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Writing - Commentary

From Eternity to Here - The Quest for the Ultimate Theory of Time





                        Time is always an integral part of our lives, one that is very familiar and often taken for granted. Plus, we rarely consider it in a technical or physical sense. Yet what exactly is time at the fundamental level? Why does it have a direction – indeed an ‘arrow’, from ‘past’ to ‘future’? In his book, From Eternity to Here – The Quest for the Ultimate Theory of Time, Sean Carroll tries to find solutions to this issue.

The Arrow of Time

                        Basically, the arrow of time arises from the occurrence of various irreversible processes, like breaking an egg. These, in turn, are dependent on entropy, which (in a rough sense) measures a system’s disorder. With time, orderly systems always tend to become disordered, as stated by the Second Law of Thermodynamics. However, this implies that the universe was in a low-entropy, relatively ordered state early on. Why should it be so? This is the question that Carroll seeks to answer in his book.

                        Explaining time is more complicated than it may seem at first glance. As Carroll’s explanation above has hinted, cosmology plays a vital role. Other important areas of physics involved include thermodynamics and quantum mechanics. Altogether, most of the book is actually devoted to giving a background about these areas (although in time-related context). To fully explain the book will impractical for our purposes, so I will only give a simplified description.

                        Carroll starts his narrative by clarifying the similar but ultimately different meanings of time we typically refer to: 1. a sequential label of specific moments; 2. a measure of duration between events; and 3. a medium through which we move through. The first two regards time from the perspective of an external observer, the third through an internal observer. If we wish to find a model for time, it will have to account for all of these perspectives.

                        Irreversible processes, entropy, and their role in generating the “arrow of time” are then explained in more detail. Carroll also mentions that time is defined by the Big Bang, an event that created the observable universe. The farther we are from it, the more time has passed. These combined gives the arrow of time, and helps separate the fixed past from the changeable future. Yet from a physical point of view, such a difference between the two is not quite warranted. That is, unless we assume that the past had low entropy, as it was near the Big Bang.

                        Strictly speaking, we do not know what happened at the moment of the Big Bang exactly. Regardless, once it did occur, the universe became a hot, dense system that continuously expanded. Towards the future, expansion continues at an accelerating rate, and stellar objects may break down to give empty space. Throughout this history of the universe, entropy increased from the initial low entropy state. The big question is why the entropy should be low to start with at this beginning?

Explaining Low Entropy

                        To move forward, a deeper understanding about physics is required. Carroll first explains briefly how special and general relativity works. The former is crucial in understanding spacetime (the combined property of space and time). The latter explains how gravity is simply just curved spacetime.

                        Using this background, Carroll considered two questions related to the arrow of time. One is whether time travel back to the past is possible. One method includes creating “Closed Timelike Curves”, a spacetime arrangement where the future loops to the past. Another is through wormholes, literally shortcuts between two positions. Yet both ultimately seem to be impossible under by natural laws, so the arrow of time is preserved.

                        While going back to the past seems impossible, is it possible to reverse time? Obviously, the laws of physics allow us to predict the future (to an extent), given current conditions. Can we use them to reconstruct the past? Or rather, are they reversible? That seems possible, as long as some of the systems’ states are transformed or reversed in addition to time. In addition, there is another required condition: conservation of information. In reality, information can be lost, thus the past cannot always be reconstructed from the present. So while the laws of physics themselves are reversible, actual macroscopic situations are not. This indicates the presence of an arrow of time as well.

                        Carroll then returns to entropy, using its more statistical definition to account for the arrow of time. Simply put, entropy is the number of states a system can be arranged microscopically to give the same macroscopic state. The greater the entropy, the more number of states there are. This explains why entropy increases: a system is more likely to evolve into the numerous high-entropy states. Unfortunately, this applies in both directions of time: it is equally likely for any current state to have evolved from high-entropy states. Again, we assume that entropy was low in the past, which is called a “Past Hypothesis”. Although we now understand more about entropy, we still get the same question.

                        Carroll highlights how entropy and our lives are related. For one thing, it helps define our memories. Along with information, it also helps define a crucial aspect of all life: maintaining order and avoiding (high-entropy) thermal equilibrium (for a time at least).

                        Next, an interesting idea was introduced: recurrence. If one waited long enough, any system will eventually return to its initial state. In our world where entropy increases, it means that entropy will eventually decrease as well. This infers that our universe is an entropy fluctuation from an eternal high-entropy state of thermal equilibrium. However, Carroll showed that this scenario appears to be impossible, so we still have to account for our low entropy beginnings.

                        The importance of quantum mechanics in the arrow of time is then explored. In general, certain quantum properties seem to give rise to irreversibility, and hence maybe time’s direction. Carroll believes quantum mechanics – given its fundamental character – will be involved in explaining time. Though how it actually will be involved is unknown.

                        In the rest of the book, Carroll shows several things. He takes a look at black holes, which seems to be created in an irreversible way. As it involves both entropy and gravity, black holes can provide clues as to how they work together. That could then show how the universe and time developed.

                        He then pointed out that we should expect our universe to have high entropy and exist as diluted, almost empty space.  But our universe is nowhere near the expected high entropy state. So we need to account for our unlikely initial low entropy state. The idea of inflation could do so at first glance. Simply put, we can appeal to the eternal inflation of a setting with various pockets of expanding space. Some of these pockets can then eventually reach a state similar to the Big Bang, creating a universe within a larger Multiverse setting. The problem is that, inflation itself requires low-entropy initial conditions, and so fails to answer our question.

No Definite Answer, Multiple Possible Solutions

                        Having built up this hefty background in related physics areas, Carroll tries to tackle his question: Why was entropy low early on, hence account for the arrow of time? Not surprisingly, Carroll does not have any definite answer, though there are quite a few suggestions. One solution he had worked on himself involves baby universes. It assumes an eternal, high-entropy Multiverse where quantum fluctuations create isolated baby universes and entropy increases without limit. Each baby universe starts in a low entropy state, hence the arrow of time. Despite of this, Carroll reminds us that other likely options are on the table. All of them - or something entirely new – could possibly be the real answer.

                        In the end, Carroll addresses the frustration that readers may be feeling. After going through fourteen chapters to comprehend the problem, the last chapter yielded no definite explanation of time. He stated that this imbalance is intentional, as we do not really know what the solution should be. Hence it is appropriate to focus on understanding more about the problem itself. Readers may also feel that the Multiverse idea he mentioned may not exactly seem like science. Indeed, as it is not observable, we cannot do experiments to falsify it. Carroll’s response is that we simply need to further develop our Multiverse theories so that they make falsifiable predictions. At the very least, it should be possible in principle to do so.

                        Carroll concludes by saying that we now have many ideas and data about the universe from various areas. With these, it is time that we seriously attempt to produce theories for explaining time.

Familiar yet Different

                        Although it is presented in plain terms, this book is definitely not a light read. Time is something that we are very familiar with, yet it proved to be very different at the basic level in physics. More than once, I felt intrigued how a book on time can involve so many seemingly unrelated concepts. Personally, I cannot form any opinions or preferences about how to explain time. Certainly I would need more information and get a better grasp of the ideas involved to do so. To further complicate things, there are many things that are uncertain in this area. At this stage, I agree with Carroll that we should focus on understanding the problem, rather than the solutions.

                        I do not doubt that progress can be made in explaining time. Still, I do often wonder how far we can actually go. Eventually, as we delve deeper and deeper into the fundamental causes, perhaps we may hit upon the big question: Why is there something rather than nothing? This challenging question may never be answered, as it never ends. If we discover a new law, we can always ask what lies behind it. Of course, our understanding of physics is nowhere near this level, so it is purely a philosophical concern. The religiously inclined may protest, by inferring God or other theological reasons to explain everything’s existence. But that would beg the question of why these influences exist, so nothing is solved overall.

                        It occurred to me that some may regard this quest for explaining time as unnecessary. Even if we can eventually explain the arrow of time, it seems to have few practical consequences on our lives. That may be so. But from another perspective, understanding time is part of our quest to comprehend our surroundings. Additionally, it could increase our understanding about other aspects, as Carroll had stated in the book. These reasons, at least, should make our search for the ultimate theory of time worthwhile.

                                                                                                                                                                  Mike Ko
                                                                                                                                                                  ( 1,776 words )


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