03 February, 2010

History of science: Gulio Racah kicks ass

As many other branches of atomic, molecular, and chemical physics, the field of chemical stereodynamics emerged from nuclear physics. Basically all this business about how molecules rotate after a chemical reaction, or how to boost the reaction efficiency controlling the reagents – it was already there, in nuclear reactions and spectroscopy.

In 30's some people were involved in studying double gamma decays, when a nucleus goes from some excited state A to state B and then to the ground state C, emitting two gamma-quanta. Already in 1940 Dunworth proposed that there might be some correlations between the directions of emission of these two quanta. Here "correlations" mean that, for instance, if we detect the first gamma-ray from the upper side of the nucleus, the second one most likely will be emitted to the left, less likely to the bottom, and never to the right.

Dunworth was not able to check this experimentally, but induced Donald Hamilton to theoretically prove that there should be some correlations. A few years later Edward Brady and Martin Deutsch from MIT found an experimental evidence of gamma-gamma correlations in the decay of Co(60) and Sc(46) nuclei.

In the meantime theorists published lots of papers, step by step trying to improve approximations and thoroughly elaborate the problem. This lasted for more then a decade, from 1940 to 1951, when a brilliant article by Gulio Racah was published.

He starts it literally as: "Hey guys, last ten years I was interested in something else, but I just bumped into you and I see that you are really stuck. Well, the problem becomes very easy if you use the tensor operator algebra that I developed about a decade ago..."

And, of course, the problem turned out to be solvable in a very natural and straightforward manner. Now tensor operators is an important tool, not only in nuclear physics, but anywhere in physics in general. We all use it and like it.

Take care,


UPD: By the way, I never knew that Gulio Racah was a cousin of Ugo Fano...

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