From Eternity to Book Club: Chapters Fifteen and Sixteen

By Sean Carroll | April 20, 2010 3:07 pm

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.


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!

  • Tom Allen

    The border of science and philosophy is a fun place to be! I love this wild stuff. It seems to me that you believe that de Sitter space is the natural state of (at least the parent) universe and that provided that de Sitter space can give rise to baby universes, then we can find our place in eternity.

    You show this schematically in Figure 87. A couple of questions about the figure:

    1. The vertical axis is labeled “time”. Does the horizontal extent represent physical space? Or maybe entropy (in 2-D?)

    2. The spawned universes are shown as coevally coming from the parent. I assume this is just for representational convenience.

    Do you believe that de Sitter space is a necessary condition for birthing a baby universe, or merely more likely given the longevity of such a state? Might it be possible that conditions for instantiating the high energy inflaton could be more favorable in a younger universe, maybe even in the context of a black hole? (I’m itching to coin the term “de Baby Sitter space.”}

    And finally, thank you for this delicious book. It’s just the sort of intellectual repast that I enjoy, even though I am far from sated.

  • Sean

    The horizontal extent is just space; the circles are supposed to represent all of space. And yes, it’s very hard to draw a reliable picture on a 2-d piece of paper.

    I think that de Sitter space is where we are going, so a challenge for cosmology is how we can escape that fate. The problem with high-energy de Sitter space is that it’s a low entropy configuration, so you have to explain why you’re ever in such a state in the first place.

    Glad you liked the book!

  • Oded

    Hi Sean

    I have a question that belongs to the previous chapter – you said cosmologists would say that inflation would robustly follow from a high energy dense region of space, creating a fresh low entropy universe – the only problem was, the high energy space was in itself very low entropy, with high entropy being actually empty space.
    My question is – what about a big crunch? If the universe were right now to be overtaken by gravity, so that everything starting collapsing into each other, but still in an entropy increasing, full of black holes, messy way, then would inflation robustly follow the result of that? Because that configuration is of course very high entropy. What am I missing?

    I’ll also add that I loved the book, absolutely wonderful fun, keeping me excited as I now plow through my first undergraduate year in physics, calculating uniform spherical charges and such. :)

  • Sean

    That would be high entropy, but there’s no reason to think that inflation would follow. In classical GR you’d just expect to get a singularity. In quantum gravity who knows, but I don’t think there’s any reason to thing that space would jump to a lower-entropy state and start inflating.

    Glad you liked the book!

  • Mantis

    Sean: “I haven’t heard any other broad-stroke idea that sounds like a sensible dynamical origin for the arrow of time.”

    Personally I fail to see how your “multiverse did it” explanation of the arrow of time is any better then the “god did it” approach (even though I’m an atheist). Both multiverse and god are similarly ill-defined concepts completely inaccessible to empirical science.

  • David Brown

    “… baby universes pinching off from a multiverse …” What might be some empirical predictions stemming from baby universes pinching off from a multiverse? If the Fredkin-Wolfram information process is the unique, physically valid computational method for M-theory, then Fredkin-Wolfram digital physics might make 3 decisive empirical predictions. Google “nks forum” and look at the “Applied” section. Google “feynman fredkin wolfram” for the conceptual basis of the theory.

  • Another Sean

    A wonderful book and most valuable contribution-thank you. The dimension of time is so psychologically powerful- so pervasive. Even after more than 100 years from the birth of relativity, the fact that time itself is the subject of scientific scrutiny is something I still psychologically resist. Your book, as well as Gell-Mann’s thoughts, help explain why.
    If you ever write a second edition, I’d love some thoughts on how time relates to holography. It seems like the holorgram needs to be placed at a boundary in time. Indeed you talked about the fact that we live close to a temporal boundary in the past which, like the earth in space, provides us a sense of direction. This makes sense to me. But try as I might, taking the step of imagining a boundary in future time proves difficult. Particularly if the universe is open. What are we to make of a boundary in time that is always receeding? It doesn’t seem like much of a boundary.

  • Peter

    Hi Sean

    I enjoyed reading your book. I especially liked your way to explain complex facts and that you clearly distinguish between established and speculative ideas. Sadly, in the epilogue you help to spread a common misunderstanding about Christians. We Christians do not oppose Darwin’s theory because we think that we matter, but because we think that God and what he tells us in the bible matters. This is just the opposite of what you write. Likewise, in the medieval worldview it’s not an honour to live near the centre of the universe. Belonging to the lower spheres means being imperfect and mortal, while the perfect and eternal beings live in the outer spheres. This also shows that God matters, not human beings.

  • Jacob Russell

    A free drink for explaining dark energy! Wow…now I know i’M gonna have to buy your book!

    … seriously, mucho thanks for taking the time to demonstrate the wonder of real science to a wider public…. if more people can understand science… maybe there’s hope for poetry!

  • eigen value
  • Ahmad

    >”so I frequently gather up my physics books and papers and bring them to a restaurant or coffee shop for a change of venue.”

    That’s funny – I do that all the time too. I cannot spend too much time in one place reading about things that take you many places. It.. itches.

    Recently though, I have found coffee shops and the like to be rather limiting of ones imagination, even more so than a room with four white walls, because they impose a particular atmosphere on the thinker. The park is now my favorite hangout. It is a little overwhelming at first (for a large enough park), but I heartily recommend it.

    As for losing weight without physical exercise, I am living proof that a man can lose 30 pounds in a couple of weeks via unhealthy manner of thought.


  • Brando

    I loved the last paragraph. There are MANY of us out there who do not have a proper science education (beyond our own exploration) nor work in scientific fields, yet have the same curiosities about the Universe and our place in it. Scientific enthusiasm amongst the population seems lacking these days, however I think it can be very contagious and easy to spread with the right combination of factors. You are the soldiers and we are glad to support your endeavors in our own ways and perhaps sometimes live vicariously through them.

  • Ray

    Not sure how active this thread is at this point.
    Anyway, I’m curious how well established the baby universe idea is at this point. I seem to recall that you can use holography to show that a new expanding universe corresponds to a black hole forming and evaporating in the parent universe, but I also remember reading somewhere that this can’t be a pure state. I suppose this makes sense since there’s only so much information that you can fit in a black hole, while the entropy of an expanding universe is unlimited.

    Is it at all helpful that a lot of the hawking radiation coming off of an evaporating black hole in an emptyish universe will cross a cosmological horizon before it interacts with anything?

    Compliments on the book. I very much enjoyed reading it. It reads kind of like a cross between pop-science and a philosophy book but still provides enough technical info to teach something new to those of us who have had years of rigorous training.

  • Sean

    The baby-universe idea is nowhere near established! There are some arguments that it’s reasonable, and some equally good arguments that it’s not going to happen. We don’t really understand quantum gravity well enough to say for sure at the moment.

    I don’t think it’s helpful that the Hawking radiation doesn’t interact much — at least, I don’t see what it would help with.

    Glad you liked the book!

  • Cindy W

    Hi Sean,

    I have hardly any science background and can only barely grasp most of what you write about but I have been fascinated by the following questions my entire life and I bet a day doesn’t go by where I don’t (very unscientifically!!) contemplate the following:

    What truly is infinity? Does infinity bother you?
    Isn’t it true that if we can describe something (ie the expanding universe), then what is beyond that “expanding thing”?
    Is it true that it is not possible to understand what comes afterwards?
    Or….what actually precipitated the Big Bang? Isn’t it true that no matter what the theory is of what happened or what was at pre-Big Bang, there still has to then be a precipitating event that made the pre-pre Big Bang even happen?

    Do these questions “bother” cosmos scientists?


  • Sean

    I personally am not at all bothered by infinity. The universe might ultimately turn out to be infinite, or not; I actually suspect that it will be.

    There doesn’t have to be anything that “made” the Big Bang happen. The universe could just very well last forever.

    Of course it doesn’t matter what our preferences are — we’ll have to see what Nature actually chooses to do.


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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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