An experimental physicist who shared the 1952 Nobel Prize in Physics with Felix Bloch for their independent discovery of nuclear magnetic resonance, Purcell also contributed substantially to both observational and theoretical astrophysics. He shared the 1988 Tinsley Prize of the American Astronomical Society with Harold I. Ewen for their independent discovery of the emission of the HI 21-cm line by interstellar hydrogen. Ewen and Purcell's discovery was published in Nature (168, 356, 1951), along with reports from J. Pawsey in Australia and J. Oort in Leiden, whom Purcell had asked to confirm his own. This discovery opened up a new window in the then-young field of radio astronomy, important as spectral lines always are, and enabled fundamental work on the structure of our own and other galaxies, as well as on the temperature, density, and dynamics of the interstellar medium.
Over the decade 1969-1979, Purcell published a number of articles on the physics of interstellar dust grains. In one he showed that the Kramers-Kronig relation between the real and imaginary parts of the refractive index of interstellar space implies that "in many models or mixtures of models that reproduce the whole (interstellar) extinction curve, the total volume occupied by grain substance will be largely independent of grain size and not strongly dependent on grain composition." Thus, the fraction of volume occupied by dust in the galactic plane must be at least 8.6 × 10-27, a conclusion that depends only upon causality. In an area of research filled with so many imponderables as the ISM, it is reassuring to have such a rigorous result to reflect upon.
In other contributions, Purcell pointed out that various mechanisms such as molecule formation at selected sites on the surface of a dust grain can, through its recoil effect, spin up the grain to energies vastly exceeding the kT expected from collisions with surrounding gas molecules. Recently, a number of authors have shown that suprathermal rotation is critical to the alignment of dust grains in the galactic magnetic field, overcoming long-standing problems with the classic Davis-Greenstein mechanism.
With Carl Pennypacker, Purcell developed a method for computing the optical properties of grains by representing the grain as a discrete array of dipoles for which one calculates a self-consistent field. Unlike classical Mie theory, this method can be applied to grains of arbitrary shape and inhomogeneity. It was first applied to astronomy by Paul Shapiro in a study of nonspherical grains of magnetite.
No doubt Purcell's interest in interstellar dust grains was stimulated by his mastery of electromagnetic calculations, demonstrated in his classic text Electricity and Magnetism, Vol. II (McGraw Hill 1965, 1985). His associates at Harvard University, where he served as the Gerhard Gade University Professor, valued his penetrating physical insights, particularly into the nature of quantum mechanics, where others are satisfied to use the formalism to get the results they need. Purcell was restless until he could enunciate precisely the correspondence between the equation and the process. Quantum paradoxes delighted him.
Purcell functioned happily in the land of the physics of everyday life. Paul Shapiro remembers him waving a small piece of wood in the air as Paul entered his office. He was helping black paint to dry on the wood so he could use it in an apparatus he had designed to demonstrate to students that light is made of particles. The apparatus focused the light of a candle and internally reflected it so that the emergent flux was so low that a photomultiplier would record single quanta. The paint had to be especially black to assure that reflections from the walls of the apparatus wouldn't reduce the attenuation. These and other experiments were the subjects of dozens of articles under the title "The Back of the Envelope" that Purcell published in the American Journal of Physics between 1983 and 1988.
In his later years, Purcell suffered a number of illnesses which kept him house-bound. He always welcomed visitors who could bring him news from the frontiers of research. He invariably expressed wonder that nature—be it pulsar rotation or rainbows—works the way it does because of the laws of physics.
His penetrating intellect ranged far beyond his lab. A member of the President's Science Advisory Committee from 1957 to 1960 and from 1962 to 1965, he had an important role in formulating national policy in intelligence gathering and space technology. He rejoiced at the successes of the Hubble Space Telescope, but grumbled frequently about what he perceived to be overemphasis on man in space, reserving special scorn for proposals to send humans through interstellar space to the stars. As he was fond of pointing out, a rocket traveling at a speed required to reach the stars in a lifetime—say 0.1 c—would be intensely bombarded with 5 MeV protons-the inevitable consequence of running into those H atoms he and Ewen had observed in space. Instead of attempting such projects, he argued, we should use the photons nature gives us, traveling at c, to yield the information we seek. He favored every effort to discover whether life could be found elsewhere in the Galaxy. As a consequence, he was a strong supporter of SETI.
Purcell was a physicist of the old school. He pursued the subject out of pure curiosity, not caring for honors or remuneration, and as a result left those who knew him invigorated in their desire to discover something. Nevertheless, he won the Nobel Prize for magnetic resonance, and membership in the great academies of the world. But Nicolaas Bloembergen, his first graduate student and himself a Nobelist, tells us that, in the last year of his life, Purcell confided that he considered his and Ewen's discovery of the 21-cm line and later contributions to radio astronomy at least as significant as his NMR work. The AAS should be proud that it awarded him and Ewen the Beatrice M. Tinsley Prize in 1988 for his work in astronomy.
Nature published a brief obituary of Purcell, and a more extensive one should appear in Biographical Memoirs of the National Academy of Sciences.