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'T Hooft and Voltman Awarded Nobel Prize in Physics; Zewail Wins Nobel Prize in Chemistry

In October, the 1999 Nobel Prize for Physics was awarded to Gerardus 't Hooft of the University of Utrecht and Martinus Veltman, formerly of the University of Michigan and now retired, for their work toward deriving a unified framework for all the physical forces. Their efforts, part of a tradition going back to the 19th century, centers around the search for underlying similarities or symmetries among disparate phenomena, and the formulation of these relations in a complex but elegant mathematical language. A past example would be James Clerk Maxwell's demonstration that electricity and magnetism are two aspects of a single electro-magnetic force.

Naturally this unification enterprise has met with various obstacles along the way. In this century quantum mechanics was combined with special relativity, resulting in quantum field theory. This theory successfully explained many phenomena, such as how particles could be created or annihilated or how unstable particles decay, but it also seemed to predict, nonsensically, that the likelihood for certain interactions could be infinitely large.

Richard Feynman, along with Julian Schwinger and Sin-Itiro Tomonaga, tamed these infinities by redefining the mass and charge of the electron in a process called renormalization. Their theory, quantum electrodynamics (QED), is the most precise theory known, and it serves as a prototype for other gauge theories (theories which show how forces arise from underlying symmetries), such as the electroweak theory, which assimilates the electromagnetic and weak nuclear forces into a single model.

But the electroweak model too was vulnerable to infinities and physicists were worried that the theory would be useless. Then 't Hooft and Veltman overcame the difficulty through a renormalization comparable to Feynman's. They succeeded in renormalizing a non-Abelian gauge theory. Getting the non-Abelian electroweak model to work was a formidable theoretical problem. An essential ingredient in this scheme was the existence of another particle, the Higgs boson, whose role is to confer mass upon many of the known particles. For example, interactions between the Higgs boson and the various force-carrying particles result in the W and Z bosons (carriers of the weak force) being massive (with masses of 80 and 91 GeV, respectively) but the photon (carrier of the electromagnetic force) remaining massless.

With Veltman's and 't Hooft's theoretical machinery in hand, physicists could more reliably estimate the masses of the W and Z, as well as produce at least a crude guide as to the likely mass of the top quark. Happily, the W, Z, and top quark were subsequently created and detected in high energy collision experiments, and the Higgs boson is now itself an important quarry at places like Fermilab's Tevatron and CERN's Large Hadron Collider.

The 1999 Nobel Prize in Chemistry was awarded to Ahmed H. Zewail of Caltech, for developing a technique that enables scientists to watch the extremely rapid middle stages of a chemical reaction. Relying on ultra-fast laser pulses, "femtosecond spectroscopy" can provide snapshots far faster than any camera-it can capture the motions of atoms within molecules in the time scale of femtoseconds.

An atom in a molecule typically performs a single vibration in just 10-100 femtoseconds, so this technique is fast enough to discern each and every step of any known chemical reaction. Shining pairs of femtosecond laser pulses on molecules (the first to initiate a reaction and the second to probe it) and studying what type of light they absorb yields information on the atoms' positions within the molecules at every step of a chemical reaction. With this technique, Zewail and his colleagues first studied (in the late 1980s) a 200-femtosecond disintegration of iodocyanide, observing the precise moment at which a chemical bond between iodine and carbon was about to break.

Since then, femtochemistry has revealed a whole new class of intermediate chemical compounds that exist less than a trillionth of a second between the beginning and end of a reaction. It has also provided a way for controlling the courses of chemical reaction and developing desirable new materials for electronics. It has provided insights on the dissolving of liquids, corrosion and catalysis on surfaces (see Physics Today, October 1999, p. 19); and the molecular-level details of how chlorophyll molecules can efficiently convert sunlight into useable energy for plants during the process of photosynthesis.

Veltman and Zewail are fellows of the APS. Zewail was awarded the APS Earle Plyler Prize in 1993 and the Herbert Broida Prize in 1995. Although not an APS member, nonetheless, Gerardus 't Hooft received the 1979 Dannie Heineman Prize.

-Philip F. Schewe, AIP Public Information


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