Sometimes when Nature poses a problem of seemingly insurmountable difficulty, she simultaneously reveals a subtle phenomenon in an unexpected quarter that opens the door to a clever solution. In the nineteenth century, for example, the French philosopher Auguste Comte stated unequivocally that the composition of stars was something not merely unknown but forever unknowable. Comte could not have foreseen that in 1860, just three years after his death, dark absorption lines would serendipitously be discovered in starlight and would be used to read the elements of star stuff as clearly as words in a book.
A century later a less remote yet far more fundamental mystery was resolved in a similarly unexpected way. In 1952 the great mathematician and physicist Hermann Weyl, a colleague of Einstein’s at the Institute for Advanced Study in Princeton, published Symmetry, a beautifully illustrated little book for the general public, in which he declared categorically that in all physics nothing has shown up indicating an intrinsic difference of left and right. Just as all points and all directions in space are equivalent, so are left and right. The true difference between left and right, Weyl thought, was unknowable. But in 1957, just two years after his death, a team of physicists at the National Bureau of Standards in Washington, D.C., found that the nuclei of cobalt 60 atoms, when cooled to a sufficiently sluggish state and lined up in a magnetic field, display such a geometric property: their spins can be distinguished as left- handed or right-handed. This discovery caused a revolution in the understanding of elementary particles and immediately earned a Nobel Prize for the young Chinese-American theoreticians, Chen Ning Yang and Tsung-Dao Lee, who had suggested the experiment.
A modern example of the unexpected ways in which nature’s innermost secrets come to light concerns the determination of the strengths of magnetic fields in the inaccessible regions between atoms in solid materials--a world as remote as the interiors of stars. Since internal magnetic fields affect the behavior of electric currents, such measurements are urgently needed by the designers of integrated circuits and other electronic devices. However, these measurements are extremely difficult to make with conventional techniques. (Nuclear magnetic resonance, for example, reveals only the magnetic fields in the immediate vicinity of nuclei, not those that fill the outlying spaces between atoms. Worse, the technique doesn’t work at all in metals, because metals shield the microwave signals that carry out the information.) Where the dimensions of the materials are microscopic, as in the billionth-of-a-meter-thick structures on the surfaces of computer chips, the internal magnetic fields have defied measurement altogether--at least until now. But an elegant solution to this apparently intractable problem may be at hand. It is provided by the little-known technique of muon spin rotation, which makes use of an exotic misfit, called the muon, in the elementary particle zoo. Since the muon’s name derives from the Greek letter µ (pronounced mew), the method is abbreviated µSR.
