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On the Process of Space Quantization" in Physical Review, Vol. 49, Second Series, January 1-June 15, 1936 pp. 324-328
American Institute of Physics, Lancaster 1936 - FIRST EDITION OF RABI'S DESCRIPTION OF HIS THEORETICAL IDEA OF HOW TO MEASURE NUCLEAR SPIN, an idea that would lead to the determination of the signs of magnetic moments via his magnetic resonance method, the most significant improvement in molecular and atomic beam techniques to date" (Gonolis, Lindau Nobel Laureate Meetings). The Nobel Prize Committee agreed and awarded Rabi the 1944 Nobel Prize in Physics "for his resonance method for recording the magnetic properties of atomic nuclei") (Nobel Portal). Experiments in molecular beam resonance evolved from the fusion of two very different fields: the first, molecular beam experiments and the second, "speculation about space quantization when the axis of quantization is changed" (Ramsey, Early Years, 1). The scientific community knew that "nuclei have intrinsic spins and magnetic moments. The magnetic moment has both a magnitude and a sign. [But] in 1935, the sign was missing. The sign of a magnetic moment can be either plus or minus: if the spin and the magnetic moment have the same space-quantized direction, the sign of the moment is plus; if these directions are opposed, the sign is minus? [But] as beamlets of a particular atom were refocused into the detector, the same pattern was observed regardless of whether the sign of the atom's moment was plus or minus. The problem of determining the signs was something like trying to determine whether someone's right hand or left hand is pushing the front-door buzzer" (Rigden, Rabi, Scientist, 92). Theoretical in nature, Rabi's paper analyzes experiments carried out in Otto Stern's Hamburg lab. As Stern had written: "The purpose of [the experiments] had been to answer a question that went back to the days of the old quantum theory, the days when the idea of space quantization strained credulity. The question was, Can an atom that is 'clinging' to a magnetic field with some particular space-quantized orientation be shaken loose? Can an atom be made to change its orientation" (ibid). The Hamburg group had determined that "when the direction of the magnetic field is changed quickly enough, the atoms, on passing from one field to another, will reorient. It was in this reorientation process that Rabi saw the possibility of determining the signs of nuclear magnetic moments" (Rigden). In considering the Hamburg group's thoughts on reorientation, Rabi began to think that magnetic moments might "tend to align either parallel or antiparallel to an external magnetic field, and tend to behave somewhat like tops, precessing about the direction of the magnetic field, with a frequency that depends on the magnetic field strength and the atom's nuclear magnetic moment" (APS Physics 1, 2, July 2006). The idea in this paper, one of few he authored alone, came to Rabi as he walked up a hill on the Riverside campus: "One day I was walking up the hill on Claremont Avenue and I was thinking about it [the sign of the nuclear magnetic moment] kinesthetically with my body. Now, yes, I was thinking about this as follows: here's the moment and it's wobbling around in the direction of the field and [to find] the sign was to find out in which sense it was wobbling. To do this, I have to add another field which goes with it or against it. This is the idea, just concretely. The whole resonance method goes back to this. His intuition was sound, and atoms did reorient in such a way that the signs of their magnetic moment could be determined" (Rigden, 93). Rabi's theory supposed that the effects of the spins of the nuclei, along with those of the electrons had to be considered in weak magnetic fields where the nuclear and electron angular momenta were significantly coupled together (Zeller). In his own words, Rabi believed it theoretically "possible to measure the sign of nuclear magnetic moment vector with respect to the spin vector" (Rabi, 324). Rabi published this paper, then worked with his doctoral fellows to prove his theory. Their results, published late in th [Attributes: First Edition]
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