In 1929, Isidor Isaac Rabi began teaching quantum mechanics at Columbia University. Over the next decade his research group used a technique called molecular beam resonance to study the magnetic properties of atoms and molecules. At the time of Rabi's experiments, physicists knew that the atomic nucleus is composed of two types of particles, positively charged protons and neutral particles called neutrons. Surrounding this nucleus in a kind of fuzzy cloud are negatively charged electrons. Physicists also had discovered that electrons, protons, neutrons--and in many cases the nuclei themselves--behave as though spinning about their axes, just like planets. This results in a property called spin angular momentum, which has both magnitude and direction. Such a spinning particle generates a magnetic field and associated "magnetic moment"--acting like a tiny bar magnet with north and south poles. When placed in a strong external magnetic field, the "magnetic moment" of a nucleus tends to align with (parallel to) or against (antiparallel to) the external field. Parallel alignment corresponds to a lower energy state than antiparallel alignment.

Rabi's experiments involved passing a beam of lithium chloride molecules through a vacuum chamber and manipulating the beam with different magnetic fields. By studying how the magnetic field affected the path of the molecules, he could learn about the magnitudes of the magnetic moment of the nucleus. With the appropriate stimulus, he predicted, the magnetic moments of the nuclei could be induced to flip, or change their orientation relative to the magnetic field. In 1937, following a suggestion by Dutch physicist Cornelius J. Gorter, Rabi and his group added a new wrinkle to their experiments: They bathed the molecular beam in radio waves--electromagnetic signals in the radiofrequency, or broadcasting, range--while varying the magnetic field strength.

They did this because of another feature of spinning particles. In an external magnetic field, atomic particles spin with a wobble called precession, much like a child's top wobbles about the vertical when it is tipped slightly. The magnetic moment of an atomic nucleus wobbles at a characteristic frequency that depends on its type (hydrogen vs. lithium, for instance) and also on its environment. For example, increasing the magnetic field strength increases this frequency, while decreasing the field strength lowers it.

Rabi and his team adjusted the magnetic field strength until they induced the magnetic moments of the nuclei to flip, which occurs when the frequency of the radio signal matches the nuclei's characteristic precessional frequency. When this match--the resonance frequency--occurs, a nucleus absorbs energy from the radio signal that is precisely equal to the difference between its two energy states and thus jumps to the higher state. A flip also occurs when a nucleus emits that energy in falling back from the higher to the lower energy state. Whether the nucleus was excited to the higher energy state or falling to the lower one, Rabi could detect the transition. His technique is now called magnetic resonance or, more precisely, molecular beam magnetic resonance.

Rabi's group applied the new technique to deduce unprecedented details of the internal interactions of molecules. They discovered a series of resonances within a single molecule that allowed them to "see" how individual atoms are bound together and how their nuclei are affected by neighboring atoms. These extraordinary experiments and the development of molecular beam magnetic resonance as a technique for studying the magnetic properties and internal structure of molecules, atoms, and nuclei resulted in Rabi winning the 1944 Nobel Prize in physics.

Within a few months of these experiments, Rabi's group would try a variation: manipulating the radio frequency rather than the strength of the magnetic field. This method--which spreads the resulting signals into a spectrum, much as visible light is spread by passing through a prism--is the basis for radio frequency spectroscopy, which would revolutionize chemical analysis and prove a vital component in the development of magnetic resonance scanning as a tool for medical diagnosis.