Sean Carroll

Author: Sean Carroll

  • Repost: Quantum Interrogation

    Sorry for the radio silence around here of late. I don’t know about anyone else, but I’ve been traveling like a mad person. The good news is that I just got back from UC Davis, where I had the chance to meet John Conway for the first time in person.

    The bad news is: no time for blogging. But I recently received an email pointing out that some links have died in an old post, which I proceeded to update. And that gave me the idea of stooping to a classic blogospheric move in times of sparse content: reposting old stuff! So here is the post in question, from several years ago. If people don’t complain too loudly, maybe we’ll dig up some more ancient blogging and bring it back to the surface.

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    Quantum mechanics, as we all know, is weird. It’s weird enough in its own right, but when some determined experimenters do tricks that really bring out the weirdness in all its glory, and the results are conveyed to us by well-intentioned but occasionally murky vulgarizations in the popular press, it can seem even weirder than usual.

    Last week was a classic example: the computer that could figure out the answer without actually doing a calculation! (See Uncertain Principles, Crooked Timber, 3 Quarks Daily.) The articles refer to an experiment performed by Onur Hosten and collaborators in Paul Kwiat‘s group at Urbana-Champaign, involving an ingenious series of quantum-mechanical miracles. On the surface, these results seem nearly impossible to make sense of. (Indeed, Brad DeLong has nearly given up hope.) How can you get an answer without doing a calculation? Half of the problem is that imprecise language makes the experiment seem even more fantastical than it really is — the other half is that it really is quite astonishing.

    Let me make a stab at explaining, perhaps not the entire exercise in quantum computation, but at least the most surprising part of the whole story — how you can detect something without actually looking at it. The substance of everything that I will say is simply a translation of the nice explanation of quantum interrogation at Kwiat’s page, with the exception that I will forgo the typically violent metaphors of blowing up bombs and killing cats in favor of a discussion of cute little puppies.

    So here is our problem: a large box lies before us, and we would like to know whether there is a sleeping puppy inside. Except that, sensitive souls that we are, it’s really important that we don’t wake up the puppy. Furthermore, due to circumstances too complicated to get into right now, we only have one technique at our disposal: the ability to pass an item of food into a small flap in the box. If the food is something uninteresting to puppies, like a salad, we will get no reaction — the puppy will just keep slumbering peacefully, oblivious to the food. But if the food is something delicious (from the canine point of view), like a nice juicy steak, the aromas will awaken the puppy, which will begin to bark like mad.

    It would seem that we are stuck. If we stick a salad into the box, we don’t learn anything, as from the outside we can’t tell the difference between a sleeping puppy and no puppy at all. If we stick a steak into the box, we will definitely learn whether there is a puppy in there, but only because it will wake up and start barking if it’s there, and that would break our over-sensitive hearts. Puppies need their sleep, after all.

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  • Bespoke Life

    Synthetic Cell Craig Venter and colleagues have achieved a remarkable milestone: they designed a genome, and brought it to life. More specifically, they’ve synthesized a chromosome consisting of over a million DNA base pairs, and implanted it in a bacterial cell to replace the cell’s original genome. That cell then reproduced, giving birth to offspring that only had the synthetic genome. See the Venter Institute press release, discussion in Nature (pdf), more discussion at Edge, and some background from Carl Zimmer. Update: and here is the paper.

    Who knows exactly what this means as yet — but it’s important! You can argue if you like about whether it’s really “artificial life” — that argument has already started, and already seems boring. There are also speculations about designing microorganisms to help us solve problems like global warming or (let’s say) massive oil spills. Not completely crazy speculations, either. But there’s a long way to go before anything like that is coming off a biological assembly line. And eventually we’ll be going much further than that, beyond designer microorganisms into much weirder terrain. This isn’t a culmination, it’s just a start.

  • Non-Normalizable Probability Measures for Fun and Profit

    Here’s a fun logic puzzle (see also here; originally found here). There’s a family resemblance to the Monty Hall problem, but the basic ideas are pretty distinct.

    An eccentric benefactor holds two envelopes, and explains to you that they each contain money; one has two times as much cash as the other one. You are encouraged to open one, and you find $4,000 inside. Now your benefactor — who is a bit eccentric, remember — offers you a deal: you can either keep the $4,000, or you can trade for the other envelope. Which do you choose?

    If you’re a tiny bit mathematically inclined, but don’t think too hard about it, it’s easy to jump to the conclusion that you should definitely switch. After all, there seems to be a 50% chance that the other envelope contains $2,000, and a 50% chance that it contains $8,000. So your expected value from switching is the average of what you will gain — ($2,000 + $8,000)/2 = $5,000 — minus the $4,000 you lose, for a net gain of $1,000. Pretty easy choice, right?

    A moment’s reflection reveals a puzzle. The logic that convinces you to switch would have worked perfectly well no matter what had been in the first envelope you opened. But that original choice was complete arbitrary — you had an equal chance to choose either of the envelopes. So how could it always be right to switch after the choice was made, even though there is no Monty Hall figure who has given you new inside information?

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  • Esoteric Knowledge

    You may have heard that a major climate bill — the “American Power Act,” sponsored by John Kerry and Joe Lieberman — is trundling through Congress. Its prospects for passage are highly unclear; it’s a giant mess of a bill, which would have important consequences for any number of sectors in the economy, and the country’s attention is largely focused elsewhere at the moment. (A substantial fraction is focused on Justin Bieber, but I don’t really blame him.)

    So what does the bill say? Here’s the very short version, from our sister blog 80 Beats:

    The carbon emissions targets are: 17 percent below 2005 levels by 2020, and 83 percent below 2005 levels by 2050. That’s made to match the goals in the House bill that passed in 2009. In addition, the bill proposes putting a price on carbon.

    Somewhat longer version from Think Progress here. Or of course you could just read the bill yourself (pdf). Only 987 pages! Most of which read like this:

    23 ‘‘(B) WITHHOLDING ALLOWANCES.—
    24 ‘‘(i) IN GENERAL.—Notwithstanding
    25 subparagraph (A), subject to the condition
    1 described in clause (ii), the Administrator
    2 shall withhold from distribution under this
    3 paragraph a quantity of emission allow-
    4 ances equal to the lesser of—
    5 ‘‘(I) 14.3 percent of the quantity
    6 of emission allowances allocated under
    7 section 781(a)(1) for the relevant vin-
    8 tage year; and
    9 ‘‘(II) 105 percent of the emission
    10 allowances of the relevant vintage year
    11 that the Administrator anticipates will
    12 be distributed to merchant coal units
    13 and long-term contract generators
    14 under subsections (c) and (d).

    There are good reasons why bills are written in turgid legal language; but it means that very few concerned citizens are going to be curling up with a good piece of legislation in the evening. That’s okay; we have multiple high-profile media outlets that are here to help us understand the complexities of these important changes to how our country does its business. I mean, right?

    Sadly, no, as a wise person once said. CNN had a sit-down interview with Kerry and Lieberman last night, and here’s what we get:

    Last night, John Kerry and Joe Lieberman appeared on John King’s CNN program to promote their climate bill, the American Power Act. The transcript is fairly lengthy, but at no point does King ask them to explain the provisions of their bill. Instead, he begins by asking whether they have 60 votes, tries to get them to explain why John McCain isn’t on the legislation, and then asks them to comment on the Sestak-Specter race in Pennsylvania. In fact, the clip the John King show posted online (which I embedded above) doesn’t even mention the climate bill.

    Isn’t there room in the media landscape for just one TV news channel that would take seriously the responsibility of actually providing their viewers with useful information? It might be a small, niche market, but if the Golf Channel can thrive, surely it’s an experiment worth trying? I refuse to believe that providing useful information is of necessity such a tedious and boring activity that it can’t be made interesting, no matter how hard we try. We need to get Stephen Spielberg and Jay Rosen in a room together to figure out how to make a news channel that would honestly inform people in an entertaining way. Have them call me.

  • Inaugural Cosmic Variance Whisky Tasting

    One of the consequences of having a blog is that people occasionally offer to send you free stuff. Not out of the goodness of their hearts, for the most part; rather, because they’d like some free publicity in return. Usually it’s a book of some sort, and usually I just decline; I can’t possibly get through all the books I hope to read on my own, much less other books that people want me to read.

    So when I received an email from Kimberly Moniz at SHIFT Communications asking if I’d be interested in receiving a free sample to possibly mention on the blog, I almost replied automatically in the negative. But then the nature of the product sunk in, and I paused — this wasn’t a book, this was booze! Specifically, Canadian Club whisky.

    I’ll admit that I’m an occasional Scotch drinker, but not much of a Canadian whisky fan. To be honest, the mention of Canadian Club conjures images of something my grandmother would have been drinking (while smoking her Pall Malls), although that seems to be changing. I suspect the marketing people recognize that, and thought it would be good to freshen their image among a younger, hipper crowd. And what better way to do that than by reaching out to science blogs? (Especially ones that occasionally rhapsodize about the perfect martini.) This is some new-media marketing savvy I can get behind. Also, free booze.

    But our honor is not sold so cheaply — we’re not going to provide free advertising just because someone sends us some loot. We have our scruples, and everything we post must adhere to the guiding principles of our Mission Statement. But then I remembered that our Mission Statement says we post about whatever we feel like posting about. Still, we like to convey at least the illusion of integrity.

    So I hit upon the perfect solution: talk about the whisky, but do so through the lens of Science! That is, we would accept the free booze, but only under the understanding that we would subject it to a rigorous taste-test in comparison with other comparable whiskies, apply the time-honored techniques of the experimental method to the results, and publish whatever they might turn out to be. Kim was up for this adventure, so we set the wheels in motion. Results below the fold.

    whiskies

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  • Good Sentences

    Timothy Ferris, in The Science of Liberty:

    In 1900 there was not a single liberal democracy in the world (since none yet had universal suffrage); by 1950 there were twenty-two.

    Tyler Cowen at Marginal Revolution has an ongoing series of posts in which he highlights “good sentences.” At first the conceit bugged me a bit, as how good can a single sentence be? It’s not like you have space to develop a sensible argument or anything.

    But that’s the point, of course. A really good sentence packs a wallop because it fits an enormous amount into very few words. One technique for doing that is to exhibit an underlying assumption that is a remarkable claim in its own right. If I were to have tried to make the point that Ferris makes above, it would have been something like this:

    Liberal democracies were established in fits and starts over a period of hundreds of years. The first major steps happened in countries like Britain, the United States, and France, where aristocratic systems were replaced (with different amounts of violence) by rule by popular vote. But I would argue that a true liberal democracy is one that features universal suffrage — every adult citizen has a right to participate. By that standard, there weren’t any liberal democracies in existence in the year 1900; but fifty years later, there were twenty-two.

    Makes the point, but it’s a somewhat ponderous collection of mediocre sentences, rather than a single one of immense power. That’s the difference between someone who writes things, like me, and a true writer. I’m trying to learn.

    Ferris’s book seems excellent, although I’ve just started reading it. It has a provocative thesis: the Enlightenment values of liberal democracy and scientific reasoning didn’t simply arise together. The emergence of science is rightfully understood as the cause of the democratic revolution. That’s the kind of thing I’d be happy to believe is true, so I’m especially skeptical, but I’m looking forward to the argument.

  • Congratulations to Heywood and Moira!

    It’s that time of year again. Young graduate students, having toiled for several years at the feet of Science, are kicked out of the nest to take their places among the ancient and honorable community of scholars. If you will forgive the mixed metaphors.

    This week we had a double-decker celebration: both Heywood Tam and Moira Gresham successfully defended their Ph.D. theses. Congratulations to both!

    Heywood was stuck with me as an advisor, but he seems to have turned out okay. We worked together on a number of papers that looked into models of Lorentz violation, including issues of extra dimensions and stability. More recently we’ve been finishing a couple of papers on fine-tuning in the early universe — coming soon to a preprint server near you! In the Fall Heywood will leave the dry heat of SoCal for the damp heat of Florida.

    Moira’s advisor was Mark Wise, but we also interacted quite a bit. She and I collaborated with Heywood and Tim Dulaney on a couple of aether papers, and she and Tim recently wrote a really interesting paper on anisotropic inflation. But she promises that her next project will be completely Lorentz-invariant. And she’ll be doing it from Ann Arbor, where she’ll be joining the Michigan physics department as a member of the Society of Fellows.

    Always bittersweet when students graduate; it will be a loss to Caltech when the leave, but it’s great to see people launch their independent research careers. Best of luck to both Moira and Heywood!

  • Guest Post: Caleb Scharf on the Shadow Biosphere

    Caleb ScharfWe’ve been talking about life quite a bit here recently at Cosmic Variance, and it’s always fun to talk about areas in which one has absolutely no professional expertise. But it’s also fun to bring in experts, which is why we’re happy to welcome Caleb Scharf as a guest blogger. Caleb is Director of Astrobiology at Columbia University, author of a textbook on the subject, an recently jumped into blogging. In this post he reminds us that we’re still learning a lot about the forms of life right here on Earth — knowledge that will be invaluable as we search for it elsewhere.

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    It’s a real privilege to be able to write a guest blog for Cosmic Variance and to take a little side trip from my regular postings to Life, Unbounded – the science of origins.

    The modern search for life in the universe encompasses everything from exoplanets and astrochemistry to geophysics and paleontology. Underlying and motivating the investigations in these fields – collectively labeled astrobiology – there are some fundamental assumptions, but do they make sense?

    In recent weeks one might be forgiven for thinking that a shadowy biosphere surrounds us, aliens are poised to dismantle civilization, and that time traveling species are flitting in and out of view like barflies on a Saturday night. It’s a little disconcerting, does the Kool Aid have something special in it this Spring?

    Unfortunately I think that all of these headline grabbing items miss the real story of what life is, here on Earth and potentially further afield. The idea of ‘shadow biospheres’ or multiple origins of terrestrial life sounds intriguing, and certainly helps bring focus to the fact that we can be very blinkered in our outlook. It also steers attention away from a more interesting and demonstrably real point.

    microbes In the past couple of decades we have found a shadow biosphere, except that far from lurking in the cracks it turns out to be the biggest, most critical, biosphere on the planet. An astonishing 99.9% of life on Earth cannot be coerced to grow in a lab, and so we have overlooked it. Microbial life – single-celled bacteria and our ancient cousins the Archaea – is not just the stuff under your fingernails, it is what makes multi-cellular life like us function, and it helps govern the grand chemical cycles of our planet, from the continents to the oceans to the atmosphere. Such organisms have, over three to four billion years, evolved into an eye popping array of microscopic machines, the ultimate nano-bots. They can extract energy and raw materials from, it seems, almost any environment. A particularly good example is Desulforudis audaxviator – discovered 2.8 km down in a South African gold mine in a pocket of isolated water. Little audaxviator lives all alone when the vast majority of microbial life is utterly reliant on colonial symbiosis. It earns a living by mopping up the molecular detritus left after radioactive decay in the uranium rich rocks dissociates water and bicarbonates. That’s a very, very neat trick.

    Twenty or thirty years ago we barely understood that such life existed on this planet. Now we are beginning to see that the longevity of our biosphere owes itself to precisely this crowd of ‘shadowy’ organisms. A truly wonderful paper was published a couple of years ago in which Falkowski, Fenchel and Delong laid out the big picture for life on Earth. In essence, they argue that single-celled microbial life is the manifestation of an even deeper truth; the core planetary gene set. This is the set of recipes for metabolism, or how to harvest a planet for energy, and we all rely on them. The result of billions of years of natural selection, these genes are widely dispersed across the microbial biosphere. This is true to such an extent that should 99% of life be wiped out by an asteroid collision, supervolcano, or dirty telephone receiver, the information for the molecular machinery that drives all organisms will be safely preserved in the surviving 1%. The living world does not end, it just reboots. Because of this, carbon-based life is a far more robust phenomenon than we could have ever imagined. It is the ultimate, Google-like, cloud computer.

    Still though, isn’t this also a blinkered view of what might constitute life? Well, sure, but there’s another fact to consider. When we look out into the universe we find that the chemistry of our life – carbon based molecular structures – is not just occasional, it’s ubiquitous. Carbon is a fabulous player; simple molecules, rings, chains, polymers, sheets, crystals, and great clumps of sooty particles abound. Some is produced directly from the huge outflows of cooling gas from old stars, much forms in the thick nebulae and proto-stellar cocoons that eventually give rise to planets. Thousands of recognizable organic molecules, including amino acids, are found in the treacly mix of some meteorites – the remains of our own ancient solar system. This is a chemical bonanza that must have played a role in setting the stage on the young planet Earth. If this is blinkered then stick a blindfold on me.

    So life on Earth is tough and tenacious, and the building blocks are everywhere. Is this enough reason to think that a similar blueprint exists in other places across the universe? Well, it’s definitely motivation to go looking, and to go looking for the kind of exotica that we already know, rather than inventing new ones. Is this reason enough to think that ‘intelligent’ life exists somewhere else? That’s a tough call. Life on Earth did remarkably well for the past 3.5 billion years without us around, I don’t think there is anything that indicates we are more than an evolutionary oddity (albeit an incredible one). It’s a big universe though, with plenty of room for oddities, even if they turn out to be extremely familiar.

  • You Can’t Derive Ought from Is

    (Cross-posted at NPR’s 13.7: Cosmos and Culture.)

    Remember when, inspired by Sam Harris’s TED talk, we debated whether you could derive “ought” (morality) from “is” (science)? That was fun. But both my original post and the followup were more or less dashed off, and I never did give a careful explanation of why I didn’t think it was possible. So once more into the breach, what do you say? (See also Harris’s response, and his FAQ. On the other side, see Fionn’s comment at Project Reason, Jim at Apple Eaters, and Joshua Rosenau.)

    I’m going to give the basic argument first, then litter the bottom of the post with various disclaimers and elaborations. And I want to start with a hopefully non-controversial statement about what science is. Namely: science deals with empirical reality — with what happens in the world. (I.e. what “is.”) Two scientific theories may disagree in some way — “the observable universe began in a hot, dense state about 14 billion years ago” vs. “the universe has always existed at more or less the present temperature and density.” Whenever that happens, we can always imagine some sort of experiment or observation that would let us decide which one is right. The observation might be difficult or even impossible to carry out, but we can always imagine what it would entail. (Statements about the contents of the Great Library of Alexandria are perfectly empirical, even if we can’t actually go back in time to look at them.) If you have a dispute that cannot in principle be decided by recourse to observable facts about the world, your dispute is not one of science.

    With that in mind, let’s think about morality. What would it mean to have a science of morality? I think it would look have to look something like this:

    Human beings seek to maximize something we choose to call “well-being” (although it might be called “utility” or “happiness” or “flourishing” or something else). The amount of well-being in a single person is a function of what is happening in that person’s brain, or at least in their body as a whole. That function can in principle be empirically measured. The total amount of well-being is a function of what happens in all of the human brains in the world, which again can in principle be measured. The job of morality is to specify what that function is, measure it, and derive conditions in the world under which it is maximized.

    All this talk of maximizing functions isn’t meant to lampoon the project of grounding morality on science; it’s simply taking it seriously. Casting morality as a maximization problem might seem overly restrictive at first glance, but the procedure can potentially account for a wide variety of approaches. A libertarian might want to maximize a feeling of personal freedom, while a traditional utilitarian might want to maximize some version of happiness. The point is simply that the goal of morality should be to create certain conditions that are, in principle, directly measurable by empirical means. (If that’s not the point, it’s not science.)

    Nevertheless, I want to argue that this program is simply not possible. I’m not saying it would be difficult — I’m saying it’s impossible in principle. Morality is not part of science, however much we would like it to be. There are a large number of arguments one could advance for in support of this claim, but I’ll stick to three.

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  • Einstein Should Be Grateful He Didn't Have Email

    I’m reading an interesting new book, Bursts by Albert-Laszlo Barabasi. It’s just released today, but I scored an advance copy by virtue of sharing the same publisher. The basic idea is simple: human behavior obeys power laws! That is, things we occasionally do tend to be clustered together, rather than simply occurring with uniform probability. I can’t vouch for either the truth or usefulness of the claims put forward in the book; we all know that power laws can be slippery things. But the stories related along the way are pretty amusing. (And there’s a very spiffy web page.)

    I’ll admit that I jumped right to a chapter in the middle that relates the correspondence between Einstein and Theodor Kaluza in the year 1919 and thereabouts. Kaluza had just come up with the idea that electromagnetism could be unified with gravity by hypothesizing an extra dimension of space — a scenario now known as Kaluza-Klein theory, which underlies all the contemporary excitement about extra dimensions of space. Many crackpots like to assert that our contemporary system of scientific publishing is overly ossified and hierarchical, and that a modern-day Einstein would never be appreciated; the truth is close to the opposite, as back in those days you really needed endorsement from someone established to get your papers published. So Kaluza wrote to Einstein, who was originally enthusiastic about the idea, and they had a flurry of correspondence. Eventually (as I now know) Einstein cooled on the idea, and Kaluza left physics to concentrate on pure mathematics. A couple of years later, after getting nowhere with his own attempts to unify gravity and E&M, Einstein turned back to Kaluza’s approach, and wrote him again, offering to present his paper to the academy.

    The book’s interest is actually in the “burstiness” of the correspondence — a flurry of letters back and forth in 1919, then silence, then the conversation resumed in 1921. I was struck by this paragraph, relating the growth of Einstein’s celebrity after the eclipse expedition of 1919 provided evidence supporting general relativity.

    [Einstein’s] sudden fame had drastic consequences for his correspondence. In 1919, he received 252 letters and wrote 239, his life still in its subcritical phase, allowing him to reply to most letters with little delay. The next year he wrote many more letters than in any previous year. To the flood of 519 he received, we have record of his having managed to respond to 331 of them, a pace, though formidable, insufficient to keeping on top of his vast correspondence. By 1920 Einstein had moved into the supercritical regime, and he never recovered. The peak came in 1953, two years before his death, when he received 832 letters and responded to 476 of them.

    Can you imagine what Einstein would have faced in the email era? One thing is for sure: he was a champion correspondent. He composed approximately 14,500 letters, more than one per day over the course of his adult life.

    Not for the first time, Einstein makes me feel like a slacker.