fórmulas) que recuerdo parecido al de la carrera. Es decir, que el autor trabaja en la flecha del tiempo más que en el tiempo en sí, cosa que a alguno podría desilusionar. Para leer sobre la propia física del tiempo sugiero encarecidamente ¿Y si el tiempo no existiera??, de Carlo Rovelli. Obra maestra.

Este que nos ocupa hoy es fantástico. muy entretenido pero no es una lectura ligera; hay que hacer trabajar a la neurona para salir de este mejor de lo que se entró. Muy recomendable.

[Nilesh Jasani · Rating 3 out of 5]

Prof Carroll deploys a novel tool - the passage of time - to elaborate on the same scientific theories from a new vantage point. He succeeds to a large degree given his mastery of the subjects and ability to explain complex things.

A large part of the book is spent on introducing the thermodynamic time. Professor explains lucidly how nothing in the Newtonian or even the relativistic physics point to an arrow of time that so fundamentally define our universe, let along one moving so uniformly in the forward direction. Entropy is used to indicate how the flow of time leads to a more chaotic world, and then the lectures do a stupendous backflip to prove why the same theories cannot justify the low entropy starting point even as they comprehensively indicate the future higher entropy states. (like a deck of cards where it is possible to explain on shuffle a random future state but still something more is needed to explain why the states pre-shuffles have some orders).

In a way, the important thermodynamic sections tend to be too simplistic at times and too repetitive at others. The author does well to show how the world is continuously moving towards a probablistic state which is one of the kind that is most common as a group (that is extremely random or chaotic) despite creating temporary orderly situations in microspheres through galaxies, stars, life etc. These sections are diluted by too many elementary sections defining entropy and other basic statistical and scientific concepts.

The backflip talked above is used nicely to make a switch to the cosmological time post the halfway point. The occasional, sudden digressions to psychological, mythical and philosophical "time"s are largely redundant and add little to the topic. The sections that describe relativity are not much different from those by many others in popular science books over the last few decades. The book makes some good, interesting points towards the end when it integrates the relevant latest findings from particle physics as well as astronomy. The sections on multiverses are where the Professor is at his best - in explaining something extremely complicated in a delightfully easy and still forceful way.

Overall, the lectures suffer from an uneven pace but otherwise provide some rich new knowledge.

[Tretiakov Alexander]

I've learned the formula for entropy. The author explains it so well that I think I'll remember it forever now.
He makes a good point that quantum rules must apply to the macroscopic world and how the Copenhagen interpretation that involves "observers" doesn't make any sense. Maybe the multiverse is the only sensible theory after all. All of this to say that there is a big gap right now in physics.
Lastly, a good point about the "past hypothesis". The Big Bang could have had large entropy, but it didn't, and it's not clear why.

[Mitchell McGill · Rating 3 out of 5]

A reasonably good introduction to the topic, but Carroll's work is very weak philosophically speaking. It's not just that this is physics-- and therefore there's the annoying unwillingness to reference anything philosophical-- but that the explanations themselves lack the kind of rigor a genuine truth seeker would desire. If all you want is a basic introduction to the physics of the problem, this text is fine. If you're interested in the logic itself as it applies to causality, process, movement-- then you're better off investing your time in something explicitly philosophical-- you'll get the physics there too.

[Mohamed Kamal · Rating 5 out of 5]

Time is a fascinating and mysterious concept that has captivated human imagination for centuries. The book 'Mysteries of Modern Physics: Time' is a must-read for anyone who wants to dive deeper into this captivating subject. I was blown away by the way the author weaves together complex theories and ideas with everyday experiences, making the concepts easily accessible to a wide audience. From Einstein's theory of relativity to the mysteries of quantum mechanics, this book offers a captivating journey through the evolution of our understanding of time.

What sets this book apart is its attention to detail, thorough research, and engaging writing style. I was hooked from the first chapter and couldn't put it down until I reached the end. Whether you're a seasoned physicist or just a curious reader, this book will leave you with a new appreciation for the complexities and wonders of time. I highly recommend this book to anyone who is looking to deepen their understanding of the world around us.

[Jim · Rating 3 out of 5]

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

[Alex Shrugged · Rating 4 out of 5]

I took off a star because it seemed like he threw in a couple of lectures that seemed superfluous. I suppose he had to stretch it out to a standard 24 lectures. Otherwise, great.

Note that I have been over this material from various other Great Courses lectures. This series of lectures focuses on the issues of time, how to define it (if one can), and why it goes in only one direction. In other words, why can I remember what I ate for dinner yesterday, but I can't remember what I'm going to eat for dinner tomorrow? He also beats the 2nd law of thermodynamics to death. Probably all for the best.

I might return to this material again.

[Stargazer · Rating 5 out of 5]

This was one of the most fun and educational books, I've read in a while. Time itself is a fascinating topic, discussion of which touches upon areas within physics, neuroscience and even philosophy. Some of the lectures include relatively basic stuff within cosmology, but everything is discussed with the aim of understanding the arrow of time, which made it interesting. Furthermore, Sean Carroll masters the skills of making complex concepts such as the second law of thermodynamics or inflation accessible to people without physics background. Most of all, the lectures reignited my love of science - there is so much we still don't know, but that is perhaps what makes it so fascinating.

[Jack · Rating 5 out of 5]

I love Sean Carroll’s work. His books are great and this course was really enlightening. Sometimes it seemed like a mix of science, philosophy and fiction, but through 24 sessions he managed to hold it together. I think it would have worked better if he had at least shown some of the equations that he talked about. I’m sure the solutions would have been beyond me, but at least I would have been able to get a feel for what the moving parts are. Still, it was well worth the time. I just bought his latest book about Time. It’s called From Eternity to Here. Looking forward to it.

[Bob Collins · Rating 5 out of 5]

Dr. Carroll covers time from a number of viewpoints, but primarily from the viewpoint of quantum mechanics. Although there is no coherent theory as to why there is such a concept as time - why do we remember yesterday, but not tomorrow - there are a number of interesting hypotheses, and Dr. Carroll covers many of them and does a good job of explaining a complex, difficult subject with examples, analogies and scaffolding the information, that is continually building the subject from easier concepts to more difficult ones.

[Bill · Rating 5 out of 5]

This is the fifth or tenth such book/course I've read/heard. I'm slowly starting to understand some of this, but very slowly.
Of these non-PhD-accessible explainers, Carroll's course is second only perhaps to Carlo Rovelli's work in terms of clarity and enjoyment to me.
He is a clear and entertaining speaker. A lot is covered in 12 hours of lectures. I will say that I probably put closer to 15 hours in because I had to back up and listen to several parts twice. But no fault of Carroll's. I plan on going through most of it again sometime in the near future.

[Yilmaz Aksoy · Rating 5 out of 5]

Bu kitabı audible'da dinledim. Biraz popüler fizik takip etmiş biri için çok karmaşık değil. Felsefi problemlere de değiniyor ve bu sırada felsefenin çok içine girmiyor. Böyle yapması iyi de oldu bence, yoksa ipin ucu çok kolay kaçabilirdi. Derslerin (24 adet) son kısmı da spekülasyona dayalı teorilerden oluşuyor ama bunu zaten kendisi de söylüyor. Evet, bilimde de veri yokken, uç noktalardaki sorularla uğraşırken bu yöntemler kullanılıyor. Zaman problemini anlamaya çalışırken fiziğin son 150 yıllık tarihini de oldukça güzel ve takip edilebilir bir şekilde anlatıyor.

[Benjamin · Rating 4 out of 5]

This course is very good -- it gives an excellent background to the scientific history of the study of entropy and time-reversibility. It is not nearly as good at Carroll's own book "from eternity to here" -- absolutely read that book (or listen to it) if you haven't already! -- but it does describe things not found in the book and is absolutely worth attending if you have already read that book.

[Donald · Rating 3 out of 5]

Not for the faint of heart, or those who are not already quite familiar with Physics. Carroll moves VERY quickly into quantum mechanics and general relativity. While there is no math, he assumes a tremendous amount of understanding and familiarity with the vocabulary and concepts here. If you are a fan of cosmology and have read extensively, you will likely enjoy these lectures. Beginners, and even intermediates, look elsewhere.

[Dave Symmonds · Rating 3 out of 5]

Good information, just couldn’t get into It. I got too bogged down in the properties of thermodynamics, algorithms, and other groundwork laid for later in the lecture, that I was kind of burned out by the time I got to the actual physics of time.

[Jonathan · Rating 3 out of 5]

not what i expected, i was expecting more about time travel, Einstein, and the other aspects of time. I guess i should have realized when the topic is mysteries of modern physics that it was more physics based. I guess to each their own, but doesn't the concept of entropy seem a bit arbitrary.

[Kevin Hanks · Rating 4 out of 5]

Whoa. crazy subject. Seemed at times nothing but an explanation on the 2nd law of thermodynamics (entropy increasing over time). That being said, I learned more about entropy and the arrow of time than I ever have in my life.

[Lacer · Rating 4 out of 5]

An interesting lecture series about why we have time. Looking at entropy, neuroscience, thermodynamics, quantum mechanics and cosmology. I've read quite a few popular physics books, but I still managed to learn some new stuff here.

[Trenton · Rating 5 out of 5]

This is an instant favorite of mine. I loved every moment. It is full of fun science and interesting facts about the universe, spacetime, the past hypothesis, Laplace's demon, entropy, and much more.

[Chandra Prakash · Rating 5 out of 5]

What is Time? What would happen in a universe without time?
These sort of questions are answered by Prof. Sean Carrol. He is great at explain these things. We can only observe the order of time in a universe with entropy, it is entropy which determines the order of time. In a universe without time, there won't happen anything it will be just frozen so the question doesn't even arise from what will happen in a universe without time, as we ourselves will be frozen nothing to observe. And if by universe began with big bang it had to get time,
Time travel is also very interesting topic discussed, I think after reading this book that it is our consciousness that travels in time, our body can't go back as it would voilate 2nd law of thermodynamics.

[Sjoerd van Heijst · Rating 5 out of 5]

Excellent course on modern Physics as well as a course in “Time”. Time itself remains a mystery but it does take away some of the mystical areas and sheds lights on the parts we do understand.

[mmmm · Rating 5 out of 5]

Quite accessible!

[Travis · Rating 3 out of 5]

This was an interesting lecture series to listen to, especially for all of the real science explanations of out there things like time travel or parallel universes. But honestly, I never really understood the central problem that he spent so much time going over: that time has a direction. I just don't understand the physicist perspective that somehow there shouldn't be a direction to time and/or that processes should run equally well forward or backward; the objection sounded like it was based on human standards rather than objective reality. So at times it felt like the argument was going in loops, but it was still interesting to listen to.

[Joseph L. · Rating 3 out of 5]

Watch a detailed review along with my favorite ideas and takeaways at:
https://youtu.be/WjfoUueTJMo

[Tom · Rating 5 out of 5]

Best Great Courses audiobook I've listened to yet. Super interesting subject and it really blew my mind when I listened to it.