The smooth and growing deviation from theHermitian starting point A = 0 ends at a certain critical A(crit) where the two energiesmerge. Next, they form a conjugate pair which moves further in the complex plane.The PT symmetry of the system becomes spontaneously broken. The phenomenonof this type has been detected by the various methods in the spectra of many differentPT symmetric Hamiltonians
The force goes to zero with the conducting boundary going to infinite distance.
It is very simple to show using electrostatics (Maxwell theory) that an asymmetric capacitor can be accelerated by an internal AC/DC source, in air or vacuum and inside a conducting box. The explanation is indeed related to the existence of a conducting box or enclosure.It is so simple that the demonstration is written in four slides, including the title:http://www.ing.unitn.it/~fontana/electrostatic_levitation.pdfThe force goes to zero with the conducting boundary going to infinite distance.