London

Veltman (left) and 't Hooft: Dutch double success. Credit: UNIV. MICHIGAN

This year's Nobel Prize for Physics has gone to Geradus 't Hooft of the University of Utrecht and Martinus Veltman, emeritus professor at the same university, who showed how the complex mathematical structure of the theory of the electroweak interaction need not be an obstacle to predictive calculations.

They showed how to perform precise calculations of the physical properties of the new particles predicted by this and related theories, many of which have now received experimental confirmation. “The major advances in particle physics of the past 25 years depend on their work,” says Christopher Llewellyn-Smith of University College London.

Initial progress with the theory of the electroweak interaction was hampered by the same problem that had plagued the development of quantum electrodynamics (QED) — how to cope mathematically with the short-lived particles and anti-particles that quantum theory predicts will surround a ‘naked’ central particle.

For QED, the problem was solved by the introduction of a ‘renormalization’ procedure: the experimentally determined values of mass and charge for the particle are used as ‘renormalized’ input parameters.

But the electroweak theory does not possess the symmetry simplicity of QED. The particles mediating the electromagnetic interaction are the massless, non-interacting photons; for the electroweak interaction, however, they are massive and interacting. The mathematical difficulties arising from these additional properties were such that many theorists at the time doubted the physical viability of the electroweak theory.

Veltman, optimistic that the calculational difficulties could be overcome, developed the computational tools and showed that many of the more problematic aspects of the electroweak theory effectively cancelled each other out.

According to Llewellyn-Smith, “Veltman was the driving force keeping interest in such theories alive.” 't Hooft, Veltman's student at the time, had the insight that if the masses of particles such as W and Z were a natural consequence of the theory, rather than input parameters, then the theory became self-consistent and tractable.

These combined developments were key, and the theory of the electroweak interaction became, by the early 1970s, a practical — and very successful — theoretical tool for performing precise calculations. Calculations of physical quantities associated with the W and Z particles and the top quark show agreement with recent measurements.