School of Natural Sciences
Three years ago, just before it was scheduled to shut down in preparation for its second phase, the Large Hadron Collider (LHC) discovered the Higgs boson. Its discovery was expected, having been predicted nearly sixty years earlier by Peter Higgs, but the LHC-produced particle is bizarre and puzzling.
“There are all sorts of issues, theoretical issues surrounding the Higgs, which are very mysterious,” says Nima Arkani-Hamed, Professor in the School of Natural Sciences. “The Higgs is the first truly new kind of elementary particle that we have discovered in four decades, and it is really a strange object.” Point-like with no properties aside from a mass of 125 gigaelectronvolts (GeV), the Higgs particle does not have any charge or spin. It is also the only known particle with the ability to interact with itself.
Faced with such a compelling cliffhanger, Arkani-Hamed headed to China. “I thought the most effective thing I could do to push this part of physics forward is to try to make sure that the next machine happens,” says Arkani-Hamed. “I thought I could usefully engage a large pool of talent among young people who are experts in how all of these colliders work, who also were looking for something to do in the gap period between when the LHC shut off and turned on again.”
by Danian Hu
Addressing an international audience in 2004, Professor Dong Guangbi, an erudite historian of science, summarized Chinese physics development over the previous century, and he argued that the country from which Chinese physicists and physics benefited most was the United States of America.2 Dong’s argument was supported by the background of the seven “most creative Chinese physicists.”
Five out of these seven received doctorates in America and four of the five— Chou, Wu, Yang, and Lee—were former Members of the Institute for Advanced Study, indicating the dominating American influence and the significant role of IAS in Chinese development. This essay supports Dong’s thesis with additional evidence revealed in my preliminary survey of Chinese physicists schooled in America during the first half of the twentieth century.
The first Chinese physicist to graduate from an American college was most likely Yuanli Hsia (夏元瑮, 1883–1944), one of a few in the first generation of Chinese physicists. Sponsored by the Guangdong Provincial Government, Hsia came to study at Yale University. Upon his graduation in 1907, Hsia went on to the University of Berlin where he studied with Max Planck and Heinrich Rubens before his return to China in 1912. He then served six years at Peking University as dean of the School of Sciences. Remarkably, Hsia did not accept Einstein’s theory of relativity before 1919 when he returned to Berlin and met Einstein through Planck. Hsia’s early resistance to relativity seemed to be partially influenced by his Yale professor Henry Bumstead. After studying with Einstein during 1919–1921, however, Hsia became an active and enthusiastic relativist who delivered numerous speeches and published many articles in China, expounding and advocating Einstein’s theories. He produced in 1921 the first Chinese translation of Einstein’s only popular book, Relativity: The Special and General Theories.
by Johannes M. Henn
What do the motion of the planets in our solar system, the energy levels of the hydrogen atom, and the interactions between subatomic particles have in common? Surprisingly, they are all governed by the same hidden symmetry principles.
Symmetry is a very important notion in physics, for mainly two reasons. On the one hand, systems with a lot of symmetry are usually easier to solve and study, so that key properties can be understood analytically. On the other hand, and more fundamentally, in the development of physics, symmetry principles have often been a successful guiding principle toward theories relevant for describing nature. An example is Einstein’s equivalence principle that led to the development of general relativity.
What is the hidden symmetry underlying the motion of the planets, such as the Sun and the Earth? The answer to this question is important for the Kepler problem, i.e., the question of how to predict the position and velocity of two bodies, given some initial conditions. (It should be noted that physicists often use the word “problem” not in the standard meaning, which has a negative connotation; rather, it should be thought of in a positive sense, as an interesting challenge.) The motion is governed by Newton’s laws, which tell us, in particular, that the gravitational force between two objects depends only on their relative distance. From this, it follows that the orbits lie in a plane. However, observing the trajectories more closely, one sees that they form ellipses that do not precess with time. In other words, the orientation of the ellipses does not change, and hence the orbits are closed. This regularity is a hint for a hidden symmetry, which in turn implies a constant of motion. Indeed, a certain vector, named after Laplace-Runge-Lenz (LRL), does not change with time (see figure).1 It points toward the perihelion of the ellipse, i.e., the point of the orbit where the Earth comes closest to the Sun, and its conservation explains the regularity of the orbits that we observe.
In 2006, Edward Witten, Charles Simonyi Professor in the School of Natural Sciences, cowrote with Anton Kapustin a 225-page paper, “Electric-Magnetic Duality and the Geometric Langlands Program,” on the relation of part of the geometric Langlands program to ideas of the duality between electricity and magnetism.
Some background about the Langlands program: In 1967, Robert Langlands, now Professor Emeritus in the School of Mathematics, wrote a seventeen-page handwritten letter to André Weil, a Professor at the Institute at the time, in which he proposed a grand unifying theory that relates seemingly unrelated concepts in number theory, algebraic geometry, and the theory of automorphic forms. A typed copy of the letter, made at Weil’s request for easier reading, circulated widely among mathematicians in the late 1960s and 1970s, and for more than four decades, mathematicians have been working on its conjectures, known collectively as the Langlands program.
Witten spoke about his experience writing the paper with Kapustin and his thoughts about future directions in mathematics and physics in an interview that took place in November 2014 on the occasion of Witten’s receipt of the 2014 Kyoto Prize in Basic Sciences for his outstanding contributions to mathematical science through his exploration of superstring theory. The following excerpts are drawn from a slightly edited version of the interview conducted by Hirosi Ooguri, Member (1988–89) and Visiting Professor (2015) in the School of Natural Sciences, which was published in the May 2015 issue of Notices of the American Mathematical Society (www.ams.org/notices/201505/ rnoti-p491.pdf).1
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.