# Darwin term and Zitterbewegung

I've noticed that in the discussion of the fine structure of Hydrogen atom standard QM texts claim that the Darwin term, which corrects energy of $\ell=0$ (or $s$-) states only, is related to the zitterbewegung of the relativistic electron.

I'm quite familiar with the zitterbewegung phenomenon, but it is not clear to me how it leads to the correction to the Hamiltonian in the form: $$H_\text{Darwin}=\frac{\hbar^2}{8m^2c^2}\nabla^2 V$$

Furthermore, this article: http://arxiv.org/abs/atom-ph/9607005 claims that the Darwin term has its origin in the spin-orbit coupling rather than zitterbewegung. Who is right?

Question:

1. What is the physical origin of the Darwin term?
2. What is the history behind the Darwin term?
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The Darwin term can be obtained from the low energy approximation ($|p|^2/m^2<<1$) of the Dirac equation of the electron in a central field. An elegant way to perform this task is by means of the Foldy-Wouthuysen Transformation. The same approximation leads also to the other terms detected in the Hydrogen atom fine structure including the spin-orbit term.

However, the relation to the zitterbewegung is also correct, at least in the heuristic level. Please see a derivation in the following work by Klaus Capelle (Equation 4). The explanation is that due to the zitterbewegung high frequency motion, the central potential gets smeared on the average and acquires the extra Darwin term.

Charles Darwin together Gordon, were the first to compute the exact energy levels of the Hydrogen atom using the Dirac equation in 1928, just two years after Dirac discovered his equation. Darwin also performed the low energy approximation to understand the differences from the nonrelativistic treatment.

A little piece of history about the Darwin term can be found in footnote 7 of the following lecture notes which is quoted in the following:

"Sir Charles Galton Darwin (1987-1962), a British physicist, was a grandson of the Charles Darwin of evolution fame. Sir Charles was the first, with Gordon, to work out the exact energy levels of Hydrogen according to the Dirac equation and thereby discovered the eponymous term in the levels. He worked out the Lagrangian and Hamiltonian for classical motion of several interacting charges correct to O($v^2/c^2$). He also worked on statistical mechanics (Darwin-Fowler method). Later in life he took part in the Manhattan project."

Darwin's work was referred to in the original Foldy-Wouthuysen article: Phys Rev. 78 no.1, 29-36, 1950.

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Thank you for the very complete answer! – QuantumDot Sep 7 '12 at 0:22