Sean Carroll

Category: Science

  • Talking About LHC Safety

    In the latest Science Saturday at Bloggingheads, Jennifer and I zip from the breathtakingly topical — the Large Hadron Collider, what it’s good for, and why you shouldn’t be scared of it — to the profoundly eternal — calculus, and why people are scared of it.

    I’ve finally settled on a proper response to questions about whether the LHC will destroy the world. (After the initial response, I mean.) It comes in two parts.

    Part One: To an inquisitive person who is asking in good faith, who has heard a bit about disaster scenarios involving black holes swallowing the Earth, and wants to know why we’re so sanguine about the possibility of global cataclysm. Sure, we can explain that it’s really unlikely you would even make a black hole, and even if you do, that all of physics as we understand it predicts that such a black hole would evaporate away, and that this has been carefully studied.

    But the better, and punchier, response is simply: there’s nothing the LHC will do that the Universe hasn’t previously done many times over. This is, of course, the conclusion of the recent paper by Giddings and Mangano. It’s long been understood that the energies attained by high-energy cosmic rays are vastly larger than those created by the LHC; the collisions at CERN aren’t the most energetic in the universe, they’re just the most energetic ones created by human beings, that’s all. But the alarmist brigade, desperate for continued relevance, came up with a loophole: what if black holes are created, but ones from cosmic rays simply escape the Earth’s gravity, while those created at the LHC sit around and eat us up? What G&M show is that, even if that were possible, cosmic rays bumping into to white dwarfs and neutron stars would have created black holes that did get stuck, and would have eaten them up. But the fact is, we see plenty of white dwarfs and neutron stars in the sky; so that’s not a danger. It has nothing to do with any arrogant presumption that we understand physics at high energies, or the evolution of microscopic black holes; it’s simply that there is no scenario in which such black holes are created without having other observable effects. (See a nice write-up of this by Michael Peskin in the new APS online review magazine, entitled simply Physics.)

    Part Two: Experience reveals that, even if you carefully explain why there is no allowed scenario in which black holes swallow the Earth, some stubborn folks will still say “But you can never be perfectly sure! So why take the risk?” For them, it makes sense to switch from science and turn to an analogy. (Experience also reveals that people would rather nitpick at the ways in which the analogy is imprecise, rather than addressing its point; but we soldier on.)

    So imagine that you go home to cook dinner — you boil some water, drop in some pasta, and pull a jar of tomato sauce from the fridge. But wait! Are you sure you want to open that jar? After all, if you ask any hyper-careful science-type of person, they will tell you that they can’t be absolutely sure that this innocent-looking sauce doesn’t actually host a virulent mutated pathogen. It’s possible — not very likely, but possible — that when you open that jar, a deadly virus will kill you dead, and proceed to eliminate all human life over the course of the next two weeks.

    What is the chance of this happening? Very, very small. But not strictly, absolutely zero. And, we must admit, the consequences of being wrong are very bad indeed. And frankly, how much enjoyment are you going to get from that jar of tomato sauce, anyway? The only logical and moral choice would be to forgo the sauce entirely, and just enjoy your pasta plain.

    Except this is nuts, of course. Even if the consequences of an action would be truly cataclysmic, sometimes the chances of it happening are so low that we have to take the risk. Or, more accurately, we choose to take the risk, because science never proves anything beyond any possible doubt, and we can’t help but take such improbable risks all the time. Maybe there is a fleet of invisible alien spaceships hovering over head, testing our commitment to unlocking the secrets of the universe, and they will destroy the Earth with their alien death-lasers if we don’t turn on the LHC. There’s a chance!

    And that’s a chance I don’t want to take.

  • Live-Blogging the LHC Startup!

    9:20 am Pacific Time: Let’s be clear. Tonight’s start-up is a symbolic event, not a physics event; as I understand it, the beam will only be circulating in one direction, so there won’t even be any collisions. Still, it’s a very important symbolic event! The first time the beam goes through the entire machine. So, just for fun, here will be a running commentary throughout the day, with links and musings and all that makes the blogosphere special. Co-bloggers are welcome to chime in, and any particle physicists out there who want to say something about the LHC are welcome to comment or email.

    9:45 am (Pacific), Sean: Feel free, in the comments, to make predictions about what the LHC will discover (ultimately, not today). Here are mine. Crackpots not welcome. And seriously, folks — black-hole/world-ending jokes are only funny the first million times.

    1:14pm (EST), Mark: Here at Cornell there’s going to be a public forum this evening with refreshments, chats with physicists, two talks (by Yuval Grossman and Peter Wittich) and with various instruments and components of the detector on display.

    10:26am (PDT), JoAnne: Actually, it is the end of the world as we know it. I will never again have to write a paper detailing the signatures of some crazy new Terrascale theory, wondering if there is any chance of connection to reality. I will never again have to plot a cross section as a function of the Higgs mass. In fact, I will never again have to do a loop over the Higgs mass in a code. I will never again wonder how electroweak symmetry is broken, how the hierarchy between the electroweak and gravity fundamental scales is maintained, whether there is a WIMP dark matter particle, or whether supersymmetry or extra spatial dimensions actually exist. Fundamental questions and roadblocks that have plagued us for literally decades will finally be answered and we will at last be able to move forward instead of spinning our wheels. Yes, indeed, the world will be truly different.

    10:47am (Pacific), Sean: Of course we are not the only blog covering this. The US/LHC Blogs have lots of information, and Tommaso Dorigo offers some inside scoop. There is also main CERN page for the event, and one for press releases.

    12:02pm (Pacific), Sean: The real excitement of the LHC startup is, of course, that it’s an excuse to party. Mike in comments already mentioned the Fermilab pajama party. Here at Caltech, where it’s not quite so ridiculously late at night, we’re having pizza and beer. And (for the wimps who can’t stay up), a lunch BBQ tomorrow. Everyone should feel free to put together their own party! Suggested soundtrack. (Dammit, I’m violating my own rules.)

    12:54pm (Pacific), Sean: I’ve asked some experts to chime in. Here is Gordy Kane, University of Michigan:

    The Standard Model(s) of particle physics and cosmology are wonderful established descriptions of the world we see. They leave out a lot we would like to understand, from dark matter and the matter asymmetry of the universe, to WHY the forces and particles (quarks and leptons) are what they are. LHC won’t tell us much more about the world we see and how it is made, but the discoveries there will point the way to “WHY”. It’s a WHY machine.

    The discovery that makes sense is supersymmetry, i.e. the superpartners of some of the Standard Model particles. There’s a lot of indirect phenomenological evidence that indeed some superpartners will be seen at LHC, such as the unification of the forces at very short distances, the absence of large new effects at the LEP and Tevatron colliders, and the very good indirect evidence for a light Higgs boson. A supersymmetric world is also one where we can understand how the electroweak symmetry is broken and how the matter asymmetry arises, and it has a dark matter candidate. I estimate ten or twenty gluinos and a lot of Higgs bosons will be produced in October this year (but not seen unless we are very lucky about the decay signatures). IF the LHC indeed establishes the world is supersymmetric, there is a great bonus – we can write string theories at the Planck scale where the laws of nature should be written and calculate predictions for LHC experiments and dark matter from them, and we can extrapolate data from LHC and dark matter experiments to the Planck scale to see what theories are suggested. Without that window we might never learn the underlying theory from which everything emerges.

    It’s very lucky that our technologies and our society allowed us to afford and to build the LHC to study nature so deeply (another anthropic idea?). It’s very unlikely (because of technological and financial and cultural limits) that we can ever have a further facility to extend this study, so we’re very lucky that a framework like string theory has emerged, one that addresses all the basic questions, at the same time we may be able to get from LHC the data that can test and establish it.

    1:24 pm (PDT), JoAnne: The History Channel (US cable TV) is airing
    The Next Big Bang at 8 PM this evening. The show details our expectations for the LHC and features David E. Kaplan of Johns Hopkins as well as many other of your favorite physicists, so don’t forget to tune in!

    1:58pm (Pacific), Sean: Ph.D. Comics weighs in.

    6:53pm (EST), Mark: BBC World News America, starting in a few minutes on the East coast, and repeated later, will have a piece on the LHC.

    4:05pm (Pacific), Sean: Prize for the best paper title goes to Mihoko Nojiri, arXiv:0809.1209.

    The Night before the LHC
    Authors: Mihoko M. Nojiri

    Abstract: I review recent developments on the use of mT2 variables for SUSY parameter study, which might be useful for the data analysis in the early stage of the LHC experiments. I also review some of recent interesting studies. Talk in SUSY08.

    4:25pm (Pacific), Sean: There will be a live webcast from CERN beginning at 11pm Pacific, with the actual beam scheduled for half an hour later. But right now you can click the link, and listen to a pre-packaged CERN video. You can also watch the startup on EVO, if you know what that means (or care to learn).

    4:50 pm (PDT), JoAnne: Yours truly has just been recruited for a 5 minute live radio interview on KCSB (the station is on the UC Santa Barbara campus) at 7:30 tomorrow morning. I guess David Gross has the good sense to be asleep at that hour! In any case, I’ll be sure to drink some coffee first, lest I spew some gibberish on blackholes.

    6:55pm (Pacific), Sean: Sorry, the “live” blogging took a hiatus while I was talking to Hal Eisner, a TV reporter (“extraordinaire,” he asks me to add) from the local Fox affiliate. He, quite rightly, was hectoring me mercilessly in an attempt to explain the purpose of the LHC at a level accessible to six-year-olds. (He also tried very hard to get me to say “God particle,” which I mostly resisted.)

    What is the purpose? It’s to discover the laws of nature, of course, or at least extend our knowledge of them. But that doesn’t always quite cut it for people. I think it would suffice to the aformentioned six-year-old; kids are naturally curious, but adults have it beaten out of them by a relentlessly pragmatic world. Among other things, the LHC represents a tremendous triumph of the basic inquisitiveness of the human species.

    7:20 pm (PDT), JoAnne: There’s a host of First Beam Day activities planned for tomorrow across the US. Check the listings for an event near you. Here in SF Bay area, swissnex, the annex of the Consulate General of Switzerland in San Francisco, is throwing a party tomorrow night in coordination with SLAC and LBNL. Much fun will be had by all!

    7:53pm (Pacific), Sean: If you’re wondering whether the Large Hadron Collider has destroyed the world yet, see here.

    If you’re wondering whether physics is more or less tawdry than politics, see here.

    8:17pm (Pacific), Sean: The right response to end-of-the-world chatter is to change the subject — it’s just crackpottery, not a legitimate scientific debate. But damn, you have to be impressed with the vigor of the meme. Far and away the first thing that comes to mind when a person on the street hears “giant atom-smasher in Switzerland” is “might destroy the world.” How do we combat that? What is the one idea we would like to pop into people’s minds when they hear that phrase, and how do we get it there?

    11:52pm (EST), Mark: Gotta sleep, but will try to tune into BBC Radio 4’s Big Bang Day when I wake up!

    9:26pm (Pacific), Sean: Reporting now from the High Energy Physics conference room here at Caltech. In an hour and a half we’ll open a live feed to our colleagues at CERN, who will be updating us on what happens. Of course, the best answer is simply “all systems nominal.” The only way a detector will actually see anything (as I understand it) is if the beam is not focused perfectly from the start, which is perfectly possible. If the beam is well-behaved, it will just zip through.

    But of course, there are many steps along the way, and “first protons circumnavigating the accelerator” is as good a “turn on” event as any. Folks in the know have assured me that CERN will not be hosting multiple “trust us, this is the real start” events — this is it.

    9:48 pm (PDT), JoAnne:
    From looking at our comments, it’s clear that some folks are still genuinely frightened by the LHC. This should not have happened. The LHC is one of the most exciting scientific journeys in our lifetimes! We should all watch it in wonder and be amazed at its discoveries.

    Many a thoughtful, carefully analyzed and written scientific treatise has appeared which thoroughly disproves the claim that the LHC will destroy the Earth. But these aren’t published or mentioned or taken seriously by the press…. (HELP – I’m sounding like a Republican!)

    So, let me present a different, non-scientific, but emotional argument. We physicists are human beings too. We have children, parents, siblings, friends, etc, that we care deeply about. We care about this planet and its future and the future of our families. There are literally thousands of physicists, worldwide, involved in the LHC. If there was a serious concern, the scientists themselves would have stepped forward.

    As for me, one of my best arguments is that my bottle of 1990 LaTour remains in my cellar. I’m going to pull it out when we achieve collisions at the next accelerator after the LHC! Oh – and the fact that I’ve just spent the last 8 months undergoing intensive, arduous treatment for cancer so that I too can have a future and be a part of the LHC.

    10:00 pm (PDT), John:

    B minus two hours. Oh yeah! We’ve waited a long time for this.

    11:03pm (Pacific), Sean: Action is heating up, although the pizza has yet to arrive. So I’m going to start paying attention to the “real world.” I’ll come back if any disasters occur.

    11:30 pm (PDT), John:

    Looks like CERN has stuck a PR video in place of the live webcast…not too surprising…maybe the site got hammered, or they have that up until it starts.

    The SPS is cycling nicely. That’s what they’ll use to inject the beam in 30 minutes.

    11:37 pm (PDT), JoAnne: This is the error message I’m getting:

    Due to a huge interest for this live video feed of the LHC First Beam day, you may not be able to see the live video stream and we apologise for this.
    Please try reloading the page, come back later, or check the other connection options available on this page.
    Many thanks for your interest in CERN and the LHC!

    The folks at CERN should have planned for heavy traffic – I’ve waited 25 years for this and I’m disappointed.

    11:48 pm (Pacific), Sean: Getting updates from CERN. No disasters, but there was apparently a tiny glitch with one of the collimating magnets, which has now been fixed.

    The current beams are low energy (450 GeV, lower than the Tevatron at Fermilab). They want to ramp up to 5,000 GeV (5 TeV) by the end of October — on October 21st, there is a get-together featuring heads of state, and they would love to have actual high-energy collisions by then.

    They will be circulating the beam in both directions — just not at the same time, at least today.

    The computing system involves about a hundred thousand processors — soon to be upgraded to a few hundred thousand. Data flies from CERN to Caltech at about 40 GB per second, which they also want to upgrade by a factor of ten.

    11:58 am (Pacific), Sean: The webcast is limited to 2000 connections! Who’s the rocket scientist behind that?

    Midnight (Pacific), Sean: First beam! Or so they say. (See below.)

    12:03 am (PDT), John:

    Woo hoo! Did it work?

    I think it actually starts in a few minutes. The press kit says

    9:00 Live satellite broadcast and webcast begin with an introduction from the commentators in the CERN Control Centre, an animation showing the passage of a beam through the LHC, and highlights of the LHC operators’ daily meeting where they lay out the procedure for getting the first beam circulating in the LHC.

    9:06 Coverage begins of the first attempt to circulate a beam in the LHC. Lyn Evans, LHC project leader, will narrate the proceedings from the CERN Control Centre. Video of accelerator operators at work in the CCC will alternate with views of the LHC apparatus in its tunnel 100 meters underground.

    12:08 (Pacific), Sean: Well, there was a video countdown. No human being has actually confirmed yet…

    12:11 am (PDT), JoAnne: Only 2000 connections? No wonder nobody can get on! With all the hype they should have planned better than this….

    12:22am (Pacific), Sean: Robert Aymar, CERN Director General … is speaking in French. Translation: in a few minutes we will let the beam zip through the LHC, sector by sector. (They stick absorbers in the way of the beam at certain points, just to check things in each sector before letting it go.) Sounds like the whole thing will take some time.

    Liveblogging closer to the source from Adam Yurkewicz, and from David Harris.

    I can’t update our blog because too many people are trying to read it!

    12:33am (Pacific), Sean: First beam for real! We saw it! Not yet all the way around, as per previous update.

    12:36am (Pacific), Sean: BBC reporter: “Ooh! This is exciting!”

    12:38am (Pacific), Sean: Okay, I think the beam they had was … actually still in the injector, not the LHC. Because now there is really beam in the LHC! Still not all the way around.

    12:40am (Pacific), Sean: Carlo Rubbia seen wandering around the LHC control room.

    12:46am (Pacific), Sean: They removed another absorber, and now the beam has reached CMS! I think that’s 3 octants from the beginning.

    1:02am (Pacific), Sean: They’ve made it about half way around, and are preparing a beam dump. Sadly, our reserved time on the videoconference has run out, as has my stamina, so I’m heading home. They’re predicting that a full circle will be achieved in the next half-hour or hour.

    See you tomorrow!

    1:12 am (PDT), JoAnne: The beam is at Point 8, which is 3/4 of the way around! Thanks to SkyNews for the feed!

    1:18 am (PDT), JoAnne: Now the beam is at ATLAS, 7/8 of the way through. They are giving ATLAS some events (not collisions, but beam halo and beam gas). Lyn Evans, LHC project manager, was heard to say that he’s going to win his bet, whatever that is.

    1:23 am (PDT), JoAnne: BEAM! We have BEAM! All the way round! Now they’re doing it again.

    1:43 am (PDT), JoAnne: SkyNews has just interviewed folks in the control rooms for each of the 4 experiments. All of the detectors turned on without trouble and are excited to be getting beam halo and beam gas events. LHCb and ATLAS saw the muons from the beam dump!

    Now that the beam has safely travelled through the full accelerator, it’s time for some shut-eye.

    7:38 am (PDT), JoAnne: Turns out that the live radio interview was with KCBS here in the Bay Area (which makes much more sense than KCSB in Santa Barbara – our communications department got that wrong!) and just finished. They mainly asked questions about the operation of the accelerator, what comes next, etc. They did ask if the research was open and if all the results would be public or if some of it would be kept secret. And, yes, the subject of those pesky blackholes came up…

    9:34 am (Pacific), Sean: As commenters have noted, Google has caught the fever:

    But here is something better: the signal from ATLAS when beam first went through.

    Click for the full glory!

  • Friday Silly Science

    Women are from Maserati, men are from Lamborghini. At least, that’s what science tells us:

    To test the theory that high-performance cars get people hot, Moxon had 40 men and women listen to recordings of the three Italian exotics and a Volkswagen Polo. Everyone had significantly more testosterone after hearing the exotics, and all of the women were turned on by the Maserati. The guys, on the other hand, were drawn to the Lamborghini…

    As for the Polo? Everyone had less testosterone after listening to it.

    The effect could simply be related to Italian cars vs. German cars, of course, rather than the high-performance engines. No word on how Porsches would stack up against Grecavs. Clearly more research needs to be done.

    Note that the Wired blog post is entitled “Science Proves Exotic Cars Turn Women On,” but the study indicates that men are turned on as well. A fast car is equal-opportunity sexy.

  • Seeing the Sky with Different Eyes

    I just got back from the Cosmo-08 conference in Madison, which was great fun (and I’m sorry I had to miss the last couple of days). But just because I’m traveling, doesn’t mean that science stops happening. It just means I might be late in blogging about it, if I were moved to do so, which in this case I am.

    The big news is that the Gamma-Ray Large Area Space Telescope, a satellite observatory launched back in June, reached two milestones: (1) it got a name change, from GLAST to the Fermi Gamma-Ray Space Telescope (on the theory that not enough things are named after Fermi), and (2) they released the first new picture of the gamma-ray sky! And here it is; click for higher resolution.

    You can clearly see the Galactic plane, of course, as well as a few objects that shine brightly in gamma-rays — a handful of pulsars, and one distant blazar. GLAST Fermi will be cranking out the science over the next several years, from down-and-dirty astrophysics to searches for annihilating dark matter. See Andrew Jaffe, Phil Plait, and symmetry breaking.

    Meanwhile, some of the folks who brought you the Bullet Cluster have now come up with MACSJ0025.4-1222 (or MAC-Daddy-J, as pundits have suggested).

    It’s a similar story — two giant clusters of galaxies smacked into each other, allowing their dark matter to separate from the hot ordinary matter in between. Gravitational lensing lets you figure out where the dark matter is, while X-ray observations reveal the ordinary matter. The Bullet Cluster was pretty darn convincing, but it’s a scientific truism that nothing ever happens just once, so it’s nice to see that magic repeated.

    Finally, I wanted to mention something that is somewhat old news, but that somehow had escaped my attention until Dan Hooper‘s nice talk at Cosmo-08. That is the WMAP Haze, a phenomenon originally noticed by Douglas Finkbeiner. The WMAP satellite, trying to observe primordial temperature perturbations in the cosmic microwave background, measures several different frequencies, to help correct for foregrounds. The CMB isn’t the only thing that emits microwaves, but nearby dusty astrophysical sources generally depend on frequency in different ways, so one can try to remove their effects by seeing how the maps at different frequencies compare with each other. Some people, apparently, are actually interested in those dusty foregrounds, so they try not only to remove them, but to understand them. And Finkbeiner claims that when we remove all of the foregrounds that we know how to explain, and mask out the parts of the galaxy that are simply too bright to deal with, we are left with this:

    There is some mysterious emission near the center of the galaxy, dubbed the “WMAP haze.” There is an explanation on the market, which is what Hooper’s talk was about — the haze could come indirectly from dark matter! Dark matter particles annihilate, so this story goes, giving off a bunch of lighter particles, including electrons and positrons. These electrons and positrons swirl around in the galactic magnetic field, giving off synchrotron radiation, which is what we see as the haze.

    True? Plausible? Crazy? I don’t know. The good news is that the dark matter model required to make it work is not thrown together just to fit this result — it’s a fairly vanilla model of weakly-interacting massive particles. The bad news is that it’s hard to understand these dusty foregrounds, and difficult to be sure that you’ve accounted for all of the mundane ones.

    The great news is that this is exactly the kind of thing that GLAST Fermi will test, by looking for the high-energy gamma rays that should also be emitted by annihilating dark matter. So stay tuned for some possibly exciting dark matter news, right around the corner.

  • The First Quantum Cosmologist

    Many of you scoffed last week when I mentioned that Lucretius had been a pioneer in statistical mechanics. (Not out loud, but inwardly, there was scoffing.) But it’s true. Check out this passage from De Rerum Natura, in which Lucretius proposes that the universe arises as a quantum fluctuation:

    For surely the atoms did not hold council, assigning order to each, flexing their keen minds with questions of place and motion and who goes where.

    But shuffled and jumbled in many ways, in the course of endless time they are buffeted, driven along, chancing upon all motions, combinations.

    At last they fall into such an arrangement as would create this universe…

    Lucretius, along with Democritus and Epicurus, was an early champion of atomism — the idea that the tremendous variety of substances we see around us arise from different combinations of a few kinds of underlying particles. He was also a materialist, believing that the atoms obeyed laws, not that they received external guidance. So a problem arose: how could all of that regular atomic motion give rise to the complexity we see around us? In response, Lucretius (actually Epicurus — see below) invented the “swerve” — an occasional, unpredictable deviation from regular atomic behavior. And then, he points out, if you wait long enough you will swerve your way into the universe.

    It’s a good idea, and one that has been re-invented since then. Boltzmann, another famous atomist, hit upon the same basic scenario. Here is Boltzmann in 1897:

    There must then be in the universe, which is in thermal equilibrium as a whole and therefore dead, here and there relatively small regions of the size of our galaxy (which we call worlds), which during the relatively short time of eons deviate significantly from thermal equilibrium. Among these worlds the state probability increases as often as it decreases. For the universe as a whole the two directions of time are indistinguishable, just as in space there is no up or down.

    However, just as at a certain place on the earth’s surface we can call “down” the direction toward the centre of the earth, so a living being that finds itself in such a world at a certain period of time can define the time direction as going from less probable to more probable states (the former will be the “past” and the latter the “future”) and by virtue of this definition he will find that this small region, isolated from the rest of the universe, is “initially” always in an improbable state.

    Boltzmann imagines the universe as a whole (or what we would call the “multiverse”) is in thermal equilibrium, about which he knew a lot more than Lucretius. But he also understood that the Second Law was only statistical, not absolute. Eventually there would be statistical fluctuations that took the thermal gas and turned them into something that looks like our universe (which, as far as Boltzmann knew, was just the galaxy).

    We are now smart enough to know that this kind of scenario doesn’t work, at least in its unmodified form. The problem is that fluctuations are rare, and large fluctuations are much more rare; a universe-size fluctuation would be rare indeed. Who needs 100 billion galaxies when one will do? Or even just one observer? This objection was forcefully put forward by none other than Sir Arthur Eddington in 1931:

    A universe containing mathematical physicists [which is obviously the correct anthropic criterion — ed.] will at any assigned date be in the state of maximum disorganization which is not inconsistent with the existence of such creatures.

    These days, we throw away the rest of the mathematical physicist and focus exclusively on the cognitive capacities thereof, and call the resulting thermodynamic monstrosity a Boltzmann Brain. The conclusion of this argument is: the universe we see around us is not eternal in time and bounded in phase space. Because if it is, over the long term we really would just see statistical fluctuations, and we would most likely be lonely brains. So either the universe is not eternal — so that it doesn’t have time to fluctuate ergodically throughout phase space — or its set of states is not bounded — so that it evolves forever, but doesn’t sample every possible configuration.

    Sorry about that, Lucretius. You’ll be happy to know that we’re still struggling with these same issues. Except that you’re dead and famously railed against the irrationality of belief in life after death. So probably you don’t care.

  • Superhorizon Perturbations and the Cosmic Microwave Background

    And another paper! Will the science never end?

    Superhorizon Perturbations and the Cosmic Microwave Background
    Adrienne L. Erickcek, Sean M. Carroll, Marc Kamionkowski (Caltech)

    Abstract: Superhorizon perturbations induce large-scale temperature anisotropies in the cosmic microwave background (CMB) via the Grishchuk-Zel’dovich effect. We analyze the CMB temperature anisotropies generated by a single-mode adiabatic superhorizon perturbation. We show that an adiabatic superhorizon perturbation in a LCDM universe does not generate a CMB temperature dipole, and we derive constraints to the amplitude and wavelength of a superhorizon potential perturbation from measurements of the CMB quadrupole and octupole. We also consider constraints to a superhorizon fluctuation in the curvaton field, which was recently proposed as a source of the hemispherical power asymmetry in the CMB.

    This is a followup to our paper on the lopsided universe, although the question we’re tackling is a little different. Remember that the point there was that we imagined some sort of ultra-long-wavelength perturbation, much larger than the size of the visible universe, and we asked how that would change the amplitude of small-scale perturbations in one direction of the sky as compared to the other.

    In the new paper, we actually address a more basic question: what about the induced temperature anisotropy itself? So instead of looking at the power asymmetry (how does the amplitude of fluctuations in one direction compare to that in the opposite direction), we’re looking at the temperature asymmetry (how does the temperature in one direction compare to the temperature in the other). In fact, we’re looking at the “dipole” asymmetry — not small-scale fluctuations, but the large-scale hemispherical pattern.

    Ordinarily, we simply ignore the dipole asymmetry, for a good reason: you get a dipole just from the ordinary Doppler effect, even if there are no intrinsic fluctuations in the CMB. If you have both, it’s hard to disentangle one from the other. But we were considering a supermode that was pretty substantial, and it became an issue — if the predicted dipole was much larger than what we actually observe, it would be hard to wriggle out of.

    Except — it exactly cancels. That’s what the new paper shows. (And another paper the next day, by Zibin and Scott, comes to the same conclusion.) We were surprised by the result. There are clearly competing effects: we do have a peculiar velocity, so there is a Doppler effect, and there is an intrinsic anisotropy from the primordial density perturbation (the Sachs-Wolfe effect), and there is also something called the “integrated Sachs-Wolfe effect” from the evolution of the gravitational field between us and the CMB. And they all delicately cancel. We came up with a plausible hand-waving explanation after the fact, but it was the grungy calculations that were more convincing.

    Nevertheless, the supermode idea is still constrained — the dipole cancels, but there are higher-order effects (quadrupole and octupole) that are observable. Karl Popper would be proud.

  • Dark Matter and Fifth Forces

    I promised (myself) that I would post something every time I submitted a paper, but have been falling behind. An exciting glimpse into How Science Is Done!

    So here is arxiv:0807.4363:

    Dark-Matter-Induced Weak Equivalence Principle Violation
    Sean M. Carroll, Sonny Mantry, Michael J. Ramsey-Musolf, Christopher W. Stubbs

    A long-range fifth force coupled to dark matter can induce a coupling to ordinary matter if the dark matter interacts with Standard Model fields. We consider constraints on such a scenario from both astrophysical observations and laboratory experiments. We also examine the case where the dark matter is a weakly interacting massive particle, and derive relations between the coupling to dark matter and the coupling to ordinary matter for different models. Currently, this scenario is most tightly constrained by galactic dynamics, but improvements in Eotvos experiments can probe unconstrained regions of parameter space.

    The idea of a long-range “fifth force” is a popular one, although it’s hard to make compelling models that work. In this paper we focused in on one particular idea: imagine that there were a new long-range force that directly coupled only to dark matter. (An old idea: see Frieman and Gradwohl, 1993.) After all, there is a lot more dark matter than ordinary matter, and we don’t know much about the physics in the dark sector, so why not? But then we can also imagine that the dark matter itself interacts, via the weak interactions of the Standard Model, with ordinary matter — i.e., that the dark matter is a Weakly Interacting Massive Particle (WIMP). Then, through the magic of quantum field theory, the fifth force would automatically interact with ordinary matter, as well.

    So we scoped out the possibilities and wrote a short paper; a longer one that goes into more details about the field theory is forthcoming. The punchline is this graph:

    You can think of the horizontal axis as “strength with which the new force couples to ordinary matter,” and the vertical axis as “strength with which the new force couples to dark matter.” Then you have various experimental constraints, and a band representing a range of theoretical predictions. The excluded blue region to the right, labeled ηOM, comes from direct searches for fifth forces coupled to ordinary matter, by measuring tiny composition-dependent accelerations of test bodies in the lab. The excluded red region on top, labeled β and involving only dark matter, comes from purely astrophysics, namely the fact that dark matter and ordinary matter seem to move in concert in the Sagittarius tidal stream. The diagonal green region at top right which doesn’t actually independently exclude anything, labeled ηDM, comes from searching for anomalous accelerations in the direction of the galactic center, where the source would mostly be dark matter. If the experimental sensitivity improves by enough, that constraint will become independently useful. The yellow diagonal band is the prediction of our models, in which the fifth force only interacts with ordinary matter via its coupling to WIMP’s. The length comes from the fact that the direct coupling of the new force to WIMP’s is a completely free parameter, and the thickness comes from the fact that the WIMP’s can couple to ordinary matter in different ways, depending on things like hypercharge, squarks, etc.

    It was a fun paper to write — a true collaboration, in that none of the authors would ever have written a paper like this all by themselves. Part of our goal was to use particle physicist’s techniques on a problem that gets more attention from astrophysicists and GR types.

    [Update: this part of the post is edited from the original, as will become clear.] Amusing technical sidelight: the way that you actually get a coupling between the fifth force and Standard Model particles can depend on details, as we show in the paper. For example, if there are “sfermions” (scalar partners with the same quantum numbers as SM fermions) in the theory, you can induce a coupling at one loop. But if you stick just to the WIMP’s themselves, the coupling first appears at two loops:

    You certainly need at least one WIMP loop (that’s χ), by hypothesis. You might think that you could just have a single SU(2)L or U(1) hypercharge gauge boson connect that loop to the Standard Model fermion ψ, but that vanishes by gauge invariance; you need two gauge bosons, and thus two loops. But the the interaction you are looking for couples left- and right-handed fermions, so you need to insert a Higgs coupling. At low energies the Higgs gets a vacuum expectation value, and acts like a mass term, converting the left-handed fermion into a right-handed fermion, which is what you want.

    In the original version of this post (and in the original version of our paper), I claimed that you would need a three loop diagram in the case where the dark matter had zero hypercharge (so you had to use SU(2)L gauge bosons, which couple only to the left-handed fermions). It was just the diagram shown above, with an extra gauge boson connecting the final leg to the segment between the existing gauge bosons. Fortunately, Tim Tait and Jacques Distler convinced us otherwise, in the comments of this very blog. (Fortunately for the integrity of the scientific method, anyway; for us personally, we would rather have figured it out ourselves.) You can read my version of an explanation here. The internet works!

  • Quantum Diavlog

    Remember when I asked for suggested topics for an upcoming Bloggingheads discussion with David Albert about quantum mechanics? The finished dialogue is up and available here:

    I would estimate that we covered about, say, three percent of the suggested topics. Sorry about that. But perhaps it’s better to speak carefully about a small number of subject than to rush through a larger number.

    And I think the dialogue came out pretty well, if I do say so myself. (And if not me, who?) We started out by laying out our respective definitions of what quantum mechanics “is,” in terms that should be accessible to non-experts. (One user-friendly answer to that question is here.) Happily, that didn’t take up the whole dialogue, and we had the chance to home in on the real sticky issue in the field: what really happens when we observe something? This is known as the “measurement problem” — it is unique to quantum mechanics, and there is no consensus as to what the right answer is.

    In classical mechanics, there is no problem at all; you can observe anything you like, and if you are careful you can observe to any precision you wish. But in quantum mechanics there is no option of “being careful”; a physical system can exist in a state that you can never observe it to be in. The famous example is Schrodinger’s cat, trapped in a box with some quantum-mechanical killing device. (Someone must write a thesis on the ease with which scientists turn to bloodthirsty examples to illustrate their theories.) After a certain time has passed, the cat exists in a superposition of states: half alive, half dead. It’s not that we don’t know; it is really in a superposition of both possibilities at once. But when you open the box and take a look, you never see that superposition; you see the cat alive or dead. The wave function, we say, has collapsed.

    This raises all sorts of questions, the most basic of which are: “What counts as `looking’ vs. `not looking’?” and “Do we really need a separate law of physics to describe the evolution of systems that are being looked at?”

    In our dialogue, David does a good job at laying out the three major schools of thought. One, following Niels Bohr, says “Yes, you really do need a new law, the wave function really does collapse.” Another, following David Bohm, says “Actually, the wave function doesn’t tell the whole story; you need extra (`hidden’) variables.” And the final one, following Hugh Everett, says “You don’t need a new law, and in fact the wave function never really collapses; it just appears that way to you.” This last one is the “Many Worlds Interpretation.”

    I want to actually talk about the pros and cons of the MWI, but reality intervenes, so hopefully some time soon. Enjoy the dialogue.

  • What Will the LHC Find?

    With the Large Hadron Collider almost ready to turn on, it’s time to prepare ourselves for what it might find. (The real experts, of course, have been preparing themselves for this for many years!) Chad Orzel was asked what we should expect from the LHC, and I thought it would be fun to give my own take. So here are my judgments for the likelihoods that we will discover various different things at the LHC — to be more precise, let’s say “the chance that, five years after the first physics data are taken, most particle physicists will agree that the LHC has discovered this particular thing.” (Percentages do not add up to 100%, as they are in no way exclusive; there’s nothing wrong with discovering both supersymmetry and the Higgs boson.) I’m pretty sure that I’ve never proposed a new theory that could be directly tested at the LHC, so I can be completely unbiased, as there’s no way that this experiment is winning any Nobels for me. On the other hand, honest particle phenomenologists might be aware of pro- or con- arguments for various of these scenarios that I’m not familiar with, so feel free to chime in in the comments. (Other predictions are easy enough to come by, but none with our trademark penchant for unrealistically precise quantification.)

    • The Higgs Boson: 95%. The Higgs is the only particle in the Standard Model of Particle Physics which hasn’t yet been detected, so it’s certainly a prime target for the LHC (if the Tevatron doesn’t sneak in and find it first). And it’s a boson, which improves CERN’s chances. There is almost a guarantee that the Higgs exists, or at least some sort of Higgs-like particle that plays that role; there is an electroweak symmetry, and it is broken by something, and that something should be associated with particle-like excitations. But there’s not really a guarantee that the LHC will find it. It should find it, at least in the simplest models; but the simplest models aren’t always right. If the LHC doesn’t find the Higgs in five years, it will place very strong constraints on model building, but I doubt that it will be too hard to come up with models that are still consistent. (The Superconducting Super Collider, on the other hand, almost certainly would have found the Higgs by now.)
    • Supersymmetry: 60%. Of all the proposals for physics beyond the Standard Model, supersymmetry is the most popular, and the most likely to show up at the LHC. But that doesn’t make it really likely. We’ve been theorizing about SUSY for so long that a lot of people tend to act like it’s already been discovered — but it hasn’t. On the contrary, the allowed parameter space has been considerably whittled down by a variety of experiments. String theory predicts SUSY, but from that point of view there’s no reason why it shouldn’t be hidden up at the Planck scale, which is 1015 times higher in energy than what the LHC will reach. On the other hand, SUSY can help explain why the Higgs scale is so much lower than the Planck scale — the hierarchy problem — if and only if it is broken at a low enough scale to be detectable at the LHC. But there are no guarantees, so I’m remaining cautious.
    • Large Extra Dimensions: 1%. The idea of extra dimensions of space was re-invigorated in the 1990’s by the discovery by Arkani-Hamed, Dimopolous and Dvali that hidden dimensions could be as large as a millimeter across, if the ordinary particles we know and love were confined to a three-dimensional brane. It’s a fantastic idea, with definite experimental consequences: for one thing, you could be making gravitons at the LHC, which would escape into the extra dimensions. But it’s a long shot; the models are already quite constrained, and seem to require a good amount of fine-tuning to hold together.
    • Warped Extra Dimensions: 10%. Soon after branes became popular, Randall and Sundrum put a crucial new spin on the idea: by letting the extra dimensions have a substantial spatial curvature, you could actually explain fine-tunings rather than simply converting them into different fine-tunings. This model has intriguing connections with string theory, and its own set of experimental predictions (one of the world’s experts is a co-blogger). I would not be terribly surprised if some version of the Randall-Sundrum proposal turned out to be relevant at the LHC.
    • Black Holes: 0.1%. One of the intriguing aspect of brane-world models is that gravity can become strong well below the Planck scale — even at LHC energies. Which means that if you collide particles together in just the right way, you could make a black hole! Sadly, “just the right way” seems to be asking for a lot — it seems unlikely that black holes will be produced, even if gravity does become strong. (And if you do produce them, they will quickly evaporate away.) Fortunately, the relevant models make plenty of other predictions; the black-hole business was always an amusing sidelight, never the best way to test any particular theory.
    • Stable Black Holes That Eat Up the Earth, Destroying All Living Organisms in the Process: 10-25%. So you’re saying there’s a chance?
    • Evidence for or against String Theory: 0.5%. Our current understanding of string theory doesn’t tell us which LHC-accessible models are or are not compatible with the theory; it may very well be true that they all are. But sometimes a surprising experimental result will put theorists on the right track, so who knows?
    • Dark Matter: 15%. A remarkable feature of dark matter is that you can relate the strength of its interactions to the abundance it has today — and to get the right abundance, the interaction strength should be right there at the electroweak scale, where the LHC will be looking. (At least, if the dark matter is thermally produced, and a dozen other caveats.) But even if it’s there, it might not be easy to find — by construction, the dark matter is electrically neutral and doesn’t interact very much. So we have a chance, but it will be difficult to say for sure that we’ve discovered dark matter at the LHC even if the accelerator produces it.
    • Dark Energy: 0.1%. In contrast to dark matter, none of the energy scales characteristic of dark energy have anything to do with the LHC. There’s no reason to expect that we will learn anything about it. But again, maybe that’s because we haven’t hit upon the right model. It’s certainly possible that we will learn something about fundamental physics (e.g. supersymmetry or extra dimensions) that eventually leads to a breakthrough in our understanding of dark energy.

      • (more…)

  • Quake!

    I had just stepped out of the shower yesterday (getting a bit of a late start, yes) when the building began to shake. We’re on the ninth floor of a twelve-story building in downtown Los Angeles, so it was quite exciting there for a while — the ground shook for maybe twenty seconds, the cat scampered under the bed, and an item or two had to be rescued from imminent spillage off of bookshelves. (Our cat has her own blog, so it usually takes quite a shock to drag her away from the internets.)

    But a minor earthquake overall, just 5.4 on the Richter scale. No significant damage, even closer to the center (we were about 30 miles away). The interesting thing is that within seconds after the event you could hop to the US Geological Survey page to find a map of all the world’s recent earthquakes, and then home in on this one. Obviously most of the information is computer generated, although the main page for the earthquake does reassure you that “This event has been reviewed by a seismologist.”

    So you can check out the Shake Map, of course:

    We’re right on top of the dot labeled “Los Angeles.” But you can also find Google maps, travel times for the shocks,

    and of course — waveforms!

    Earthquakes are so much better with science. The only downside is that I spent the immediate aftermath looking for the kitty rather than drying my hair, so I went through the rest of the day with the dreaded “earthquake hair.”