Sean Carroll

Author: Sean Carroll

  • Time Travel in Lost: The Metaphorics of Predestination

    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.

    (more…)

  • Making Coffee

    My new-espresso-machine wave function has not yet collapsed. In the meantime, via Cynical-C, here are two videos from Intelligentsia Coffee in Venice (CA, not Italy). Making espresso, and making siphon (or “syphon,” apparently) coffee.

    Suffice it to say that my level of coffee-making care doesn’t really compete.

  • From Eternity to Book Club: Chapter Two

    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!

  • Coffee Rituals

    We’re long overdule for an open-type thread around here, so let me provide the excuse by asking one of the world’s great questions: what’s the best way to make coffee?

    I’m an eclectic coffee drinker; I like espresso but also enjoy a really good cup of American coffee, and I prefer coffee black but am willing to adulterate it with milk if I suspect the quality is not going to be that high. (Sugar under no circumstances.) For the past few years I’ve relied on the lowest-effort method I know of that is guaranteed to produce a good cup: freshly-ground dark roast beans, placed in a simple cone filter and hot water poured right in. Practically instant coffee, but a result that can be as good as the beans allow.

    S1CO But I’d like to start mixing more espresso into my home coffee experience, so I’m in the market for a new espresso machine. If I were a physicist of means, I might go for a work of art like the Elektra Micro Casa Lever on right. Or would I? This is a spring-action lever machine, which is to be contrasted with the manual levers, not to mention the automatics and super-automatics, and then there’s the matter of boilers, switches, heat exchangers … a complete mess. The pumps are certainly elegant, but I’d also like something that is functional and doesn’t require constant pampering. So I am in the unusual position of being frozen with indecision about what kind of espresso machine to get. Any opinions out there?

    The ground rules here are:

    1. There’s no such thing as right or wrong; different people have different tastes, for which different approaches are appropriate.
    2. Answers with specific comparisons of advantages and disadvantages are more useful than simple insistence on truth.

    I do understand that this is the internet and rules are unlikely to be followed, but I feel I should try.

    Obviously not all advice on such a topic is too be trusted. The Engineer’s Guide to Drinks thread featured a sobering (as it were) number of people who think a “martini” should just be chilled gin rather than a proper cocktail, and were proud to admit it in public! So caveat lector. And if you want to talk about something other than coffee, be our guest.

  • Andrew Lange

    lange_-_size All of Caltech, and the cosmology community worldwide, is mourning the death of Andrew Lange. He was one of the world’s leading scientists, co-leader of the Boomerang experiment that provided the first precise measurement of the first acoustic peak in the cosmic microwave background. He took his own life Thursday night.

    It’s hard to convey how unexpected and tragic this news is. Very few people combined Andrew’s brilliance as a scientist with his warmth as a person. He always had a sparkle in his eye, was enthusiastically in love with science and ideas, and was constantly doing his best to make Caltech the best possible place, not just for himself but for everyone else around him. He was one of the good guys. The last I spoke with him, Andrew was energetically raising funds for a new submillimeter telescope, organizing conferences, and helping plan for a new theoretical physics center. We are all walking around in shock, wondering how this could happen and whether we could have done anything to prevent it. Caltech has had several suicides this year — hard to make sense of any of them.

    The message from Caltech President Jean-Lou Chameau is below the fold. For any local readers, there’s contact info if you would like to talk to counselors for any reason.

    (more…)

  • Engineer’s Guide to Drinks

    It’s Friday and science keeps getting in the way of blogging, but here’s one of the more useful posts you’ll ever get from us: via FlowingData, the Engineer’s Guide to Drinks. Yes, there is a full-sized pdf at the original post.

    Engineer's Guide to Drinks

    That’s a slightly crazy Martini — 2:1 gin to vermouth. I think the pendulum has swung too far towards “dry,” but please.

  • The Truth Still Matters

    Over at the Intersection, Chris Mooney is concerned that we haven’t had a science/religion tiff in what, days? So he wants to offer a defense of organizations like the National Center for Science Education, who choose to promote science by downplaying any conflicts between science and religion. For example, the NCSE sponsors a Faith Project, where you can be reassured that scientists aren’t nearly as godless as the newspapers would have you believe.

    In the real world, scientists have different stances toward religion. Some of us think that science and religion are (for conventional definitions of science and religion) incompatible. Others find them perfectly consistent with each other. (It’s worth pointing out that “X is true” and “People exist who believe X is true” are not actually the same statement, despite what Chad and Chris and others would have you believe. I’ve tried to emphasize that distinction over and over, to little avail.)

    In response to this situation, we uncompromising atheists have a typically strident and trouble-making idea: organizations that bill themselves as “centers for science education” and “associations for science” and “academies of science” should not take stances on matters of religion. Outlandish, I know. But we think that organizations dedicated to science should not wander off into theology, even with the best of intentions. Stick with talking about science, and everyone should be happy.

    But they’re not happy; Chris and others (Josh Rosenau at Thoughts from Kansas is a thoughtful example) think that the NCSE can be more effective if it proactively tries to convince people that science and religion need not be incompatible. As an argument toward this conclusion, Chris attempts to horrify us by offering the following hypothetical conversation between a religious believer and an NCSE representative:

    Religious believer: I know you say that evolution is good science, but I’m afraid of what my pastor says–that accepting it is the road to damnation.

    NCSE: As a policy, we only talk about science and to not take any stance on religion. So we couldn’t comment on that.

    Religious believer: I do have one friend who accepts evolution, but he stopped going to church too and that worries me.

    NCSE: All we can really tell you is that evolution is the bedrock of modern biology, and universally accepted within the scientific community.

    Religious believer: And I’m worried about my children. If I let them learn about evolution in school, will they come home one day and tell me that we’re all nothing but matter in motion?

    NCSE: ….

    To which I can only reply … um, yeah? That doesn’t seem very bad at all to me. Do we seriously want representatives of the NCSE saying “No, the claim that accepting evolution is the road to damnation is based on a misreading of Scripture and is pretty bad theology. If we go back to Saint Augustine, we see that the Church has a long tradition of…” Gag me with a spoon, as I understand the kids say these days.

    Of course, we could also imagine something like this: (more…)

  • From Eternity to Book Club: Chapter One

    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.

    (more…)

  • 24 Questions for Elementary Physics

    This weekend at Caltech we had a small but very fun conference: the “Physics of the Universe Summit,” or POTUS for short. (The acronym is just an accident, I’m assured.) The subject matter was pretty conventional — particle physics, the LHC, dark matter — but the organization was a little more free-flowing and responsive than the usual parade of dusty talks.

    One of the motivating ideas that was mentioned more than once was the famous list of important problems proposed by David Hilbert in 1900. These were Hilbert’s personal idea of what math problems were important but solvable over the next 100 years, and his ideas turned out to be relatively influential within twentieth-century mathematics. Our conference, 110 years later and in physics rather than math, was encouraged to think along similarly grandiose lines.

    And indeed people had done exactly that, especially ten years ago when the century turned: see representative lists here and here. I asked the organizers if anyone was taking a swing at it this time, and was answered in the negative. I was scheduled to give one of the closing summaries, and this sounded more interesting than what I actually had planned, so naturally I had to step up.

    Here are the slides from my presentation, where you can find some elaboration on my choices.

    hilbert1

    And here’s the actual list:
    (more…)

  • Reading Courses

    In the past I’ve often been listed as the nominal professor for various graduate students taking “reading courses,” which basically meant “I’m going to be doing my research, but there’s some university requirement that says I must be registered for a certain number of courses each term, so please sign my sheet.” But this term I have two students doing honest-to-goodness reading courses — trying to learn some specific material that isn’t being offered in any structured course offered at the moment.

    And — it’s great! Anyone have their favorite suggestions/anti-suggestions for reading courses? The method I chose was the following: the student and I consult on a course of readings for the term. Every week, the student reads through the relevant material. Then once a week we meet, and I sit in my chair and take notes as the student gives an informal lecture, as if they were the professor and I was the student.

    Obviously good for me, since I get to brush up on some things that I knew really well some time ago but haven’t thought about recently. And the students get to dig into something they really care about. But the somewhat-unanticipated bonus is that the students get fantastic practice in teaching and giving talks. Since it’s just one-on-one, we can stop at any moment for me to point something out or for them to ask a question. And I can expound upon my theories of chalkboard etiquette, such as the need to speak out loud every single symbol you write on the board. Over the course of a single hour, I can see the student’s presentation skills improve noticeably (from “good” to “even better”).

    The world being what it is, it’s not possible for every course to be taught with just one student and one professor. But despite all the very real advances in technology and pedagogical theory, I still believe that the best teaching happens with two people sitting at opposite ends of a log (or equivalent), passing words and ideas back and forth. Everything else is just trying to recreate that magic.