American physicist (1915–)

Born in Greenville, South Carolina, Townes was educated at Furman and Duke universities in his home state and at the California Institute of Technology, where he obtained his PhD in 1939. He worked at the Bell Telephone Laboratories (1939–47) before he took up an appointment at Columbia University, New York, where he became a full professor in 1950. He moved to the Massachusetts Institute of Technology in 1961 and then served as professor of physics at the University of California at Berkeley from 1967 until 1986.

In 1953 Townes designed the first maser (microwave amplification by stimulated emission of radiation). The maser works on the realization in quantum theory that molecules can only adopt a certain number of discrete characteristic energy states and that in their movement from one energy level to another they emit or absorb precisely determined amounts of radiation. Townes knew that the ammonia molecule (NH₃) could occupy one of two energy levels, the difference between them equaling a particular energy of a photon.

The question Townes went on to ask is, what happens if the ammonia molecule absorbs the photon of the appropriate frequency while it is at its higher energy level? Albert Einstein had answered the question in 1917 and shown that the molecule would fall to its lower state emitting a photon of the same frequency. There would then in fact be two photons of the right frequency to repeat the process and produce four photons. This is a rapid and powerful amplification producing a narrow beam of radiation with a single frequency. In this case the radiation emitted would have a frequency of 1.25 centimeters and thus fall in the microwave band, hence the name maser.

In any normal sample of ammonia only a few of the molecules would be in the higher energy state. Townes's problem was to devise a technique for the separation of molecules of the higher energy level (‘population inversion’) and he did this using a nonuniform electric field; molecules in the higher state were repelled and focused into the resonator while those in the lower state were attracted to it. By 1953 Townes had a working model of a maser.

Masers quickly found use in atomic clocks, receivers in radio telescopes, and numerous other uses. The maser led to the development of the laser, where light is amplified rather than microwave radiation and which was known in its earlier days as an ‘optical maser’. Townes, in fact, published a paper with A. L. Schawlow in 1958 showing the theoretical possibility of the laser. He was, however, beaten in the race to construct it by Theodore Maiman in 1960.

For his work on the maser Townes shared the 1964 Nobel Prize for physics with the Russians Nicolai Basov and Aleksandr Prokhorov, who had independently produced a maser in the Soviet Union in 1955.

Charles Townes (born 1915) was a physicist whose work concentrated on the development of high-resolution spectroscopy of gasses in the microwave region of the electromagnetic spectrum. He shared the Nobel Prize in Physics in 1964 for his work leading to the development of the maser and his research and ideas were instrumental in the development of the laser by Theodore Maiman. Townes was elected to the National Academy of Sciences in 1956.

Charles Hard Townes was born on July 28, 1915, in Greenville, South Carolina. As a youth he was interested in the biological and natural sciences. He was a gifted scholar who skipped the seventh grade. Townes entered Furman University in his hometown at age 16 and became interested in physics. He received two degrees from Furman - a Bachelor of Arts in modern languages and a Bachelor of Science in physics. He then went to Duke University, from which he received a Master's degree in physics in 1937. He wrote his masters thesis on van der Graaf generators and continued his studies of French, Italian, and Russian. He completed his education at the California Institute of Technology, where he researched the spin of the carbon-13 nucleus and was awarded a Ph.D. in physics in 1939.

Early Research and Achievements

The next eight years were spent at Bell Telephone Laboratories, where Townes worked as a researcher. While living in New York City, he also took classes at the Julliard School of Music and enjoyed the cultural attractions of the city. During World War II he did extensive work on radar bombing and systems design, as well as some of the early work in radio astronomy. After the war he made critical contributions in the development of high-resolution spectroscopy of gasses in the microwave region of the electromagnetic spectrum. He continued this work when he joined the Columbia University faculty in 1948.

Maser Breakthrough

The arrival of the radar in World War II gave rise to extensive use of electronic devices in scientific research. The area that interested Townes the most was the use of microwaves (low frequency radiation) to investigate the structure of matter. To carry out this sort of investigation effectively, oscillators that could produce very short wavelength radiation were needed. But by the late 1940s it had become clear that it would never be possible to build an ordinary oscillator that would be able to generate radiation of wavelength less than one millimeter.

Townes made use of the phenomenon of stimulated emission in his first attempt to produce an oscillator that would suit his purpose. This phenomenon, which had been known to physicists since at least 1917 when Albert Einstein showed its existence, is one through which atoms under the influence of an applied electromagnetic field emit photons. It was in 1951 that Townes had the breakthrough idea for his maser and outlined the plans on the back of an envelope while waiting for a restaurant to open. Townes reasoned that to be able to amplify very short wavelength radiation, action on the molecular scale would be required. He conceived of a way that an ensemble of molecules would be able, through stimulated emission, to produce a self-excited oscillator that could amplify signals. The molecules had to be in what is known as an excited state - namely, they had to contain a large amount of energy; they also had to be unstable. Electromagnetic waves would stimulate the molecules to release their extra energy at the same frequency and phase as the stimulating electromagnetic energy. If the right number of molecules were present, this energy would convert into electromagnetic energy very quickly, and coherent (that is, in phase) amplification would become possible.

Masers Explained

Townes called this device a "maser" - an acronym for microwave amplification by stimulated emission of radiation - and he built the first one in 1954 with H. J. Zeiger and James P. Gordon at Columbia University. This maser operated on ammonia gas. The gas is collimated by a small hole into a vacuum, where it acts like a beam of molecules. These molecules are in two energy states; therefore, depending on their energy, the molecules get deflected in different directions by an applied electromagnetic field. The molecules with the higher energy get deflected into a chamber known as a resonant cavity. If the number of molecules that gets deflected into the cavity is high enough, then amplification occurs. Because of the sharpness and invariance of the interactions in the ammonia beam and the accuracy with which they can be measured, this particular type of maser functions extremely well as a standard of time or of frequency.

With the collaboration of A. L. Schawlow, Townes described the conditions necessary for the operation of masers in different wavelength regions - namely, the infrared, visible, and ultraviolet portions of the spectrum. Such devices were known as optical masers, and the first one was built in 1960 by Theodore H. Maiman.

Townes' development of the maser proved to be critical in modern experimental research. Maser amplifiers have a very high signal-to-noise ratio. They come extremely close to amplifying a single photon of radiation, since they approach the maximum accuracy allowed by the uncertainty principle in measuring the phase and the energy of a given particle. (The uncertainty principle sets a limit on how accurately the energy and the phase of a particle can be measured simultaneously.) Masers are thus extremely useful in experiments performed on a quantum level. In addition, they are useful in long-distance radar and microwave communications and in the reception and detection of weaker signals in radio astronomy.

In 1964 Townes was awarded the Nobel Prize for the crucial work in quantum electronics that led him to develop the maser. The award was shared with two Russian scientists, N. G. Basov and Aleksandr Prokhorov who had independently developed somewhat similar maser-like devices.

Masers to Lasers

While working on the maser in 1957, Townes and physicist Arthur L. Schawlow were both looking for ways to produce extremely concentrated beams of light. At the time, lasers were thought to have possible pure scientific uses. Townes and Schawlow were granted the patents in 1960 on the laser (light amplification by stimulated emission of radiation) technology, but they never profited personally. Townes was a consultant for, and Schawlow an employee of Bell Telephone Laboratories.

Academic and Scientific Career

From 1950 to 1952 Townes was the director of Columbia's Radiation Laboratory. From 1952 to 1955 he was also the chairman of the physics department at Columbia. In 1959 he took a leave of absence to work as vice-president and director of research of the Institute for Defense Analysis in Washington, D.C., where he dealt primarily with issues concerning national defense and foreign policy. In 1961 he became a physics professor at the Massachusetts Institute of Technology. He left MIT in 1966 to become a University Professor of Physics at University of California at Berkeley. He retired from that institution in 1986.

Further Reading

A quantum mechanical explanation of maser and laser devices can be found in Robert Eisberg's Quantum Physics (1974). An explanation of the work Townes did toward the Nobel Prize can be found in the Nobel Foundation's publication Nobel Prizes 1964, which also contains a biographical summary. Townes' own writing includes Making Waves (Masters of Modern Physics) (1995); Microwave Spectroscopy (1955) with A. L. Schawlow. He also served as editor for Quantum Electronics (1960) and Quantum Electronics and Coherent Light (1965). Information is also available on the World Wide Web (circa 1997) http://www.nforce.com/projects/inventure/book/book-text/104.html

1915–, American physicist and educator, b. Greenville, S.C. He was educated at Furman Univ., Duke, and the California Institute of Technology (Ph.D., 1939), was on the technical staff of the Bell Telephone Laboratories (1939–48), and taught at Columbia (1948–59). After serving as vice president and director of research of the Institute for Defense Analyses, Washington, D.C., he was provost of the Massachusetts Institute of Technology (1961–66). Townes is known for his work on the theory and application of the maser, on which he obtained the fundamental patent, and other work in quantum electronics connected with both maser and laser devices. He shared the 1964 Nobel Prize in Physics with N. G. Basov and A. M. Prokhorov for contributions to this field.