We investigate the interaction of metastable 2S hydrogen atoms with a perfectly conducting wall, including parity-breaking S-P mixing terms (with full account of retardation). The neighboring 2P 1/2 and 2P 3/2 levels are found to have a profound effect on the transition from the short-range, nonrelativistic regime, to the retarded form of the Casimir-Polder interaction. The corresponding P state admixtures to the metastable 2S state are calculated. We find the long-range asymptotics of the retarded Casimir-Polder potentials and mixing amplitudes, for general excited states, including a fully quantum electrodynamic treatment of the dipole-quadrupole mixing term. The decay width of the metastable 2S state is roughly doubled even at a comparatively large distance of 918 atom units (Bohr radii) from the perfect conductor. The magnitude of the calculated effects is compared to the unexplained Sokolov effect. , research on related matters has found continuously growing interest over the last decades [5][6][7][8]. In the non-retarded regime (close range), the interaction energy scales as 1/Z 3 with the atom-wall distance Z, while for atom-wall distances large in comparison to a typical atomic wavelength, the interaction energy scales as 1/Z 4 (see Chap. 8 of Ref.[9]). The leading term is given by virtual dipole transitions, while multipole corrections have recently been analyzed in Ref. [10]. The symmetry breaking induced by the wall leads to dipole-quadrupole mixing terms, which lead to admixtures to metastable levels [11,12]. While this effect has been analyzed in the non-retarded van-der-Waals regime [11,12], a fully quantum electrodynamic calculation of this effect would be of obvious interest. This fact is emphasized by the curious observation of a long-range, and conceivably super-long-range (micrometer-scale) interaction of metastable hydrogen 2S atoms with a conducting surface (the so-called Sokolov effect, see Refs. [13][14][15][16]). It is not far-fetched to suspect that this effect could be due to a quantum electrodynamically induced tail of the dipole-quadrupole mixing term in the atom-wall interaction. Namely, for the hydrogen 2S atom, the neighboring 2P 1/2 and 2P 3/2 levels are removed only by the Lamb shift and finestructure, respectively, while it is known that virtual states of lower energy can induce long-range tails in atomwall interactions, as well as in the Lamb shift between plates (see Refs. [17][18][19][20][21][22][23][24][25][26]). The large admixtures typically induced in atomic systems when a metastable level couples to nearly degenerate states of opposite parity suggest that a closer investigation of the hydrogen system is warranted. Atomic units with = 4πǫ 0 = 1 and c = 1/α are used throughout this Rapid Communication, where α is the fine-structure constant. The electron charge is explicitly denoted as e unless stated otherwise.Retardation of the atom-wall interaction.-The quan-