I choose to read this while I read Time Reborn by Lee Smolin. In my review of that book, I discuss time and my thoughts around it. I was drawn to Smolin’s book because I wanted to understand the concept of the block universe more clearly. I have since learned that Smolin is on the extreme end of experts in cosmology when it comes time, and I came to Carroll for a more “conventional” point of view. I have never read a book by Carroll. Although, I do listen to his podcast, Mindscape, on occasion, and it was this that taught me how great a communicator Carroll is of complex concepts.
That all led me to reading this book, the second book in less than a week that wasn’t on my (very packed) list of books to read this week. That is why I am forcing myself to pace myself with this book. It is split into four parts, and I will stop after each part to discuss my thoughts when I have the time, all the while reading other things in the down time. It isn’t so much a review as it is a review of the topics and my attempt to articulate what I have learned. You can go to each part from the Table of Contents below.
Table of Contents
- Time, Experience, and the Universe
- Time in Einstein’s Universe
- Entropy and Time’s Arrow
- From the Kitchen to the Universe
Part 1: Time, Experience, and the Universe
Carroll doesn’t spend much time defending the concept of the Block Universe. The entire reason I wanted to read this was summarized in a paragraph. The idea of a block universe versus a single framed (presentism) universe is, Carroll explained, largely accepted. Whether it is exactly true from a philosophical perspective, isn’t what is important. All that matters is that the laws of physics as we understand them are best explained by the block universe. Therefore, it is most sensible to think of it as reality. Forgive me here, and moving forward, as I may mistake what he says, but I will do my best to translate it as well as I can.
Instead, Carroll begins by discussing what time is, from a physicists perspective and the role of entropy in the flow of time. Part of me wants to reread all of this because it is so well explained. It isn’t just fascinating, it is engaging. It seems, based on one read through, that our concept of time and memory are influenced by the tendency of the universe to reach “chaos” (increased entropy). We can’t remember the future because we lack the necessary information to do it. We can create what we perceive as memories because all the possible scenarios (given the information we have, which is limited) that might have happened have condensed down into one single order of events. If we had complete information and cognitive abilities, one could predict the future, essentially remembering it, but we can’t. I find this concept particularly fascinating. It helps us try and understand why things seem to flow from one point to the next.
My fascination will likely fuel a reread because I know how I like to talk about ideas that intrigue me, and I want to do it justice. For years, I’ve held the idea of the block universe from one PBS documentary, but I always feared I misremembered it because of limited understanding. Lucky for me, Carroll makes the learning experience a joy to undertake.
Part 2: Time in Einstein’s Universe
Staying still in space will increase how much we move through time, but moving through space slows down our movement through time. Light cones of “experience” are all the possible directions life could extend from or to that event. If we consider a single event, we could try and identify all other points at a constant time, or a time where things happen simultaneously with that event, separating the past and future. Carroll explains that with relativity, we can identify the “light cone”, that is all the things that can effect this event, into the events past and future. However, there still exists a piece of the universe outside this events “light cone” which is to say, not a part of this events past or future. Our instinct is to identify far away events that we say happen simultaneously with this event, but this distinction is a matter of personal choice, as Carroll puts it.
I am writing this first bit as I listen to him explain it because I don’t really understand it, and this is my way of taking notes. It seems he is saying, if an event is independent of an event on earth, both now and in the past and future, we can’t correlate the two. Why then, can’t you repeat the process at the boundary of each “light cone” where you can identify simultaneous moments at different corrections of space? Then you could get a precise estimate of the now?
To answer this, I looked for a youtube video to help explain it! This really helps explain why simultaneous actions are subjective. The video begins by considering two observers, A and B. A is stationary, but B is moving. The act of moving, slows down B’s movement through time, relative to A. So while time for A acts normally (straight up), the time axis for B, again relative to A, is slower (tilted).
I think the best way to think about this is with B’s time being like the hypotenuse of a right triangle. Both A and B are moving along their line of time at the same “speed”, but from A’s perspective, the path B is taking is longer than its path (up the side instead of along the hypotenuse). Then, its perception of B is that it is making its way slower than itself.
When we think about it in regards to simultaneity, everything perpendicular to the time axis of A is what it experiences as happening “now” while everything perpendicular to B’s time axis is happening “now”. The problem is, as the video shows, that these lines of “now” do not match. Therefore, the entire idea of now is relative.
As Carroll approaches the end of this part, he begins to discuss time travel and wormholes. However, first he talks about the nature of predictions and the idea of the block universe. In regards to the philosophical debate over the block universe versus only the now existing, he says the following:
There is an interesting philosophical debate over which is the more fruitful version of reality; to a physicist, however, they are pretty much indistinguishable.Sean Carroll
The laws of physics work like a computer, receiving an input and predicting the future or the past occurrences. In this case, knowing everything about the present means knowing everything about the past, and, by extension, everything about the future. Nevertheless, he makes an important lesson that we make assumptions that seem reasonable using our knowledge of the world, but assumptions can be wrong (if still the best assumption available).
Part 3: Entropy and Time’s Arrow
I nearly finished this a week ago, but Buzzwordathon came along and took up all my time. I finally got back around to it. I’ve been away from this material, so I may struggle getting back into the material. Luckily, the basic concept here is simple. Carroll suggests we observe the arrow time as a matter of entropy. As we move forward in time, entropy is known to increase. We have information about what we consider the past, but what seperates these “memories” from vague predictions of the future is an understanding that entropy was lower in the past. Of course, this idea was explained in the past. What Carroll does here is do an in depth discussion about entropy to help us understand how it works and why.
Carroll equates the arrow of time to reversibility or more specifically, conservation of information. The trick is so much of our information about the world is broad and what seems irreversible has more to do with our broad scale abilities to interpret it. Let me break this down using a similar example. Entropy, as Carroll describes it, is the tendency of particles to reach an equilibrium state. This notion of states is really a bulk measurement of some substance, say a gas. However, that same process can be tracked by using the precise mechanical information of each individual molecule of gas. In that way, the tendency of this gas towards higher entropy is reversible. Part of this is because entropy is a measurement defined by how we constrain it (energy of a gas, of the world, etc.). In fact, entropy is not entirely restricted to moving to higher states.
The reason entropy increases isn’t a fundamental nature of the world because it needs to be at a higher state. Think of a system like Earth where we have to wonder how such low entropy states could be reached (assuming you would call this low entropy). Of course, that is all explained by energy being inputed to fuel localized increases, further highlighting the nature of how we define our system. The fact is, however, that even though these lower states of entropy exist and are attainable, there are simply a lot less of them. Particles can be thought to change to different levels of entropy randomly, and because there are so many more states of high entropy than low, the system as a whole will tend towards higher entropy.
This concept begot a very interesting (in my opinion) concept by Bohr. Essentially, if we break entropy down to a matter of probability, we might wonder if the low entropy state of the universe was in fact a part of a larger universe that has experienced a brief, random, drop in entropy. What we experience as the observable universe can be thought as what has happened since that drop. It is very unlikely, but given an infinite amount of time, these impossibilities will happen. It raises the question of what time would be like, say during the period of decrease in entropy. Carroll proposes it would seem the same because time isn’t a consequence of the increase in entropy but rather that there is a distinctly different level of entropy in one direction. In the end, Carroll dismisses this idea because it has other problems. Basically, even though our universe is possible, it is apparently more likely that localized drops in entropy would produce intelligent disembodied brains floating in space far more frequently than the series of events that were necessary to lead to us. Assuming we are representative for the average life form in the universe, we can’t accept this idea. There is a lot of unpack here, and I am not doing it justice. Nevertheless it was fascinating.
Carroll then goes on to discuss cause and effect, and I think it is best conveyed with a couple quotes:
Ultimately, our ability to “choose” how to act in the future is a reflection of our ignorance concerning the specific microstate of the universe; if Laplac’es Demon were around, he would know exactly how we are going to act…
We have no trouble believing in a past condition that restricts our current microstate. The microscopic laws of physics draw no distinction between past and future, and the idea that one event “causes” another or that we can “choose” different actions in the future in a way that we can’t in the past is nowhere to be found therein.Sean Carroll, From Eternity to Here, Page 185
This really relates to the idea of entropy and the state of every particle involved. We have enough information about the “particles” that relate to the past to recognize what lead to what. However, we don’t have the necessary information to constrain what leads to what in the future. We can track certain paths, and even make broad scale predictions, but it is flawed because it is not a complete estimation. Leplace’s demon is a hypothetical observer with complete knowledge.
Personally, I am very much determinist, and this really highlights why. We can look back and see how a causes b, and how said outcomes are out of our control. Why then is it so taboo to suggest c will cause d, thereby constraining what we can and cannot do.
This is where Carroll pivots to consider quantum mechanics. If we suggest reversibility is a question of information, quantum mechanics directly challenges that. I don’t want to get into the finer details, largely because it is hard to understand even if I thought I could convey the meaning. Just rest assure Carroll takes the time to introduce the reader to this concept. Essentially, observing particles creates a random event. Granted, there are equations to constrain how they act (i.e. deterministic), but they are still random. Does quantum mechanics introduce a fundamental arrow of time into the laws of physics? Carroll suggests no. Avoiding the details, he breaks it down to this:
We’re…coarsegraining, just as we did in (classical) statistical mechanics to define macrostates corresponding to various microstates. The information about our entanglement with the messy external environment is analogous to the information about the…molecule(s) in box of gas.Carroll, From Time to Eternity, page 255
The way I take this to mean is we are looking at a localized region without understanding the finer interactions between it, the outside world, and ourselves. Admittedly, I don’t entirely grasp this idea. It seems that with more information we could better understand how we lead to the collapse of a wave into a precise location. All that was to end with the same basic concept that the laws are reversible on a microscopic scale, and the irreversibility comes from broad scale attempts to constrain it.
Part 4: From the Kitchen to the Multiverse
Up until now, I’ve tried to provide an in depth (if scattered) overview of each part of this book. My goal has been to understand the material well enough to articulate it. Unfortunately, I am running short on time and energy, so this will be more succinct. Here Carroll digs into cosmological concepts like the big bang and the loss of information with black holes or inflation and dark matter. He tries to answer the question of why the universe started with low entropy because it is the fundamental reason for our perception of time, as he sees it. He spends a great deal of time, here and in past sections, providing the reader with fundamentals, and it is hard not to lose focus. He is walking us through the appropriate science to understand his conclusion, but all the while I’m trying to jump ahead and see what his point is. I advise future readers (possibly myself, of at least this last part) to focus on the details at hand.
I am leaving this last section with a lesser understanding because of my inability to read through this multiple times and take notes as I go. A lot of it goes back to previously discussed topics. It isn’t enough to say the universe is the way it is because it is. We seemingly have an early universe that defies the laws of physics as we now know them. Therefore, he tries to tackle this problem from a number of different approaches. I think that is representative of this book as a whole which has a fairly clear take away but plenty of alternatives to consider. The answer may be a multiverse that produces a number of different beginnings, or perhaps its baby universes that spring up from a higher entropy universe. It might be an a result of inflation.
In his epilogue, he takes time to discuss how he approached the book, but he also takes time to discuss the nature of science and physics like this that is on the edge. Its okay for science to make fart fetched predictions (like a multiverse, which he explains isn’t a theory unto itself), but it must be a part of a larger testable theory. Sometimes, those tests take time to develop. Other times, they fade away. In either case, the process is what is exciting, the act of testing and learning more and more about the nature of time.
Overall, this book was fascinating. It is dense, but part of that comes from the ample review that Carroll so helpfully provides. All of it is accompanied by clear conclusions of his points that should make it easier for the average reader to understand. 4.5/5 stars.