Saturday, January 24, 2015
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The personal side of science

Wednesday, February 4, 2015
Center for Student Services 360 (Workshop Space) – Center for Student Services

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Fiona Harrison Awarded High-Energy Astrophysics Prize

The 2015 Rossi Prize has been awarded to Fiona Harrison, the Benjamin M. Rosen Professor of Physics at Caltech, for her "groundbreaking work on supernova remnants, neutron stars, and black holes enabled by NuSTAR." The award is the top prize in high-energy astrophysics.

Harrison is the principal investigator of NASA's NuSTAR (Nuclear Spectroscopic Telescope Array) mission. The telescope, launched in June 2012 under NASA's Small Explorer program, is the most powerful high-energy X-ray telescope ever developed. By focusing high-energy X-rays, NuSTAR is able to study some of the hottest, densest, and most energetic phenomena in the universe, including black holes, collapsed stars, and supernovae remnants. NuSTAR is conducting a census of the black holes in our cosmic neighborhood, examining the origins of high-energy particles in active galaxies, and mapping the remains of supernovae to better understand how stars explode and chemical elements are formed.

The citation for the Rossi Prize notes that Harrison's "assembly and leadership of the extraordinary NuSTAR team has opened a new window on the Universe."

"The exciting scientific results from NuSTAR are the culmination of close to two decades of work by a talented and dedicated team," says Harrison. "It is a privilege to work with them, and an honor to be recognized through the Rossi Prize."

Harrison came to Caltech as a research fellow in 1993 after earning her PhD in physics at UC Berkeley. She joined the faculty at Caltech in 1995 and was named the Benjamin M. Rosen Professor in 2013. Harrison is a member of the National Academy of Sciences and a fellow of the American Academy of Arts and Sciences and the American Physical Society. In 2013, she won a NASA Outstanding Public Leadership Medal, and she was recently elected as an Honorary Fellow of the Royal Astronomical Society.

The AAS High Energy Astrophysics Division awards the Rossi Prize annually in honor of physicist Bruno Rossi, an authority on cosmic-ray physics and a pioneer in the field of X-ray astronomy. It recognizes significant contributions to high-energy astrophysics, with particular emphasis on recent, original work. Harrison will accept the award and present a plenary lecture at the 227th annual meeting of the American Astronomical Society, which will be held in Kissimmee, Florida, in January 2016.

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Wednesday, February 18, 2015
Center for Student Services 360 (Workshop Space) – Center for Student Services

HALF TIME: A Mid-Quarter Meetup for TAs

Unusual Light Signal Yields Clues About Elusive Black Hole Merger

The central regions of many glittering galaxies, our own Milky Way included, harbor cores of impenetrable darkness—black holes with masses equivalent to millions, or even billions, of suns. What is more, these supermassive black holes and their host galaxies appear to develop together, or "co-evolve." Theory predicts that as galaxies collide and merge, growing ever more massive, so too do their dark hearts.

Black holes by themselves are impossible to see, but their gravity can pull in surrounding gas to form a swirling band of material called an accretion disk. The spinning particles are accelerated to tremendous speeds and release vast amounts of energy in the form of heat and powerful X-rays and gamma rays. When this process happens to a supermassive black hole, the result is a quasar—an extremely luminous object that outshines all of the stars in its host galaxy and that is visible from across the universe. "Quasars are valuable probes of the evolution of galaxies and their central black holes," says George Djorgovski, professor of astronomy and director of the Center for Data-Driven Discovery at Caltech.

In the January 7 issue of the journal Nature, Djorgovski and his collaborators report on an unusual repeating light signal from a distant quasar that they say is most likely the result of two supermassive black holes in the final phases of a merger—something that is predicted from theory but which has never been observed before. The discovery could help shed light on a long-standing conundrum in astrophysics called the "final parsec problem," which refers to the failure of theoretical models to predict what the final stages of a black hole merger look like or even how long the process might take. "The end stages of the merger of these supermassive black hole systems are very poorly understood," says the study's first author, Matthew Graham, a senior computational scientist at Caltech. "The discovery of a system that seems to be at this late stage of its evolution means we now have an observational handle on what is going on."

Djorgovski and his team discovered the unusual light signal emanating from quasar PG 1302-102 after analyzing results from the Catalina Real-Time Transient Survey (CRTS), which uses three ground telescopes in the United States and Australia to continuously monitor some 500 million celestial light sources strewn across about 80 percent of the night sky. "There has never been a data set on quasar variability that approaches this scope before," says Djorgovski, who directs the CRTS. "In the past, scientists who study the variability of quasars might only be able to follow some tens, or at most hundreds, of objects with a limited number of measurements. In this case, we looked at a quarter million quasars and were able to gather a few hundred data points for each one."

"Until now, the only known examples of supermassive black holes on their way to a merger have been separated by tens or hundreds of thousands of light years," says study coauthor Daniel Stern, a scientist at NASA's Jet Propulsion Laboratory. "At such vast distances, it would take many millions, or even billions, of years for a collision and merger to occur. In contrast, the black holes in PG 1302-102 are, at most, a few hundredths of a light year apart and could merge in about a million years or less."

Djorgovski and his team did not set out to find a black hole merger. Rather, they initially embarked on a systematic study of quasar brightness variability in the hopes of finding new clues about their physics. But after screening the data using a pattern-seeking algorithm that Graham developed, the team found 20 quasars that seemed to be emitting periodic optical signals. This was surprising, because the light curves of most quasars are chaotic—a reflection of the random nature by which material from the accretion disk spirals into a black hole. "You just don't expect to see a periodic signal from a quasar," Graham says. "When you do, it stands out."

Of the 20 periodic quasars that CRTS identified, PG 1302-102 was the best example. It had a strong, clean signal that appeared to repeat every five years or so. "It has a really nice smooth up-and-down signal, similar to a sine wave, and that just hasn't been seen before in a quasar," Graham says.

The team was cautious about jumping to conclusions. "We approached it with skepticism but excitement as well," says study coauthor Eilat Glikman, an assistant professor of physics at Middlebury College in Vermont. After all, it was possible that the periodicity the scientists were seeing was just a temporary ordered blip in an otherwise chaotic signal. To help rule out this possibility, the scientists pulled in data about the quasar from previous surveys to include in their analysis. After factoring in the historical observations (the scientists had nearly 20 years' worth of data about quasar PG 1302-102), the repeating signal was, encouragingly, still there.

The team's confidence increased further after Glikman analyzed the quasar's light spectrum. The black holes that scientists believe are powering quasars do not emit light, but the gases swirling around them in the accretion disks are traveling so quickly that they become heated into glowing plasma. "When you look at the emission lines in a spectrum from an object, what you're really seeing is information about speed—whether something is moving toward you or away from you and how fast. It's the Doppler effect," Glikman says. "With quasars, you typically have one emission line, and that line is a symmetric curve. But with this quasar, it was necessary to add a second emission line with a slightly different speed than the first one in order to fit the data. That suggests something else, such as a second black hole, is perturbing this system."

Avi Loeb, who chairs the astronomy department at Harvard University, agreed with the team's assessment that a "tight" supermassive black hole binary is the most likely explanation for the periodic signal they are seeing. "The evidence suggests that the emission originates from a very compact region around the black hole and that the speed of the emitting material in that region is at least a tenth of the speed of light," says Loeb, who did not participate in the research. "A secondary black hole would be the simplest way to induce a periodic variation in the emission from that region, because a less dense object, such as a star cluster, would be disrupted by the strong gravity of the primary black hole."

In addition to providing an unprecedented glimpse into the final stages of a black hole merger, the discovery is also a testament to the power of "big data" science, where the challenge lies not only in collecting high-quality information but also devising ways to mine it for useful information. "We're basically moving from having a few pictures of the whole sky or repeated observations of tiny patches of the sky to having a movie of the entire sky all the time," says Sterl Phinney, a professor of theoretical physics at Caltech, who was also not involved in the study. "Many of the objects in the movie will not be doing anything very exciting, but there will also be a lot of interesting ones that we missed before."

It is still unclear what physical mechanism is responsible for the quasar's repeating light signal. One possibility, Graham says, is that the quasar is funneling material from its accretion disk into luminous twin plasma jets that are rotating like beams from a lighthouse. "If the glowing jets are sweeping around in a regular fashion, then we would only see them when they're pointed directly at us. The end result is a regularly repeating signal," Graham says.

Another possibility is that the accretion disk that encircles both black holes is distorted. "If one region is thicker than the rest, then as the warped section travels around the accretion disk, it could be blocking light from the quasar at regular intervals. This would explain the periodicity of the signal that we're seeing," Graham says. Yet another possibility is that something is happening to the accretion disk that is causing it to dump material onto the black holes in a regular fashion, resulting in periodic bursts of energy.

"Even though there are a number of viable physical mechanisms behind the periodicity we're seeing—either the precessing jet, warped accretion disk or periodic dumping—these are all still fundamentally caused by a close binary system," Graham says.

Along with Djorgovski, Graham, Stern, and Glikman, additional authors on the paper, "A possible close supermassive black hole binary in a quasar with optical periodicity," include Andrew Drake, a computational scientist and co-principal investigator of the CRTS sky survey at Caltech; Ashish Mahabal, a staff scientist in computational astronomy at Caltech; Ciro Donalek, a computational staff scientist at Caltech; Steve Larson, a senior staff scientist at the University of Arizona; and Eric Christensen, an associate staff scientist at the University of Arizona. Funding for the study was provided by the National Science Foundation.

Written by Ker Than

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Watching Black Holes Merge
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Growing Snow in Pasadena

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Growing Snow in Pasadena
Credit: Ken Libbrecht


All snowflakes are broadly categorized as either columns, which are long and thin, or plates, which are flat. Needles are considered a type of column and form only when the temperature is near 23 degrees Fahrenheit and the humidity level is high.

Credit: Ken Libbrecht


Columns like these—whether hollow or solid—typically appear at around 23 degrees Fahrenheit. Unlike needles, they can appear at varying levels of humidity and also at temperatures below 8 degrees Fahrenheit.

Credit: Ken Libbrecht

Stellar Dendrites

Plate-like snowflakes, such as this stellar dendrite, typically appear under conditions of very high humidity—but only when the temperature is near 5 degrees Fahrenheit. For the record, Libbrecht employs no Photoshop trickery to create his colorful images. Instead, he shines colored lights in from different angles behind the crystal. The ice acts like a complex lens and bends the light to accentuate structural details.


Credit: Ken Libbrecht

Stellar Plates

Snowflakes such as this stellar plate are common at lower levels of humidity, usually at temperatures between 26 degrees Fahrenheit and freezing, or below −14 degrees Fahrenheit. Why plates fail to form between these temperature ranges—or, more generally, why snow crystals grow into such different shapes at different temperatures—remains a scientific mystery.


Credit: Ken Libbrecht

Sectored Plates

Libbrecht created this sectored plate snowflake in his lab when the weather outside was frightful—for snowflakes, anyway. "It was summer and about 95 degrees outside when this crystal was growing," he said. Inside the lab, this flake grew at temperatures between 5 and 8 degrees Fahrenheit, and varying levels of humidity.

Credit: Ken Libbrecht

Capped Columns

The capped column starts out as a columnar crystal growing near 21 degrees Fahrenheit. If the crystal then falls into colder air, plates may grow from both ends of the column. The final crystal looks like a spool of thread with a hexagonal center column.


Credit: Ken Libbrecht

Bullet Rosettes

The capped bullet rosette forms in much the same way as a capped column but has at least three sections sprouting from a common center. This one grew columns at 21 degrees Fahrenheit, then Libbrecht lowered the temperature to 8 degrees Fahrenheit to create conditions favorable to the formation of plates on the ends.


For a city so proud of its mild, sunny winters that it created the Rose Parade to tout its climate, Pasadena—or, at least, Caltech—has become, somewhat paradoxically, renowned for its snowflakes.

Every year, when flurries begin falling across the country, members of the media begin flocking to speak with Caltech physicist and snowflake guru Ken Libbrecht, who literally wrote the book—or, rather, books—on the science and beauty of the frozen crystals. In his lab, he creates countless snowflakes to investigate and explain how subtle changes in temperature, pressure, and humidity give rise to an impressively varied menagerie of intricate snowflake forms.

In recent years, we have showcased his research in articles that explain the physics of snowflake formation in clouds and even how to grow your own snowflakes at home. This year, we are pleased to share some of his most recent micrographs documenting the fascinating and beautiful results of his investigations.

Displayed below are some of Libbrecht's images that show representative samples of some common types of snowflakes: needles, stellar dendrites, stellar plates, columns, sectored plates, capped columns, and bullet rosettes.

To learn more, visit Libbrecht's comprehensive site. It features sections on the science, aesthetics, and history of snowflakes—and it also answers the age-old question of whether any two snowflakes are alike. (Spoiler alert: they aren't.)

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Prime Numbers, Quantum Fields, and Donuts: An Interview with Xinwen Zhu

In 1994, British mathematician Andrew Wiles successfully developed a proof for Fermat's last theorem—a proof that was once partially scribbled in a book margin by 17th-century mathematician Pierre de Fermat but subsequently eluded even the best minds for more than 300 years. Wiles's hard-won success came after digging into a vast web of mathematical conjectures called the Langlands program. The Langlands program, proposed by Canadian mathematician Robert Phelan Langlands in the 1960s, acts as a bridge between seemingly unrelated disciplines in mathematics, such as number theory—the study of prime numbers and other integers—and more visual disciplines such as geometry.

However, to get the ideas he needed for his history-making proof, Wiles only scratched the surface of the Langlands program. Now Xinwen Zhu, an associate professor of mathematics at Caltech, is digging deeper, looking for further applications of this so-called unifying theory of mathematics—and how it can be used to relate number theory to disciplines ranging from quantum physics to the study of donut-shaped geometric surfaces.

Zhu came to Caltech from Northwestern University in September. Originally from Sichuan, China, he received his bachelor's degree from Peking University in 2004 and his doctorate from UC Berkeley in 2009.

He recently spoke with us about his work, the average day of a mathematician, and his new life in California.


Can you give us a general description of your research?

My work is in mathematics, related to what's called Langlands program. It's one of the most intrinsic parts of modern mathematics. It relates number theory—specifically the study of prime numbers like 2, 3, 5, 7, and so on—to topics as diverse as geometry and quantum physics.


Why do you want to connect number theory to geometry and quantum physics?

Compared to number theory, geometry is more intuitive. You can see a shape and understand the mathematics that are involved in making that shape. Number theory is just numbers—in this case, just prime numbers. But if we combine the two, then instead of thinking about the primes as numbers, we can visualize them as points on a Riemann surface—a geometric surface kind of represented by the shape of a donut—and the points can move continuously. Think of an ant on a donut—the ant can move freely on the surface. This means that a point on the donut has some intrinsic connections with the points nearby. In number theory it is very difficult to say that any relationship exists between two primes, say 5 and 7, because there are no other primes between them, but there are points between any two points. It is still very difficult to envision, but it gives us a more intuitive way to think about the numbers.

We want to understand certain things about prime numbers—for example, the distribution of primes among all natural numbers. But that's difficult when you're working with just the numbers; there are very few rules, and everything is unpredictable. The geometric theory here adds a sort of geometric intuition, and the application to quantum field theory adds a physical intuition. Thinking about the numbers and equations in these contexts can give us new insights. I really don't understand exactly how physicists think, but physicists are very smart because they have this intuition. It's just sort of their nature. They can always make the right guess or conjecture. So our hope is to use this sort of intuition to come back to understand what happens in number theory.


Mathematicians don't really have lab spaces or equipment for experiments, so what does a day at the office look like for you?

Usually I just think. And unfortunately, it's usually without any result, but that's fine. Then, after months and months, one day there is an idea. And that's how we do math. We read papers sometimes to keep our eyes on what the newest development is, but it's probably not as important as it is for other disciplines. Of course, one can also get new ideas and stimulation from reading, so we keep our eyes on what's going on this week.


A two-part question: How did you get first get interested in math in general, and how did you get interested in this particular field that you're in now?

My interest in math began when I was a child. People can usually count numbers at a pretty early age, but I was interested in math and could do calculations a little bit quicker and a bit younger than others. It came naturally to me. Also, my grandfather was a chemist and physicist, and he always emphasized the importance of math.

But to be honest, I didn't really know anything about this aspect of the Langlands program until I was in graduate school at Berkeley. My adviser, Edward Frenkel, brought me into this area.


What are you most excited about in terms of your move to Caltech?

I think this is, of course, a fantastic place. The undergraduates here are very strong, and the graduate school is also very good, so I'm also very excited to work with all of those young people. Also, the physics department here is very good, and as I said, quantum field theory has recently provided promising new ways to think about these old problems from number theory. Caltech professors Anton Kapustin and Sergei Gukov have played central roles in revealing these connections between physics and the Langlands problem.


Is there anything else that you're looking forward to about living in Pasadena?

I'm from Sichuan [province in China], and one thing that I miss is the food. It's hot and spicy, and now it's also kind of popular in the U.S. And there are very good Szechwan restaurants in the San Gabriel Valley. Actually, maybe the best Szechwan food in the U.S. is right here.


Aside from your research and professional interests, do you have any other hobbies?

Yes, I've been playing the game Go for more than 20 years. It's a board game that is kind of like chess. It's interesting, and it's very complicated. Many years ago, you'd play with a game set and one opponent, but now you can also play it online. And that's good for me because after moving from place to place, it's hard to consistently find someone to play with.

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India Becomes TMT Partner

Leaders of the Thirty Meter Telescope (TMT) project announced on Tuesday that the government of India has signed on as a full partner in the construction of what will be the world's largest ground-based telescope.

A collaboration of institutions in the United States, including Caltech and the University of California, along with institutions from Canada, Japan, India, and China, are working together to build the TMT observatory on Mauna Kea in Hawaii. TMT is expected to begin operation in the early 2020s.

Members of the TMT International Observatory Board call India's commitment to the project "most welcome and essential" and say that it "will enhance science collaborations for the next generation."

"India's commitment is an exciting step in making our shared vision of the Thirty Meter Telescope a reality," says Ed Stone, the David Morrisroe Professor of Physics and Executive Director of the TMT International Observatory. "We look forward to a close collaboration with Indian colleagues in launching a new age of astronomy."

To learn more about TMT—as well as Caltech's involvement in the project—visit our TMT Updates page.


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Marvin L. "Murph" Goldberger


Marvin L. "Murph" Goldberger, Caltech president and professor of theoretical physics, emeritus, passed away on November 26, 2014. He was 92.

A Chicago native, Goldberger received his bachelor of science degree from the Carnegie Institute of Technology (now Carnegie Mellon University) in 1943 and, in 1948, his PhD in physics from the University of Chicago, where he later served as a professor of physics. In 1957, he was named Higgins Professor of Mathematical Physics at Princeton University, where he remained until 1978, when he was named Caltech's fifth president and a professor of theoretical physics.

During his tenure as president, Goldberger helped to spearhead the development of the first 10-meter telescope at the W. M. Keck Observatory in Hawaii—that telescope and its twin are the largest optical telescopes in the world—and he worked to secure the support of the Arnold and Mabel Beckman Foundation to build the Beckman Institute. "Murph played a key role in helping me convince Arnold Beckman to fund the Caltech proposal for an institute that would develop technology to support research in chemistry and biology," says Harry Gray, Arnold O. Beckman Professor of Chemistry and founding director of the Beckman Institute.

While Goldberger was in office, Caltech's endowment more than doubled. In addition, the Institute's teaching standards were revised and the curriculum was restructured, and the undergraduate houses were renovated.

"The loss of Murph Goldberger is one that affects us deeply here at Caltech, but also reverberates throughout the nation's scientific leadership and the international physics community," says Caltech president Thomas F. Rosenbaum. "Murph was a man whose excitement about physics was contagious, and his enjoyment of life and caring for individuals deeply felt. He held to a vision to push the Institute to new heights of discovery and educational distinction, realized through decisive interventions."

Goldberger left Caltech in 1987 to assume the directorship of the Institute for Advanced Study in Princeton. In 1991, he returned to California to become a professor of physics at UCLA; in 1993, he moved to UC San Diego, where he was a professor of physics and later dean of the university's Division of Natural Sciences.

Goldberger, a particle physicist who worked on the Manhattan Project, derived, with theoretical physicist Sam Bard Treiman, the so-called Goldberger-Treiman relation, which gives a quantitative connection between the strong and weak interaction properties of the proton and neutron.

The recipient of numerous awards and academic honors, including the Dannie Heineman Prize for Mathematical Physics, the Presidential Award of the New York Academy of Sciences, the Leonard I. Beerman Peace and Justice Award, and honorary degrees from Carnegie Mellon, the University of Notre Dame, Hebrew Union College, the University of Judaism, and Occidental College, Goldberger was a member of the National Academy of Sciences, the American Physical Society, the American Association for the Advancement of Science, the American Academy of Arts and Sciences, the American Philosophical Society; served as cochairman of the National Research Council; and was a member of the Council on Foreign Relations, the Institute on Global Conflict and Cooperation International Advisory Board, and the President's Science Advisory Committee.

He also was active in international security and arms-control issues and served on the Supercollider Site Evaluation Committee of the National Academies of Sciences and Engineering and as a member of the Media Resource Service Advisory Committee of the Scientists' Institute for Public Information. In addition, he was a member of JASON, an independent group that advises the United States government on matters of science and technology policy.

Goldberger was predeceased by his wife, Mildred Goldberger, in 2006. He is survived by his sons, Joel and Sam, and three granddaughters, Nicole, Natalie, and Natasha.

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Tuesday, December 2, 2014
Guggenheim 101 (Lees-Kubota Lecture Hall) – Guggenheim Aeronautical Laboratory

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