Monthly Archives: February 2011

Rotation curves, dark matter, and modified gravity

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Astronomers like to plot pairs of observables on xy planes. Sometimes the result is a blobby mess of uncorrelated points. Sometimes the plotter is thrilled to discover a correlation that turns out to be astronomically useful, astrophysically significant, or both.

Edwin Hubble’s galactic redshift–distance relation is perhaps the most famous astronomical xy plot. Ejnar Hertzsprung and Henry Norris Russell’s diagram of stellar luminosity and spectral type is perhaps the most beautiful.

In 1977 Brent Tully and Richard Fisher discovered another important correlation. For a sample of galaxies that have disks, they plotted each galaxy’s intrinsic luminosity against each galaxy’s total emission in atomic hydrogen’s 21-cm spectral line. On a log–log plot, the points arrange themselves on a straight line with surprising neatness.

Tully and Fisher assembled their relation using disk galaxies whose distances from Earth had already been determined. But if you don’t know the distance to a disk galaxy, you can use the Tully–Fisher relation to calculate it: Just measure the 21-cm linewidth, look up what the galaxy’s intrinsic luminosity should be, and then work out the distance from the galaxy’s apparent brightness.

The Tully–Fisher relation is not only astronomically useful, it’s also astrophysically significant. The 21-cm linewidth reflects how fast stars are orbiting a galaxy’s center of mass; the intrinsic luminosity reflects how much luminous mass a galaxy contains. The more massive the galaxy is, the faster the stars orbit.

Galaxies consist of more than just stars. Besides interstellar gas, they also contain—or seem to contain—dark matter. How the luminous mass is distributed varies considerably from galaxy to galaxy. Some disk galaxies have wide, flat disks, small central bulges, and relatively little interstellar gas. Others, like M102 shown here, have big, gas-filled central bulges. Remarkably, the Tully–Fisher relation is valid not only over several orders of magnitude in total luminosity, but also over several orders of magnitude in bulge size.

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Hubble’s diagram directly reflects how the universe is expanding. The Hertzsprung–Russell diagram, on the other hand, reflects how the physics of gravity, nuclear interactions, and fluid dynamics interact and play out in stars of different mass and chemical composition. Regardless of whether the Tully–Fisher corresponds to a direct physical correlation or to something more involved, if you think you understand how matter and gravity interact to shape galaxies, your theory should be able to reproduce it.

But to connect the theories (which deal with mass) with the observations (which deal with light), you have to make assumptions about the mass distributions of stars—at least in the case of disk galaxies whose nondark, baryonic mass is dominated by stars. Fortunately, some galaxies’ interstellar media contain a lot more baryonic matter than their stars do. From a sample of such gassy galaxies, you can create an accurate baryonic Tully–Fisher relation (BTFR), in which total baryonic mass replaces intrinsic luminosity.

MOND the gap

That’s what the University of Maryland’s Stacy McGaugh has done in a paper in press at Physical Review Letters. McGaugh’s paper is causing quite a stir. According to his calculations, a form of modified gravity known as MOND can reproduce the gassy galaxies’ BTFR without invoking dark matter. “Gassy Galaxies Defy Dark Matter” is the catchy headline that drew readers to a story on the BBC’s homepage.

The MOND modification is straightforward in form. At the high values of gravity-induced acceleration found in our solar system, gravitational force is proportional to the acceleration, just as Isaac Newton specified. But in the outskirts of galaxies, where gravity-induced acceleration is far weaker, gravitational force is proportional to acceleration squared.

McGaugh also finds that the reigning cosmological paradigm, ΛCDM, can also yield the gassy galaxies’ BTFR, but only with seemingly contrived fine-tuning.

Moreover, in a galaxy where MOND prevails, the stellar rotation is determined only by the amount of baryonic matter. The BTFR’s neat correlation is accounted for by the robustness of the measurements. But in a galaxy where ΛCDM prevails, stellar rotation is determined by the amount of dark matter and baryonic matter. The BTFR’s observed neatness becomes harder to explain because it would seem to require two values to fluctuate conspiratorially.

What do McGaugh’s results mean for astronomy and physics? First, they don’t refute the existence of dark matter. As McGaugh acknowledges in his paper, whereas ΛCDM has difficulty reproducing the gassy galaxies’ BTFR, MOND has difficulty reproducing its equivalent for clusters of galaxies. Both theories fall short.

You should also keep in mind what’s required to calculate a BTFR. In a MOND galaxy, you can integrate the gas mass, calculate the gravitational potential, and obtain the stellar rotation directly. In a ΛCDM galaxy, you can’t see the dark matter, nor do you know how the baryonic and dark matter interacted in the past to create today’s galaxies. To me, the surprise and significance of McGaugh’s paper is not that ΛCDM struggles, but that MOND succeeds.

But even if MOND or some other form of modified gravity supplants dark matter, it would amount, for the most part, to swapping one embodiment of our ignorance (a new form of matter) for another (a new type of gravity). Neither dark matter nor modified gravity connect directly to fundamental physics. Until they do, we remain in the dark.

Charles Day

A physicist at the gym

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When I go to my gym, I sometimes see people lifting weights in physically inappropriate ways. By “physically inappropriate” I don’t mean that they exhibit poor, injury-inducing form or that they use weights that are dangerously heavy or ineffectually light. I mean that they don’t, to use the language of physics, do much work.

First defined in the 1830s by Gaspard-Gustave Coriolis, work entails moving a force’s point of application. If you’re handling a barbell, dumbbells, or other free weights, doing work means raising the weight’s center of mass against the pull of Earth’s gravitational field.

But if you don’t raise the weight, if you move it about in a horizontal plane, your workout will be workless and quite possibly worthless. One near-workless routine I’ve witnessed involves hoisting a weight above your head while simultaneously bending at the knees, thereby ensuring that weight remains at the same height.

There’s no doubt that the woman in this video is doing work as she raises her own center of mass and those of two 45-pound plates and a 45-pound bar.

Although knowing the concept of work is helpful in the gym, calculating how much work you do when you lift weights can be disheartening. The work required to lift 135 pounds five times through a distance of 1 meter is 3 kJ or just 0.7 nutritional calories.

Charles Day

Atmospheric railways

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One of the joys of the internet in general and Wikipedia in particular is that your curiosity can take you along a chain of links to discover new and interesting things.

Last week, for a reason I’ve now forgotten, I wanted to know more about the River Lea, a tributary of the River Thames that meets its more illustrious parent in London’s Docklands. A chain of links that sprang from the Lea’s Wikipedia entry took me eventually to a page about the topic of this blog post: atmospheric railways.

In 1843 Joseph and Jacob d’Aguilar Samuda established the shipbuilding firm Samuda Brothers near the mouth of the Lea. Among the vessels built at Samuda Brothers’ yard was the Fuso, one of the Imperial Japanese Navy’s first armored battleships. Heihachiro Togo, who would later lead Japan’s navy to victory over Russia’s in the Battle of Tsushima, was on hand to observe the ship’s construction, having recently graduated from the Thames Nautical Training College.

But I digress. The Samuda brothers, I learned, were also proponents of atmospheric railways, whose trains are not pulled by fuel-carrying locomotives but sucked along by a vacuum.

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The section of disused track shown here gives some idea of how atmospheric railways worked. One of the carriages in the train was equipped with a piston that hung from its undercarriage. The piston fitted snugly inside a pipe that ran between the rails. Ahead of the train, a pumping station created a vacuum in the pipe; behind the train, the pipe was opened, allowing atmospheric pressure to push the piston forward—hence the name.

Thanks to their central pumping stations, atmospheric railways were cleaner than their coal- or wood-fired contemporaries, and lighter. But the technology didn’t catch on. The piston rod that connected the source of the propulsion to the train had to pass through the pipe without losing the vacuum as the train moved forward. Meeting that requirement in the 19th century proved challenging. Moving the trains on and off sidings and onto the mainline was also difficult because of the railways’ closed-loop topology.

Reading about atmospheric railways, I was reminded of a talk entitled “The Energy Problem: What Can a Physicist Do?” that Steven Chu gave at the April 2007 meeting of the American Physical Society in Jacksonville, Florida. At that time, Chu directed Lawrence Berkeley National Laboratory. He became secretary of the US Department of Energy a year later.

In his talk, Chu reviewed progress toward developing energy sources that don’t emit climate-warming gases into the atmosphere. Of all the points he made, the one that has stuck in my mind is the versatility of chemical energy. Unlike the vacuum that propelled atmospheric railways, the chemical energy that fueled steam locomotives was convenient to store and transport.

To catch on, whatever energy source powers the trains and cars of the future will also have to be convenient to store and transport.

Charles Day

Pitch standards, playback speeds, and metronome marks

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On 16 February 1859 the French government passed a law that fixed the frequency of the A note above middle C at 435 Hz. Besides the benefits of uniformity, the new standard sought to end a growing problem: pitch inflation.

Violins, pianos, and other stringed instruments sound livelier when their strings are tightened to raise the pitch. The tightening amplifies the harmonics. High, bright notes sound thrilling to an audience, but they’re harder to sing. Pitch inflation was troubling opera singers, whose complaints helped bring about the French law.

Although the French standard didn’t catch on everywhere, the idea of a standard did—sort of. In 1995 the International Organization for Standardization chose 440 Hz for its A-note standard, but orchestras around the world have not unanimously adopted it.

Strained singers aside, does it matter if the New York Philharmonic favors A = 442 Hz whereas the London Philharmonic favors A = 440 Hz? Perhaps not if you lack perfect pitch and hear the music in a concert hall. But pitch standards do matter when it comes to recorded music.

On 2 March 1959 Miles Davis and his band went into Columbia Records’ 30th Street studio in New York City to record the first four tracks of Kind of Blue. They recorded the rest of the album on 22 April. Kind of Blue was an immediate critical and commercial success. It remains the best-selling jazz album of all time.

In 1992 Mark Wilder, senior mastering engineer for Sony Music Studios, undertook a new remastering of the famous album. During the project he discovered an error. The three-channel tape deck used to record the 2 March session had been running slowly. Thus, the first four tracks, which occupied side one of the original vinyl LP, ended up faster and sharper than Davis had intended. Wilder corrected the problem for his remaster.

I’m not sure how much Ludwig van Beethoven worried about pitch standards, or even whether pitch could be accurately measured in his day, but the great composer did care how fast his music sounded.

In 1817, two years after the mechanical metronome had been patented, Beethoven went back to his musical scores and marked the tempi at which each movement should be played. To some conductors, Beethoven’s tempi seem too fast, but to others, like Carlos Kleiber shown here, the tempi are exhilarating.

Charles Day

Aeroecology: Transcending boundaries among ecology, meteorology, and physics

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The title comes from a session at the American Association for the Advancement of Science’s annual meeting, which was held in Washington, DC, and ended yesterday.

The annual AAAS meetings are somewhat odd. At the other meetings I attend, scientists give talks for other scientists. At AAAS meetings, scientists give talks for science journalists. As you might expect, AAAS talks are accessible and informative for nonexperts. However, if your job, like mine, entails keeping up with advances in a particular field, you’ll learn more from the sessions devoted to topics outside your field—which is why I spent Saturday afternoon learning about the ecology of Earth’s aerosphere.

Belying the sweeping scope of its title, the aeroecology session was about using radar, lidar, and other imaging modalities to track and count bats, birds, and airborne insects. The narrow focus did not lessen my interest. Rather, I learned that just gathering ecologically relevant radar data, let alone using them, is a fascinating problem that’s rewarding to solve.


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The map, which you can enlarge by clicking on it, shows the locations of the 159 high-resolution Doppler radars operated by the National Oceanic and Atmospheric Administration. Together, the networked radars—known as NEXRAD—provide meteorologists with a detailed and near-complete view of precipitation over the continental US.

The network is in the midst of a major upgrade. Old radars that use only horizontally polarized waves are being replaced with more advanced radars that use both horizontally and vertically polarized waves. By rapidly switching between the two polarizations, the new radars can tell whether precipitation is rain, sleet, hail, or snow.

They can also tell the difference between precipitation and airborne fauna. Indeed, meteorologists routinely filter out from their radar maps the signals from swarms of bats and other animals. Ecologists, however, are using the data to better understand how colonies of insectivorous bats and birds wax and wane in response to climate and other factors.

In her talk, ecologist Winifred Frick of the University of California, Santa Cruz, described using radar data to determine what local weather conditions influence when colonies of Brazilian free-tailed bats debouch from their caves for their evening feed.

But what of the physics promised in the title? One of the session speakers, Phillip Chilson of the University of Oklahoma in Norman, is a physicist. Among the problems he tackles is how to get as much information as possible from NEXRAD. Dual polarization radar, he said, could be used to distinguish long skinny insects, such as dragonflies, from round squat insects, such as bumblebees.

At the end of the session, the speakers fielded questions from their audience. One questioner asked about the challenges of working in an interdisciplinary team. Language—or rather jargon—was cited as one barrier to effective teamwork, but its negative effect was more than offset by the benefits of interdisciplinary teamwork.

Frick recalled that before she worked with Chilson, she tended to regard the atmosphere as a static medium, a place for bats and other aerosphere-dwelling animals to fly and feed in. From Chilson and his colleagues, she learned to appreciate the atmosphere’s dynamic properties—which, as it turns out, are important to the bats, birds, and insects that she studies.

Charles Day

Molecules to materials

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I was happy the other day to receive a thick, info-packed newsletter from Tohoku University’s Advanced Institute for Materials Research. As one of Japan’s World Premier International Research Centers, AIMR has a twofold mission: to draw researchers from around the world and to do world-beating research.

The newsletter’s title, “Molecules to Materials,” succinctly and accurately describes one of AIMR’s aims. It also got me thinking.

The notion of making materials from molecules isn’t new. In the late 19th and early 20th centuries Nottingham University’s Frederick Kipping developed a family of polymerized siloxanes, which he called silicone.

Silicone consists of crosslinked chains of alternating silicon and oxygen atoms. Attached to each silicon atom are two alkyl groups, such as methyl shown here.

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The methyl groups mediate the crosslinking. By altering the length of the chains and the type of methyl group, Kipping and his successors have created a huge range of silicone materials—from free-flowing liquids to hard, stiff resins, all of which share siloxane’s resistance to UV degradation and corrosion, low toxicity, and low coefficient of friction. My wife has a silicone madeleine pan. “It’s great,” she says.

Superhybrid materials

Following Kipping’s example, chemists, material scientists, and physicists at AIMR and elsewhere are trying to make molecular materials that have useful properties. In principle, molecules are versatile building blocks. Chemical substitutions can tailor both a molecule’s properties and how the molecule binds to other molecules.

Making molecular material that does what you want is tricky. The bonds that hold the molecules to each other can alter the molecules’ properties. And even if you can identify a molecular arrangement that yields the properties you want, there’s no guarantee that the resultant material is either robust or makable.

AIMR’s Tadafumi Adschiri has devised a clever way to fabricate molecular materials. Like other researchers, he assembles his materials from nanoparticles whose size and chemistry have been picked to yield a material with a particular set of properties. And as Kipping was, he’s interested in combining inorganic and organic molecules into hybrid polymeric materials.

To both make the nanoparticles and combine them with polymers, Adschiri uses supercritical fluids as solvents. Above its critical point—that is, the temperature and pressure at which liquid and gas phases coexist—a substance becomes a much better solvent. Supercritical carbon dioxide, for example, will readily dissolve caffeine, hence its use in the fanciest and most effective method of decaffeination.

Fluids that are immiscible below the critical point will readily mix when they’re both supercritical. Adschiri exploits that feature to combine inorganic nanoparticles, whose ingredients dissolve in subcritical water, with organic nanoparticles and polymers, whose ingredients dissolve in subcritical oil.

Among the “superhybrid materials” that Adschiri makes with his supercritical process is a film that conducts heat but not electricity and the flexible, high-refractive-index film shown here.

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The info-backed newsletter that inspired me to write this post opens with a message from AIMR’s director, Yoshinori Yamamoto, entitled “Green Materials.” Yamamoto wants his institute to be in the forefront of developing new materials for energy harvesting, energy saving, and environmental cleanup.

Of course, it’s better if you don’t have to clean up the environment in the first place. By using supercritical water and oil, Adschiri greenly avoids using environmentally hostile organic solvents.

Charles Day

The history of Physics Update

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When Physics Today surveys its print readers and logs the habits of its online readers, the Physics Update department routinely comes top or near the top in popularity. Many online readers evidently like the department so much they bypass the homepage and go directly to the Physics Update page.

In its current incarnation Physics Update serves 250-word summaries of research papers. The summaries appear online on Mondays and Thursdays as soon as they emerge from editing. Not all the online summaries appear later in the monthly print issue, the exceptions being summaries of papers written up at greater length in the Search and Discovery department.

Physics Update has always striven to cover interesting research, but its format, audience, and editorial home have all changed over the years. You might be surprised to learn that the department made its debut in 1990—not as part of Physics Today, but as a service for science journalists.

Devised and written by Phil Schewe of the American Institute of Physics’s media relations department, Physics News Update, as it was called, came out once a week and featured 50-word summaries of news stories about physics. Here’s the first issue, dated 28 September 1990.

FIFTH FORCE experiments are increasingly giving negative results. New measurements have demonstrated that a fifth force could be no more than about a trillionth as strong as gravity. (Science News, 22 September, p. 183)

THE MAGELLAN SPACECRAFT map of Venus (2% of the surface so far) provides images 10 to 100 better than previous radar surveys. Strange tectonics–plentiful volcanoes and weird fracture systems–are at work. (NY Times, 26 September, p. A1)

SUPERCONDUCTORS BEYOND 123: Robert Cava of AT&T Bell Labs reviews recent research on the thallium and bismuth superconductors and discusses prospects for higher critical temperatures. (Scientific American, August 1990)

GRAVITATIONAL-WAVE ASTRONOMY may truly come into being in the next few years with the advent of new facilities such as the Caltech-MIT Laser Interferometer Gravitational Wave Observatory. So far only indirect evidence for gravitational waves can be inferred, in this case from the decaying mutual orbit of the two neutron stars in the pulsar system PSR 1913+16. (Mosaic Magazine, Summer 1990, published by the NSF)

SOLAR CELLS may be better at converting sunlight into heat than into electricity, some researchers believe. The record efficiency for conversion to electricity, a tandem GaAs and Si cell at Sandia, is 31%. The theoretical limit is thought to be 40%. But thermal conversion may be more efficient and, besides, a great deal of the world’s energy consumption goes toward the production of heating anyway. (New Scientist, 22 September, p. 48)

METEOR COMPOSITION: unlike earth rocks which in the course of time have been melted and recrystallized into different forms, meteors have remained largely unchanged from when the solar system began and even before. Diamonds found in some meteors may have been produced by chemical vapor deposition, a process (also used in making diamonds artificially) that might occur in the outer layers of stars and supernovas. (New Scientist, 15 September, p. 46)

The first issue was exceptional in that all the items came from the popular press. But within a year, papers in Physical Review Letters, Nature, and other journals provided the raw material, along with press releases from labs and talks at meetings.

The distribution method also changed. The first issues were sent by fax to a modest number of science journalists. Later ones were sent by e-mail to the journalists and to an increasing number of scientists in the physical sciences who appreciated the short, punchy summaries.

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Physics Update first appeared in the pages of Physics Today in the magazine’s February 1995 issue, whose cover is reproduced above. The issue was the fourth produced under editor-in-chief Steve Benka. Wanting to provide his readers with more physics news, Steve took Phil’s Physics News Update, edited the items for the magazine’s readership, and ran them on one yellow-tinted page up front.

The new department proved popular with readers and also with advertisers, who’d pay extra to appear on the facing page. Phil and his colleague, Ben Stein, wrote most of the items. Steve wrote some, too. Physics Update ran on page 9 for nearly 10 years.

For the June 2006 issue, Physics Today underwent a major redesign, acquiring not just a facelift but also two new departments, Quick Study and Back Scatter. As part of the redesign, Physics Update migrated to the end of the Search and Discovery department. Introducing the redesign in an editorial, Steve wrote

Thus, all the current research news coverage is logically brought together in one place: In-depth stories of results deemed important by the community are followed by brief notices of research deemed interesting by our editors.

The last significant changes to Physics Update occurred two years ago. In June 2008, Physics Update became the truly online department it is today, accessible to all and updated twice a week. Around the same time, AIP’s media relations department decided Phil’s and Ben’s outreach efforts were best spent doing other things, among them AIP’s Inside Science News Service. Since August 2008, all Physics Update items have been written by Physics Today editors.

In chatting with Phil today, we both realized that his very first Physics News Update of 21 years ago resembled a modern blog. All it lacks are links. If you visit the website of the American Physical Society, you’ll spot, on the left-hand side, a list of short items that could well be called Physics News Update.

Charles Day

Let’s talk about lampreys!

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Lampreys look like eels, but they’re anatomically simpler than other fish. Lacking jaws, lampreys instead have sucker-like mouths lined with concentric rings of teeth. The likelihood that lampreys are the closest surviving descendant of the first vertebrate has motivated numerous studies, including some that involve physics.

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My first encounter with lampreys was in 2004. For Physics Today‘s June issue of that year I wrote a news story about the UV-sensing ability of the lamprey’s pineal gland. All vertebrates have pineal glands. Ours is about the size of a lentil and sits deep inside our brain. Its principal function appears to be the secretion of the hormone melatonin, which regulates circadian rhythms.

The lamprey’s pineal lies close to the top of its head—close enough to directly sense light. Biologists knew since the 1960s that the lamprey’s pineal could sense UV. In 2003 Kyoto University’s Mitsumasa Koyanagi and his colleagues set about trying to find out what the lamprey does with its UV-sensing ability.

Using a mix of methods drawn from biochemistry, genetic engineering, and spectroscopy, Koyanagi and his colleagues discovered that the lamprey’s pineal gland contains an unusual bistable pigment. The pigment’s bistability enables the lamprey to sense the ratio of visible to UV light. Because that ratio varies with depth in water, Koyanagi speculated that the pineal helps the lamprey gauge its depth.

Thymus-like organs

I noticed more lamprey-related research earlier this month. Thomas Boehm of the Max Planck Institute of Immunobiology and Epigenetics in Freiburg, Germany, and his colleagues found strong evidence that lampreys have thymus-like organs in their gills.

If confirmed, the finding is significant because it could shed light on the evolution of our immune systems. Unlike other animals, vertebrates have an adaptive immune system that continuously trains pathogen-killing T-cells to respond to new threats. The thymus is where the training takes place. Lampreys have a simple version of the adaptive immune system but before Boehm’s work no one had identified a lamprey organ that resembles the thymus.

My curiosity roused, I looked for the latest research on lamprey ancestry and turned up a paper from last fall. A team led by Philip Donoghue of Dartmouth Medical School in New Hampshire and Kevin Peterson of Bristol University in the UK analyzed the genetic sequences of the microRNAs found in lampreys and in another kind of jawless fish, the hagfish.

Known as phylogenetics, the statistical analysis of DNA and RNA exploits techniques such as Bayesian inference and Markov chain Monte Carlo that are used and in some cases were invented by physicists.

When Donoghue, Peterson, and their collaborators compared the microRNAs of lampreys and hagfish with each other and with other fish, they made two interesting discoveries. First, lampreys and hagfish are branches of the same evolutionary tree, not separate branches as some biologists had argued.

Second, the anatomical simplicity of lampreys and hagfish is not the result of their being more closely related to a primitive ancestor than sharks, cod, and other fish are. Rather, the simplicity results from the jawless fish having lost features and functions over time, just as snakes lost their limbs.

Intriguingly, it now looks as though the closest common ancestor to all today’s vertebrates was more, not less, complex than the lamprey.

Charles Day

Proust, Wagner, and the spacetime of gardens

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In Swann’s Way, the first volume of Marcel Proust’s 1.5-million-word novel In Search of Lost Time, the narrator recalls the church in his hometown of Combray. After describing the church’s tapestries, crosses, and other treasures, the narrator muses that

all these things made of the church for me something entirely different from the rest of the town; a building which occupied, so to speak, four dimensions of space—the name of the fourth being Time—which had sailed the centuries with that old nave, where bay after bay, chapel after chapel, seemed to stretch across and hold down and conquer not merely a few yards of soil, but each successive epoch from which the whole building had emerged triumphant.

At the end of act I of Richard Wagner’s 5.5-hour opera Parsifal, Gurnemanz, the oldest of the Grail knights, leads Parsifal, who’ll later become the youngest, from the forest and into the hall of Monsalvat Castle, telling him,

You see, my son, time
Changes here to space.

Should physicists be surprised that a writer and a composer should have appeared to grasp something like Albert Einstein’s notion of spacetime? I don’t think so. Of all the weirdnesses in Einstein’s special relativty—differentially aging twins, shrinking yardsticks, mass-increasing projectiles—spacetime seems the most natural and palatable.

When I first encountered spacetime, as a physics undergraduate at Imperial College London, I remember thinking that the presence of ct as the fourth coordinate alongside x, y, and z made clear, readily acceptable sense. In retrospect, I perhaps should have thought more deeply about the concept.

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But you don’t have to be a giant of civilization like Proust (shown here), Wagner, and Einstein or an undergraduate physicist as I was to appreciate the dimensional kinship of space and time.

Yesterday I learned from my wife that a careful gardener sees her plot in four dimensions. She plants her flowers and vegetables based on where and when she wants them to appear.

Charles Day

The freedom to choose physics

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Thor, the six-year-old son of my friends Anne and Vince, likes to wear orange. Emma, the (nearly) six-year-old daughter of my friends Laura and Neil, likes to wear pink.

Thor and Emma popped into my mind yesterday when I came across a paper in the Proceedings of the National Academy of Sciences. The paper’s authors, Stephen Ceci and Wendy Williams of Cornell University, scrutinized studies from the past 20 years to answer the question: Why are women underrepresented in physics and other math-intensive fields of science?

Perhaps to some readers’ surprise, Ceci and Williams concluded that

despite frequent assertions that women’s current underrepresentation in math-intensive fields is caused by sex discrimination by grant agencies, journal reviewers, and search committees, the evidence shows women fare as well as men in hiring, funding, and publishing (given comparable resources). That women tend to occupy positions offering fewer resources is not due to women being bypassed in interviewing and hiring or being denied grants and journal publications because of their sex. It is due primarily to factors surrounding family formation and childrearing, gendered expectations, lifestyle choices, and career preferences—some originating before or during adolescence—and secondarily to sex differences at the extreme right tail of mathematics performance on tests used as gateways to graduate school admission underrepresentation.

As Ceci and Williams point out, the impact of raising a family affects the prospects of all women scientists, not just those in math-intensive fields. Improving the support available to scientists with young families, adjusting tenure process would help to remedy that source of underrepresentation.

But what to do about “gendered expectations, lifestyle choices, and career preferences”? The question is tricky because, as Ceci and Williams state,

to the extent that women’s choices are freely made and women are satisfied with the outcomes, then we have no problem.

Indeed, studies cited by the two authors show that adolescent girls prefer careers that focus on people rather than things. That preference, according to the studies, accounts for women’s large presence in biology and medicine and their small presence in math-intensive fields.

But to what extent is that preference the outcome of free choice? I don’t doubt that young Thor made his unusual and distinctive wardrobe choice by himself and not under the influence of either society or his parents. Only Dutch sports fans favor orange garments.

Young Emma’s preference for pink, on display in this photo, seems more rooted in contemporary American society than in her free choice. All five of my young nieces favor or used to favor pink or its close relative purple.

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Paradoxically, pink preference could be grounds for optimism. If physics were marketed to girls as skillfully and vigorously as pink clothing is, maybe the underrepresentation in our ranks would begin to diminish.

Don’t get me wrong. By talking about orange, pink, and purple clothing, I don’t mean to be flippant. I agree with Ceci and Williams:

To the extent that [career] choices are constrained by biology and/or society, and women are dissatisfied with the outcomes, or women’s talent is not actualized, then we most emphatically have a problem.

Charles Day

PS Knowing how punctilious young children are about their ages, I should point out that Emma was four years old when the photo was taken.