Monthly Archives: August 2011

Rocks and bones

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The availability of scientific papers online makes it easier not only to find the latest research but also to trace back through time a paper’s academic ancestry, which may or may not lie in the same field. That boon to the curious came to mind this morning when I encountered a paper in JASA Express Letters.

The paper is by Katsunori Mizuno of Doshisha University in Kyoto, Japan, and his colleagues. What caught my eye was its title: “Propagation of two longitudinal waves in a cancellous bone with the closed pore boundary.”

Cancellous bone, in case you didn’t know, is the spongy type of osseous tissue that constitutes the inside parts of long bones, especially at the ends, and of vertebrae. Cortical bone is the other type of osseous tissue. It provides cancellous bone with a solid casing and constitutes the inside and outside of small bones.

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Both types osseous tissue regenerate at their surfaces, but because of its porous structure, cancellous bone is more readily weakened than is cortical bone when regeneration fails to keep up with wear and tear. The two images show samples of cancellous bone taken from vertebrae. The top one comes from a 21-year-old man; the bottom one comes from a 65-year-old woman.*

Mizuno and his colleagues are trying to develop an ultrasound-based method for assessing the condition of bones. Although doctors already have methods based on x rays at their disposal, an ultrasound diagnostic would likely be cheaper and more convenient.

Water-saturated rocks

Not knowing much about the propagation of ultrasound in bones, I was surprised to learn from the antecedents to Mizuno’s paper that his line of research springs, in part, from a paper about water-saturated rocks. In 1956 Maurice Biot published a theoretical description of what happens when sound waves travel through a fluid-filled porous medium.

At that time Biot worked for an oil company. I’m not sure whether his paper helped anyone to find oil deposits. It did, however, help to establish the field of poromechanics. It also predicted that porous, fluid-filled media support two distinct compressional waves: the normal shear wave and a so-called slow wave that arises from refraction and mode conversion at the solid–fluid boundaries.

Biot’s prediction was verified in 1979 by another oil-company researcher, Thomas Plona, who detected the slow wave when he sent ultrasound pulses through a medium that consisted of sintered glass spheres immersed in water.

Judging by the papers that Mizuno and his coauthors cite, it took another two decades before anyone recognized the relevance of Biot’s slow wave to bone tissue. In 1995 Atsushi Hosokawa and Takahiko Otani detected the fast and slow waves in a sample of cancellous bone from a cow. What’s more, they showed that the normal waves propagate through bone, whereas the slow waves propagate through the soft, viscous material that fills the pores.

Before Biot’s theory can be turned into an in vivo diagnostic, more research is needed into how ultrasound propagates in whole, intact bones, whose structure is not isotropic, as Biot’s theory presumes. Mizuno and his collaborators took a further step in that direction by examining a sample of cancellous bone that retained its casing of cortical bone.

Having learned something of Biot’s theory, I became curious about the man himself. If you are too, you can read about his distinguished career in his obituary, which appeared in Physics Today in May 1986.

Charles Day

* The images come from “Plasticity and toughness in bone,” an article by Robert Ritchie, Markus Buehler, and Paul Hansma, which appeared in Physics Today‘s June 2009 issue.

Bringing bits of an asteroid back to Earth

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The kidney-shaped asteroid 25143 Itokawa (shown below) has a length of 535 m and a mass of 3.5 × 1010 kg. Its eccentric 556-day orbit takes it just within Earth’s orbit and just beyond Mars’s orbit.

On 25 November 2005 the Japanese space probe Hayabusa touched down on Itokawa, scooped up a sample of the dusty surface, and headed back to Earth. On 13 June last year Hayabusa reentered Earth’s atmosphere. Before the probe burned up, it jettisoned its sample return capsule, which made a safe parachute landing at the Woomera Test Range in the South Australian outback.

Today, in a series of six papers that appear in Science, the Hayabusa science team reports what it learned from subjecting the Itokawa dust grains to a series of physical and chemical tests. The team’s main finding—that asteroids and meteorites are made of the same, ancient material—confirmed prevailing theory. What came as a surprise—at least to me—is how much the asteroid has changed, and continues to change, even as it sits in a seemingly passive orbit.

And of course, bringing back a sample from any solar system object represents a stunning technological feat—and for Japan’s space agency, JAXA, a triumphant milestone in the history of space exploration.

Asteroids and meteorites

Asteroids are rocky objects of various types and sizes that orbit the Sun in a wide belt that lies mostly between Mars and Jupiter. Meteorites are rocky objects of various types and sizes that fall to Earth. Both classes of object are thought to be made up of ancient material that did not end up in Mercury, Venus, Earth, or Mars.

By tracing meteorite trajectories, astronomers had already deduced that meteorites mostly likely come from the asteroid belt. Comparing asteroids’ remotely sensed spectra with meteorites’ chemical compositions had suggested that the two classes of space rock are closely related.

The Hayabusa team has proven directly that Itokawa, which belongs to one of the most common classes of asteroid, the stony S-class, has the same composition as the most common type of meteorite, the chondrite. Evidently, meteorites are asteroids or bits of asteroid that are knocked off their orbital perches and make their way to Earth.

But the team could do more than clinch the case for kinship. From the composition and structure of some of the grains, the researchers deduced that the grains had been heated in the past to 800 °C and had cooled to 600 °C at a rate of 0.5 °C per millennium. If, as seems likely, the heat source was internal and originated in the decay of aluminum-26, then the slow cooling rate implies that Itokawa was once 20 km in diameter—40 times its current size! Writing in one of the papers, Tomoki Nakamura of Tohoku University and his collaborators concluded that

the Itokawa parent S-class asteroid was originally much larger, experienced intense thermal metamorphism, and was then catastrophically disaggregated by one or more impacts into many small pieces, some of which re-accreted into the present greatly diminished, rubble-pile asteroid.

Further evidence of catastrophic impact comes from cracks in some of the grains. But, according to the team’s analysis of the grains’ three-dimensional structure, the impacts, presumably from meteorites, were not energetic enough to melt the grains.

Itokawa also shows evidence of another kind of bombardment: from particles in the solar wind and from cosmic rays. The solar wind contains helium, neon, argon, and other noble gases. When those atoms strike Itokawa, they become implanted at depths of about 1 μm. The more energetic cosmic rays not only penetrate more deeply, they also transmute sodium, potassium, and other elements into noble gases.

The two sources of implanted nobel gases—the solar wind and cosmic rays—have different isotopic compositions. By analyzing the noble gas content of a grain, it’s therefore possible to deduce where a given isotope originated. When the researchers detected in a micron-sized gathered from Itokawa’s surface an isotope created by a cosmic ray at an original depth of 1 m, they reached a remarkable conclusion. To quote from the paper written by Tohoku University’s Keisuke Nagao and his collaborators:

Our results suggest that Itokawa is continuously losing its surface materials into space at a rate of tens of centimeters per million years. The lifetime of Itokawa should be much shorter than the age of our solar system.

The Hayabusa team is made up of researchers from about 20 different institutions, mostly in Japan. But the lead authors of the six papers all come from Tohoku University in Sendai. Although Sendai is far enough inland to have escaped the 11 March tsunami, the accompanying earthquake damaged many buildings there. The prompt publication of the Hayabusa results, despite the earthquake and its continuing aftermath, represents another triumph.

Charles Day

Speculative fact

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One of my favorite science fiction authors is Iain Banks. Most of his science fiction (he writes literary, Earthbound fiction, too) involves the Culture, a civilization of spacefairing humanoids and artificial intelligences who cohabit a technically advanced utopia. Conflict typically arises when the Culture meddles, albeit with good intentions, in lower civilizations.

Bank’s most recent novel, Surface Detail (2010), concerns the political, diplomatic, and moral ramifications of virtual hells, to which the disembodied personalities of criminals and other miscreants are sentenced to endure perpetual, gruesome torment.

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Unlike some of his fellow science fiction writers, Banks does not have much of a background in science. He studied English, philosophy, and psychology at the University of Stirling in his native Scotland. To keep up with developments in science and, one presumes, to gain inspiration for the technology in his novels, he reads New Scientist.

As someone already immersed in the world of science and news about science, I don’t normally read New Scientist, but my wife has a subscription. Taking a look at a recent issue yesterday evening, I noticed a news story by Amanda Gefter entitled “Time need not end in the multiverse.”

Gefter’s story describes a problem in the theory of multiple universes or multiverses:

In any infinite multiverse, everything that can happen, will happen—an infinite number of times. That has created a major headache for cosmologists, who want to use probabilities to make predictions, such as the strength of the mysterious dark energy that is accelerating the expansion of our own universe. How can we say that anything is more or less probable than anything else?

One solution, proposed last year by Raphael Bousso, Ben Freivogel, Stefan Leichenauer, and Vladimir Rosenhaus, evokes the end of time—literally. The abstract of their paper reads:

Present treatments of eternal inflation regulate infinities by imposing a geometric cutoff. We point out that some matter systems reach the cutoff in finite time. This implies a nonzero probability for a novel type of catastrophe. According to the most successful measure proposals, our galaxy is likely to encounter the cutoff within the next 5 billion years.

Although 5 billion years is a long way off, the catastrophic demise of our home galaxy is alarming. Fortunately, as Gefter reports, a paper posted earlier this month by Alan Guth and Vitaly Vanchurin preserves predictions in the mutliverse without killing off time.

Guth and Vanchurin’s paper resolves a speculative paradox in a speculative (and controversial) theory. The same issue includes a feature article, “Catching the Sun,” which describes startup companies that “aim to harness the power of nuclear fusion more quickly and cheaply than anyone imagined.”

Covering speculative—even sensational—ideas is typical of New Scientist. Recent cover stories have included “How animals shaped our minds,” “Life on Titan,” and “Your seventh sense.” Such an emphasis risks misrepresenting the scientific enterprise, given that most scientific results are humdrum, incremental advances.

But New Scientist is magazine, not a journal. Its goal is to entertain as well as to inform. And if you feel tempted to disdain the magazine’s breathless coverage of the next far-fetched line of research, keep in mind the speculative ideas—such as antimatter, optical lasers, prions, and cloaking devices—that turned out to be correct.

What’s more, I suspect New Scientist inspires not only one established science fiction writer, but also many young students who dream about revolutionizing science.

Charles Day

Is weak measurement more than an experimental tool?

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“You cannot measure a quantum particle without disturbing it. Or can you? Weird ‘weak measurements’ are opening new vistas in quantum physics.”

Thus reads the dek that tops Adrian Cho’s excellent news story in last week’s issue of Science. Adrian brought me and other readers up to date on the applications and implications of weak measurement, a concept that debuted in a 1988 paper in Physical Review Letters by Lev Vaidman, Yakir Aharonov, and David Albert.

My first encounter with weak measurement came in 2008 when I wrote about a paper in Science by Onur Hosten and Paul Kwiat. The paper described the observational confirmation of a spin Hall effect for light, which arises when a linearly polarized beam (purple in the figure) refracts at a boundary. Interaction between the beam’s polarization or spin degree of freedom and its geometric or orbital degree of freedom causes its right-hand (red) and left-hand (blue) circularly polarized components to separate.

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The separation that Hosten and Kwiat sought was just 70 nm. To measure it, they adapted Vaidman, Aharonov, and Albert’s weak measurement approach, which I introduced in the following way:

The three theorists considered the case of a Stern–Gerlach experiment whose beams and magnets are too weak to segregate up and down spins. Ordinarily, such a setup would yield not the two well-separated spots of Otto Stern and Walther Gerlach’s famous experiment but a peanut-shaped blob.

That unhappy situation would change, the three theorists argued, if you used two polarizing filters placed before and after the magnets. Orienting the filters’ axes at 90∞ would cut off all transmission, of course. But setting them just off perpendicular would have a surprising effect: The wavefunctions of those few atoms that made it through would interfere and boost the spin-dependent displacement by orders of magnitude.

Weak measurement does not provide a free lunch, however. In graphical terms, it pushes a peaked signal further away from the origin, making its displacement easier to determine. But the technique also reduces the peak’s amplitude, making it harder to see anything at all. If an experiment’s resolution is limited only by the statistics of photon counting, the two effects cancel. That was far from the case for Hosten and Kwiat’s experiment, in which systematic errors, not a paucity of photons, limited resolution.

My story was about the spin Hall effect of light, not weak measurement. Indeed, if Hosten and Kwiat had been able to overcome jitter in their laser’s pointing direction, wobbles in the optical table, and other sources of blur, they need not have used weak measurement at all. Moreover, the experiment can be analyzed using the 19th-century physics of Augustin-Jean Fresnel and James Clerk Maxwell.

But as Adrian recounts in his story, weak measurement could be more than just a useful tool for determining small values. By barely perturbing a particle’s wavefunction—at least during the weak interaction phase—weak measurement appears to provide a means of circumventing quantum mechanics’ prohibition on tracking individual particles as they fly through an interferometer.

What’s more, in a feature article that appeared in last November’s Physics Today, Aharonov, Sandu Popescu, and Jeff Tollaksen proposed that weak measurements can be understood in terms of an alternative formulation of quantum mechanics—one in which wavefunctions propagate forward and backward in time symmetrically.

Not surprisingly, weak measurement as an interpretation of quantum mechanics has proven controversial. Aharonov, Popescu, and Tollaksen’s article provoked four letters in Physics Today, all of which took issue with it. Adrian also encountered the controversies when he reported his story.

It’s difficult for a nonexpert like me to evaluate the broader claims of weak measurement. For one thing, Aharonov, Popescu, and Tollaksen assert that their interpretation is “completely equivalent to standard quantum mechanics in so far as their predictions are concerned.” And some of the arguments, pro and con, hinge on interpretation, rather than experimentally determined facts or unimpeachable mathematical analysis.

Aharonov, Popescu, and Tollaksen suggest that their time-symmetric formulation of quantum mechanics could readily accommodate new, as-yet undiscovered physics. Even if that doesn’t turn out to be the case, it’s beyond dispute that weak measurement has proven useful in the lab.

Charles Day

The X Club

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Writing in 1864, the mathematician Thomas Archer Hirst (shown here) recounted a recent meeting in London’s Mayfair district:

On Thursday evening Nov. 3, an event, probably of some importance, occurred at the St George’s Hotel, Albemarle Street. A new club was formed of eight members: viz: Tyndall, Hooker, Huxley, Busk, Frankland, Spencer, Lubbock and myself. Besides personal friendship, the bond that united us was devotion to science, pure and free, untrammelled by religious dogmas.

A ninth member, the mathematician and physicist William Spottiswoode, joined the December meeting of the club, which became known as the X Club.

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The relationship between science and religion was severely strained in the mid 1800s. Charles Darwin’s On the Origin of Species came out in 1859. Three years later, the Anglican bishop of Natal John William Colenso published the first of his mathematical investigations into the society described in the Pentateuch, the first five books of the Bible. Based on population dynamics, food supply, transport, and other considerations, he concluded that information in the Pentateuch was unreliable.

Reacting to those and other encroachments of science and reason into religion, Pope Pius IX published a Syllabus of Errors. Its third article declared false the notion that

human reason, without any reference whatsoever to God, is the sole arbiter of truth and falsehood, and of good and evil; it is law to itself, and suffices, by its natural force, to secure the welfare of men and of nations.

The X Clubbers were not against religion. Like some of today’s scientists who believe in God, they considered biblical miracles as allegories, rather than actual feats of divine intervention. Indeed, the X Club’s position on science and religion could reasonably be described as mainstream now.

But that mainstream position rests on today’s science. Could new discoveries prove as disrupting to religious belief as Darwin’s theory of evolution was in the X Clubbers’ day? I think so. Scientists could conceivably find purely mechanistic explanations for human consciousness and the origin of life. When they do, and if a religious backlash ensues, I hope they won’t feel compelled, as the X Clubbers evidently did, to meet in private to talk about science “untrammelled by religious dogmas.”

Charles Day

Physics at San Diego Comic-Con International

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Each July fans of science fiction, fantasy, and horror convene in San Diego, California, for Comic-Con, a four-day celebration of invented realities. Originally focused on Superman, Spider-Man, and other comic books, Comic-Con now encompasses TV, film, anime, manga, toys, card and video games, e-comics, and novels.

I went to Comic-Con for the first time last month—as a member of the press. Besides enjoying the spectacle and learning from attendees about their enthusiasms and from vendors about their products, my goal was to hunt for physics in and around the conference hall.

Some Comic-Con physics lies more or less in plain view. At least two notable comic-book characters are physicists. Bruce Banner, who morphs into the Incredible Hulk, designed and oversaw the testing of a gamma bomb, a fearsome physics-based weapon. Charles Xavier, the X-Men patriarch, is the son of a nuclear physicist and earned a PhD in biophysics (in addition to PhDs in genetics and psychology). Attendees who strolled along aisle 2200 of the exhibit hall would have encountered the American Physical Society’s booth, where six members of the Physics Central team, including Laser Girl, handed out free, physics-themed comic books.

Hidden physics

But I was more interested in less obvious manifestations of Comic-Con physics. At booth MZ 18, I found one—in the person of a Rebel Alliance fighter pilot, who was staffing the fan table of San Diego Fan Force, a club for the city’s Star Wars enthusiasts.

The pilot, a young woman, turned out to be an engineer in real life. She works on the Stratospheric Observatory for Infrared Astronomy at NASA’s Ames Research Center in Moffett Field, California. What struck me was how naturally our conversation passed from the meager merits of Star Wars Episode I: The Phantom Menace to the challenge of operating an IR observatory from a modified Boeing 747. The bridge between science fiction and science fact was, I think, her dedication to both.

I found more hidden physics when I attended the live broadcast of the Nerdist podcast, which took place at 4th and B, a small concert venue in downtown San Diego. After warming up the crowd, Nerdist Chris Hardwick introduced his first guest, Wil Wheaton, who played Wesley Crusher for the first four seasons of Star Trek: The Next Generation.

Wheaton has forged a post-Trek career as an engaging and amusing writer, both online and in print. But despite his popularity among Comic-Con attendees, the audience at the Nerdist podcast had come to see two other stars of science fiction TV, Doctor Who‘s Karen Gillan and Matt Smith (shown below; thanks, Jenny, for the picture!).

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According to their biographies, neither Gillan nor Smith has much of a background in physics, or even in science. Both actors decided to pursue careers in drama while still in high school. But when a member of the audience asked Smith whom he based his portrayal of the Doctor on, Smith replied, “Albert Einstein.” The actor keeps a book of the physicist’s quotations in his bathroom for inspiration.

Star Trek, Doctor Who, and other science fiction franchises depend increasingly on sophisticated computer-generated imagery (CGI) to depict worlds that are too elaborate to re-create in any other way. Achieving verisimilitude, or at least a convincing sense of disbelief, requires adhering to familiar physical laws even in microgravity and other unfamiliar settings. How do CGI artists ensure their animations are physically plausible?

I found my answer from the extension school of the University of California, San Diego—or, rather, at the school’s Comic-Con booth. Most of the CGI that ends up on the screens of movie theaters, TVs, computers, and game consoles is created with the help of a powerful software package called Maya. “The physics is baked in,” one of the school’s staff explained to me.

Maya is made by Autodesk of San Rafael, California. Hoping to find the source of Maya’s physics power, I looked at its latest brochure, where I found this sales pitch:

Take advantage of the multi-threaded NVIDIA® PhysX® engine to create rigid-body simulations directly in the Maya viewport—and if you use PhysX in your game engine, you’ll be matching the runtime solution. The new Digital Molecular Matter plug-in from Pixelux Entertainment™ enables you to create highly realistic shattering simulations with multiple interacting materials. Meanwhile, further development of the Nucleus unified simulation framework and its associated modules means that convincing pouring, splashing, and boiling liquid effects are easier to achieve.

In case it’s not clear from the paragraph above, PhysX is a microprocessor whose architecture is designed specifically to calculate the physics that underlies realistic animation. Video games need such physics processing units (PPUs, as they’re known) to keep up with the action. Animators, I presume, need them to speed the creative process.

Although physics doesn’t pervade Comic-Con and the media it showcases, physics has a strong and diverse presence there. What’s more, like George R. R. Martin’s Westeros, Iain M. Banks’s Culture, and other richly imagined alternative worlds, physics repays those who immerse themselves in it—with fascination, if not with money.

Charles Day