The world does not present us with abstract concepts wrapped up with pretty bows, which we then must work to understand and reconcile with other concepts. Rather, the world presents us with phenomena, things that we observe and make note of, from which we must then work to derive concepts that help us understand how those phenomena relate to the rest of our experience. For subtle concepts such as entropy, this is pretty clear. You don’t walk down the street and bump into some entropy; you have to observe a variety of phenomena in nature and discern a pattern that is best thought of in terms of a new concept you label “entropy.” Armed with this helpful new concept, you observe even more phenomena, and you are inspired to refine and improve upon your original notion of what entropy really is.
For an idea as primitive and indispensable as “time,” the fact that we invent the concept rather than having it handed to us by the universe is less obvious—time is something we literally don’t know how to live without. Nevertheless, part of the task of science (and philosophy) is to take our intuitive notion of a basic concept such as “time” and turn it into something rigorous. What we find along the way is that we haven’t been using this word in a single unambiguous fashion; it has a few different meanings, each of which merits its own careful elucidation.
The book is divided into four major parts — Part One gives an overview of the issues, Part Two discusses relativity and time travel, Part Three (the longest and best part of the book) is about reversibility, entropy, and the arrow of time proper, and Part Four puts it all into a cosmological context. So Part One is somewhat out of logical order — it’s an attempt to survey the terrain and raise some ideas that will come to fruition later in the book.
The basic point of Chapter One is to examine the ways in which we use the concept of “time.” I’ll readily admit that this doesn’t sound like the sexiest idea for an opening chapter. (In my next book, an important character will be murdered within the first few pages, after which his beautiful daughter will be compelled to search for his killer in various exotic locales.) The first chapter has to serve multiple purposes — it obviously needs to provide some background for the rest of the book, but this is not a classroom where you can assume the audience will necessarily follow you to the end. So the first chapter also has to be fun and engaging, hinting at some of the mysteries to come.
In fact, I juggled the first three chapters back and forth. Chapter Two explains the basics of entropy and the arrow of time, while Chapter Three explains the basics of cosmology. At one point I had the current Chapter One placed after these two chapters, on the theory that we could be precise about definitions after we had been exposed to some of the big and exciting ideas. This was a well-intentioned theory, but not an especially good one. Test readers balked, so the current Chapter One was put back in the beginning.
Despite being about definitions and so forth, I think Chapter One turned out to be pretty interesting — indeed, I wonder now whether it shouldn’t have been longer. When you talk to people on the street about “time,” the first questions they ask tend to be along the lines of “what is time, really?” or “is time real, or just an illusion?” This chapter tries to answer those questions, or at least spell out the perspective I’ll be taking for the rest of the book. And they’re important questions, interesting in their own right, even if I breeze through them — lots of philosophical work, not to mention physics, has been addressed to these issues.
We distinguish between three ideas of time — time is a coordinate, time is what clocks measure, and time is the agent of change. These aren’t really “definitions” in any careful sense, so much as “ways we use the notion of time.” And my readers were right — it’s important to set out these different senses right from the start, as I’ve discovered that even physicists tend to blur them together in their minds.
The most important non-obvious stance I take in this chapter is to come down firmly on the side of an “eternalist” or “block universe” conception of time. The past, present, and future are equally real. Philosophers and other deep thinkers have been arguing about this for years, and I kind of dismiss the whole discussion in a couple of paragraphs. Sorry, philosophers! It’s an important issue, but we have other conceptual fish to fry.
So let me know what you thought, and what questions still remain — either about the substance of the chapter, or the stylistic choices made along the way. I’ll try to respond, although I reserve to right to say “hold that thought until we get to Chapter X.” And of course everyone else is encouraged to chime in, too.