Polarisability determines the force with which an inhomogeneous external electric field acts on the ions of an ion beam. However, it can be quite tricky to obtain accurate values for this force.

This polarisability has baffled scientists since the 1980s, when the first measurements appeared to be at odds with the theory. Today's result is in close agreement with theory.

Being a characteristic quantity of an atom's or ion’s electronic state, the polarisability of atoms or ions with several electrons had been difficult to obtain to date because simple equations for it ...

The calculation of the polarisability of the vacuum or of scattering of light by light, however, leads to divergent series. To avoid this, Dirac and Heisenberg1 suggested a convention which allows ...