Quantum Tests at the Highest Electrical Fields

An electron in orbit around a proton is not like a planet circling a star. Not only does the electron (in a quantum sense) not follow a trajectory of well-defined locations, but space itself, in the case of a hydrogen atom, teems with virtual particles such as photons and electron-positron pairs popping into and out existence owing to the energy vested in the electromagnetic field surrounding the electron and nucleus.

The presence of these virtual particles can shift the allowed energies of the electron, and measurements of this "Lamb shift" (named for Willis Lamb) constitute the most stringent test of quantum electrodynamics (QED) and indeed the highest-precision test of any physical theory. As strong as the electromagnetic field may be within the hydrogen atom, however, it is small compared to the electric field felt by the innermost electron in a uranium atom.

To get at this electron, and to test QED amid the highest possible fields, physicists at the Experimental Storage Ring (ESR) at the GSI lab in Darmstadt, Germany send a beam of uranium atoms through foils which successively strip all but one of the 92 electrons in the atom. The resultant ions, U91+, are a sort of hydrogen atom with the E field turned way up: the electric field felt by the lone electron is more than 10¹⁶ V/cm, the strongest constant field in any lab. Even the most intense laser electric field is about 10¹² V/cm. The measured value of the ground-state Lamb shift is 468 eV with an uncertainty of 13 eV, and largely agrees with QED predictions. The GSI scientists (Thomas Stoehlker, t.stoehlker@gsi.de, 011-49-615-971-2712) hope soon to achieve 1 eV precision. (Stoehlker et al., Physical Review Letters, 9 Oct; Select articles.)