Place: United States of America
Subject: biography, physics
US physicist who is remembered for discovering the Compton effect, a phenomenon in which electromagnetic waves such as X-rays undergo an increase in wavelength after having been scattered by electrons. For this achievement he shared the 1927 Nobel Prize for Physics with the British physicist C T R Wilson. Compton was also a principal contributor to the development of the atomic bomb.
Compton was born in Wooster, Ohio, on 10 September 1892, the son of a philosophy professor at Wooster College who was also a Presbyterian minister. He was educated at his father's college, graduating in 1913 and going on to Princeton University, from which he gained his PhD three years later. He became a physics lecturer at the University of Minnesota, but left after a year to take up an appointment as an engineer in Pittsburgh for the Westinghouse Corporation. He travelled to the UK in 1919 and stayed for a year at Cambridge University, where he worked under Ernest Rutherford. In 1920 he returned to the USA and went to Washington University, St Louis, as head of the physics department, moving to the University of Chicago in 1923 as professor of physics. Compton remained at Chicago for 22 years until, after the end of World War II in 1945, he returned to Washington University. He was chancellor of the university 1945-53 and then became professor of natural philosophy there, a position he held until 1961, when he became professor at large. He died in Berkeley, California, on 15 March 1962.
Compton began studying the scattering of X-rays by various elements in the early 1920s (using blocks of paraffin wax in which the carbon atoms' electrons deflected X-rays) and by 1922 he had noted the unexpected effects that the scattered X-rays show an increase in wavelength. He explained the Compton effect in 1923 by postulating that X-rays behave like particles and lose some of their energy in collisions with electrons, so increasing their wavelength and decreasing their frequency. He calculated that the change in wavelength, Δλ, is given by the equation
Δλ = (h/mc)(1 − cosθ)
where h is Planck's constant, m is the rest mass, c is the velocity of light, and θ is the angle through which the incident radiation (for example, X-rays) is scattered.
The chief significance of Compton's discovery is its confirmation of the dual wave/particle nature of radiation (later extended by Louis de Broglie in his hypothesis that matter can also have wave/particle duality). The behaviour of the X-ray, previously considered only as a wave, is explained best by considering that it acts as a corpuscle or particle - as a photon (Compton's term) of electromagnetic radiation. Quantum mechanics benefited greatly from this convincing interpretation. Further confirmation came from experiments using C T R Wilson's cloud chamber in which collisions between X-rays and electrons were photographed and analysed. They could be interpreted as elastic collisions between two particles and measurements of the particle tracks in the cloud chamber proved the correctness of the mathematical formulation of the Compton effect.
In the early 1930s several scientists were debating the nature of cosmic rays. Robert Millikan had proposed that the rays are a form of electromagnetic radiation which, if this were true, would be unaffected by their passage from outer space through the Earth's magnetic field and should strike the Earth in undeflected straight lines. Other scientists, such as the German physicists Walther Bothe (1891-1957), postulated that the ‘rays’ are made up of streams of charged particles, which, therefore, should be deflected into curved paths as they traverse the Earth's magnetic field. Compton reasoned that the argument could be resolved by making cosmic-ray measurements at various latitudes, which he carried out during a long series of trips to various parts of the world to measure - or to organize dozens of other scientists to measure - comparative cosmic-ray intensities using ionization chambers. By 1938 he had collated the multitude of results and demonstrated a significant latitude effect by the Earth's field, proving that at least some component of cosmic rays consists of charged particles. He went on to confirm these findings by showing variations in cosmic-ray intensity with the rotation period of the Sun and with the time of day and time of year. One interpretation of these results suggests an extra-galactic source for cosmic rays.
During World War II Chicago University was the prime location of the Manhattan Project, the effort to produce the first atomic bomb, and in 1942 Compton became one of its leaders (as director of the code-named Metallurgical Laboratory). He organized research into methods of isolating fissionable plutonium and worked with Enrico Fermi in producing a self-sustaining nuclear chain reaction, which led ultimately to the construction of the bombs that, used against Japan, effectively ended World War II. Compton's book Atomic Quest (1956) summarized this part of his career.
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