A story from
One of Sid Drell's pioneering accomplishments in theoretical physics at SLAC was his derivation with Anthony C. Hearn of an "Exact Sum Rule for
The Drell-Hearn paper was published in Physical Review Letters in April, 1966. Their remarkable sum rule relates a dynamical process -- polarized
photoproduction reactions on a proton or neutron, summed over energy, to a fundamental property of the nucleon -- its magnetic moment. (The sum rule was
derived independently in Russia by S. B. Gerasimov and is now known as the DHG sum rule.)
However, a year later, in 1967, Professors Gabriel Barton at the University of Sussex and Norman Dombey at Harvard University published an article in the
Physical Review claiming that even if the DGH sum rule is valid for nucleons, it is false for nuclei !
I never saw Sid so mad! Sid emphasized that his derivation with Tony Hearn was based on fundamental physics principles, - a rigorous dispersion relation
and the low energy theorem for Compton scattering. Sid said it was inconceivable that the DHG sum rule would not also hold for nuclei.
Sid asked his student at the time, Joel Primack -- now an astrophysicist at UC Santa Cruz -- to uncover Barton and Dombey's error. Joel brought me into
this topic, and we discovered a fundamental theoretical mistake -- Barton and Dombey had boosted the nuclear wavefunction from rest to a nonzero momentum
incorrectly. We showed that the boost of a nuclear wavefunction cannot be written as the product of boosts of the individual nucleons, a common
misconception in physics -- there is an additional dynamical contribution due to nuclear binding.
When one performs the boost of the nucleus correctly, not only is the DHG sum rule completely valid, but one also verifies the low energy theorem for
Compton scattering on composite systems.
Sid Drell was absolutely right. Barton and Dombey were, in fact, grateful and relieved to understand the new insights into boosting the wavefunctions of
Primack and I published two papers in Physical Review and Annals of Physics in 1968, which are primary references for computing the electromagnetic
interactions of composite systems. In our acknowledgements, we wrote "We especially wish to thank Professor S. D. Drell for his continued
encouragement and many helpful suggestions. "