Charles Hard Townes died peacefully on 27 January 2015 in Oakland, California. Coinventor of the maser and laser, he was famous as well for his research in microwave spectroscopy and astronomy.
Charlie was born on 28 July 1915 in Greenville, South Carolina, the fourth of six children. He grew up on a farm, and he often emphasized that his interest in science and nature started there, when he and his older brother, Henry (later a well-known entomologist), explored the woods and streams around their home. After graduating from Furman University in 1935, he got his master’s at Duke University and carried out his PhD work at Caltech. His thesis, completed in 1939 under William Smythe, was on the separation of isotopes and the determination of their nuclear spins. He then took a position at Bell Labs in New York, where he worked on secondary electron emission from surfaces bombarded with ions.
During World War II, Charlie worked on shortwave aircraft radar. Profiting from the technical know-how he had gained, in 1945 he began exploring the properties of molecules with high-resolution microwave spectroscopy, initially at Bell Labs and, starting in 1948, as an associate professor at Columbia University. Over the next seven years, Charlie and his students and postdocs carried out breakthrough studies on molecular structure and on spins and quadrupole moments of atomic nuclei. That research culminated in the seminal 1955 textbook Microwave Spectroscopy by Charlie and Arthur Schawlow, his postdoc and brother-in-law.
One emerging goal was the push to millimeter and even IR wavelengths, where many molecules have their rotational–vibrational transitions. However, the lack of electronic oscillators made progress difficult. But the general idea of exploiting atomic and molecular transitions as natural oscillators, along with wave amplification through stimulated emission, led to the maser in the early 1950s. Since Albert Einstein’s 1917 paper, numerous physicists have considered the possibility of exploiting stimulated emission. Inversion population had been demonstrated by Willis Lamb and Robert Retherford in 1950 and by Edward Purcell and Robert Pound in 1951. However, because of inherent losses in the apparatus, none of the researchers demonstrated a practical level of gain sufficient for applications.
Charlie told the story of how he had a “eureka” moment in 1951 while sitting on a park bench in Washington, DC: A device with substantial gain could be developed by employing a Paul quadrupole focuser to generate an intense beam of ammonia molecules in the excited state and feeding the beam through a resonant cavity whose highly conducting walls would help to supply the necessary positive feedback.
Indeed, in 1954 Charlie, student James Gordon, and postdoc Herbert Zeiger reported the operation of the first NH3 maser and demonstrated its potential as oscillator, amplifier, and clock. Independently, Aleksandr Prokhorov and Nikolai Basov at the Lebedev Physical Institute in Moscow proposed a maser device of a similar design.
The foundational work by Charlie, Prokhorov, and Basov instantly created the new field of quantum electronics. For their achievements, they were awarded the 1964 Nobel Prize in Physics.
The demonstration of the first practical maser, together with a conceptual paper in 1958 with Schawlow describing how an “optical maser” could be constructed by embedding the active medium in a Fabry–Perot open resonator, led the way to the plethora of short-wavelength lasers that have since been invented. In the early 1960s, Charlie and his students used a ruby laser for research in the new field of nonlinear optics. Among Charlie’s discoveries during that period were stimulated Brillouin scattering and self-trapping of optical beams.
Charlie was drawn to new, unexplored territories of scientific inquiry. Once he had done the initial exploring, he was happy to move on and leave the wonderful playgrounds to others; he did so in 1955 when he left microwave spectroscopy and in the early 1960s when he moved his attention away from the laser.
After the 1957 “Sputnik shock,” Charlie felt a strong duty to help his country with scientific and technical advice. He spent 1959–61 in Washington, DC, as vice president of the Institute for Defense Analyses. There he founded the JASON advisory group. Later Charlie served on the Apollo program advisory committee and the science advisory committees to four US presidents. In the early 1980s he chaired the MX missile basing committee under President Ronald Reagan.
After several years as provost of MIT, Charlie returned to basic research in the late 1960s. He joined the University of California, Berkeley, and worked in microwave and IR astrophysics. Charlie had been interested in astronomy for some time. With his students—including Arno Penzias, who later shared the Nobel Prize for the discovery of the cosmic microwave background—and colleagues at the US Naval Research Laboratory, Charlie put his first tunable ruby maser from 1956 to good use as a sensitive amplifier for radio astronomy. In a paper for the 1957 General Assembly of the International Astronomical Union, he laid out a detailed list of interesting radio and millimeter transitions of atoms and molecules. That paper was a prescient preview of the emerging field of astronomical molecular spectroscopy. At Berkeley, Charlie and William Welch pioneered the field with their detection of ammonia and water in interstellar space in 1969. Their work demonstrated that interstellar clouds are much denser than previously thought and that many species of molecules can form.
Charlie’s work on detecting rapidly moving ionized gas clouds at the center of the Milky Way provided the first evidence for a concentration of mass there of about 4 million solar masses. By 1982 Charlie and his team were fairly certain that the object was a massive black hole, whose existence in galactic nuclei had been proposed by Donald Lynden-Bell and Martin Rees. With increasing resolution and precision, detailed studies of orbiting stars have since confirmed Charlie’s conclusion.
At Berkeley, Charlie also pioneered the development of IR spatial interferometry, which kept him busy until a few years ago. By extrapolating radio-interferometry techniques to a wavelength of 10 µm and replacing electronic oscillators with carbon dioxide lasers, Charlie resolved and mapped the IR emission from dusty stars at a level of detail far surpassing that possible even with the largest ground-based telescopes.
Charlie was devoted to his family and was a deeply spiritual person. He was an active member of the local church communities wherever he lived and strongly felt that science and religion are not in opposition but constitute different but related routes for exploring and understanding the universe. His levelheadedness, fairness, optimism, and humanity were rooted in that spirituality. To his colleagues and students, Charlie was a role model and a revered mentor. He deeply cared for his students. The intensity and vibrancy in his research group was legendary, driven by Charlie’s relentless curiosity and boundless energy. It was an enormous privilege to be a member of the Townes group. The passing of Charles Townes marks the end of an era. The community of physicists will miss this great man and colleague.
