Friday, May 15, 2009

Fermi's Golden Rule Is Actually Dirac's!

In the latest issue of Am. J. Phys. (June 2009), there is a letter to the Editor on the historical accuracy of attributing the name of things in physics. A letter by Taco Visser of Delft University has a very interesting historical account of the attribution of the time-dependent perturbation "Golden Rule" to Fermi.

In fact, in his classic text Nuclear Physics,3 Fermi gives two results, one for first order transitions and one for second order transitions. He coins the names “Golden Rule #1” and “Golden Rule #2,” but does not give a derivation. For this he refers to Schiff’s textbook. Clearly the names suggested by Fermi were hugely successful. So successful even that Schiff adopted this terminology in a later edition of his book,5 in which he writes “Eq. 35.14… is so useful that it was called ‘Golden Rule No. 2’ by E. Fermi.” Nevertheless, Schiff was well aware that this formalism originated elsewhere. In another footnote he mentions its discoverer: Paul Dirac. More than 20 years before Fermi’s book appeared, Dirac published a beautiful and comprehensive treatment of quantum mechanical perturbation theory6 in which the first order result is presented and applied to absorption and emission of radiation. In older texts, for example the book by Kramers7 or that by Condon and Shortley,8 Dirac is given full credit for his work. After Fermi published his book, that habit seems to have gone out of style. But Fermi is in no need of extra accolades; the key formula of perturbation theory is really Dirac’s Golden Rule.

Read the article by Jackson that is referred to in the letter to see other examples of mis-attribution of the wrong names in physics.


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