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Charles H. Townes (1915-2015)

Charles Hard Townes, co-inventor of the maser and laser, world-famous for his research in microwave spectroscopy and microwave/infrared astronomy, died peacefully on 27 January 2015 in Oakland, California, a few months before his 100th birthday.

Charlie Townes was born on 28 July 1915 in Greenville, South Carolina. He grew up on a farm, and often emphasized that his interest in science and nature started there, when he and his older brother Henry explored the woods and streams around their home. After graduating from the local college, Furman University, he started his physics graduate education at Duke, and then carried out his PhD work at Caltech. His thesis (1937-1939) under W.R. Smythe was on the separation of isotopes and the determination of their nuclear spins. In 1939 he took a position at AT&T Bell Laboratories in New York, to understand secondary electron emission from surfaces bombarded with ions. Here he soon also met and married his beloved wife and partner of 74 years, Frances (née Brown), and together they founded and brought up their family of four daughters, Linda, Ellen, Carla and Holly.

During World War II, Charlie was assigned to work on shortwave aircraft radar. After the War and profiting from the technical knowhow he had gained, as well as from hardware developed for the shortwave radar, Charlie began in 1945 an innovative program of exploring the properties of molecules with high resolution, microwave spectroscopy, initially at Bell Labs and, starting in 1948, as Associate Professor at Columbia University. Over the next seven or so years, Charlie and his students and postdocs carried out breakthrough studies on molecular structure, as well as on spins and quadrupole moments of atomic nuclei. This research culminated in the seminal 1955 text book “Microwave Spectroscopy” by Charlie and his postdoc and brother-in-law, Art Schawlow.

Next he wanted to push to millimeter and even infrared/optical wavelengths, where many molecules have their rotational transitions. However, the lack of electronic oscillators owing to the small size of resonant cavities made progress difficult. This is where the general idea of exploiting atomic and molecular transitions as natural oscillators, in conjunction with wave amplification through stimulated emission, known theoretically since Einstein’s 1917 paper, led to the breakthrough of the maser in the early 1950s. A number of physicists had previously considered the possibility of exploiting stimulated emission, and inversion population had been demonstrated in two laboratory experiments. However, a practical demonstration of substantial gain useful for applications had not been obtained, due to inherent losses in the apparatus. Charlie told the story of how he conceived the conceptual solution to these problems in a ‘Eureka’ moment sitting one morning on a bench in Washington’s Franklin Park in 1951. By employing a ‘Paul’ quadrupole focuser (invented by the German Nobelist Wolfgang Paul) to generate an intense beam of NH3 molecules in the excited state of one of the inversion transitions and feeding this beam through a resonant cavity, whose highly conducting walls helped to supply the necessary positive feedback, Charlie was convinced that a device with substantial gain could be developed. And indeed, after about two years Charlie with student James Gordon and postdoc Herb Zeiger reported the successful operation of the first NH maser (M icrowave A mplification by S timulated E mission of R adiation) in 1954 and demonstrated its enormous potential as oscillator, amplifier and precise clock. He pointed out that the maser was the prototype of all quantum-limited amplifiers, whose sensitivity is ultimately limited only by the uncertainty principle for number and phase. Independently and at about the same time, Alexandr Prokhorov and Nikolay Basov at the Lebedev Institute in Moscow also proposed a maser device operating on a similar design. This pioneering work instantly created a new field, ‘quantum electronics’, which to this day continues to be one of the most exciting research areas of fundamental physics research. The 1954/1955 demonstration of the first practical maser, together with a conceptual paper in 1958 with Art Schawlow describing how an ‘optical maser’ could be constructed by embedding the active medium of excited states in an open, Fabry-Perot type resonator, opened the path to the plethora of short-wavelength lasers that have since been invented. In the early 1960s Charlie and his students used the ruby laser, the first working laser invented in 1960 by Theodore Maiman, for research in the new field of nonlinear optics. Among his discoveries during this period were the new phenomena of stimulated Brillouin scattering and self-trapping of optical beams.

For these achievements, Townes, Prokhorov and Basov were awarded the 1964 Nobel Prize in Physics. The laser has since become a household word for millions of people and is an excellent example of how basic research can create technology that influences humanity at large. The laser plays an important role in modern communication, industrial production, energy creation, in medicine and much more, and is the ultimate tool of precision and stability.

As Charlie himself emphasized, he was drawn to new, unexplored territories of scientific inquiry. Once he had done the initial exploring, he was quite happy to move on. So he did in 1955, when he left microwave spectroscopy, and so he did in the early 1960s when he moved his attention away from the laser and its many applications. After the 1957 ‘Sputnik shock’ Charlie felt a strong duty to help his country, when called upon, with scientific and technical advice. He decided to spend 1959-1961 in Washington as Vice President of the Institute of Defense Analysis. There he founded and later was a member of the JASON advisory group. Over the years Charlie had many important and influential roles as government advisor. To name a few, he was member on the Apollo-Program Advisory Committee to NASA, as well as member of the Science Advisory Committee to four US Presidents. In the early 1980s he chaired the MX Missile Basing Committee under President Reagan.

After several years in science administration as Provost of MIT, Charlie decided in 1966/1967 to return to basic research and change fields again. This time he moved out to the University of California and began an entirely new and soon world-leading effort in microwave and infrared astrophysics in the Berkeley Physics Department. Charlie had toyed with the idea of taking up radio astronomy as early as 1945. Together with his students (one of whom was Arno Penzias, who later shared the Nobel Prize for the discovery of the Cosmic Microwave Background) and colleagues at the Naval Research Lab, Charlie put his first tunable ruby maser in 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 had laid out a detailed list of interesting radio and millimeter transitions of atoms and molecules. This paper initially had little impact, but in hindsight was a prescient preview of the field of astronomical molecular spectroscopy, which would soon start blooming. In Berkeley Charlie and his colleague Jack Welch pioneered this development by detecting in 1969 NH3 and H2 O in interstellar space (remarkably with H2 O acting as a natural maser). This demonstrated that interstellar clouds are much denser than previously thought, and, as a result, many species of molecules can form. We now know that the presence of dust and molecules is critical for the formation of stars, as they can cool interstellar gas to low temperature, such that gravity can overcome thermal motions.

Another important highlight of his Berkeley astronomy research was the detection of rapidly moving ionized gas clouds at the center of the Milky Way. This work provided the first evidence for a mass concentration there of a few million times the mass of the Sun. By 1982 Charlie and his team were fairly certain that this object likely was a massive black hole, whose existence in galactic nuclei had been proposed by Donald Lynden-Bell and Martin Rees in 1971, following the discovery of the quasars. Detailed studies of orbiting stars with ever better resolution and precision have since fully confirmed and strengthened Charlie’s conclusion. A final example of these remarkable Berkeley astrophysics years is the development of infrared spatial interferometry, which kept Charlie busy until a few years ago. Extrapolating radio interferometry techniques to a wavelength of 10 μm, and replacing electronic oscillators by CO2 lasers, Charlie used this novel technique to resolve and map the infrared 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 this 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.

Photo: Lawrence Berkeley National Laboratory

Obituary written by: Reinhard Genzel (MPI Für Extraterr. Physics)

BAAS Citation: BAAS, 2015, 47, 015