Integrating innovation and climate with economic growth

William D. Nordhaus and Paul M. Romer have designed methods for addressing some of our time’s most basic and pressing questions about how we create long-term sustained and sustainable economic growth.

At its heart, economics deals with the management of scarce resources. Nature dictates the main constraints on economic growth and our knowledge determines how well we deal with these constraints. This year’s Laureates William Nordhaus and Paul Romer have significantly broadened the scope of economic analysis by constructing models that explain how the market economy interacts with nature and knowledge.

Technological change

Romer demonstrates how knowledge can function as a driver of long-term economic growth. When annual economic growth of a few per cent accumulates over decades, it transforms people’s lives. Previous macroeconomic research had emphasised technological innovation as the primary driver of economic growth, but had not modelled how economic decisions and market conditions determine the creation of new technologies. Paul Romer solved this problem by demonstrating how economic forces govern the willingness of firms to produce new ideas and innovations.

Romer’s solution, which was published in 1990, laid the foundation of what is now called endogenous growth theory. The theory is both conceptual and practical, as it explains how ideas are different to other goods and require specific conditions to thrive in a market. Romer’s theory has generated vast amounts of new research into the regulations and policies that encourage new ideas and long-term prosperity.

Climate change

Nordhaus’ findings deal with interactions between society and nature. Nordhaus decided to work on this topic in the 1970s, as scientists had become increasingly worried about the combustion of fossil fuel resulting in a warmer climate. In the mid-1990s, he became the first person to create an integrated assessment model, i.e. a quantitative model that describes the global interplay between the economy and the climate. His model integrates theories and empirical results from physics, chemistry and economics. Nordhaus’ model is now widely spread and is used to simulate how the economy and the climate co-evolve. It is used to examine the consequences of climate policy interventions, for example carbon taxes.

The contributions of Paul Romer and William Nordhaus are methodological, providing us with fundamental insights into the causes and consequences of technological innovation and climate change. This year’s Laureates do not deliver conclusive answers, but their findings have brought us considerably closer to answering the question of how we can achieve sustained and sustainable global economic growth.

The Laureates:

• William D. Nordhaus, born 1941 in Albuquerque, USA. Ph.D. in 1967 from Massachusetts Institute of Technology, Cambridge, USA. Sterling Professor of Economics, Yale University, New Haven, USA.
• Paul M. Romer, born 1955 in Denver, USA. Ph.D. in 1983 from University of Chicago, USA. Professor at NYU Stern School of Business, New York, USA.

The Prize amount: 9 million Swedish krona, to be shared equally between the Laureates

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A major challenge has been how to make such comb sources smaller and more robust and portable. In the past 10 years, major advances have been made in the use of monolithic, chip-based microresonators to produce such combs. While the microresonators generating the frequency combs are tiny — smaller than a human hair — they have always relied on external lasers that are often much larger, expensive, and power-hungry.

Researchers at Columbia Engineering announced today in Nature that they have built a Kerr frequency comb generator that, for the first time, integrates the laser together with the microresonator, significantly shrinking the system’s size and power requirements. They designed the laser so that half of the laser cavity is based on a semiconductor waveguide section with high optical gain, while the other half is based on waveguides, made of silicon nitride, a very low-loss material. Their results showed that they no longer need to connect separate devices in the lab using fiber — they can now integrate it all on photonic chips that are compact and energy efficient.

The team knew that the lower the optical loss in the silicon nitride waveguides, the lower the laser power needed to generate a frequency comb. “Figuring out how to eliminate most of the loss in silicon nitride took years of work from many students in our group,” says Michal Lipson, Eugene Higgins Professor of Electrical Engineering, professor of applied physics, and co-leader of the team. “Last year we demonstrated that we could reproducibly achieve very transparent low-loss waveguides. This work was key to reducing the power needed to generate a frequency comb on-chip, which we show in this new paper.”

Microresonators are typically small, round disks or rings made of silicon, glass, or silicon nitride. Bending a waveguide into the shape of a ring creates an optical cavity in which light circulates many times, leading to a large buildup of power. If the ring is properly designed, a single-frequency pump laser input can generate an entire frequency comb in the ring. The Columbia Engineering team made another key innovation: in microresonators with extremely low loss like theirs, light circulates and builds up so much intensity that they could see a strong reflection coming back from the ring.

“We actually placed the microresonator directly at the edge of the laser cavity so that this reflection made the ring act just like one of the laser’s mirrors — the reflection helped to keep the laser perfectly aligned,” says Brian Stern, the study’s lead author who conducted the work as a doctoral student in Lipson’s group. “So, rather than using a standard external laser to pump the frequency comb in a separate microresonator, we now have the freedom to design the laser so that we can make the laser and resonator interact in new ways.”

All of the optics fit in a millimeter-scale area and the researchers say that their novel device is so efficient that even a common AAA battery can power it. “Its compact size and low power requirements open the door to developing portable frequency comb devices,” says Alexander Gaeta, Rickey Professor of Applied Physics and of Materials Science and team co-leader. “They could be used for ultra-precise optical clocks, for laser radar /LIDAR in autonomous cars, or for spectroscopy to sense biological or environmental markers. We are bringing frequency combs from table-top lab experiments closer to portable, or even wearable, devices.”

The researchers plan to apply such devices in various configurations for high precision measurements and sensing. In addition, they will extend these designs for operation in other wavelength ranges, such as the mid-infrared where sensing of chemical and biological agents is highly effective. In cooperation with Columbia Technology Ventures, the team has a provisional patent application and is exploring commercialization of this device.

Now, a research collaboration involving scientists in the Chemical Sciences Division at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has zeroed in on a low-temperature chemical mechanism that may have driven the formation of multiple-ringed molecules — the precursors to more complex chemistry now found in the moon’s brown-orange haze layer.

The study, co-led by Ralf Kaiser at the University of Hawaii at Manoa and published in the Oct. 8 edition of the journal Nature Astronomy, runs counter to theories that high-temperature reaction mechanisms are required to produce the chemical makeup that satellite missions have observed in Titan’s atmosphere.

The team also included other researchers at Berkeley Lab, the University of Hawaii at Manoa, Samara University in Russia, and Florida International University. The team used vacuum ultraviolet light experiments at Berkeley Lab’s Advanced Light Source (ALS), together with computer simulations and modeling work to demonstrate the chemical reactions that contribute to Titan’s modern-day atmospheric chemistry.

“We provide evidence here for a low-temperature reaction pathway that people have not thought about,” said Musahid Ahmed, a scientist in Berkeley Lab’s Chemical Sciences Division and co-leader of the study at the ALS. “This gives rise to a missing link in Titan’s chemistry.”

Titan may yield clues to the development of complex chemistry on other moons and planets, including Earth, he explained. “People use Titan to think about a ‘pre-biotic’ Earth — when nitrogen was more prevalent in the early Earth’s atmosphere.”

Benzene, a simple hydrocarbon with a six-carbon single-ring molecular structure, has been detected on Titan and is believed to be a building block for larger hydrocarbon molecules with two- and three-ring structures that, in turn, formed other hydrocarbons and aerosol particles that now make up Titan’s atmosphere. These multiple-ring hydrocarbon molecules are known as polycyclic aromatic hydrocarbons (PAHs).

In the latest study, researchers mixed two gases — a short-lived two-ring PAH known as a naphthyl radical (C10H7) and a hydrocarbon called vinylacetylene (C4H4) — at the ALS, and produced three-ring PAHs in the process. Both of the chemicals used to drive the reaction are inferred to exist on Titan based on what is known about the chemical makeup of its atmosphere.

The ALS experiments jetted away the end products of the reactions from a small reaction chamber. Researchers used a detector known as a reflectron time-of-flight mass spectrometer to measure the mass of molecular fragments produced in the reaction of the two gases. Those measurements supplied details about the chemistry of the three-ring PAHs (phenanthrene and anthracene).

While the ALS experiments used a chemical reactor to simulate the chemical reaction and a beam of vacuum ultraviolet light to detect the products of the reaction, supporting calculations and simulations showed how the chemicals formed in the ALS experiments do not require high temperatures.

PAHs like the chemicals studied at the ALS have properties that make them particularly difficult to identify in deep space, Kaiser said. “In fact, not a single, individual PAH has been detected in the gas phase of the interstellar medium,” which is the material that fills the space between stars.

He added, “Our study demonstrates that PAHs are more widely spread than anticipated, since they do not require the high temperatures that are present around carbon stars. This mechanism we explored is predicted to be versatile and is expected to lead to the formation of even more complex PAHs.”

And because PAHs are considered as precursors to forming molecular clouds — the so-called “molecular factories” of more complex organic molecules that can include the precursors to life as we know it — “This could open up theories and new models of how carbon-containing material in deep space and in the rich atmospheres of planets and their moons in our solar system evolve and originate,” he said.

Alexander M. Mebel, a chemistry professor at Florida International University and co-leader of the study, carried out calculations that showed how the reactants can naturally come together and form new compounds at very low temperatures.

“Our calculations revealed the reaction mechanism,” Mebel said. “We showed that you don’t need any energy to drive the reaction of naphthyl and vinylacetylene, so the reaction should be efficient even in the low-temperature and low-pressure atmospheric conditions on Titan.”

A key to the study was in the detailed modeling of the reactor cell where the gases were mixed.

Mebel noted that modeling of the energies and simulations of the gas-flow dynamics in play within the reactor help to monitor reaction progress inside the reactor, and allowed researchers to tie theoretical results closely with experimental observations.

The modeling work, which helped to predict the chemicals produced in the reactions based on the initial gases and the temperature and pressure of the heated chamber where the gases were mixed and struck with the vacuum ultraviolet beam, was led by the research team at Samara University.

“This verification of the model, by comparing it with experiments, can also be helpful in predicting how the reaction would proceed in different conditions — from Titan’s atmosphere to combustion flames on Earth.”

An aim of the continuing research, Kaiser said, is to unravel the details of how carbon-containing compounds with similar structures to DNA and RNA can develop even in extreme environments.