Atomic Physics

The understanding of the hydrogen atom, consisting of an electron circulating a proton, was one of the milestones on the way to the development of quantum physics (see Figure). Nowadays the hydrogen atom is quantitatively described by Quantum Electrodynamics (QED) which is considered the most accurately tested and best confirmed of all physics theories.

These high precision tests have, however, been confined to light atoms. The special feature at GSI is that one can study hydrogen-like uranium ions, atoms which were stripped of all but one electron; the electron now experiences an electric field which is a million times stronger than in hydrogen. In these extremely strong electromagnetic fields QED had not been tested so far. This challenge was taken up by the atomic physics group at GSI.

Fully stripped U92+ ions from the heavy-ion synchrotron SIS were stored in the ESR; they picked up an electron in the gas jet target and the binding energy of this electron - containing the so-called Lamb shift as a measure of the QED contributions - was measured over the years with ever increasing accuracy.

For the ultimate test of QED the experiments still have to be improved by an order of magnitude. This example shows how fundamental theories can be tested in a so far unexplored regime utilizing highly ionised heavy-ion beams.