Grad Student Solves 30-Year-Old Physics Problem

By Erin Weeks

Sometimes an age-old question just needs a fresh set of eyes.

That was the case in Duke’s physics department, where a graduate student and professor recently resolved a calculating dilemma that has vexed computational physicists for decades.

Emilie Huffman, second year PhD student in physics

Emilie Huffman, second year PhD student in physics

Emilie Huffman is a second-year PhD student from Charlotte, North Carolina. Last spring she began working with Shailesh Chandrasekharan, an associate professor and the director of graduate studies in physics, on what’s known as a sign problem.

Chandrasekharan is a theoretical nuclear and particle physicist who specializes in solving sign problems, which arise when one uses certain computational algorithms to calculate the behavior of large numbers of particles called fermions.

“Almost all the matter we know of are made with fermions,” Chandrasekharan said. “As building blocks of matter, it’s very important to be able to do calculations with them.”

But calculations of such complexity get tricky, and sign problems make it easy for wrong results to surface.

“It’s a very broad problem that affects almost all fields of physics involving quantum mechanics with strong correlations, where Monte Carlo methods are essential to perform calculations,” Chandrasekharan said.

Some in the field have simply moved on since the 1980s, leaving interesting questions plagued by sign problems unexplored. Other scientists have found workarounds and approximations. Very few, including Chandrasekharan, have tried to figure out solutions through the years. Huffman began work to expand on one of her advisor’s solutions, involving a grouping concept called fermion bags, and apply them to a new class of problems.

“She finally figured out a nice formula,” Chandrasekharan said. “Although the formula is quite simple and elegant, I couldn’t guess it.”

“In physics, often there’s a truth, and if you’re hitting on the right truth, everything starts falling into place.” Chandrasekharan says that’s what happened when he began applying Huffman’s formula to a class of problems.

Their paper appeared recently in the journal Physical Review B’s Rapid Communications.

“Now that I have a solution, I can begin to apply it,” Huffman said. Starting with condensed matter physics, Huffman plans to apply her solution to various questions that have been stymied by sign problems. “I can use this solution to study properties of graphene,” she said, referring to the single-layer carbon that has been touted as the strongest material in the world. Many puzzles remain in the field, especially involving multi-layer graphene sheets.

Wherever she turns her attention next, it’s clear Huffman has a promising career ahead.

Citation: “Solution to sign problems in half-filled spin-polarized electronic systems,” Emilie Huffman and Shailesh Chandrasekharan. Physical Review B Rapid Communications, March 12, 2014. DOI: 10.1103/PhysRevB.89.111101.

 

The Catastrophic Origins of Our Moon

This still from a model shows a planet-sized object just after collision with earth. The colors indicate temperature. (Photo: Robin Canup)

This still from a model shows Earth just after collision with a planet-sized object. The colors indicate temperature. (Photo: Robin Canup)

By Erin Weeks

About 65 million years ago, an asteroid the size of Manhattan collided with the Earth, resulting in the extinction of 75% of the planet’s species, including the dinosaurs.

Now imagine an impact eight orders of magnitude more powerful — that’s the shot most scientists believe formed the moon.

One of the leading researchers of the giant impact theory of the moon’s origin is Robin Canup, associate vice president of the Planetary Science Directorate at the Southwest Research Institute. Canup was elected to the National Academy of Sciences in 2012, and she’s also a graduate of Duke University — where she returned yesterday to give the fifth Hertha Sponer Lecture, named for the physicist and first woman awarded a full professorship in science at Duke.

According to the giant impact hypothesis, another planet-sized object crashed into Earth shortly after its formation 4.5 billion years ago. The catastrophic impact sent an eruption of dust and vaporized rock into space, which coalesced into a disk of material rotating around Earth’s smoldering remains (see a very cool video of one model here).  Over time, that wreckage accreted into larger and larger “planetesimals,” eventually forming our moon.

Physics professor Horst Meyer took this photo of Robin Canup, who was his student as an undergraduate,

Robin Canup (Photo: Horst Meyer, who taught Canup as an undergrad at Duke)

Scientists favor this scenario, Canup said, because it answers a number of questions about our planet’s unusual lunar companion.

For instance, our moon has a depleted iron core, with 10% instead of the usual 30% iron composition. Canup’s models have shown the earth may have sucked up the molten core of the colliding object, leaving the dust cloud from which the moon originated with very little iron in it.

Another mystery is the identical isotopic signature of the moon and the earth’s mantle, which could be explained if the two original bodies mixed, forming a hybrid isotopic composition from the collision.

Canup’s models of the moon’s formation help us understand the evolution of just one (albeit important) cosmic configuration in our galaxy. As for the rest out there, she says scientists are just beginning to plump the depths of how they came to be. Already, the models show “they’re even crazier than the theoreticians imagined.”

Touring the Planet’s Most Powerful Gamma Ray Source

By Erin Weeks

When students pass by its unassuming building along Circuit Drive, many have no idea the world’s most intense gamma ray laser lies tucked away in the heart of Duke’s campus.

The Duke Free Electron Laser Laboratory, or DFELL, attracts physicists from across the country to study everything from nuclear security to the death of stars. Gamma rays are the most energetic members of the electromagnetic spectrum, and they can tell us a great deal about physics at the nuclear level. Cosmic events, like massive star collapse, produce high gamma radiation, but few natural sources exist on earth — so DFELL provides a window into the dense and infinitesimal world of the atomic nucleus. A group of students organized by Duke’s Society of Physics Students recently toured the lab to learn more. All photos were taken by Jonathan Lee.

FEL_00

DFELL’s gamma rays are generated by the collision of electron beams with photons racing around a large, oval storage ring, like a race track. Here, half the group gathers next to the straight leg of the oval’s left side.

FEL_01

Physics professor Ying Wu, the associate director for accelerators & light sources at DFELL, explains how electrons colliding with photons are like bowling balls crashing into an oncoming train. At left, you can see the red magnets that help guide the beams.

FEL_02

Though hidden to onlookers, this room houses a cavity mirror that helps to steer the beam of electrons with incredible precision. Even minute vibrations can knock the laser beam out of alignment, so the mirror assembly sits on reinforced concrete with shock absorbers.

FEL_04

Behold the Blowfish. This detector system boasts 88 “spines,” which can precisely measure what happens when intense gamma rays excite a nucleus, causing it to spit out particles. The spines are comprised of liquid-scintillator cells, which produce photons triggered by radiation, and photomultiplier tubes, which convert the photons to electrical signals.

FEL_03

“Blowfish’s goal in life is to make precision measurements of photonuclear reaction cross-sections,” says Grayson Rich, a graduate student at UNC who studies nuclear and neutrino physics at DFELL with Duke professor Phil Barbeau. Blowfish’s proximity to DFELL’s gamma rays “allows for really rigorous evaluation of theories for how nuclear systems behave.”

Finding Order in Insect and Orc Swarms

Ouellette's model of insect swarming

Ouellette’s model of insect swarming

By Olivia Zhu

Dr. Nicholas Ouellette looks for the organization in disorder.

Ouellette, associate professor in the mechanical engineering department at Yale University, studies collective motion in animal systems. On February 17, he presented his models of swarming of Chironomus riparius, the non-biting midge, as part of Duke’s Physics Colloquium. Ouellette ultimately hopes to pin down fundamental laws of biology through his physics research.

In the lab, Ouellette has found that Chironomus insects swarm in a columnar, teardrop shape in the center of their container. They only live in their flying state for two to three days, during which they mate, lay eggs and die. During this period, swarming affords them protection from predators and the opportunity to mate.

Ouellette and his lab have devised various methods of modeling the insects’ swarming. They found that the insect density remains constant, and that the “scattering,” or collisions of insects, mirrors that of an ideal gas over long periods of time. Interestingly, the graph of individual insect speed follows a Maxwell-Boltzmann distribution, even though the lab did not track the usual factors that create such a distribution, like temperature.

The most pressing question Ouellette would like to answer is which factors create a swarm—he has determined that close insect-insect repulsion contributes to swarming, but distant insect-insect attraction does not. To pursue this question, Ouellette is testing how many insects it takes to make a swarm.

Wildebeest stampede modeled in The Lion King

Wildebeest stampede modeled in The Lion King

Other animals that exhibit collective motion are mackerel, wildebeests and starlings. Some familiar examples of collective motion modeling are visible as the Orcs storm the castle in Lord of the Rings and as the wildebeests charge the canyon in The Lion King.

Why Meteorites Are So Hard to Find in North Carolina

by Erin Weeks

Note: This is the first in a multi-part series following Nick Gessler’s course on meteorites and the history of the solar system. Astronomy enthusiasts should check out the North Carolina Museum of History’s Astronomy Days happening this weekend, Saturday, January 25.

Visitors to Nick Gessler's lab can touch the moon -- literally -- and a whole lot of meteorites (Photo: Eric Ferreri)

Visitors to Nick Gessler’s lab can touch the moon — literally — and a whole lot of meteorites (Photo: Eric Ferreri)

A meteorite hasn’t been discovered in North Carolina in over 80 years, but Professor Nick Gessler dreams that his students will be the first to break that streak.

On the first day of his class at Duke, Gessler hauled in three tables worth of meteorites, chunks of rock and metal that hurdled through outer space for millions, even billions of years before surviving the descent into Earth’s atmosphere. Among his collection are pieces broken off from the moon, Mars and the dramatic Chelyabinsk meteorite that lit up Russia and YouTube in February 2013.

“That’s what they look like,” Gessler said, gesturing toward one table. “They’re ugly rocks.”

That’s not entirely fair. Past their melted, black crusts, most of the meteorites glint with flecks of iron. Some thin slices, when finely polished and held up to a light source, resemble stained glass. But ugly or no, the rocks have captivated human interest since people first observed them falling from the sky. Now, they’ve attracted Duke students in wide-ranging fields, from English and history to mathematics and engineering, united by a common interest in the extraterrestrial.

Gessler has a couple of theories about why North Carolina has seen a dearth of meteorite discoveries. First, farming practices have changed — farmers found many meteorites while tilling their land and dislodging the stony debris. These days, farmers rely on heavy machinery unfazed by rocks.

Second, Gessler thinks it’s possible that, despite our astronomical advances, light pollution may have clouded our chances of seeing minor meteors falling to earth.

“Maybe people just don’t look up at the sky like they used to,” he said.

Throughout this course, it seems certain his students will be looking up. Each has been assigned a North Carolina meteorite to research — they’ll ferret out old newspaper clippings and find when and where it fell, whether the land is private and what the likelihood of finding remnants now might be. Gessler will instruct them on the art and science of meteorite hunting, and eventually the students may put their skills to work in the field.

And who knows — some of them might even find the meteorites beautiful.

Supernova Explosion in M82: Exciting, but No Neutrinos

The M82 galaxy before (top) and after (bottom) its new supernova on Jan. 22 (Photo: UCL/University of London Observatory/Steve Fossey/Ben Cooke/Guy Pollack/Matthew Wilde/Thomas Wright)

The M82 galaxy before (top) and after (bottom) its new supernova on Jan. 22 (Photo: UCL/University of London Observatory/Steve Fossey/Ben Cooke/Guy Pollack/Matthew Wilde/Thomas Wright)

 

By Erin Weeks

In the early morning hours of January 22, the Earth turned spectator to a celestial event the likes of which hadn’t been seen in nearly three decades. The explosive death of a white dwarf star in Messier 82 (M82), a nearby galaxy, quickly ignited the astronomy world.

The supernova is exciting for a number of reasons that other outlets have well outlined — but unfortunately for Kate Scholberg, neutrinos are not one of them. Scholberg, a Duke University physics professor, studies the mysterious, nearly-massless particles at Super-K, a detector located deep in the mountains of Japan. Super-K was designed to spot neutrinos as they speed through Earth, revealing information about their sources, which can include the sun, cosmic rays, and supernovae.

“M82 is too far away for us to see any neutrinos from it,” Scholberg wrote in an email. “It’s about 11.4 million light years from us, meaning that the chance of seeing even a single neutrino from a core-collapse supernova in current detectors is probably a few percent or less (of course, we’ll look).”

Even if it were close enough, she said, this supernova doesn’t appear to be the type that generates a lot of neutrinos. White dwarves are common, dense stars whose supernovae are triggered when they consume new matter and experience runaway nuclear reactions. It takes a far more massive star to undergo core collapse, when a star’s unstable center caves in under the its own gravity.

“Core-collapse supernovae are the ones that spew neutrinos,” she wrote. “So, not much hope we’ll see anything in neutrinos, alas.”

Scholberg coordinates SNEWS, the SuperNova Early Warning System, which will sound the alarm at the first sign of a neutrino-generating supernova. SNEWS and Scholberg will have to keep waiting for the next big blast, but the white dwarf supernova in M82 is still great news for physicists who study another hot topic — dark energy.

Astronomers spotted the supernova unusually early, meaning they’ll be able to collect a wealth of data before it hits peak brightness in a couple of weeks. It may be possible then to see the supernova with just a set of binoculars.

A six-photo composite of the starburst galaxy M82 (Photo: NASA, ESA, and The Hubble Heritage Team [STScI/AURA])

A six-photo composite of the starburst galaxy M82 (Photo: NASA, ESA, and The Hubble Heritage Team [STScI/AURA])