July 2009

Guest Post: Evalyn Gates on Cosmic Magnification (or — Invasion of the Giant Blue Space Amoebas)

Evalyn Gates Scientists like to argue, contra Walt Whitman, that understanding something increases our appreciation of its beauty, rather than detracting from it. The image below, as Evalyn Gates explains, is a perfect example. Evalyn is an astronomer at the University of Chicago, and the author of a great new book on the science of gravitational lensing, Einstein’s Telescope: The Hunt for Dark Matter and Dark Energy in the Universe (Amazon, Barnes & Noble, Powell’s). This post is an introduction to how gravitational lensing gives us some of the most visually arresting and scientifically informative images in all of astronomy.

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I had the pleasure of meeting up with Sean and some other old friends at the World Science Festival in NYC last month, and over champagne at the opening night reception (science has its benefits) Sean graciously invited me to write a guest post on gravitational lensing. It’s a broad topic, mainly because lensing is proving to be such an incredibly useful tool for many areas of cosmology and astronomy, but I have to admit that the visual beauty of the images produced by lensing is part of the appeal for me.
I’m also enamored of the visceral connection between these images and lensing phenomena that all of us encounter in daily life – and the access into a complex theory that this connection affords. The giant arcs, Einstein Rings, and multiple copies of a single distant galaxy or quasar that have now been observed in hundreds of images are concrete visualizations of otherwise abstract concepts of general relativity – they effectively trace out the warps in spacetime created by massive objects, revealing the outline of the cosmos much as the technique of “rubbing” can reveal the writing on an ancient gravestone.

This image, from a recent paper by Adi Zitrin and Tom Broadhurst is both scientifically and visually irresistible:

zitrinbroadhurstfigure1.jpg

First, the image itself is really cool. The bright white/yellow galaxies are members of a cluster known as MACS J1149.5+2223, while the blue amoeba-like objects that appear to be invading the cluster are actually five images of a single distant (z ~ 1) spiral galaxy.

This galaxy has been lensed by the warp in spacetime created by the cluster. Light from the galaxy, which lies almost directly behind the center of the cluster but much farther away from us, travels along several curved paths through the cluster lens, producing multiple magnified images of the galaxy. The inset box shows a computer generated model of the unlensed source galaxy, enlarged by a factor of four so that the details, including the spiral arm structure, are visible. Without the lensing power of the cluster, we would see this galaxy as a single small blue smudge.

In general, lensing will both magnify and distort (shear) images of a background source. This lens is fairly unique in that we see large but relatively intact images of the spiral galaxy, which implies that the mass distribution in the central region of the cluster must be nearly uniform. The images in the upper left (#1) and lower right (#2) are especially striking. #1 is magnified but very minimally distorted, while #2, the largest image with a magnification of over 80, seems to be curling its tentacles about one of the galaxies in the cluster.

A close look also reveals the negative parity (mirror symmetry) of the remaining three images – the spiral arms appear to circle in the opposite direction – as expected from lensing. The total magnification of the distant galaxy (the sum of all five images) is about 200, the largest known to date – supporting the authors’s claim that this is “the more powerful lens yet discovered.”

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Looking for Dark Matter in the Moon’s Shadow

moonshadow.jpg Here’s an extremely clever and fun idea (via arxiv blog). A while back the PAMELA experiment claimed to see an excess of high-energy positrons in cosmic rays — a signal that could come from imperfectly-understood astrophysical objects such as pulsars, or might be produced by something more exotic like dark matter annihilations. Some damper on enthusiasm for this idea was introduced by new results from the Fermi observatory, but it wasn’t completely conclusive, since Fermi’s detectors can’t actually distinguish between positrons and electrons.

So now Pierre Colin and collaborators have hit upon a cute way to distinguish between electrons and positrons: treat the magnetosphere of the Earth like the interior of a giant particle detector. Ever since cloud chambers, physicists have put magnetic fields in their detectors to help distinguish between positively charged particles and negatively charged particles, which get pushed in opposite directions. Well, the Earth has a magnetic field, so maybe we can use that. The problem is that the positrons and electrons would still all hit a telescope such as MAGIC, so the fact that they were deflected by the magnetic field wouldn’t be very relevant.

But Colin et al. suggest a trick: using the Moon’s shadow. Let’s imagine that the excess positrons really are coming from dark matter annihilating in the galactic center. When the moon is near the position of the galactic center in the sky, it will block out some of those particles, casting a shadow on ground-based telescopes. That’s already interesting, but the fun part is that positrons and electrons will be deflected by the Earth’s magnetic field, so the positron shadow will be in a slightly different position than the electron shadow! Using that effect, it may be possible to distinguish between the signals.

I am completely unable to judge how feasible this actually is. But the idea is sufficiently imaginative, I’m sure rooting for it.

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Social Mediation

People writing books, I have to imagine, are much like people with babies. This newborn thing has been the center of their life, and will continue to be, for some time; and one naturally presumes that the rest of the world shares one’s fascination with it. This presumption, alas, may not always be true.

You may have heard that I have a book coming out — pushed back to January, unfortunately. I haven’t shown any hesitation in blogging about substantive questions related to the topic of the book, nor do I see any reason to. And once it comes out I do want to do some sort of book club so that people can ask questions and have a conversation about what’s in the various chapters. So there will be no shortage of book-related stuff here on the blog.

But there is a whole ‘nother level of bookish miscellany — admiring the illustrations, having blurbs come in for the back cover, setting up public talks, and all that. Now we’re pretty much into baby-picture territory; it might not be completely safe to assume that everyone else is as fascinated by all this as I am. But you don’t want to deprive those who are, right? So I’m sending all that stuff here:

That will shield you from the worst of my enthusiasms. A bit, anyway.

Not that I’m at all sure that this is the right thing to do. Back in my day, we didn’t have all these fancy social networks to play around in; you had your blog, and that was it. Now there’s been a bit of proliferation, and there’s no question that it’s changing the landscape. It can obviously be annoying to try to follow too many things at once, but on the other hand it’s nice to have more appropriate tools for distinct tasks. In the old days, I wouldn’t think much of writing a blog post with an amusing link and little else. Now I will just put that on my Twitter feed. So there are fewer blog posts overall, but the average amount of substance per post is higher. Is this an improvement? Not really sure.

A lot of bloggers have Twitter feeds where they link to every one of their blog posts, which seems backwards to me. (So I usually don’t subscribe to those folks — nothing personal.) I once asked (on Twitter) whether people thought that was a useful service, and I received strong opinions on either side — but then I noticed that everyone who was in favor of linking to every blog post on Twitter was a blogger who linked to every one of their blog posts on Twitter. So I resist. But then again, I synchronize my Twitter feed to my Facebook status updates, which is considered unforgivably gauche in some circles. So who am I to complain?

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Does Philosophy Make You a Better Scientist?

Steve Hsu pulls out a provocative quote from philosopher of science Paul Feyerabend:

The withdrawal of philosophy into a “professional” shell of its own has had disastrous consequences. The younger generation of physicists, the Feynmans, the Schwingers, etc., may be very bright; they may be more intelligent than their predecessors, than Bohr, Einstein, Schrodinger, Boltzmann, Mach and so on. But they are uncivilized savages, they lack in philosophical depth — and this is the fault of the very same idea of professionalism which you are now defending.

It’s probably true that the post-WWII generations of leading physicists were less broadly educated than their pre-war counterparts (although there are certainly counterexamples such as Murray Gell-Mann and Steven Weinberg). The simplest explanation for this phenomenon would be that the center of gravity of scientific research switched from Europe to America after the war, and the value of a broad-based education (and philosophy in particular) has always been less in America. Interestingly, Feyerabend seems to be blaming philosophers themselves — “the withdrawal of philosophy into a `professional’ shell” — rather than physicists or any wider geosocial trends.

But aside from whether modern physicists (and maybe scientists in other fields, I don’t know) pay less attention to philosophy these days, and aside from why that might be the case, there is still the question: does it matter? Would knowing more philosophy have made any of the post-WWII giants better physicists? There are certainly historical counterexamples one could conjure up: the acceptance of atomic theory in the German-speaking world in the late nineteenth century was held back considerably by Ernst Mach‘s philosophical arguments. On the other hand, Einstein and Bohr and their contemporaries did manage to do some revolutionary things; relativity and quantum mechanics were more earth-shattering than anything that has come since in physics.

The usual explanation is that the revolutionary breakthroughs simply haven’t been there to be made — that Feynman and Schwinger and friends missed the glory days when quantum mechanics was being invented, so it was left to them to move the existing paradigm forward, not to come up with something revolutionary and new. Maybe, had these folks been more conversant with their Hume and Kant and Wittgenstein, we would have quantum gravity figured out by now.

Probably not. Philosophical presuppositions certainly play an important role in how scientists work, and it’s possible that a slightly more sophisticated set of presuppositions could give the working physicist a helping hand here and there. But based on thinking about the actual history, I don’t see how such sophistication could really have moved things forward. (And please don’t say, “If only scientists were more philosophically sophisticated, they would see that my point of view has been right all along!”) I tend to think that knowing something about philosophy — or for that matter literature or music or history — will make someone a more interesting person, but not necessarily a better physicist.

This might not be right, though. Maybe, had they been more broad and less technical, some of the great physicists of the last few decades would have made dramatic breakthroughs in a field like quantum information or complexity theory, rather than pushing harder at the narrow concerns of particle physics or condensed matter. Easy to speculate, hard to provide much compelling evidence either way.

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arxiv Find: The Local Density of Dark Matter

One of the big hopes of particle- and astro-physicists over the next few years is to experimentally pin down the nature of dark matter. In a perfect world, we’ll make the dark matter particle at the LHC, observe gamma rays produced when dark matter annihilates in the galaxy, and detect it directly in experiments here on Earth. The world isn’t always perfect, but sometimes it’s even better, so everyone is sitting on the edges of their seats waiting to hear what the experiments tell us.

For the direct-detection strategy here on Earth, we build giant detectors and wait for ambient dark-matter particles to interact with something in the detector. If the dark matter is a weakly interacting massive particle (WIMP), that’s not so hard; the difficult part is distinguishing a purported signal from various backgrounds. To know what the signal should be, of course, we need to know how many dark matter particles are zipping through the laboratory. It should be a good number: roughly speaking, there would be about one weak-scale-sized dark matter particle per coffee-cup-volume in the universe, and in our galaxy these particles will typically be trucking along at around 300 kilometers per second.

Still, you’d like an accurate estimate of how much dark matter there is supposed to be in your detector. That’s what Riccardo Catena and Piero Ullio claim to have provided:

A novel determination of the local dark matter density
Authors: Riccardo Catena, Piero Ullio

Abstract: We present a novel study on the problem of constructing mass models for the Milky Way, concentrating on features regarding the dark matter halo component. We have considered a variegated sample of dynamical observables for the Galaxy, including several results which have appeared recently, and studied a 7- or 8-dimensional parameter space – defining the Galaxy model – by implementing a Bayesian approach to the parameter estimation based on a Markov Chain Monte Carlo method. The main result of this analysis is a novel determination of the local dark matter halo density which, assuming spherical symmetry and either an Einasto or an NFW density profile is found to be around 0.39 GeV cm$^{-3}$ with a 1-$sigma$ error bar of about 7%; more precisely we find a $rho_{DM}(R_0) = 0.385 pm 0.027 rm GeV cm^{-3}$ for the Einasto profile and $rho_{DM}(R_0) = 0.389 pm 0.025 rm GeV cm^{-3}$ for the NFW. This is in contrast to the standard assumption that $rho_{DM}(R_0)$ is about 0.3 GeV cm$^{-3}$ with an uncertainty of a factor of 2 to 3. A very precise determination of the local halo density is very important for interpreting direct dark matter detection experiments. Indeed the results we produced, together with the recent accurate determination of the local circular velocity, should be very useful to considerably narrow astrophysical uncertainties on direct dark matter detection.

So they’re claiming the density is about .39 GeV per cubic centimeter (where one GeV is about the mass of the proton), whereas the standard figure is something closer to .30 GeV per cubic centimeter. More importantly, they claim to trust their estimate to a precision of about 7%, while the usual number is supposed to be uncertain by a factor of 2 or 3.

I’m not expert enough to judge whether they are right, but it would certainly be very impressive to pin down the density to such high precision. They do assume spherical symmetry, however, which I suspect is not a very good assumption. There are ongoing arguments about how lumpy the distribution of galactic dark matter really is, and I can easily imagine that lumpiness can distort the local density by much more than 7%. But work like this is going to be very important in interpreting the results, if (when?) we do directly detect the dark matter.

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Newton, P.I.

When I was studying for my Ph.D., a fellow grad student and I asked our advisor if he could think of one single characteristic that was common to all of the best scientists he knew. Without too much hesitation, he answered: “Hard work.” That certainly wasn’t the answer we wanted to hear — you mean there isn’t some secret recipe to being brilliant? And of course hard work is not nearly enough to elevate you to the ranks of the world’s great scientists. But now that I have marinated for some time in the juices of experience myself, I see the truth of what he was getting at; there are a lot of smart people out there, so it makes sense that what elevates a few of them above their peers is an extraordinary focus on their work and a great amount of simple effort.

So it should come as no surprise that Isaac Newton, the greatest physicist of all time, was a relentless worker. In his days at Cambridge, when he focused on the workings of the natural world, he would spend as little time as possible on anything that drew him away from the researches in his rooms. Over the couple of years he was writing the Principia Mathematica, he took things to extremes, going for extended periods without food or sleep. (He also, apparently, died a virgin. Extremes come in many guises.)

Most contemporary physicists have heard that Newton eventually left Cambridge and more or less turned his back on scientific research, to take up activities in later life that we associate with varying degrees of disreputability: alchemy, religious studies, taking a bureaucratic position at the Royal Mint, using the Royal Society to attack his scientific rivals. Lots of us shrug and agree that many older scientists do all sorts of crazy things, and don’t wonder too much about the details.

levenson-newtoncounter-us-cover1.jpgHappily, Tom Levenson (of The Inverse Square, and one of our honored guest bloggers) has provided us with a fascinating peek into a telling episode in Newton’s later life — his career as a criminal investigator. Not really “P.I.”, as Newton was acting in his capacity as a government official, the Warden of the Mint. The story is closer to something from Law and Order or CSI — remarkably close, in fact. In Newton and the Counterfeiter, Levenson tells the tale of how Newton took up what should have been a cushy sinecure, and ended up devoting his extraordinary Newtonian powers to the pursuit and prosecution of one William Chaloner, the counterfeiter of the title. Poor Chaloner, suffice it to say, never knew what hit him.

I should say right up front that this is not a book about physics. Some time back Tom asked me to read some pages from his draft, to make sure the physics was coming out right, but he assured me that physics played a very minor role in the book. That baffled me a bit, because — well, it is Isaac Newton, right? But this is a work of biography and intellectual history, and offers a fascinating “street-level view” of the dawn of the Age of Reason. I can recommend it without hesitation to anyone who likes good stories, which I presume is just about anyone.

The book does begin with some stage-setting about Newton’s scientific work in Cambridge — it is Isaac Newton, right? But it picks up when our protagonist finally wrangles a position in London as Warden of the Mint. Not supposed to be a taxing job; one of the attractions for Newton was that he was going to have plenty of time available for his research. Mostly, at that time, on alchemy and religion — one of the enlightening chapters looks at how Newton actually went about his alchemical work, which is both engrossing and baffling to the modern reader.

History did not cooperate. The 1690’s was a transformative time for the English currency system, including the introduction of paper money, trade imbalances with the Continent, massive debts run up by William III’s wars in France, and an epidemic of counterfeiting and “coin-clipping,” by which people would shave off the edges of silver coins and melt them down to make new ones. In response, the Mint eventually gave in and undertook a comprehensive re-coinage — a program that was on track to become a complete fiasco until Newton stepped in. Remember that he was not simply an abstract theorist (although he was that); Newton was an extraordinarily careful experimenter, and he turned his practical side to the problem of re-coinage, with spectacular results.

But the real fun comes in when Newton takes on Chaloner, one of the most notorious counterfeiters of the day. I don’t want to give away too much, because you really should buy the book. Suffice it to say that where Newton was gifted with an extraordinary intellect and a relentless work ethic, Chaloner was gifted with what we would today call “balls.” No scheme was too audacious to be undertaken, no lie was too grandiose to be told, no collection of co-conspirators was too extensive to be betrayed or turned against each other. Chaloner was a colorful character, whose story would have made entertaining reading no matter what era he was born into. But he made one unforgivable mistake: he attracted the particular ire of Isaac Newton, who turned the full force of his powers to tracking this miscreant down and bringing him to justice. Chaloner’s own gifts notwithstanding, it was not a fair fight.

We tend to look at successful people and imagine that they are defined by their sphere of success. It’s hard for us today to think of Isaac Newton as anything other than a scientist. But he was good at what he did, whether it was piecing together the mysteries of classical mechanics or paying informers to spy on suspected criminals. Gil Grissom would approve — maybe not of all his methods, but certainly of his results.

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