Atom catcher: With lasers and magnets, Waseem Bakr traps atoms for study under the microscope

By Bennett McIntosh

THE COLDEST SPOT on the Princeton campus is a cluster of a few thousand atoms suspended above a table in Waseem Bakr’s laboratory. When trapped in a lattice of intersecting lasers at just millionths of a degree above absolute zero — and roughly one-millionth the density of air — atoms become very still, enabling Bakr, an assistant professor of physics, to study them through a microscope.

At these frigid temperatures and ultralow densities, atoms begin to act very strangely. They function less like individual particles and instead behave like waves that blur and overlap, losing their individual identity and trading the physics of the everyday world for the laws of quantum mechanics. The resulting state, known as a degenerate Fermi gas, can yield insights into new states of matter that someday may lead to applications such as superconductors and quantum computers.

Bakr uses a system of lasers and magnetic fields to cool and trap the ultracold atoms in a crystal-like lattice made from light. He then manipulates and observes the atoms using a quantum-gas microscope, a device that he helped invent during his graduate studies with Markus Greiner at Harvard University, and further improved when he was a postdoctoral researcher with Martin Zwierlein at the Massachusetts Institute of Technology.

“We use lasers to create artificial crystals in which we place these quantum-mechanical atoms where the spacing between atoms is 10,000 times larger than what you find in real crystals,” Bakr said. “We are essentially engineering the behaviors of atoms using light.”

Bakr and his team first heat a block of lithium to 800 degrees Fahrenheit to liberate individual atoms that then fly into a long tube. There, the particles collide head-on with a laser beam pointed in the opposite direction, which rapidly slows and cools them. The atoms then flow into a chamber where the intersection of several laser beams creates an electromagnetic field that confines the atoms in an “optical trap.” The trap allows the fastest-moving (and warmest) atoms to escape, further cooling the ultracold gas. The resulting cluster of atoms, Bakr said, is “the coldest stuff you can find in the universe.”

Using the microscope, Bakr can agitate a single atom to watch the disturbance propagate, or he can rearrange the entire system to simulate a different material. “If I decide I want to study graphene today,” he said, “I can arrange my lasers to make a graphene-like lattice, and suddenly the physics that I’m looking at are very different.” This precise control could hold the key to another advance, he said. “If you have 1,000 atoms, and you have control over every single atom and their interactions, these are the basic building blocks of a quantum computer,” Bakr said.

Cold atoms

Trapped by lasers and magnets, lithium atoms form a fluorescent red ball at the center of this image. In this initial stage of laser cooling, about 1 billion atoms are brought from a temperature of 350 degrees Celsius to a thousandth of a degree above absolute zero.

Bakr and his team are using ultracold atoms to study the behavior of superfluids with imbalanced spin populations. In a paper published in the August 24, 2016, issue of Physical Review Letters, Bakr and his team showed that the two-dimensional gas separates into two phases, a superfluid in the center of the trap and a  normal gas at its periphery, like the phase  separation that happens when mixing oil and  water. “Observing this phase separation is the first step in a search for exotic types of superfluidity that were predicted over 50 years ago,”

The Bakr lab’s work is supported by grants from the Air Force Office of Scientific Research, the National Science Foundation and the Alfred P.  Sloan Foundation.

Exploring collective interactions of matter and antimatter

STRIP AWAY ELECTRONS FROM THEIR ATOMS and you get a plasma — a collection of negatively charged electrons and positively charged ions. But at high energies around compact cosmic objects such as black holes, quasars and pulsars, curious plasmas may form that, instead of ions, contain positrons, the antimatter counterparts of electrons.

Scientists are searching for ways of distinguishing this type of plasma from others, both in astrophysical environments and in laboratories on Earth. Julia Mikhailova, an assistant professor of mechanical and aerospace engineering, and Matthew Edwards, a graduate student in her lab, together with Professor of Astrophysical Sciences Nathaniel Fisch, found that, contrary to earlier claims, an electron-positron plasma would scatter some wavelengths of light surprisingly intensely via a process called Brillouin scattering.

This fundamental insight into the unusual behavior of matter-antimatter plasmas, published in the journal Physical Review Letters Jan. 8, 2016, may help to find such plasmas in space, or validate methods for creating them in the lab. The work was funded in part by the National Science Foundation and the National Nuclear Security Administration. –By Bennett McIntosh

Researcher probes the secret life of electrons

ELECTRONS DART within and between atoms far too quickly for current imaging techniques to observe their motion. To capture fast-moving objects without a blur, a photographer can use a camera flash to light up a scene for an instant. Julia Mikhailova, an assistant professor of mechanical  and aerospace engineering, hopes to capture electron motion in a similar way, but her camera flash must last only a few attoseconds — just millionths of a trillionth of a second.

Mikhailova and her team use lasers and plasmas, which are collections of charged particles, to create attosecond pulses of light. “With these pulses one can observe the action inside atoms and molecules,” Mikhailova said. These observations could help researchers predict electron behavior, leading to a better understanding of everything from chemical reactions to superconductivity.

Lasers cannot by themselves produce such brief pulses. Instead, scientists scatter high-powered laser light off a stream of gas. Too much energy from the laser, however, can strip the electrons in the gas from their atoms to make a plasma that no longer scatters light in the right way.

Mikhailova and her team, with funding from the National Science Foundation, are taking a different approach. Instead of using a gas target, they aim a much higher-powered laser at a solid glass disc, creating a dense plasma at its surface. The laser light — an oscillating electromagnetic field — accelerates the electrons in the plasma toward nearly the speed of light. At these speeds, Newton’s physics breaks down and relativity takes over, causing the release of light in the form of attosecond pulses.

To improve the technique, Matthew Edwards, a graduate student in Mikhailova’s lab, ran simulations of plasma-laser interactions on Princeton’s TIGRESS high-performance computer cluster. The results, published Sept. 16, 2016, in the journal Physical Review Letters, showed that mixing laser light with light at harmonic frequencies — which are multiples of the original laser’s frequency — increases the efficiency of the process.

In a paper published Feb. 24, 2016, in the journal Physical Review A, the researchers proposed an extremely efficient two-target system: the first target generates light with harmonic frequencies, which hits the second target to generate attosecond pulses. With such powerful pulses, a new domain becomes visible, Mikhailova said. “We may be able to really see what the electron is doing as it ‘orbits’ in the atom.” –By Bennett McIntosh

Of swords, stars and superconductors

Robert Cava weaves physicists’ dreams into exotic new materials

By Bennett McIntosh

ROBERT CAVA PULLS A LONG CURVED steel blade from its ornate sheath, revealing a rippling pattern of light and dark metal. The sword is a Japanese katana made from steel of such legendary strength and sharpness that it was said to be able to cut a hair as it fell to the ground.

In his office where the shelves are lined with mineral samples and crystal structures, Cava, Princeton’s Russell Wellman Moore Professor of Chemistry, recounts the sword’s role in his destiny. He’d entered the Massachusetts Institute of Technology (MIT) wanting to study applied physics. “Someone at MIT interpreted ‘applied physics’ as being about materials science,” he said. “So, I ended up in a freshman seminar about samurai swords.”

Samurai swords derive their mythic properties from distinctive arrangements of iron and carbon atoms, Cava learned. His fascination with the atomic structure of the ancient metal turned into a career arranging atoms into materials for a more modern age: batteries, superconductors and materials with strange and exotic properties that could become the basis for future electronic devices.

In the 1970s, when Cava was a student, these technologies were far off in the future. Captivated by the science of materials, Cava stayed at MIT to earn his Ph.D. in ceramics. “Now I know how to make toilet bowls,” Cava quipped. In fact, ceramics have a wide range of electrical properties that make them useful in computers, televisions and communications devices. After graduation and a postdoctoral fellowship at the National Bureau of Standards, Cava took a job at Bell Laboratories, the research arm of the then-dominant AT&T telephone company.

Renowned for hiring the best and giving its researchers intellectual freedom, Bell Labs was at the time brimming with new ideas. “Collaborations were built by sitting with random people in the cafeteria,” Cava recalled. One day in 1986, one of these collaborators invited Cava to a seminar on high temperature superconductors, which were newly discovered materials that conducted electricity with no energy loss, but required less of the expensive refrigeration conventional superconductors needed.

Sitting in the seminar, Cava contemplated how the atoms in the new materials could be arranged to improve their performance. “I went back to the lab and four days later I had made a better superconductor,” he said. He would co-author more than 30 papers on superconductors in 1987 alone. One former colleague, Bertram Batlogg, now at the Swiss Federal Institute of Technology in Zurich (ETH Zurich), recalled being so excited about one discovery that they wrote the paper in one night, fueled by “European-strength coffee and fresh home-baked cornbread.”

In 1996, as AT&T broke up and spun off Bell Labs, Cava moved to Princeton, where he established a reputation of being able to weave physicists’ dreams into exotic new materials. When his collaborators in the Department of Physics come to him with theoretical predictions, he can often make a material that exhibits the desired properties. Some of the new materials he has conjured are topological insulators, materials that act like superconductors on their surface but conduct no electricity at all under the surface. “It is dark magic,” said B. Andrei Bernevig, a Princeton professor of physics and a frequent collaborator of Cava’s.

Robert Cava

Chemistry professor Robert Cava can sometimes be spotted walking through Frick Chemistry Laboratory in one of the costumes he dons for his first-year general chemistry course. (Photo by Sameer A. Khan/Fotobuddy)

Cava is more understated in explaining what he calls his “chemical intuition.” The properties of a material depend as much upon the geometrical arrangement of its atoms as on the specific kinds of atoms. Cava approaches designing a new material first by finding the right geometry — how many other atoms of each kind should each atom connect to, and in which orientations — and then finding the right atoms to fit this geometry. All the while, he bounces ideas off his students and collaborators. “In the end, science is very personal,” Cava said.

The move to Princeton from Bell Labs brought more than new collaborators and projects. “At Princeton, I have to be more than a scientist,” Cava said. He had to become a teacher and, often, a performer, to engage the 100-plus students in his first-year general chemistry course.

To share the inspiration he has felt every day since his first materials-science class, Cava peppers his lessons with references to ancient alchemists and demonstrations of the power of their discoveries. Slicing a pumpkin — often adorned with a Harvard cap — with his Samurai sword is perhaps the tamest demonstration. “He’s always blowing something up, or lighting something on fire,” said Marisa Sanders, a Ph.D. student in Cava’s lab.

The antics cross over to his lab, where the lab rules encourage taking experimental risks, and where he can sometimes be spotted walking the lab in a Darth Vader costume, which he wears when he administers final exams, to, as he puts it, “relieve some of the students’ tension.” He is a patient teacher, willing to sit for hours with students to work through a difficult problem or an unexpected result.

But, especially in materials chemistry, such logical teaching only goes so far. The most harebrained ideas will either succeed or teach in their failure, according to Cava. “If he thinks something is not going to work, he won’t tell you not to do it,” said Elizabeth Seibel, a doctoral student in Cava’s lab. “But he might make a bet with you.”

When Cava isn’t conjuring crystals, he pursues his first scientific love, astronomy. Growing up on Long Island during the 1960s Space Race, Cava swapped his model-train set for another student’s home-built telescope just to get a good look at the moon. Now his students laugh at him for having so many telescopes in his garage that there is no room for a car. “I love to sit under the night sky and appreciate how beautiful the universe is,” he said. He shares this love with others, setting up a telescope outside the chemistry building to share eclipses and solar flares with colleagues and students. “It’s something that a bunch of us from the lab really look forward to,” Seibel said.

Beyond his passions for chemistry and astronomy, Cava hopes his mentorship and example help his students find something they love to do. “You have to be passionate about something,” he said. “In the end, you don’t want to look back and think, ‘I didn’t do anything with my life.’” He certainly will not have to worry about that.

CITIES: Resilient • Adaptable • Livable • Smart

Innovations in building materials, design, water systems and power grids are helping to make cities more livable, say researchers in Princeton’s School of Engineering and Applied Science

By Bennett McIntosh

Cities. They sprawl and tangle, juxtaposing ancient public squares and glistening skyscrapers. They provide homes for half of humanity, and economic and cultural centers for the rest.

It has taken us thousands of years to build today’s urban centers, and yet, they’re expected to double in land-area in just the next few decades. “Half the urban infrastructure we will be using in 2050 has not yet been built,” said Elie Bou-Zeid, a Princeton associate professor of civil and environmental engineering.

Though this growth is inevitable, the way these cities will expand is not. Rather than repeat the sprawling and uncoordinated development patterns of the past, researchers like Bou-Zeid and others in Princeton’s School of Engineering and Applied Science are exploring new ways to build urban infrastructures to serve our growing population, changing civilization and warming planet.

These intelligent cities will require buildings that heat and cool themselves on a limited energy budget. They’ll require bridges and other infrastructure built with the flexibility to adapt to a changing global climate and rising sea levels. And they’ll require innovations in the networks that supply cities with water and energy. These ideas — from new building materials to continent-spanning electrical grids — have the potential to shift urban development away from the present-day jumble of strip malls, suburbs and shantytowns toward the resilient cities of the future.

Clever buildings

The basic unit of these smarter, resilient cities is the intelligent building. Assistant Professor Forrest Meggers, who has a background in architecture and engineering, has a number of plans for making buildings smarter about how they heat and cool their indoor spaces. Often these heating and cooling systems involve water, which readily absorbs heat that is then shed through evaporation.

In one structure called the Thermoheliodome, the interior is coated with mirrors at odd angles to reflect heat toward water-cooled pipes. In another, the interior cools itself with evaporation through an external membrane that traps liquid water while allowing water vapor to escape. By demonstrating the effectiveness of these innovative ideas, Meggers, who has a joint appointment in the School of Architecture and the Andlinger Center for Energy and the Environment, hopes to show other architects that it is possible to make more effective and more attractive heating and cooling systems.

Meggers’ structures take advantage of two different ways heat is transferred: It can be carried by molecules of warm air or water, or it can radiate like light directly from surface to surface. Thermometers, which measure air temperature, don’t capture the effects that radiative heating and cooling can have on a building’s occupants, so Meggers developed a radiative heat-sensing camera. About the size of a thermostat, the camera captures a 360-degree view that researchers can use to build a 3-D model of the radiative surfaces in any room.

To investigate urban radiant-heat exchanges, Meggers’ students took similar devices to New York City, about 50 miles northeast of Princeton. The resulting thermal photographs enabled them to see how heat lingers in alleyways and clusters around window-mounted air conditioners. By seeing the heat, architects and engineers can improve their designs for optimal energy efficiency.

Optimal cooling is the goal of one project that Meggers collaborated on with Dorit Aviv, who earned her master’s degree in architecture in 2013 and is now a doctoral student at Princeton. The building is called the Cool Oculus and is designed to keep cool in the desert heat through a combination of evaporation and shifting shape. The researchers built the Cool Oculus as a prototype on the Princeton campus, and have secured a grant from the New York-based Tides Foundation to build a full-scale model and measure its capabilities.

During a hot day, mist flows into the Oculus’ central chimney and evaporates to cool the air within, which sinks as a refreshing breeze into the building. Meanwhile, the structure’s foundation absorbs excess heat, which it releases at night when the chimney widens to expose the foundation to the cool night sky. Combined, these effects can turn 100-degree desert heat into a comfortable 75 degrees.

Inspired by nature

The Oculus moves on a daily cycle, but Sigrid Adriaenssens, an associate professor of civil and environmental engineering, has designed structures whose real-time response to heat is built into the material itself. In a transparent case above her desk, Adriaenssens displays three structures that could pass for the leaves of a cyborg Venus flytrap. They each are made of white translucent shells curving off a central metallic strut.

The resemblance to a flytrap is not coincidental. Adriaenssens designed the structures with inspiration from the waterwheel plant, an aquatic cousin of the flytrap. This shape allows the entire structure to open or close in response to a small movement of the central strut. The strut, in turn, is made of two metals that expand differently when heated, so the shell expands significantly with a small increase in temperature. Adriaenssens envisions that these shells, which were developed with funding from the Andlinger Center for Energy and the Environment, could cover a building’s entire façade. On hot days, the shells would expand and block heat from streaming in through the windows.

Structures like these, which make clever use of materials and their form, will be the key to affordable and efficient structures in future cities, according to Adriaenssens. To make these structures, Adriaenssens and her team use computer simulations to calculate the structure’s optimal form. Like optimized forms in nature, Adriaenssens’ structures often show striking curves, from spiraling earthen garden walls and arching steel footbridges, to shell-shaped pavilions with slats to keep out direct sunlight while allowing in scattered light and breezes. Nor are the spreading leaves Adriaenssens’ only dynamic structure: To protect coastlines from storm surges while keeping them visually uncluttered, she has designed thick elastic spherical membranes that will inflate and press together to hold off the waves.

Water world

Where does the water that surges around Adriaenssens’ barriers go? Where does the water that cools Meggers’ buildings come from? To plan something as complex as a future city’s water system requires not just understanding the interactions between structures like these, but understanding how the structures and the people affect each other. Such an undertaking requires cooperation between researchers from many different fields, and an understanding of the successes and failings of many different cities, according to Bou-Zeid.

Bou-Zeid first grew interested in cities when he was a mechanical engineering undergraduate at the American University of Beirut in Lebanon. “I thought I would be designing racecars or airplanes, but environmental problems that involve the interaction of humans with their surroundings are more interesting,” he said. During his graduate and postdoctoral studies, Bou-Zeid investigated how cities — with their skyscraper-created wind canyons and their innumerable sources of heat and steam — fundamentally alter the movement of air around them.

Bou-Zeid is interested in how this airflow affects an invisible but critical part of cities’ water systems: evaporation. Before a city is built, water evaporates out of plants and earth, cooling the area. But in built-up areas, dark asphalt absorbs heat. Water flows off impermeable pavement into storm systems before it has the chance to evaporate and take heat away with it, trapping heat in the buildings and the streets. This trapped heat can warm cities by 10 to 15 degrees Fahrenheit higher than the surrounding countryside. The so-called urban heat island raises energy consumption and contributes to climate change as we burn fossil fuels to cool ourselves.

Parks, greenbelts and green roofs covered in plants can solve this problem by encouraging cooling through evaporation, Bou-Zeid said. But it is not as simple as planting trees: While Baltimore’s greenbelts have cooled it significantly, drier cities like Denver and Phoenix may be better off saving water by cooling with traditional air conditioning. “How do you compare the value of a gallon of water and a kilowatt-hour of energy in different cities?” Bou-Zeid asked.

Bou-Zeid’s attempts to answer this question, and similar studies by other researchers in every aspect of the water cycle, led to the formation of the Urban Water Innovation Network. The Network, supported by a five-year grant from the National Science Foundation, includes engineers, architects and social scientists from 14 institutions who are studying how six American cities interact with water. Bou-Zeid, Princeton’s team lead for the network, is working with colleagues at the University of Maryland and Arizona State University to create software that will model everything water can do in a city. Such software could be used to predict the benefit of new water projects while accounting for local climate and geology.

The wildest possible experiments

To ensure that the urban landscape is accurately represented in such simulations, professor James Smith is leading a team of researchers from five universities in the network to produce extremely accurate maps of the rainfall and flooding in each of the cities. For Smith, the William and Edna Macaleer Professor of Engineering and Applied Science and professor of civil and environmental engineering, such studies of real cities are the only way to understand urbanization’s present and future effects.

“In cities,” he said, “the wildest possible experiments are being carried out for you.” Rivers are rerouted. Vast tracts of land are paved over. Artificial shorelines and skylines change the flow of water and air. It is up to researchers to watch and learn from these unprecedented alterations to the land and environment.

Collaboration within the network leads in surprising directions. Meggers, also a member of the network, found a way to combine his interest in efficient heat transfer with the water systems. With Sybil Sharvelle, a professor at Colorado State University, he is designing wastewater systems that recapture the heat from showers and other uses of hot water.

When the project ends in 2020, the network will release a report detailing its findings and recommendations for the cities under study. The research covers an environmentally diverse collection of cities so that the suggestions can be useful to cities across the country and, in some cases, around the world. In the meantime, the network connects researchers and government officials to craft individual recommendations on short-term projects. “We ask the policymakers what they need to know, and try to understand their constraints so that our recommendations can be implemented,” Bou-Zeid said.

It’s not the first time Bou-Zeid has worked to make small, efficient changes to cities. Simply painting black roofs white so that they reflect more light keeps buildings cooler and saves energy and money.

New York City has implemented this idea via their °CoolRoofs program, through which thousands of volunteers have painted roofs white since 2009. These efforts provided Bou-Zeid with more data than he could ever have achieved in a laboratory. He is using data from this experiment in conjunction with his models of urban air and heat flow to determine the cost and energy savings of painting roofs white.

Networks and grids

Painting roofs white is a relatively easy modification to make to a city, but other modifications require a new way of thinking. Our cities are already in need of upgrades to electricity supply and delivery systems. Going forward, our electricity will increasingly come from renewable sources such as solar and wind power, which, while better for the environment, can vary due to wind shifts and cloud cover.

With renewable energy making up only about 10 percent of power production in the United States, this variability is not yet an issue, said Warren Powell, a professor of operations research and financial engineering who studies networks such as electrical grids and transportation systems. “But I see us hitting problems at about 20 percent renewables,” Powell said.

This variability makes it hard to fully replace coal, the traditional workhorse of electricity generation, and natural-gas turbines, which can be ramped up quickly. “When the dust clears in 40 years, we’re still going to have some fossil energy,” Powell said. While large, efficient batteries could store wind and solar power and release it as needed, the marginal cost of battery storage increases as more batteries are added to the grid. “It is going to be hard to fight this curve,” he said.

Changes in how the power grid operates could help. Powell recently began a project in Brazil, where a drought has cut into Brazil’s heavy dependence on hydroelectric power. Powell has begun working with a group of Brazilian power companies to study strategies for managing the variability from the influx of wind power. Because of wind’s variability, this is not simply a matter of replacing one power source with another. Instead, Powell will be supervising the development of Brazil’s first grid model that can closely simulate the variability of wind. This model will be used to develop robust management policies and energy portfolios that would help Brazil optimize an energy system that depends heavily on  wind and solar.

New technologies deployed smartly will help, Powell said. For example, self-driving electrical vehicles can decrease congestion in dense cities and lend their batteries to the electrical grid, selling power when the city needs it most and recharging overnight from the grid’s excess capacity.

Ultimately, these changes in technologies and policy must work within the economic and social constraints of existing cities. Failing to understand and anticipate urban changes and growth leads to not just bad policy, but unenforceable policy, Bou-Zeid said. If a city tries to prevent urban growth, for example, by limiting new housing, the city will often still grow, but in unregulated and unhealthy shantytowns on the periphery. “You must accept urban expansion — you have to work with it,” Bou-Zeid said.

But the size and inertia of cities is an opportunity, too, Meggers said. “Cities have the power to make a change.”

If researchers and policymakers at Princeton and in cities around the world can collaborate, making clever use of form, physics and interacting components as a part of urban planning, then that change will be a positive one.

Better living through behavioral science

How the psychology of human behavior is helping tackle society’s biggest problems

By Wendy Plump

SUPPOSE someone approaches you on the street with the following proposition: You can receive either cash on the spot or a much larger contribution to your retirement account that likely will yield far more in the future. Do you choose the instant cash, or go with the retirement account?

The answer tells a lot about how people think, and about how public policymakers think people think.

Most people, it turns out, would choose the instant cash. Most policymakers, at least until somewhat recently, would have said that people would select the higher long-term payout of the retirement account.

Over the past two decades, policy planners from the Oval Office to the middle-school principal’s office have become aware that people often do not behave rationally, nor even in their own best interests. Understanding why people act as they do is the basis of the growing discipline of behavioral science, which is helping shape policies that tackle society’s biggest problems, from financial planning to public health.

“It is remarkable how little effort has been made to understand human behavior in policy circles,” said Eldar Shafir, the Class of 1987 Professor in Behavioral Science and Public Policy and a leader in this field of research. “Policy depends upon people doing things that the policymakers expect them to do. Yet, there has been almost no attempt to understand what people actually do, what they can do and what they want to do.”

Shafir has been working to change that along with colleagues at Princeton’s Woodrow Wilson School of Public and International Affairs. Wilson School researchers are exploring the behavioral aspects of policies that combat poverty, school bullying, discrimination and many other issues.

The idea that psychology is essential for good public policy can be traced back 100 years to American economist John Maurice Clark at Columbia University, according to Shafir. “Clark pointed out that any time you design policy, you have to understand psychology,” Shafir said. “If you don’t, your policy design and implementation will often be flawed.”

This may sound like common sense, but in the past, psychology rarely had a place at the policy table, said Daniel Kahneman, Princeton’s Eugene Higgins Professor of Psychology, Emeritus, and professor of psychology and public affairs, emeritus, and a pioneer in the field. Instead, two disciplines — economics and law — were the wells from which policymakers drew almost exclusively.

Kahneman’s work is credited with improving economic analyses by including insights from psychology, especially on human judgment and decision making under uncertainty. The citation for his 2002 Nobel Prize in Economic Sciences lauds him for “laying the foundation for a new field of research.”

Yet, Kahneman is uncomfortable taking credit for the field’s progress. Instead, he cites economist Richard Thaler of the University of Chicago. Thaler and Harvard University Law School’s Cass Sunstein co-authored a 2008 book titled, Nudge: Improving Decisions about Health, Wealth and Happiness, that ushered applied behavioral science into the public consciousness.

The book brought attention to concepts such as how to present choices to people in ways that provide a gentle prod toward making good decisions. For example, automatically enrolling new employees in a retirement-savings program and allowing them to opt out, rather than encouraging employees to opt in to the program, dramatically increases the number of people who save for retirement.

These and other insights are backed up by extensive studies of how people actually behave and make decisions in given situations. A number of Princeton researchers are involved in research in behavioral science that has direct implications for public policy:

Stopping schoolyard conflict

Early in her career, Elizabeth Levy Paluck became interested in how social norms can influence people’s behavior. In post-genocide Rwanda, she found that a media campaign to help reduce prejudice and violence drew much of its success from its emphasis on changing people’s definition of acceptable and desired behavior.

“I study social norms — informal laws that are created and enforced by people,” said Paluck, professor of psychology and public affairs in the Wilson School. “How do people in a community figure out what these laws are, and how to follow them? One theory is that we look to the behavior of certain peers for cues as to what we should be doing.”

Paluck and colleagues wondered whether highly influential students could have an outsized impact on the social norms and behaviors of other students in a school setting. They designed an intervention called the Roots program that was aimed at reducing school bullying and conflict by convincing influential students to practice positive behaviors, with the goal of reaching wider networks of peers.

With colleagues at Rutgers and Yale universities, Paluck tested this approach in a study conducted at 56 middle schools throughout New Jersey. The researchers asked students to report who they socialized with on a regular basis — both in person and online — and then used the data to identify the most connected students.

The analysis identified students who were leaders among their specific peer groups, not just those who were the most popular overall. The researchers encouraged this small set of students to take a public stand against bullying at their schools. Would these “social referents” be able to spread social change?

Paluck and her collaborators found that middle schools that instituted Roots experienced a 30 percent reduction in reported “conflict incidents,” a finding the researchers published Jan. 4, 2016, in the journal Proceedings of the National Academy of Sciences. The results suggest that behavior-change campaigns may be made more effective when they harness networks of influence to change societal norms.

Funding for the project came from the William T. Grant Foundation’s Scholars Program, the Canadian Institute for Advanced Research, Princeton’s Educational Research Section, the Russell Sage Foundation, the National Science Foundation and the Spencer Foundation.

Combating scarcity

For his research on poverty, Shafir studies the impact that deprivation has on an individual’s ability to focus intellectual energy on life tasks. His work touches on the age-old question regarding the causes and effects of poverty: Are people poor because they are not capable, or are they are not capable because they are poor?

Shafir and his team have found that poor people are often quite good at making short-term decisions about how to spend money. But the continual pressure to make ends meet can create an oppressive cognitive load on the individual, leaving little bandwidth for other tasks, including long-term planning.

This situation is compounded by the fact that small but unexpected expenses, such as a car-repair bill, can have much larger consequences for poor people than for middle-class individuals who have some slack in their monthly budget. Shafir and co-author Sendhil Mullainathan of Harvard explored research on poverty in their 2013 book, Scarcity: Why Having Too Little Means So Much. They challenge the common societal perception that poverty is the result of personal failings and recast it as the outcome of a chronic lack of resources, be it money, transportation and housing, or even time.

Understanding the drivers of behavior among the poor can guide policies that help reduce the stresses and challenges associated with poverty, Shafir said. For example, if a fast-food company were to hand out employee work schedules further in advance — as opposed to the 48-hour timeframe it typically uses — then parents would be able to dedicate fewer cognitive resources to the constant management of childcare concerns, leaving them with more resources to devote to other aspects in their lives, including their job performance.

Counteracting stereotypes

Since she came to Princeton 16 years ago, Susan Fiske, the Eugene Higgins Professor of Psychology and professor of psychology and public affairs, has been researching issues of bias, discrimination and stereotypes.

One area of study involves exploring our perceptions of people as “warm and trustworthy” and “competent” at what they do. Middle-class individuals get high ratings on both counts, while homeless people and undocumented immigrants score low on both counts. Older people are seen as trustworthy but not competent, and rich people are seen as competent but not trustworthy.

In a study published earlier this year, Fiske and graduate student Jillian Swencionis reported that people in the workplace try to appear more competent by acting cold when dealing with their superiors, while superiors play up their warmth when dealing with subordinates. Supervisors and subordinates engage in these behaviors both to disprove stereotypes about themselves and to match what they think about the other person.

Recognizing these warmth-competence tradeoffs in interactions between employees of different ranks could help improve communications within organizations. The study was published in the Journal of Experimental Social Psychology in May 2016. Swencionis was funded in part by the National Science Foundation.

“People automatically categorize other people by race and gender and age,” Fiske said. “They do this without intention, so it’s not about evil motivation when people act on these associations. It’s kind of a default. As a result, people and organizations have to engage in extraordinary efforts to counteract that proclivity.”

No matter how groundbreaking the research, it is useless to public policy unless it is available to people in a position to implement it. So, Fiske started the journal Policy Insights from the Behavioral and Brain Sciences a few years ago. The journal is affiliated with the Federation of Associations in Behavioral & Brain Sciences, which does education and advocacy work. Fiske has been the federation’s president and serves on its executive committee.

Bringing policy into the 21st century

In September 2015, President Barack Obama signed an executive order directing federal agencies to draw on emerging research from the field of behavioral science when crafting policies. Obama described the directive as a way to “bring our government into the 21st century.”

Researchers at the Wilson School and in Princeton’s Department of Psychology are helping lead the application of behavioral science to policymaking through their work in government, at think tanks and nongovernmental organizations, and at schools and institutions. The growing demand for these skills led Shafir and several colleagues to cofound ideas42, a nonprofit company devoted to creating behaviorally informed solutions to societal problems.

The Wilson School also is home to a new center launched in spring 2015 and led by Shafir that is focused on applied behavioral science research. In the fall of 2016, the Kahneman-Treisman Center for Behavioral Science & Public Policy launched its inaugural symposium. The center has more than 45 affiliated faculty members, including Alin Coman and Johannes Haushofer, both assistant professors of psychology and public affairs in the Wilson School. The center also has members from 11 departments across campus, including such diverse fields as geosciences, human values, philosophy and African American studies.

“It’s an exciting time,” Fiske said. “I’m a child of the ’60s and ’70s. So for me to be able to have an influence with data on policy is really a dream come true. We wanted to make the world a better place. It’s not so clear that we did, but there’s progress on several fronts.”

Tiny delivery capsules for new drugs

‘Jack’ Hoang Lu researches nanoparticles for drug delivery

Graduate student ‘Jack’ Hoang Lu works on engineering nanoparticles for targeted drug delivery and diagnostics in the laboratory of Robert Prud’homme, professor of chemical and biological engineering. PHOTO BY CATHERINE ZANDONELLA

Some drugs cannot be delivered via a normal pill or injection because they cannot readily dissolve in water. About 40 percent of new pharmaceuticals have this hydrophobic (water-fearing) character, and like a globule of oil in water, they are unable to reach their targets in the body.

Robert Prud’homme, professor of chemical and biological engineering, addresses this problem by putting the drugs inside of nanoparticles, each about one-thousandth of the width of a human hair, which are then covered with a polymer called polyethylene glycol. The small size enables dissolution of the drug to increase their bioavailability.

Nanoparticles can also tackle a different delivery challenge presented by a growing class of drugs called biologics that, as the name implies, are made from cellular or genetic components. The problem is that they are all water-soluble, which make them easy prey for patrolling proteins that identify them as foreign and degrade them. The solution is similar: nanoparticles protect the biologics long enough for them to carry out their mission.

In addition to increasing the time that drugs spend flowing through the body, Prud’homme’s nanoparticles are capable of targeted delivery. This is necessary in the case of toxic drugs, including many cancer treatments, which would damage healthy cells if allowed to roam freely throughout the body. Prud’homme puts appendages, called ligands, on the outside of his nanoparticles that, due to their specific shape and chemical properties, attach to their target and only to their target. In collaboration with Patrick Sinko at Rutgers University, Prud’homme’s group discovered the optimum density of an appendage called a mannose ligand, used to target cells harboring tuberculosis microbes. The unexpected result was that attaching more appendages to a nanoparticle decreased its targeting effectiveness. This demonstration of the complex interplay between engineered nanoparticles and how the human body responds to them is a major theme of the Prud’homme research team. In addition to tuberculosis, the researchers are using these principles to attack cancer, inflammation, and bacterial infections.

The project receives funding from the National Institutes of Health, the National Science Foundation, Princeton’s IP Accelerator Fund and the School of Engineering and Applied Sciences Old Guard Grant.

–By Takim Williams

Cosmic background: 51 years ago, an accidental discovery sparked a big bang in astrophysics

SPIDER

The balloon-borne spacecraft, SPIDER, prior to launch. PHOTO BY ZIGMUND KERMISH

ON NEW YEAR’S DAY 2015, A BALLOON-BORNE SPACECRAFT ascended above Antarctica and snapped crisp photos of space, unobscured by the humidity of Earth’s atmosphere. Meanwhile, a telescope located 4,000 miles to the north, in the desolate Chilean desert, scanned roughly half of the visible sky.

By air and by land, physicists have staked out the best vantage points on the globe, not for stargazing, but for peering between the stars at the thermal traces of the Big Bang.

Spread nearly evenly across the universe is a sea of invisible radiation called the Cosmic Microwave Background (CMB) that keeps space a chilly 2.7 degrees above absolute zero. The now-cold CMB, however, is a remnant of a much hotter, more violent cosmic epoch. About 13.8 billion years ago, immediately after the Big Bang, the universe was filled with a hot gas of ionized particles and radiation. As space expanded, the waves of radiation were stretched and diluted into their current low-energy state. The boiling plasma has since cooled and clumped into galaxies, stars, planets and human beings, all drifting through the faint afterimage of the first flash.

The prediction, discovery and study of the CMB 50 years ago comprise a story that is deeply intertwined with several generations of faculty at the Princeton physics department. The story continues today as University researchers probe the microwave background with the goal of understanding the past and future of our cosmos.

The discovery of the background radiation was a serendipitous one. In 1964, Bell Laboratories technicians Robert Wilson and Arno Penzias racked their brains for an explanation of the noisy signal recorded by their radio antenna. When it turned out that the “noise” was actually radiation from the CMB, the two engineers found themselves unexpectedly pulled into the growing field of modern cosmology. The detection of the CMB earned them the Nobel Prize.

Yet the discovery wouldn’t have been possible without the work of physicists at Princeton, 40 miles down the road from Bell Labs. Back then, the branch of science known as cosmology was ignored by most serious researchers. The physics community viewed the origin and development of the universe as dead-end topics, yet a few at Princeton had dared to tackle it.

At the time, P. James Peebles was a physics postdoctoral researcher at Princeton. “When I started working in this field, everyone was saying, ‘There’s no evidence. Why are you studying this?’” said Peebles, who today is the Albert Einstein Professor of Science, Emeritus. Instead, the mainstream focus was particle physics, which studies the subatomic particles that make up the universe.

Guyot Hall

David Wilkinson and Peter Roll used this experimental setup on the roof of Guyot Hall, which housed the Department of Geology (now Geosciences), to search for the CMB, at Bob Dicke’s suggestion. Wilkinson is holding a screwdriver, and Roll is almost obscured by the instrument. Photo by Robert Matthews circa 1964-65

Two Princeton professors, John Wheeler and Peebles’ mentor, Robert Dicke, decided that research on the cosmic scale should not be neglected. Since 1915, when Einstein developed the theory of general relativity to explain the behavior of large objects in space, hardly any further research had been done on gravity or the structure of the universe. This was due in part to respect for Einstein’s picture of the cosmos, and in part to the difficulty of devising fruitful experiments. “In the mid-1950s, Bob started a serious program of laboratory and extraterrestrial tests for general relativity,” said Peebles. “John started a school for the theoretical study of the subject. These changes marked a renaissance.”

While everyone else was thinking small, Dicke and Wheeler were thinking big. More specifically, Dicke was thinking about the Big Bang, a concept that dated back to the 1920s, when it was first observed that the universe is expanding. Yet extrapolating the current expansion back in time to a tiny, hot, dense state from which it all began was not a widely accepted leap. Peebles said: “Until the ’60s, the evidence that this is what happened was minimal. It was still just an idea, popular in some circles, detested in others.”

Dicke took the Big Bang theory from guesswork firmly into the realm of empirical physics when he proposed the CMB as evidence for a hot, dense beginning. Peebles recalls how Dicke almost casually set the course for his career and that of his peers: “He persuaded Dave Wilkinson and Peter Roll [Princeton physics faculty members] to build a device called a Dicke radiometer to look for this radiation, and he told me with a wave of his hand, ‘Why don’t you go think about the theory.’ And I’ve been doing it ever since.”

By 1970, the scientific community had accepted that the CMB had the properties that made it undeniable evidence for the Big Bang. Physicists then shifted their attention to more detailed scrutiny of the remnant radiation as a way of deepening our understanding of the birth of the universe, its expansion and its fate.

One area of scrutiny is whether the universe went through a period of rapid expansion, or inflation, after the Big Bang. To look for signs of inflation and to map the CMB in our region of space, NASA in partnership with Princeton and other universities launched the Wilkinson Microwave Anisotropy Probe (WMAP) satellite, named postmortem in honor of Wilkinson’s contribution to experimental cosmology.

The inflationary model predicts a particular pattern to the fluctuations of the CMB. When WMAP released its first set of results in 2003, they neatly matched the predictions of inflation. Among the many Princeton researchers who played significant roles in WMAP were Lyman Page, the James S. McDonnell Distinguished University Professor in Physics; Norman Jarosik, senior research physicist; and David Spergel, the Charles A. Young Professor of Astronomy on the Class of 1897 Foundation. Thanks to WMAP, Spergel said, “We have a coherent cosmological model that fits all the data.”

Since then, several other projects, including most recently the European Space Agency’s Planck space telescope, have mapped the CMB and provided evidence for inflation. But scientists are looking for additional evidence in the form of long undulations — called gravitational waves — stretching across the fabric of space. The remnants of these waves could be detected as a faint pattern in the CMB known as B-mode polarization. A reported detection of gravitational waves earlier this year from another project, BICEP2, created a stir in the astrophysics community but turned out to be an artifact of interstellar dust.

Detecting the remnants of gravitational waves is one of the goals of the Atacama Cosmology Telescope (ACT), an international project funded by the National Science Foundation and led by Princeton’s Suzanne Staggs, the Henry DeWolf Smyth Professor of Physics. The team includes Lyman Page, the James S. McDonnell Distinguished University Professor of Physics; Spergel, the Charles A. Young Professor of Astronomy on the Class of 1897 Foundation; and many colleagues at collaborating institutions.

The data collected during the flight of the balloon-borne SPIDER mission in Antarctica — funded by NASA, the National Science Foundation, the David and Lucile Packard Foundation, and the Natural Sciences and Engineering Research Council of Canada; and led by Assistant Professor of Physics William Jones — could also reveal evidence of these waves in the CMB.

The search for gravitational waves is just one of the ways in which the CMB provides opportunities for studying the early universe. The ACT collaboration is also looking for evidence of dark energy, a mysterious force that is speeding up the expansion of the universe, and answers to even bigger questions about the cosmological model. It is clear that the CMB is an important tool for the foreseeable future of cosmology. However faint, it illuminates the distant past, which in turn illuminates the future.

–By Takim Williams

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New chemistry aids drug development

Tova Bergsten

Tova Bergsten PHOTO CREDIT: C. TODD REICHART

DRUG DEVELOPMENT OFTEN INVOLVES modifying the chemical structure to get the right combination of properties, such as stability and activity. Working in the laboratory of John Groves, the Hugh Stott Taylor Chair of Chemistry, undergraduate Tova Bergsten and graduate student Xiongyi Huang developed a practical and versatile method for altering molecules that could have wide application in drug synthesis and basic research. The method involves using a manganese catalyst to convert carbon-hydrogen bonds into chemical structures known as azides, which are useful for modifying the properties of drugs.

“Since this was my first long-term lab experience, I learned quite a bit,” Bergsten said. “It was eye-opening to be involved in the experimenting, writing and publishing side of a paper. I plan to continue with scientific research, and what I’ve learned through this experience will definitely be useful for my future work.”

The research, which was supported by the National Science Foundation, was published in the Journal of the American Chemical Society on April 14, 2015.

–By Tien Nguyen

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Son-o-MERMAID takes to the waters

Son-o-MERMAID

Son-o-MERMAID takes to the waters. PHOTO BY FREDERIK SIMONS

SEISMIC WAVES CAUSED BY EARTHQUAKES can tell us a lot about the makeup of the Earth’s crust and mantle. Yet we lack seismic readings from the regions under the world’s oceans, which cover 70 percent of the planet’s surface. To address this data gap, Associate Professor of Geosciences Frederik Simons and colleagues developed ocean-going autonomous buoys called MERMAIDs (Mobile Earthquake Recording in Marine Areas by Independent Divers) and, in a paper published on Aug. 20, 2015, in Nature Communications, reported that the divers can recognize earthquakes and transmit seismograms more or less in real time.

The divers are equipped with a hydrophone to detect acoustic signals generated by seismic waves. The MERMAID drifts as deep as 2,000 meters under the surface until it detects an earthquake. Then it ascends to transmit the recorded waveform and its GPS position.

Simons and colleagues at the University of Rhode Island are now working on the next-generation buoy, which they call Son-o-MERMAID. After its maiden voyage three years ago was disrupted by Hurricane Sandy, the float is once again being tested and will be ready for deployment in the next few months. Compared to its progenitor, the new float has better position awareness and real-time communication capabilities because part of the instrument is always above water, and, in addition to batteries, has solar panels that power a vertical array of hydrophones.

The research is supported by the A.H. Phillips Instrument Fund at Princeton University and by the National Science Foundation.

–By Catherine Zandonella

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