School of Natural Sciences
By Robbert Dijkgraaf
Light is the great unifier. John Wheeler, the beloved Princeton physicist, used to draw the universe as a big capital U with a little eye on one leg, signifying that we, human beings, are the eyes of the universe looking back at itself. The universe after many, many billions of years formed human life on planet Earth, and we use light to observe and understand the universe.
The growing understanding of the nature of light through the centuries is the perfect metaphor for science: it is an eye-opener. Almost 350 years ago, Isaac Newton, as a young man, put a prism in a beam of light and unraveled its various colors. This was the beginning of a long story. Around the year 1800, the astronomer William Herschel was the first to measure the temperature of light. He made the startling discovery that the rainbow does not stop at red, but actually continues, invisibly, as infrared light, which we cannot see but can feel as a sensation of warmth.
What is light? Physics has a simple answer: an electromagnetic wave. Exactly 150 years ago, the Scottish physicist James Clerk Maxwell discovered the laws that describe these waves. I have a T-shirt with these equations and the text “And then there was light.” If only Maxwell had patented his equations! It would be enough to finance all research in the world.
by Nir Shaviv
How might climate be influenced by cosmic rays?
In 1913, Victor Hess measured the background level of atmospheric ionization while ascending with a balloon. By doing so, he discovered that Earth is continuously bathed in ionizing radiation. These cosmic rays primarily consist of protons and heavier nuclei with energies between their rest mass and a trillion times larger. In 1934, Walter Baade and Fritz Zwicky suggested that cosmic rays originate from supernovae, the explosive death of massive stars. However, only in 2013 was it directly proved, using gamma-ray observations with the FERMI satellite, that cosmic rays are indeed accelerated by supernova remnants. Thus, the amount of ionization in the lower atmosphere is almost entirely governed by supernova explosions that took place in the solar system’s galactic neighborhood in the past twenty million years or so.
Besides being messengers from ancient explosions, cosmic rays are extremely interesting because they link together so many different phenomena. They tell us about the galactic geography, about the history of meteorites or of solar activity, they can potentially tell us about the existence of dark matter, and apparently they can even affect climate here on Earth. They can explain many of the past climate variations, which in turn can be used to study the Milky Way.
The idea that cosmic rays may affect climate through modulation of the cosmic ray ionization in the atmosphere goes back to Edward Ney in 1959. It was known that solar wind modulates the flux of cosmic rays reaching Earth—a high solar activity deflects more of the cosmic rays reaching the inner solar system, and with it reduces the atmospheric ionization. Ney raised the idea that this ionization could have some climatic effect. This would immediately link solar activity with climate variations, and explain things like the little ice age during the Maunder minimum, when sunspots were a rare occurrence on the solar surface.
The theoretical astrophysicist and Princeton University professor is well known for his work on NASA’s 2001 Microwave Anisotropy Probe—he conceptualized the mission and deciphered the radio telescope’s data to measure the age of the universe, the shape of the universe, and the abundance of ordinary matter, dark matter, and dark energy. A 2001 MacArthur fellow and fall 2014 Visitor and former Member (1985–88) in the School of Natural Sciences, Spergel received the 2015 Dannie Heineman Prize for Astrophysics with Marc Kamionkowski for their investigation of the fluctuations of the cosmic microwave background, work they did when Kamionkowski was a Member (1991–95) in the School of Mathematics.
What makes you curious? I think that we are all curious. I have been fortunate to have the opportunity to have the time to explore some of the questions that have fascinated me.
Whom do you most admire and why? Galileo had the insight to point his telescope at the moon and then at Jupiter. He discovered new worlds and reshaped not only astronomy but our understanding of our place in the universe. He actively popularized his results, did both theoretical and observational work, and stood up (and suffered) for his beliefs.
Outside of your own, which field interests you most? I have always been fascinated by history and prehistory. I am fascinated by how humans have responded (or failed to respond) to challenges in the past.
How do you determine your focus? I try to identify important problems where I am likely to have a significant impact. Since there are many bright people working in astrophysics, I try to identify problems where I have a different perspective, access to new data, or a set of new tools that let me address the question in a novel way. I try to avoid areas where other scientists are doing very similar things with similar approaches.
What is the most surprising thing you’ve learned? Our universe is remarkably simple and remarkably strange. With only a handful of numbers (the universe’s age, the density of atoms, the density of matter, the amplitude of the variations in density and its scale dependence), we can describe all of its basic properties. Yet, the universe is very strange: atoms make up only 5 percent of the density of the universe. Dark matter, most likely composed of a yet undiscovered new particle, comprises most of the mass in our galaxy. Most of the energy in the universe is in the form of dark energy, energy associated with empty space.
Doubts Arise Over Claims of Evidence for Cosmic Inflation
“Space Ripples Reveal Big Bang’s Smoking Gun,” read the New York Times headline last March 17. In a seemingly momentous news conference at the Harvard–Smithsonian Center for Astrophysics, researchers using a BICEP (Background Imaging of Cosmic Extragalactic Polarization) telescope at the South Pole announced that they had detected the first direct evidence for cosmic inflation, a theory about the very beginnings of the universe first proposed in 1979.
The BICEP announcement claimed that the first images of gravitational waves, or ripples in spacetime, had been detected, a tantalizing and long hoped-for connection between quantum mechanics and general relativity. The landmark claim ignited the field and led to talk of a new era of cosmology.
At the Institute for Advanced Study, Raphael Flauger, Member (2013–14) in the School of Natural Sciences, began looking closely at the data. The year prior, Flauger had analyzed the first round of cosmic microwave background data released by the Planck satellite, a mission of the European Space Agency, which the BICEP team had used in its findings.
The Institute’s thirteenth annual Prospects in Theoretical Physics (PiTP) summer program for graduate students and postdoctoral scholars, which focused on string theory, was truly extraordinary in that it overlapped with Strings 2014. This is one of the field’s most important gatherings, which the Institute hosted with Princeton University, convening international experts and researchers to discuss string theory and its most recent developments. Six hundred attendees gathered for Strings 2014, which made it one of the largest Strings conferences since their inception in 1995.
Strings 2014 talks, which covered topics from B-mode cosmology and the theory of inflation to quantum entanglement, the amplituhedron, and the fate of spacetime, may be viewed at https://physics.princeton.edu/strings2014/Talk_titles.shtml. The program for PiTP and videos of its string theory talks may be viewed at https://pitp2014.ias.edu/schedule.html.
As part of the PiTP program, the Institute showed a screening of Particle Fever, a new film that follows six scientists, including the Institute’s Nima Arkani-Hamed, during the launch of the Large Hadron Collider and fortutiously captures the discovery of the Higgs particle. Peter Higgs, who predicted the existence of the particle fifty years ago, gave one of his first seminars on the topic at the Institute in 1966.