Fans of the hit TV series Lost are awaiting the big event next week: the premiere of Season Six on Tuesday night. The show is famous for its mysteries and plot twists, so this year has a special status: it’s the final season, where everything that’s going to be revealed will be revealed. That might not be absolutely everything, but it should be a lot.

Lost has always played with time and narrative — characters’ backstories were told through elaborate flashbacks, lending a richness of nuance to their behavior in the main story. But time travel as a plot device was established as a central theme during Season Five. One happy consequence was the invention of Lost University, through which fans could learn a little about physics and other real-world subjects underlying events in the show.

Naturally, scientifically-minded folks want to know: how respectable is the treatment of time travel, anyway? We are, as always, here to help. My short take: Lost is a TV fantasy, not a documentary, and it doesn’t try all that hard to conform to general relativity or the other known laws of physics. But happily, the most important of the Rules for Time Travelers is very much obeyed: there are no paradoxes. And more interestingly, the spirit of the rules is obeyed, and indeed put to good narrative effect. The potential for time-travel paradoxes helps illuminate issues of free will vs. predestination, a central theme of the show. And what more can you ask for in a time-travel story than that?

Details below the fold, full of spoilers. (Not for the upcoming season, of course.) See also discussions from io9, Popular Mechanics, and Sheril.

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Welcome to this week’s installment of the From Eternity to Here book club. Today we look at Chapter Two, “The Heavy Hand of Entropy.”

[By the way: are we going too slowly? If there is overwhelming sentiment to move to two chapters per week, that would be no problem. But if sentiment is non-overwhelming, we'll stick to the original plan.]

Excerpt:

While it’s true that the presence of the Earth beneath our feet picks out an “arrow of space” by distinguishing up from down, it’s pretty clear that this is a local, parochial phenomenon, rather than a reflection of the underlying laws of nature. We can easily imagine ourselves out in space where there is no preferred direction. But the underlying laws of nature do not pick out a preferred direction of time, any more than they pick out a preferred direction in space. If we confine our attention to very simple systems with just a few moving parts, whose motion reflects the basic laws of physics rather than our messy local conditions, there is no arrow of time—we can’t tell when a movie is being run backward…

The arrow of time, therefore, is not a feature of the underlying laws of physics, at least as far as we know. Rather, like the up/down orientation space picked out by the Earth, the preferred direction of time is also a consequence of features of our environment. In the case of time, it’s not that we live in the spatial vicinity of an influential object, it’s that we live in the temporal vicinity of an influential event: the birth of the universe. The beginning of our observable universe, the hot dense state known as the Big Bang, had a very low entropy. The influence of that event orients us in time, just as the presence of the Earth orients us in space.

This chapter serves an obvious purpose — it explains in basic terms the ideas of irreversibility, entropy, and the arrow of time. It’s a whirlwind overview of concepts that will be developed in greater detail in the rest of the book, especially in Part Three. As a consequence, there are a few statements that may seem like bald assertions that really deserve more careful justification — hopefully that justification will come later.

Here’s where I got to use those “incompatible arrows” stories I blogged about some time back (I, II, III, IV). The fact that the arrow of time is so strongly ingrained in the way we think about the world makes it an interesting target for fiction — what would happen if the arrow of time ran backwards? The straightforward answer, of course, is “absolutely nothing” — there is no prior notion of “backwards” or “forwards.” As long as there is an arrow of time that is consistent for everyone, things would appear normal to us; there is one direction of time we all remember, which we call “the past,” when the entropy was lower. It’s when different interacting subsystems of the universe have different arrows of time that things get interesting. So we look briefly at stories by Lewis Carroll, F. Scott Fitzgerald, and Martin Amis, all of which use that trick. (Does anyone know of a reversed-arrow story that predates Through the Looking Glass?) Of course these are all fantasies, because it can’t happen in the real world, but that’s part of the speculative fun.

Then we go into entropy and the Second Law, from Sadi Carnot and Rudolf Clausius to Ludwig Boltzmann, followed by some discussion of different manifestations of time’s arrow. All at lightning speed, I’m afraid — there’s a tremendous amount of fascinating history here that I don’t cover in anywhere near the detail it deserves. But the real point of the chapter isn’t to tell the historical stories, it’s to emphasize the ubiquity of the arrow of time. It’s not just about stirring eggs to make omelets — it has to do with metabolism and the structure of life, why we remember the past and not the future, and why we think we have free will. Man, someone should write a book about this stuff!

Welcome to the first installment of the From Eternity to Here book club. We’re starting at the beginning, with Chapter One, “The Past is Present Memory.”

Excerpt:

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.

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As promised, we’re going to have a book club to talk about From Eternity to Here. Roughly speaking, every Tuesday I’ll post about another chapter, and we’ll talk about it. Easy enough, right? (Chapters 4 and 5, about relativity, are pretty short and will be combined into one week.)

For the most part I won’t be summarizing each chapter — because you’ll all have read the book, so that would be boring. Instead, I want to give some behind-the-scenes insight about what was going through my mind when I put each chapter together — a little exclusive for readers of the blog. Of course, in the comments I hope we can discuss the substance of the chapters in as much detail as we like. I’m going to try to participate actively in all the discussions, so I hope to answer questions when I can — and certainly expect to learn something myself along the way.

The book is divided into four parts: an overview, spacetime and relativity, entropy and the Second Law, and a discussion of how it all fits into cosmology. You can find a more detailed table of contents here, and here is the prologue to get you in the mood. Part Three is definitely the high point of the book, so be sure to stick around for that.

So see you next Tuesday! Get reading!

Part One: Overview

• January 19: Chapter One (What is time?)
• January 26: Chapter Two (Entropy and the Second Law)
• February 2: Chapter Three (The expanding universe)

Part Two: Relativity

• February 9: Chapters Four and Five (Special and general relativity)
• February 16: Chapter Six (Time travel)

Part Three: Entropy and the Arrow of Time

• February 23: Chapter Seven (Determinism and reversibility)
• March 2: Chapter Eight (Entropy according to Boltzmann)
• March 9: Chapter Nine (Information, memory, life…)
• March 16: Chapter Ten (Recurrence and Boltzmann brains)
• March 23: Chapter Eleven (Quantum mechanics)

Part Four: Time and the Universe

• March 30: Chapter Twelve (Black holes)
• April 6: Chapter Thirteen (Evolution of the universe)
• April 13: Chapter Fourteen (Inflation)
• April 20: Chapter Fifteen and Epilogue (Explaining the arrow of time)

And here you thought the holidays were over. Silly you. Today is the greatest holiday of all: Book Release Day!

That’s right — From Eternity to Here: The Quest for the Ultimate Theory of Time is out today. That means you could head over to Amazon.com and buy the book right now:

From Eternity to Here at Amazon

Don’t worry, we’ll wait. Of course you could also buy it later, but there are benefits to having a great first day, and we’re aiming to get as many Amazon purchases as we can. So you might want to take Lee Billings’s advice:

Just drafted a micro-review of @seanmcarroll’s “From Eternity to Here”. It’s really quite good–I suggest you all buy several copies.

I’m excited, anyway. You can find various goodies on the web page, including a reprint of the prologue, an annotated table of contents, a list of upcoming events, links to blurbs and reviews and other commentary, and a collection of related articles. Heck, I even went out and made a video:

I don’t think Spielberg is checking his rear-view mirror, but my budget was a bit lower.

Looking back through my old emails, I was first talking seriously about writing a book on the arrow of time in August, 2006. The contract with Dutton was agreed upon in May, 2007. Worked on it on and off, and finally started working in earnest in mid/late-2008. I emailed the manuscript to the publisher at 2:42 a.m. on Friday, May 8, 2009. And now it’s released to the world.

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I think we can all agree that I’ve been admirably restrained with respect to talking about my upcoming book before it even appears. (Maybe you don’t think so, but believe me — I’ve been restraining myself.) Die-hards have been able to follow the excitement at the Facebook page, where fascinating details about cover blurbs and review copies have been politely sequestered.

All that is about to end soon! Yesterday I received in the mail an actual copy of the hardcover, a tangible artifact testifying to the reality of this long-anticipated event. Here it is, rubbing shoulders with a few other well-known bestsellers.

The official release date is January 7. Yes, there will be a Kindle edition; at some point later in January there will even be an audio book. And I’m certainly not going to stop you from ordering it. But my publisher tells me that what would be really great is if a bunch of people ordered it exactly on January 7. So that’s when I’ll really be encouraging you.

Even after the book is out, I don’t want to turn the blog into all book, all the time. But I do want to try a book club experiment, where we go through individual chapters, one week at a time, with me revealing some of the thought processes that went into each chapter and all of us having a back-and-forth discussion. Should be fun!

No formal book tour, but I’ll be doing a few readings and events. Check the Facebook page or book web page for more.

You know, other people talk a lot about time, too — it’s not just me. Here’s a great video from Nature, featuring a conversation between David Gross and Itzhak Fouxon about the existence of time. (Via Sarah Kavassalis.) Itzhak plays the role of the starry-eyed young researcher — he opens the video by telling us how he originally went into physics to impress girls, although apparently he has stuck with it for other reasons. Gross, of course, shared a Nobel Prize for asymptotic freedom, and has become one of the most influential string theorists around. David plays the role of the avuncular elder statesman (I’ve seen him be somewhat more acerbic in his criticisms) — but he’s one of the smartest people in physics, and his admonitions are well worth listening to. He gives some practical advice, but also advises young people to think big.

Unfortunately the video doesn’t seem to be embeddable, but you can go to the video page and click on the “David Gross” entry. (The others are good, too!)

You all know my perspective here — time probably exists, and we should try to understand it rather than replace it. But I’ll agree with David — let’s not ignore more “practical” problems, but not be afraid to tackle the big ideas!

Greetings from Down Under! Current at the CosPA conference in Melbourne, after spending a couple of days in Sydney — a brief fling through Adelaide up next.

It’s been a mixed bag so far; while I’ve had great fun interacting with people here in Australia, I’ve also been struggling with a nasty cold I picked up on the flight over. Spent yesterday mostly in bed, too fogged up to even work on my talk for Friday. But when I’ve had the strength to be up and about, it’s been a treat. Here’s an iPhone snap of the University of Sydney; that clocktower in the middle houses, appropriately enough, the Centre for Time.

One of the perks of civilization that hasn’t quite caught on in these parts is affordable internet access in hotel rooms, so don’t expect a lot of blogging over the next week or two. Instead, I can point you to a couple of recent videos. One is an extended interview for Edge, entitled Why Does the Universe Look the Way it Does? It is an interview (presented in text and video), not a carefully pre-planned document, so not all thoughts are arranged as elegantly as one might like. Here is some of the flavor:

We are in a very unusual situation in the history of science where physics has become slightly a victim of its own success. We have theories that fit the data, which is a terrible thing to have when you are a theoretical physicist. You want to be the one who invents those theories, but you don’t want to live in a world where those theories have already been invented because then it becomes harder to improve upon them when they just fit the data. What you want are anomalies given to us by the data that we don’t know how to explain.

The other one is a panel discussion on Time Since Einstein, from the World Science Festival. As the description there says, it features Roger Penrose, David Albert, and some other people it would be too exhausting to list individually. Here’s part 1 of 5:

World Science Festival 2009: Time Since Einstein, Part 1 of 5 from World Science Festival on Vimeo.

Now if only my immune system would finish off the little viral buggers inside me, I could get out and see a bit of this interesting country.

I’ve been meaning to write about this for, well, some time: how do we visualize time? What is the mental picture we have in our heads of this basic dimension of our existence? This is bound to be one the the stranger posts of mine you’ve read, but, so be it.

Looking online I find basically no research or anything written on this subject, but I am quite certain that just about everyone has some picture of time in their heads. For me, it’s quite a visual one, and past events and for that matter future ones are all attached to my mental picture of the time continuum. My notion of all history, from my own to that of the universe is inextricably linked with my internal mental images of time.

The thing is, as I have reflected on how I actually internally visualize time, I have found it to be somewhat bizarre. Or maybe not – I don’t really know because I haven’t really explored this in one-on-one conversation with others and haven’t learned from anything written out there just how different my picture is from others’. So here goes…I hope those of you out there who are intrigued or inspired by this will share their own images.

The main thing is that my mental picture of time changes depending on the time scale involved, from a microsecond to a minute to an hour, day, week, month, year, or many years. Starting at the largest time scales, those of the cosmos, when I am looking back in time over billions of years I imagine the classical, boring sort of “time as a line” progressing from left to right, straight across my mental image. As we zoom in to more recent cosmological time, though, millions of years, the line becomes more of a curve, and curving toward me. But then, very oddly (and this pattern will repeat itself) when we get to the much more recent past, say the last few thousand years, the curve is revealed to be more of a strip of sorts and moving from down and to the right (that’s the best way I can express it) toward the upper left.

It’s really strange: if I think of a time, say, 20 000 years ago, in my mental field of view it’s definitely off to the right, and as I refer to more recent times, the ribbon is such that more recent times are to the left of earlier times.

But this is not absolute: as we get to the last 2000 or so years, the earlier part is sort of coming straight at me, eventually becoming (you got it) a ribbon coming from the lower left to the upper right again. If I am considering the period from the Renaissance to the present, for example, I see a more distant past as actually more distant, off to the left, coming closer in more recent times an moving left to right. The future, on this time scale, goes off to my right sort of behind me (where I can’t see – duh!)

Okay I have probably lost at least 2/3 of the people who started reading this. Huh? Either this is so alien to how they think of time they don’t really see what I am seeing, or don’t care, or think that this is so off that wall it’s not worth reading further.

So, for the rest of you, the next part is where it gets kind of interesting. My mental timeline/ribbon, which has been snaking from left to right and back across my mental field of view, does a few more twists. As I think of the time scale of my life which began in the early 1960′s (okay, 1959), those early 60′s years are sort of again coming straight at me, becoming a left-to-right ribbon in the 70′s and then definitely right-to-left by the mid 80′s. The years from then to now flow from far away and to the right to nearby toward the left. But they don’t cross the center of my mental picture – that’s the present.

If we zoom in further, say to the past several years, the ribbon is a string of months going back. As I view earlier and earlier months they recede, up and to the right, and merge with the ribbon of the decades. Events, major and minor, are recallable by zooming into my past picture of then-present time. They are all there (the ones I remember, anyway), and freakily often I can remember the exact dates and times they occurred.

Last week to a few months ago is definitely on that ribbon, stretching up and to the right as we go further into the past. But then we get to yesterday, today, and tomorrow. Here the ribbon, which is segmented by lines marking the days, does a weird thing. For a given day, the ribbon starts out straight in front of me, going up, as if I had to climb it, the hours marked off by lines. The ribbon climbs up, into the darkness, reaching a peak and then, after midnight, descending down into the next day, week, month, and year, away from me, off into the distance in the left part of my mental view of time.

The future, the near future anyway (years) is definitely to the left in my mind’s eye. (And no, this whole post is not some sort of allegory of my personal political evolution…) The long term future is unpopulated by memories or images of expectations or hopes, and snakes off to the right.

All this changes when we are talking about smaller time scales. As I zoom into the present hour, to finer and finer scales it becomes more and more a straight line extending from left to right. I can zoom in from here to any micro-time scale and it stays the same. Somehow the left-right snaking curve is attached to particular memories, including my memories of historical events about which I have learned. Micro-time is so non-specific that it doesn’t trigger the snaky ribbon time view.

Another oddity about me in particular is that I actually find it hard to use a standard calendar to keep track of appointments, important meetings etc. I don’t see time on that seven-day table! But with a few anchor dates in the future, gotten from standard calendars, I can quickly calculate intervening dates and their days of the week. If I know I have an appointment on December 4, and an exam to give on December 7, I can see in my head what days they are and I do rather well remembering them. But at this moment, for example, I cannot tell you what day of the week Christmas is (though I know next January 18 is a Monday…)

I know there will be plenty of eye-rolling at this possibly boring description of my mental view of time, but, as I say, I hope it will trigger lots of you out there to share your own. If you really think about it (and I bet you probably have not) you so have *some* sort of picture in your head. What is it?

I’m in the middle of jetting hither and yon, talking to people about the arrow of time. (Wouldn’t it be great if I had a book to sell them?) Right now, as prophesyed, I’m at the Quantum To Cosmos Festival at the Perimeter Institute. They’re extremely on the ball over here, so every event is being recorded by the ultra-professional folks at TVO, and instantly available on the web. So here is the talk I gave on Saturday night — a public-level discussion of entropy and how it connects to the history of our universe.

Yes, that’s a pretty suave picture of me on the image capture. What can I say? I’m just one of those lucky folks with an effortless magic in front of the camera.

If you prefer to get your talks about entropy unadulterated by voice and motion, and don’t mind a more technical presentation, I’ve put the slides from my recent Caltech colloquium online. These are aimed basically at grad students in physics, so there is an equation or two, and the caveats are spelled out more clearly. But the punchline is the same.