And we’ve reached the final installment of the From Eternity to Here book club. Chapter Fifteen is entitled “The Past Through Tomorrow,” in an oblique allusion to Robert Heinlein, my favorite author when I was younger. We’re going to throw in the Epilogue for good measure.

Excerpt:

What we’ve done is given the universe a way that it can increase its entropy without limit. In a de Sitter universe, space grows without bound, but the part of space that is visible to any one observer remains finite, and has a finite entropy—the area of the cosmological horizon. Within that space, the fields fluctuate at a fixed temperature that never changes. It’s an equilibrium configuration, with every process occurring equally as often as its time-reverse. Once baby universes are added to the game, the system is no longer in equilibrium, for the simple reason that there is no such thing as equilibrium. In the presence of a positive vacuum energy (according to this story), the entropy of the universe never reaches a maximum value and stays there, because there is no maximum value for the entropy of the universe—it can always increase, by creating new universes.

This is the chapter where we attempt to put it all together. The idea was that we had been so careful and thorough in the previous chapters that in this one we could be fairly terse, setting up ideas and knocking them down with our meticulously-prepared bludgeon of Science. I’m not sure if it actually worked that way; one could argue that it would have been more effective to linger lovingly over the implications of some of these scenarios. But there was already a lot of repetition throughout the book (intentionally, so that ideas remained clear), and I didn’t want to add to it.

Of course my own current favorite idea involves baby universes pinching off from a multiverse, and I’m certainly happy to explain my reasons in favor of it. But there are also good reasons to be skeptical, especially when it comes to our lack of knowledge concerning whether baby universes actually are formed in de Sitter space. What I hope comes across is the more generic scenario: a multiverse where entropy is increasing locally because it can always increase, and does so both toward the far past and the far future. While there’s obviously a lot of work to be done in filling in the details, I haven’t heard any other broad-stroke idea that sounds like a sensible dynamical origin for the arrow of time. (Which isn’t to say that one won’t come along tomorrow.)

Chapter 16 is the Epilogue, where I reflect on where we’ve been and what it all means. I talk a little about why thinking about the multiverse is a very respectable part of the scientific endeavor, and how we should think about the fact that we are a very tiny part of a very big cosmos. Finally, I wanted to quote the very last paragraph of text in the book, at the end of the Acknowledgments:

I’m the kind of person who grows restless working at home or in the office for too long, so I frequently gather up my physics books and papers and bring them to a restaurant or coffee shop for a change of venue. Almost inevitably, a stranger will ask me what it is I’m reading, and—rather than being repulsed by all the forbidding math and science—follow up with more questions about cosmology, quantum mechanics, the universe. At a pub in London, a bartender scribbled down the ISBN number of Scott Dodelson’s Modern Cosmology; at the Green Mill jazz club in Chicago, I got a free drink for explaining dark energy. I would like to thank every person who is not a scientist but maintains a sincere fascination with the inner workings of nature, and is willing to ask questions and mull over the answers. Thinking about the nature of time might not help us build better TV sets or lose weight without exercising, but we all share the same universe, and the urge to understand it is part of what makes us human.

Among those people who share a fascination with the inner workings of nature, I of course include people who regularly read this blog. So — thanks!