Let the public name exoplanets

Posted on April 17, 2013 by Charles Day

The names of the five stars closest to the Sun exemplify how confusing (or historically rich) astronomical nomenclature can be.

Proxima Centauri is the closest star. The second and third closest form a binary and are known collectively as α Centauri (or Rigel Kentaurus) and individually as α Centauri A and α Centauri B. The fourth closest is Barnard’s Star. The fifth is WISE 1049−5319, which is also known as Luhman 16.

The brown dwarf binary WISE 1049−5319 appears as a yellow disk in the center of this mid-wavelength IR image. CREDIT: NASA/IPAC Infrared Science Archive

Just how the five stars got their names depends, in part, on when they were first observed. Alpha Centauri is the third brightest star in the night sky. As such, it has been named by several cultures. Its Chinese name, 南門 (Nán Mén), means “Southern Gate.” Arab astronomers called it جل القنطورس (Rijl Qanṭūris), “Centaur’s Foot.”

The name α Centauri originates in the first systematic stellar naming convention, which was devised in 1603 by the Bavarian astronomer Johann Bayer. “Centauri” (“of Centaurus”) indicates the constellation that the star belongs to; the Greek letter indicates the star’s brightness rank in the constellation.

The other stars in the top five are not visible to the naked eye and weren’t, therefore, cataloged by Bayer, whose naming convention predated the invention of the telescope by eight years. Proxima Centauri and Barnard’s Star are both red dwarfs. Proxima was named by the astronomer who discovered it in 1915, Robert Innes. Barnard’s Star was named after Edward Barnard, who was the first to measure the star’s velocity across the sky in 1916.

Remarkably, the discovery of WISE 1049−5319 was published on arXiv just last month. Kevin Luhman of Penn State University and his collaborators identified the star—which is, in fact, a brown dwarf binary—in observations made by NASA’s Wide-field Infrared Survey Explorer spacecraft. The numbers after the spacecraft’s abbreviated name are the binary’s celestial coordinates.

Names like WISE 1049−5319 are now the norm in astronomy. Space-based and ground-based observatories whose sensitivity exceeds their predecessors tend to find many new objects. Although you can’t tell from its name that WISE 1049−5319 is a brown dwarf binary, you can presume from the WISE part of the name that it’s an IR source and that it’s faint (because if it were bright, it would have been discovered and named earlier). Using the coordinates to identify sources might seem long-winded compared to a serial number, but the coordinates are astronomically meaningful, whereas a serial number wouldn’t be.

Rakhat, α Centauri Bb, or both?

I was reminded of the quirkiness of astronomical names earlier this week, when I read a news story in New Scientist entitled “Closest exoplanet sparks international naming fight.”

The dispute pits Uwingu, a startup company whose mission is to fund projects that inform the public about space science, against the International Astronomical Union, the world’s official arbiter of astronomical naming conventions and planetary nomenclature for planetary bodies.

By IAU-sanctioned convention, exoplanets are named after the the stars they orbit, with the addition of a lower-case letter: “a” designates the star; “b,” the first planet discovered; “c,” the second; and so on. Officially, the closest exoplanet to the Sun is called α Centauri Bb, but if you paid Uwingu $4.99, you could suggest a name. And if you paid $0.99, you could vote on the suggestions. Currently, the leading name for α Centauri Bb is Rakhat, which is what Mary Doria Russell called a planet that orbits the star in her 1996 science fiction novel, The Sparrow.

I think it’s great that a real planet is named after a fictional one. Granted, Rakhat conveys less astronomical information than α Centauri Bb does, but I don’t see why the two names can’t coexist. I doubt anyone would be confused.

What’s more, the IAU’s exoplanet naming convention, though simple and straightforward, does not necessarily yield neat, rational names. That’s because the stars that harbor exoplanets, like the five stars closest to the Sun, follow a mix of naming conventions. Exoplanet examples include 51 Pegasi b (the first one discovered), KIC 12557548b (the evaporating exoplanet), and HD 85512 b (a super-Earth).

Despite astronomy’s modest technological payoffs, the general public continues to fund astronomical research—thanks, in part, to the time and energy astronomers devote to engaging the public. By giving people the opportunity to name exoplanets, Uwingu is making them partners in a scientific enterprise. The IAU should support, not fight, such deep public engagement.

Standards rule OK

Posted on December 20, 2012 by Charles Day

My title comes from the chorus of a song on the Jam’s second album, This Is the Modern World (1977). Written by the band’s singer and guitarist Paul Weller, the song is a bombastically ironic attack on the enforcers of social conformity.

But if Weller were not a socially conscious rock musician and instead were a computational scientist, he might have still chanted, “Standards rule OK!” For without standards in hardware, software, and data formats, our work would be less efficient and less effective.

I first appreciated the importance of computer standards when I worked at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in the early 1990s. My field, x-ray astronomy, was just three decades old at the time. The first pioneering missions could detect only a handful of bright objects. But their successors—among them the European Space Agency’s European X-ray Observatory Satellite (EXOSAT; 1983–86) and NASA’s Einstein Observatory (1978−82)—observed thousands of x-ray emitting stars, galaxies, and other cosmic objects. Then came Germany’s Röntgen Satellite (ROSAT; 1990−99) and Japan’s Ginga (1987−91), which added to that swelling collection.

Because spacecraft telemetry is limited by bandwidth, the data gathered and beamed to Earth by satellite observatories are packaged in efficient, instrument-specific formats—15 altogether for the instruments carried by the four spacecraft listed above. In contrast with the diversity of telemetry formats, the figures that embody the data’s scientific content (and ultimately appear in research papers) typically come in a smaller set of generic flavors: images, spectra, and light curves.

Creating those figures entails background subtraction, binning, filtering, and other generic tasks. In principle, the software that, say, Fourier-transforms a data stream from EXOSAT‘s Medium Energy instrument could do the same for a data stream from Ginga‘s Large-Area Counter. But the raw formats are as different as Dutch and Japanese. If the same software is to work with data from those and other missions, the data must be translated into a common format. And that format must be flexible enough to accommodate new instruments.

My former colleagues at GSFC duly picked such a format: flexible image transport system (FITS). Originally developed for optical and radio data, FITS makes extensive use of headers and keywords. Like XML, FITS is extensible. Whenever a new detector technology comes online, new keywords and data structures are defined within the FITS framework. Granted, someone has to write an instrument-specific program that translates telemetry into FITS, but no one has to take on the more onerous job of rewriting data analysis software.

When I left GSFC in 1997, astronomers there and elsewhere used three software programs to analyze their data: Xspec (for spectra), Xronos (for light curves), and Ximage (for images). Now, 14 years later, they’re still using the same three programs for data from observatories that launched years after my departure.

FITS made its public debut in 1981 in a paper in Astronomy and Astrophysics. On 30 November of that same year, the Swedish pop group ABBA’s eighth and final album The Visitors became the first recording available on a new format, the compact disc. Although CD sales are waning, it remains a durable standard—at least I hope so. I have six Jam CDs.

This essay by Charles Day first appeared on page 96 of the September/October 2011 issue of Computing in Science & Engineering, a bimonthly magazine published jointly by the American Institute of Physics and IEEE Computer Society.

My computational education

Posted on October 3, 2012 by Charles Day

I peaked as a computational scientist in 1986. At that time, two years from finishing my PhD, I was trying to account for an aspect of the x-ray emission from a pulsar known as Hercules X-1, the first x-ray source discovered in the constellation Hercules. Hercules X-1 consists of a normal star and a rapidly spinning neutron star. So close are the two stars to each other that the neutron star’s gravity grabs material from the normal star’s outer atmosphere. By the time the purloined material reaches the neutron star, it’s a million-degree, x-ray-emitting plasma.

Every 1.7 days—a number I can’t forget!—the two stars orbit their mutual center of mass, and, if you’re watching from Earth, the neutron star is eclipsed by its companion. But just before and just after total eclipse, the neutron star’s x rays pass through the companion’s atmosphere. From a plot of x-ray emission versus time, you can probe and explain the atmosphere’s vertical structure—with the help of a computer model, that is.

Artist’s impression of an x-ray binary system from NASA’s Imagine the Universe!

And so, over several months, I created my longest-ever computer program. Excluding comments, it ran to more than 120 lines of Digital Equipment’s proprietary flavor of Fortran 77. Fortunately, I didn’t have to create a data analysis program from scratch—a postdoc had done that before me—but I did have to incorporate my model into his code. (See Dianne O’Leary’s “Computational software: Writing your legacy,” Computing in Science & Engineering, January/February 2006, page 78, to learn why that exercise builds character.)

How did I become a programmer? When I started graduate school I had no programming—or even computer—experience. To me, computers were toys for the uncool boys. But then my university offered its graduate students a course in scientific programming. And I took it.

The teacher, whose research was in the field of artificial intelligence, displayed impressive loyalty to a single ancient T-shirt. In class and around town, he’d pad about without shoes or socks like a hobbit. His course briefly touched on programming techniques, but only in the abstract. My real teachers were the postdocs in my group. They taught me a sprinkling of practical tricks and, perhaps more useful, some sound principles of programming practice.

Those learning experiences popped into my head when I read Francis Sullivan’s column about Sudoku (“Born to compute,” Computing in Science & Engineering, July/August 2006, page 88). Lacking Francis’s “compute gene,” I have no attraction whatsoever for Sudoku or any other mathematical puzzle. But I do have the physics gene, and its expression, in graduate school, was what compelled me to program.

Having solved the physics for my model, I had to implement it on a computer—which meant I had to read arcane computing manuals, swat the bugs I’d introduced into my program, and struggle with fussy compilers. Every now and again, amid the toil and frustration, would come flashes of inspiration and progress. In the end, I grew to like programming.

So if you teach computational science and your class includes compute-free students like me, give them an interesting science or engineering problem to solve. The poor things might be driven to program despite their genetic inheritance.

This essay by Charles Day first appeared on page 104 of the September/October 2006 issue of Computing in Science & Engineering, a bimonthly magazine published jointly by the American Institute of Physics and IEEE Computer Society.

A Hungarian astrophysicist advocates land reform

Posted on September 5, 2012 by Charles Day

I’ve just finished reading The Dukays, the 795-page magnum opus of the Hungarian novelist and playwright Lajos Zilahy. Written in 1949, the novel tells the story of Count István Dukay, his family, and his times. It begins in the late 1800s when Hungary, as a partner in the Austro-Hungarian Empire, ruled an area the size of modern Germany. Most of the action, however, takes place in the two decades after World War I, a conflict that led to the dissolution of the empire and, with it, the dismemberment of Hungary. Then, as is still the case today, Hungary’s borders encompassed just 27% of its prewar territory.

Dukay’s vast estates in the Great Hungarian Plain remain intact when the novel ends in 1939, but their continued existence is threatened by the land-reform movement in the person of Mihály Ursi, who first appears on page 624. At that point, the plot is following Dukay’s youngest daughter, Zia, as she recovers from a failed marriage to an Italian prince. She’s living frugally on the fictional Adriatic island of Mandria (based, I think, on Susak), where she practices her hobby of photography under a pseudonym. Ursi asks her to take his photograph after accidentally dropping his passport into the sea.

To my surprise, soon after the photography session is over, readers learn that Ursi is an astrophysicist. Biographical information about Zilahy is hard to find on the Internet, so I’m not sure whether the novelist was friends with any astrophysicists. Still, it’s at least conceivable that he might have encountered his physicist contemporaries and compatriots Leó Szilárd, Edward Teller, and Eugene Wigner in Budapest.

Ursi’s profession serves the novel in two ways. First, the calm, rational astrophysicist provides a sharp contrast to both Filippo Ozzolini, Zia’s foppish, philandering husband, and Dukay, her rich, aristocratic father—as these small details illustrate: Whereas Ozzolini smokes cigarettes in a diamond-encrusted ebony holder and Dukay keeps his cigars in a suitcase-sized humidor, Ursi doesn’t smoke at all.

Second, by choosing to make his land-reform advocate an astrophysicist, Zilahy suggests that land reform is a rational, even scientific, policy. Indeed, one of the book’s highlights is the verbatim introduction to Ursi’s eloquent polemic, The Great Fallow. In the following extract, Ursi begins to justify his interest in land reform as an inevitable consequence of his astronomical research:

When he lays down his complicated instruments, bolometers, actinometers, pyrheliometers, photometers, his prismatic grated spectroscopes, knowing that the hundred billion stars of the Milky Way represent but an infinitesimal fraction of the cosmos, the astronomer—when he says farewell to his laboratory, to the clouds of Magellan Major and Magellan Minor, to the constellation of Andromeda, to the clusters of spheres hurtling through space a distance of hundreds and hundreds of millions of light years; when he sits down to supper with the evening paper in hand—the astronomer cannot help himself if his eyes, his brain and his whole frame of reference regard the tax bill submitted by the Finance Minister, the burglary-cum-murder on Dob Street, regard even the price of kale in the column of market prices from the viewpoint of the universe as a whole.

Literary fiction hardly abounds with astronomers—which is to say I’m struggling to recall any astronomer characters in the books I’ve read. That paucity could constitute a missed opportunity. Although Ursi focuses his cosmic view on land reform and the condition of the Hungarian peasantry, other fictional astronomers could bring their minds to bear on other problems.

Real life has provided novelists with at least one model, the late astrophysicist and human rights campaigner Fang Lizhi.

The 2013–22 decadal survey in solar and space physics

Posted on August 16, 2012 by Charles Day

Last Wednesday the National Academy of Sciences held a press conference in Washington, DC, to introduce its newly completed report on priorities for the coming decade in solar and space physics. Daniel Baker of the University of Colorado chaired the committee that wrote the report. Thomas Zurbuchen of the University of Michigan was the vice chair. Together, they summarized the report’s highlights for the assembled reporters, scientists, and bureaucrats.

Like its counterparts in astronomy and planetary science, the latest solar and space physics decadal survey is more than just a shopping list of missions and facilities. Its authors begin by defining their field in a broad and inspiring way:

We live on a planet whose orbit traverses the tenuous outer atmosphere of a variable magnetic star, the Sun. This stellar atmosphere is a rapidly flowing plasma—the solar wind—that envelops Earth as it rushes outward, creating a cavity in the galaxy that extends to some 140 astronomical units (AU). There, the inward pressure from the interstellar medium balances the outward pressure of the solar plasma forming the heliopause, the boundary of our home in the universe. Earth and the other planets of our solar system are embedded deep in this extended stellar atmosphere or “heliosphere,” the domain of solar and space physics.

The report goes on to review past and present accomplishments in solar and space physics before defining the four overarching goals that guided the committee members as they drew up their final recommendations:

Determine the origins of the Sun’s activity and predict the variations in the space environment.
Determine the dynamics and coupling of Earth’s magnetosphere, ionosphere, and atmosphere and their response to solar and terrestrial inputs.
Determine the interaction of the Sun with the solar system and the interstellar medium.
Discover and characterize fundamental processes that occur both within the heliosphere and throughout the universe.

As I listened to Baker and Zurbuchen’s presentation, it became clear that two other overarching considerations informed the report. The first is a conceptual emphasis on viewing Earth’s aurorae, the solar wind, coronal mass ejections, and other heliospheric phenomena as part of a single system. It will be interesting to see whether this systemic view becomes manifest in journals, conferences, and courses. I, for one, have tended to think of solar physics as belonging more to astronomy than to heliospheric physics.

The second consideration is a realistic and—to use Baker’s word—responsible approach to costs. The committee retained Aerospace Corp, a nonprofit consultancy based in El Segundo, California, to carry out an independent cost appraisal and technical evaluation (CATE) of potential missions. For the most part, the total cost of the committee’s recommended suite of programs lies within the budget envelope that NASA provided the committee for the years 2013–22.

Physicists who remember chuckling when they first encountered the zeroth law of thermodynamics might be amused to learn that the committee’s first recommendation is also numbered zero—for good reason. As NASA and NSF, the other principal sponsor of heliospheric research, look to future missions and facilities, the committee recommends that they first complete their current program.

Among the lineup is Solar Probe Plus (shown here in an artist’s impression). The ambitious mission, whose price tag is $1.4 billion, aims to fly as close as possible to the Sun to determine how the solar corona is heated and how the solar wind is accelerated.

Diversify, realize, integrate, venture, educate

The committee’s second recommendation, numbered 1.0, is to implement an initiative that goes by the acronym DRIVE (for “diversify, realize, integrate, venture, educate”). As far as I can tell, DRIVE aims to reorganize and reinvigorate the way researchers and their students practice heliospheric science.

Surprisingly, given its high priority, DRIVE is not expensive. The committee projects that the initiative will cost at most about $50 million a year. To fulfill the goals embodied by its name, DRIVE seeks to make research opportunities more accessible to universities through small and mid-sized missions, including the shoebox-sized spacecraft called CubeSats.

Funding the analysis and interpretation of data adequately is a key element of DRIVE, as is fostering interdisciplinary approaches to heliospheric research. Indeed, the committee urges NASA and NSF to establish heliospheric science centers, where observers, theorists, and modelers can work together to solve the grand challenges of solar and space physics.

When Baker and Zurbuchen introduced DRIVE, it sounded somewhat woolly to me. Now, having read the DRIVE section of the report, I think it’s a bold and worthwhile model that could be profitably emulated in other fields, such as green energy or neuroscience. But to be effective, DRIVE will probably need a light administrative structure.

Accelerate and expand the Heliophysics Explorer program!

Recommendation 2.0 seeks to revitalize NASA’s Explorer program of modestly sized and priced spacecraft. Begun in 1958, the program, according to the committee, is “arguably the most storied scientific spaceflight program in NASA’s history.” Despite its success, which includes three Nobel prizes, funding for the Explorer program fell in 2004 and has languished since. To quote the report:

The medium-class (MIDEX) and small-class (SMEX) missions of the Explorer program are ideally suited to advancing heliophysics science and have a superb track record for cost-effectiveness. Since 2001, 15 heliophysics Explorer mission proposals have received the highest category of ranking in competition selection reviews, but only 5 have been selected for flight. Thus there is an extensive reservoir of excellent heliophysics science to be accomplished by Explorers.

Because MIDEX and SMEX missions are comparatively cheap, developing and launching more of them would not require a big outlay. The committee recommends that NASA augment the current Explorer program for solar and space physics by $70 million per year.

In addition to more money for the Explorer program, the committee also recommends establishing a faster, more nimble way of accommodating missions of opportunity—that is, missions that are conceived in response to new technologies, new scientific knowledge, or new partnership opportunities with other space agencies.

NASA: Let academia lead space science

Perhaps by coincidence, a commentary by Baker appeared in Nature two weeks before his committee released its report. Entitled “NASA: Let academia lead space science,” the commentary urged the space agency to fund more missions that are small enough in scope that university-based principal investigators (PIs) can develop and lead them.

Whether Baker’s fellow committee members endorsed his commentary is not clear. They do, however, evidently share his belief in the merits of PI-led missions. Recommendation 3.0 calls for NASA to transform its Solar Terrestrial Probes program from a large, centrally directed program to “a moderate-sized, competed, PI-led mission line that is cost-capped at approximately $520-million per mission.”

The STP program aims to elucidate the physics of the Sun’s influence on Earth, on the other bodies in the solar system, and on the interstellar medium. To avoid the risk that a competitive free-for-all would omit important aspects of STP science, the committee outlined three kinds of missions that it would like to see fly:

IMAP (Interstellar Mapping and Acceleration Probe) to characterize the zone where the Sun’s magnetohydrodynamic influence ceases to prevail in the solar neighborhood.
DYNAMIC (Dynamical Neutral Atmosphere) to study how Earth’s ionosphere and thermosphere influence, and are influenced by, processes that occur at lower and higher altitudes.
MEDICI (Magnetosphere Energetics, Dynamics, and Ionospheric Coupling) to determine how the magnetosphere-ionosphere-thermosphere system responds to solar and magnetospheric forcing.

The committee’s enthusiasm for modest missions is not unbridled, however. In the committee’s view, tackling the problem of how and why the Sun varies is a job for large, integrated missions. NASA’s Living with a Star program already includes the Solar Probe Plus and the Radiation Belt Storm Probes missions. Recommendation 4.0 is for Geospace Dynamics Constellation, a set of six formation-flying spacecraft that will characterize how the energy of geomagnetic storms is deposited and transformed in Earth’s atmosphere.

Recharter the National Space Weather Program

In March 1989 a geomagnetic storm caused the collapse of Hydro-Québec’s electricity grid. Five months later another geomagnetic storm shut down electronic trading on Toronto’s stock exchange.

Anticipating such storms—or space weather—and predicting their effects is more important, now that the world’s electrical infrastructure has expanded, the number of Earth-orbiting satellites has increased, and telecommunications have become economically and socially more important.

The current solar cycle, the 24th since records began in 1755, is set to peak next year. To monitor the cycle’s activity, the US relies on a set of spacecraft, such as the Solar and Heliospheric Observatory, whose principal purpose is basic research and whose engineering lifetimes are coming to an end.

To avoid gaps in coverage, the committee recommends that NASA, the National Oceanic and Atmospheric Administration, and the Department of Defense should plan ahead and plan together. Of particular importance, the committee says, is maintaining a permanent monitoring capability at L₁, the first Lagrange point of the Sun–Earth system. Lying between the two bodies 1.5 million km from Earth, L₁ is an ideal vantage for tracking solar activity.

The US has a comprehensive plan, the National Space Weather Program, for dealing with space weather. The trouble is, as the committee puts it, “implementation of such a program would require funding well above what the survey committee assumes to be currently available.” Accordingly, the committee recommends that the NSWP

should be rechartered under the auspices of the National Science and Technology Council and should include the active participation of the Office of Science and Technology Policy and the Office of Management and Budget. The plan should build on current agency efforts, leverage the new capabilities and knowledge that will arise from implementation of the programs recommended in this report, and develop additional capabilities, on the ground and in space, that are specifically tailored to space weather monitoring and prediction.

I haven’t read all 455 pages of the committee’s report. In venturing to summarize it, I have no doubt missed some important points and emphases. But what I have read has impressed me. Here is a plan to study the heliosphere as a system in a comprehensive, multidisciplinary, and cost-effective way. I hope its recommendations are heeded.

Analyzing students’ misconceptions about galaxies

Posted on July 6, 2012 by Charles Day

Teaching entails not only imparting knowledge and understanding, but also dispelling misconceptions. Physics teachers around the world twirl evacuated glass tubes to demonstrate that the tube’s contents (usually a feather and a coin) do indeed fall at the same rate once the complication of air resistance is absent.

It’s understandable that students have difficulty with Isaac Newton’s first law of motion. For one thing, they rarely, if ever, witness an object moving without some kind of friction retarding its progress. And it took Newton, a genius, to think beyond everyday experience and devise a simple, rigorous, and useful concept of force.

Some subjects deal with phenomena that no student encounters in or outside school. But that unfamiliarity does not mean those phenomena are free of misconceptions. It’s not unreasonable to expect electrons passing through two closely spaced slits to behave like tiny electrically charged bullets.

Dispelling misconceptions early and effectively is important, especially in physics. If your physics education was like mine, you had to master a sequence of increasingly difficult concepts. Retaining or picking up misconceptions along that path can delay or forestall attaining enlightenment.

The central regions of M51 in a composite of images from the Hubble Space Telescope and Kitt Peak National Observatory.

Last week I stumbled across an interesting preprint by Andrej Favia, Neil Comins, and Geoffrey Thorpe of the University of Maine in Orono. Favia and Comins are in the university’s physics and astronomy department; Thorpe is in the psychology department. Together, they conducted a study of 318 UMO students who had completed an introductory astronomy course. The study’s goal was to determine whether the students still held mistaken beliefs about astronomical facts and concepts.

Most of the paper by Favia, Comins, and Thorpe is devoted to explaining the method they used to analyze their survey data: item response theory (IRT). Formulated in the 1950s and 60s, IRT seeks to account not only for the possibility that testing entails measurement errors but also for the possibility that some test takers know more than others. I’m hardly an expert on testing, but the use of IRT seems appropriate for study of misconceptions.

Although Favia, Comins, and Thorpe surveyed students’ misconceptions across all areas of astronomy, they chose to present their results on galaxies first. As they note in the paper, galaxies, unlike stars and planets, are mostly outside anyone’s direct experience. The central bulge of Andromeda, the nearest spiral galaxy to our own, is visible to the naked eye on moonless nights, but its characteristic shape isn’t. Any misconceptions that students harbor about galaxies are therefore likely to have been acquired through misunderstanding or ignorance.

To avoid prejudicing the students, the researchers presented the survey as a set of statements that were explicitly identified as beliefs rather than misconceptions. Here is the full set:

the Milky Way is the only galaxy
the solar system is not in the Milky Way (or any other) galaxy
all galaxies are spiral
the Milky Way is the center of the universe
the Sun is at the center of the Milky Way galaxy
the Sun is at the center of the universe
there are only a few galaxies
the galaxies are randomly distributed
we see all the stars that are in the Milky Way
all galaxies are the same in size and shape
the Milky Way is just stars — no gas and dust
new planets and stars don’t form today

And because the researchers were interested in learning when the students picked up the misconceptions, the survey also asked students to specify for each belief if they

believed it only as a child
believed it through high school
believe it now
believed it, but learned otherwise in Introductory Astronomy
never thought about it before, but it sounds plausible or correct
never thought about it before, but think it is wrong now

IRT is evidently a powerful and sophisticated method. Analyzing their survey results, Favia, Comins, and Thorpe could determine, for example, that the misconception “we see all the stars that are in the Milky Way” is commonly held with the misconception “the galaxies are randomly distributed.” They could also tell that the way galaxies are distributed in space is the hardest concept to learn. (Not surprising, perhaps, given that the galaxy distribution is shaped primarily by the way dark matter collapses in the early universe.)

But for me, the survey’s most fascinating finding has to do with the order in which concepts are taught. Thanks to the second set of questions about timing, the researchers’ analysis yielded the optimum order for teaching the concepts to children and adolescents and a different optimum order for adults.

Both sequences end with the tricky distribution of galaxies, but the younger group starts with concepts that have to do with the visual properties of galaxies, whereas the older group starts with concepts that have to do with the unprivileged positions that the solar system and Milky Way occupy in the cosmos.

In his Lectures on Physics (1964), Richard Feynman was so worried that his students would acquire misconceptions about one of the hardest physics subjects to teach, quantum mechanics, that he introduced the Quantum Behavior chapter with a warning. Here’s an extract:

Because atomic behavior is so unlike ordinary experience, it is very difficult to get used to, and it appears peculiar and mysterious to everyone—both to the novice and to the experienced physicist. Even the experts do not understand it the way they would like to, and it is perfectly reasonable that they should not, because all of direct, human experience and of human intuition applies to large objects. So we have to learn about them in a sort of abstract or imaginative fashion and not by connection with our direct experience.

Feynman began his quantum course with electrons whizzing through closely spaced slits. Paul Dirac began The Principles of Quantum Mechanics (1930) with the superposition principle. David Griffiths began Introduction to Quantum Mechanics (1994) with the Schrödinger equation. Thanks to the methods developed by Favia, Comins, and Thorpe, teachers might discover which of these and other pedagogical entrées to difficult subjects are the most effective.

A fictional multiverse

Posted on May 30, 2012 by Charles Day

The collection of essays Science and Ultimate Reality: From Quantum to Chaos was put together to celebrate John Wheeler’s 90th birthday in 2001. Max Tegmark’s contribution, “Parallel Universes,” begins as follows:

Is there another copy of you reading this article, deciding to put it aside without ﬁnishing this sentence while you are reading on? A person living on a planet called Earth, with misty mountains, fertile ﬁelds and sprawling cities, in a solar system with eight other planets. The life of this person has been identical to yours in every respect—until now, that is, when your decision to read on signals that your two lives are diverging.

Tegmark goes on to explain why some astronomers and physicists take the idea of parallel universes, or multiverses, seriously. For one thing, if the universe is infinite—as presumed by the most popular cosmological model, the concordance or ΛCDM model—then somewhere, no matter how unlikely, a parallel “you” exists. You and your twin are most likely too far apart to ever communicate. Yet, in what Tegmark calls a level-1 multiverse, the two of you experience the same physical laws.

Tegmark’s level-2 multiverse accounts, in principle, for the suspicious fine-tuning of physical constants and initial conditions that makes life, the universe, and everything possible. If the inflationary phase that’s presumed to follow the Big Bang begat a froth of baby universes—each one occupying a bubble where different constants, particles, and dimensionality exist—then the fine-tuning is no longer suspicious. Our universe happens to be one among a diverse multitude.

Levels 3 and 4 are more esoteric. Inspired by Hugh Everett’s many-worlds interpretation of quantum mechanics, the level-3 multiverse consists of universes that correspond to quantum states embodied in wavefunctions. Contrary to the so-called Copenhagen interpretation, the quantum states don’t cease to exist when you measure a system and its wavefunction collapses. Rather, each state gives birth to a new universe in which its associated outcome exists on an equal footing with the universe that your measurement seemingly selected.

Level 4 is inspired by Wheeler himself. He wondered whether the mathematical structures that humans presume to underlie our physical understanding of the universe, complete or not, are unique. Conceivably, a multitude of different structures could exist that give rise to a multitude of different universes.

Multiverses are weird

Tegmark concludes his essay with a discussion of the pros and cons of the multiverse concept. The pros are briskly summarized in a single paragraph. Without favoring one level over another, he notes that all four succeed more or less in explaining a perceived problem with the single universe we think we inhabit.

Five times as many words are devoted to dealing with the cons, which Tegmark identifies as coming in two flavors: wasteful and weird. The “wasteful” objection amounts to evoking William of Ockham‘s famous razor. Tegmark counters that a single, infinite universe is hardly parsimonious when it comes to mass, space, or energy. What’s more, some ensembles—Tegmark cites natural integers and solutions to the Einstein field equations—can be described more simply than their components can.

As for the “weird” objection, Tegmark counters that a) weirdness is in the eye of the beholder—that is, it’s a matter of aesthetics, not science; and b) certain firmly established phenomena, such as time slowing down at high speeds and superfluid helium flowing uphill, appear weird principally because they exist under conditions that lie outside our classical, everyday world and not because of some more fundamental reason.

I don’t know whether Tegmark’s essay inspired or influenced Iain Banks, but Banks’ 2009 Transition is based on what seems like a level-1 multiverse, albeit one whose constituent universes are more accessible to one another. The plot’s principal characters belong to, or are estranged from, a morally murky organization called the Concern. Thanks to a drug called septus, members of the Concern can transition or “flit” between parallel universes, ostensibly to keep the universal peace. But as the plot unfolds, more sinister motives emerge.

Whatever your opinion of the multiverse concept, I recommend both Tegmark’s essay and Banks’s book. Both are enjoyable and thought-provoking—in our universe or any other.

Atomic rockets, space colonies, and x-ray binaries

Posted on April 12, 2012 by Charles Day

There are at least three award-winning science fiction writers who do or did research in astrophysics. Gregory Benford studies space plasma at the department of physics and astronomy at the University of California, Irvine. David Brin earned a PhD in physics at the University of California, San Diego; Hannes Alfvén was his thesis adviser. Alastair Reynolds was, until 2004, an x-ray astronomer at the European Space Research and Technology Centre in Noordwijk, the Netherlands.

It seems natural for literary physicists to write speculative fiction. The laws of nature that they study and elucidate describe not only what does happen but also what could happen. Cheap, near limitless energy from nuclear fusion is not obviously ruled out by known physics, nor are humanoid robots that could pass a Turing test or memory sticks that could plug into our brains to upload or download information. Physicists also know how physical laws might be plausibly bent, or plausibly extended, to account for uncanny phenomena and astounding technologies.

Other physicists, motivated less by literature, exercise their knowledge and imaginations to write what might be called speculative fact. To illustrate what I mean, consider the late nuclear physicist Leslie Shepherd.

Born in South Wales in 1918, Shepherd led the team that developed Britain’s high-temperature gas-cooled nuclear reactor in the 1950s and 1960s. Throughout his long career, he maintained an interest in the topic that had captured his imagination as a young boy: space travel. In two papers, “The atomic rocket” (1949, written with Rolls Royce engineer A. V. Cleaver) and “Interstellar flight” (1952), Shepherd worked out the physics of practical space travel. Achieving near light-speed travel, he proposed, would require engines powered by the annihilation of matter and antimatter.

Another writer of speculative fact was Gerard O’Neill. Nine years younger than Shepherd, O’Neill was born in Brooklyn. In 1956 he wrote a paper that outlined the storage ring, a device that traps a beam of high-energy particles from an accelerator and holds it until it can be released to smash into a second beam.

Besides particle physics, O’Neill’s other calling was the colonization of space, which was the title of an article he wrote for the September 1974 issue of Physics Today. Raw materials from the Moon and asteroids could be mined and processed to build vast cylindrical habitats whose rotation would mimic gravity. Colonists would live on the inner surfaces.

Most physicists do not devise future technologies, fictional or not. However, many of the most successful physicists have been led by their imaginations, ambitions, or both to new discoveries. In 1928 Paul Dirac incorporated special relativity into the quantum mechanics of the electron. His equations predicted negative energy states that could not be brushed under a theoretical carpet, even though the states would be manifest in two weird phenomena that had not been observed: Either electrons spontaneously switch charge from negative to positive or positively charged electrons exist.

Dirac did not ignore the problem of negative states. Three years later he published a paper in which he took the bold step of predicting the existence of antimatter. Here’s a quote from the introduction:

Following Oppenheimer, we can assume that in the world as we know it, all, and not merely nearly all, of the negative-energy states for the electrons are occupied. A hole, if there were one, would be a new kind of particle, unknown to experimental physics, having the same mass and opposite charge to an electron. We may call such a particle an anti-electron.

When I look back on my own, pre-Physics Today career as an astrophysicist, I note with regret that my most imaginative paper was the first one, written while I was still a graduate student. Its subject, Hercules X-1, is binary system that consists of an x-ray-emitting neutron star and a normal star. In “Observations of three high-state eclipse egresses of Hercules X-1,” my coauthors and I sought to explain why the atmosphere on one side of the normal star was puffed up. The combination of radiation pressure from the x rays and the Coriolis force arising from the stars’ orbit was my answer.

So whatever stage you’ve reached in your career, I urge you to emulate the likes of Brin, Shepherd, and Dirac and let your imagination wander. You might not win a Hugo Award for science fiction or a Nobel Prize for physics, but you could create something original that you’re proud of.

A killer astrophysicist

Posted on March 14, 2011 by Charles Day

My wife and I have just finished watching the first season of Luther, a police drama on BBC America. Despite its overburden of clichés (the main character, Detective Chief Inspector John Luther, has marital problems, a rookie partner, and a hot temper), the show is compelling to watch, thanks to Idris Elba’s performance as Luther and Ruth Wilson’s performance as Alice Morgan, his amoral antagonist.

In the first episode, Morgan (shown here) murders her parents and gets away with it because she has committed a flawless crime: The police unwittingly destroy incriminating evidence, just as she planned. Luther figures out Morgan’s method, but can’t convict her. For the remaining episodes, Luther and Morgan develop an uneasy relationship of mutual fascination.

Interestingly, the show’s creator, Neil Cross, made Morgan an astrophysicist. Here’s how her character is described on the show’s website:

Alice Morgan went to Oxford University aged 13, a celebrated child genius. She completed her PhD in astrophysics at the age of 18. Now working at a London university as a research fellow, she has spent her life feeling different, special and freakish. For Alice, human existence is insignificant compared to the universe, with its vast galaxies and black holes. Life is futile and senseless but still the alternative, death in all its emptiness, is even worse.

Why did Cross choose astrophysics and not some other, equally demanding pursuit for Morgan? I can think of several answers. First, of all the fields of science, astronomy is the best reported in popular media. People know just enough to be familiar and impressed by Morgan’s thesis topic: the distribution of dark matter in spiral galaxies. Spin-mediated pairing in pnictide superconductors, a hot topic in condensed-matter physics, is perhaps more intimidating as an area of study, but it’s also more obscure.

The second reason is hinted at in the description quoted above. The objects that she studies, supermassive black holes, are utterly inhuman in size, remoteness, and violence. Metabolic networks in a yeast cell are just as daunting to understand, but lack the black holes’ resonance with Morgan’s character.

As a former astrophysicist, I admit a perverse sense of pride that a show’s evil genius reads the same textbooks, publishes in the same journals, and habituates the same conferences as I used to.

Her wickedness doesn’t trouble me. I went to Cambridge, not Oxford.

Charles Day

Rotation curves, dark matter, and modified gravity

Posted on February 28, 2011 by Charles Day

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.

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.