Roberto Colella

Roberto Colella, who helped reveal the coupling of gravity to the quantum-mechanical wavefunction of particles, and who redefined “x-ray physics” in the modern era of synchrotrons, passed away after a lengthy illness on 31 March 2018, at age 82. Roberto developed innovative experimental and computational diffraction techniques that were fruitfully applied to the “phase problem” in diffraction, multiple beam dynamical diffraction of perfect crystals, and scattering from quasicrystals. He also trained a generation of x-ray physicists who now guide synchrotron science at facilities around the world.

A native of Milan, Italy, Roberto earned his doctoral degree in physics in 1958 at the University of Milan, and soon began research at the Euratom Nuclear Research Center in Ispra, Italy. In a productive collaboration with Alfonso Merlini, he investigated the effect of crystal defects on diffraction intensities and initiated his long-term interest in “forbidden” reflections in crystals. In 1967 he began a four-year stint at Cornell University as a postdoc with Bob Batterman in the early days before the construction of the Cornell High Energy Synchrotron Source (CHESS). This began a series of novel x-ray investigations covering phonon dispersion curves in vanadium, thermal diffuse scattering and Debye temperatures in metals, and detecting bond charge shifts in GaAs, all still using laboratory x-ray sources with great ingenuity.

After a brief second postdoc appointment in 1971 at Catholic University (Washington, DC), the next year Roberto moved with his family to West Lafayette, Indiana, to join the Purdue University physics department, where he established a remarkable research program in diffraction physics until his retirement in 2008. Stimulated in part by Bob Batterman’s mastery of dynamical diffraction and the growing capabilities of mainframe computers at Purdue, he developed a computational approach called the “N beam” program that could quantitatively calculate diffraction profiles from dynamically diffracting crystals when multiple reflections are simultaneously excited. This was a challenge because it involved accurately combining very strong with very weak diffracted waves, which required both improved algorithms that he developed, as well as computers with higher precision that had recently become available. The N beam program became a standard for multiple beam diffraction studies, and showed the way for using multiple beam diffraction measurements to address the phase problem.

One eventful day in 1974, his Purdue colleague Al Overhauser asked Roberto if he knew anything about neutron interferometry. “Not much” was the reply (since he had never done any kind of neutron scattering experiment before), but he could look it up. Roberto was then told about using interferometry to possibly measure the change in a neutron’s wavefunction as it rose in height above the ground: the effect of the gravitational potential on a quantum mechanical wave function. Initially incredulous, within the afternoon he had convinced himself that this would be within the technical capabilities of silicon interferometers that were already being developed elsewhere. Of course, Overhauser had asked just the right person, because Roberto was not only a leading expert on perfect crystal dynamical diffraction theory, but he had also spent those years at Cornell where there was expertise at turning boules of dislocation-free silicon into monochromators. Sensing there would be a race, they quickly produced a Physical Review Letters (1974) paper for just the proposed experiment (a feat in itself). They got a big boost when Sam Werner read their PRL and immediately offered to collaborate. The race was on!

Their stunningly successful results in 1975 followed many challenges in machining the interferometer (Roberto), solving unexpected mounting issues by using green felt from a pool table (Overhauser), and constructing a specialized rotation system at the University of Michigan’s reactor (Werner). The interference modulations were an excellent fit to theory, and this first direct demonstration of gravity’s role in quantum mechanics was widely lauded, frequently repeated, and commonly cited in textbooks such as Sakurai’s Modern Quantum Mechanics as the “COW” (Colella-Overhauser-Werner) experiment. Gravity and quantum mechanics do not simultaneously play an important role in most phenomena accessible in terrestrial physics. This experiment was the first time in physics where an observation necessarily depended upon both Planck’s quantum constant ℏ and Newton’s gravitational constant G. Several subsequent versions of this experiment carried out by Werner found that the principle of equivalence was verified to better than a percent in the quantum limit.

Roberto and collaborators illuminated two other fundamental questions with their technique. Recognizing that the COW experiment takes place in the noninertial frame of the rotating Earth, they were able to demonstrate both that precession of the neutron spin in a magnetic field changes sign upon a 360° precession (as predicted by quantum mechanics), and that an additional phase shift arises just from the rotation of the Earth, analogous to the optical Sagnac effect.

After this remarkably productive detour into neutron physics, Roberto returned to x-ray research both in his lab and increasingly at synchrotrons. In those years he and his students conducted painstaking and challenging experimental studies of quasicrystals, resonant scattering in manganates, and the subtle effect of isotopes on the lattice constants of germanium, among other topics. Sometime in 1988 he recognized that there was now a community of physicists who were not just following the traditional approach of using x-rays to characterize materials, but because of improving synchrotron sources were actually focused on how x rays mutually interact with materials in novel ways. This would include magnetic x-ray diffraction, resonant diffraction, inelastic scattering, multibeam diffraction, coherent diffraction, and other new areas. He then lobbied for the first “Gordon Conference in X-ray Physics” in 1989, giving the name “x-ray physics” to this now substantial community. While initially granted on a trial basis, the Gordon Conference (later renamed “X-ray Science”) continues to this day as an essential gathering in a dynamic field.

In Roberto’s long career at Purdue, he produced many PhDs and several postdocs who became beamline scientists and major users at synchrotron facilities around the world, including in Canada, England, France, Germany, and South Korea, as well as at the US facilities at Argonne, Brookhaven, and Cornell. In addition to this impact on the x-ray community, at Purdue University he obtained support from foundations for a successful series of annual public talks on science and religion. Finally, we note that everyone who knew Roberto and his late wife Adele learned to never pass up an invitation for an evening reception at their home, which they frequently had for their visitors. The warmth and hospitality they shared so generously with all, and especially with their students, will not soon be forgotten. He was a friend and a gentleman, par excellence.