Magnetic Moments

Brian Pippard

  • Inward Bound: Of Matter and Forces in the Physical World by Abraham Pais
    Oxford, 666 pp, £20.00, May 1986, ISBN 0 19 851971 0

It is only four years since we were treated to Abraham Pais’s authoritative study of Einstein, Subtle is the Lord, and now he presents an equally large and quite as impressive history of fundamental particle physics, in a style and at a price which calls for the warmest congratulations to him and to the Clarendon Press.

This is no book for the casual reader. The first chapters, to be sure, recount the rise of atomic and nuclear physics straightforwardly, presenting no problem for anyone with a modest appreciation of A-level physics. The years 1895-9 saw the discovery by Roentgen of X-rays, by Becquerel of radioactivity and by J.J. Thomson of the electron, while in 1900 Planck introduced the quantum. So long as this last remained separate from the others the elucidation of their properties and mutual relationships was a matter for imaginative experiment and descriptive interpretation, with little need for any but rudimentary mathematics. Almost contemporaries of the quantum, Pauli, Heisenberg, Fermi and Dirac were born into a world whose physics was soon to be dominated by Rutherford, Einstein and Bohr (Planck played little part after his initial idea). In a few years from 1925, they utterly transformed it by creating a mathematical structure that effected a merger (under entirely new management) of the concerns of Rutherford’s experiments on nuclei, Einstein’s relativity theory and Bohr’s quantum theory of the atom. From this point onwards the physics and the mathematics became inseparable.

The invisible sub-microscopic world of atoms and their nuclei rather suddenly ceased to be a visualisable world in which the particles could be imagined to move in ways little different from real-life billiard balls. Mathematics had indeed been required for exact calculation, but its primary function to a man like Rutherford was to confirm what his powerful intuition could guess by analogy. With quantum mechanics and its immediate offshoot of quantum field theory an entirely new outlook was demanded: electrons and protons ceased being real in the sense they were before. They must surely represent something which exists out there and which is manifested to the physicist in the form of flashes of light or instrument readings: but to pretend that they are actually independent little objects, bouncing against one another or caught up in orbits, gets in the way of describing the results correctly. Instead one must find a set of rules, expressed in mathematical form, whose application to a given problem will tell as much as one is permitted to know about the relation of one observation to another. Thus if one wishes to calculate the properties of a helium atom, naively pictured with two electrons attracted to the nucleus and repelling one another, only a very approximate solution can be obtained by treating them as separate particles, moving under their mutual influences. To reach an answer that agrees with experimental measurements demands putting both electrons together through the appropriate manipulations: it is as if they must be described as one particle moving in six dimensions rather than two moving in three dimensions. The theoretical physicist is no more able than anyone else to visualise six dimensions, but he knows the rules for writing down the correct equations, and if he (or his computer) works every hard, he may find the solution in the form of numbers to be checked against the experimenter’s – the wavelengths of spectral lines, for example. Bohr’s earlier quantum theory had not done this very well, but the new quantum mechanics succeeded brilliantly.

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