LOS ANGELES – Robert Pound, a Harvard physicist whose elegant experiments confirmed a key part of Albert Einstein’s theory of relativity and who helped lay the groundwork for the magnetic resonance imaging technology now widely used in medicine, died April 12 at a nursing home in Belmont, Mass. He was 90.

Pound was one of a rare breed in academia, especially the physics community: a highly respected and influential researcher who lacked a doctorate. But a keen mind and his facility for converting theoretical ideas into concrete laboratory tools provided him entree into this rarified world.

He first made a name for himself with his 1959 studies validating Einstein’s theory that gravitation can change the frequency of light, a phenomenon known as gravitational red shift. Most physicists agreed that Einstein was right, but assumed that a proof would require monitoring light over great distances, perhaps with a satellite.

Pound and his graduate student Glen A. Rebka Jr. were able to demonstrate the theory’s validity with a 75-foot tower.

Pound took advantage of a phenomenon discovered by German physicist R. L. Mossbauer, who found that certain crystals, when excited, could emit gamma rays of a very precise frequency. The crystals could also absorb gamma rays of only that exact frequency, so they could be used both as a source of electromagnetic radiation and as a detector.

Pound and Rebka mounted a crystal of iron-57 in a speaker cone at the top of the tower — created by drilling holes through each floor of Harvard’s Jefferson laboratory — and a second one at the bottom, separated by a bag of helium. Vibrations of the speaker cone created a Doppler shift in the gamma-ray emissions similar to that of a train whistle as the train approaches and recedes from an observer.

monitoring the frequency at which the Dopplered emissions counterbalanced the effects of gravity, allowing the gamma rays to be detected, the researchers were able to measure the slight gravitational red shift produced by the minutely different gravity at the top and bottom of the tower.

Pound, along with physicists Edward M. Purcell and H. C. Torrey, also played a major role in the development of nuclear magnetic resonance, a tool that is widely used by chemists for determining the structure of molecules.

Spinning nuclei generate an extremely weak magnetic field. Purcell reasoned that by immersing a liquid or a solid in a powerful magnetic field, he could bring all the nuclei into alignment — that is, with their axes all pointing in the same direction. Subsequently bathing them in radio waves and measuring their absorption at different wavelengths would then make it possible to characterize each atom’s immediate environment, i.e., what other atoms are linked to it.

Pound developed the so-called Pound box, which allowed the researchers to measure the very small absorption of the radio waves. Purcell shared the 1952 Nobel Prize in physics with Felix Bloch of Stanford University for the discovery. Pound received the National Medal of Science in 1990 for his role in the work.

Other researchers subsequently refined the technique to image the human body, where it became known as magnetic resonance imaging, or MRI.

Pound was born in Ridgeway, Canada, on May 16, 1919. His father was a physicist and mathematician. The family moved from Ontario to Buffalo, N.Y., when Robert was a child, and he became a naturalized citizen.